- Research article
- Open Access
- Open Peer Review
HbA1c levels in non-diabetic older adults – No J-shaped associations with primary cardiovascular events, cardiovascular and all-cause mortality after adjustment for confounders in a meta-analysis of individual participant data from six cohort studies
- Ben Schöttker1, 2Email authorView ORCID ID profile,
- W. Rathmann3,
- C. Herder4, 5,
- B. Thorand6,
- T. Wilsgaard7,
- I. Njølstad7,
- G. Siganos8,
- E. B. Mathiesen8,
- K. U. Saum1,
- A. Peasey9,
- E. Feskens10,
- P. Boffetta11, 12,
- A. Trichopoulou12,
- K. Kuulasmaa13,
- F. Kee14,
- H. Brenner1 and
- on behalf of the CHANCES group
- Received: 18 November 2015
- Accepted: 26 January 2016
- Published: 11 February 2016
Abstract
Background
To determine the shape of the associations of HbA1c with mortality and cardiovascular outcomes in non-diabetic individuals and explore potential explanations.
Methods
The associations of HbA1c with all-cause mortality, cardiovascular mortality and primary cardiovascular events (myocardial infarction or stroke) were assessed in non-diabetic subjects ≥50 years from six population-based cohort studies from Europe and the USA and meta-analyzed. Very low, low, intermediate and increased HbA1c were defined as <5.0, 5.0 to <5.5, 5.5 to <6.0 and 6.0 to <6.5 % (equals <31, 31 to <37, 37 to <42 and 42 to <48 mmol/mol), respectively, and low HbA1c was used as reference in Cox proportional hazards models.
Results
Overall, 6,769 of 28,681 study participants died during a mean follow-up of 10.7 years, of whom 2,648 died of cardiovascular disease. Furthermore, 2,493 experienced a primary cardiovascular event. A linear association with primary cardiovascular events was observed. Adjustment for cardiovascular risk factors explained about 50 % of the excess risk and attenuated hazard ratios (95 % confidence interval) for increased HbA1c to 1.14 (1.03–1.27), 1.17 (1.00–1.37) and 1.19 (1.04–1.37) for all-cause mortality, cardiovascular mortality and cardiovascular events, respectively. The six cohorts yielded inconsistent results for the association of very low HbA1c levels with the mortality outcomes and the pooled effect estimates were not statistically significant. In one cohort with a pronounced J-shaped association of HbA1c levels with all-cause and cardiovascular mortality (NHANES), the following confounders of the association of very low HbA1c levels with mortality outcomes were identified: race/ethnicity; alcohol consumption; BMI; as well as biomarkers of iron deficiency anemia and liver function. Associations for very low HbA1c levels lost statistical significance in this cohort after adjusting for these confounders.
Conclusions
A linear association of HbA1c levels with primary cardiovascular events was observed. For cardiovascular and all-cause mortality, the observed small effect sizes at both the lower and upper end of HbA1c distribution do not support the notion of a J-shaped association of HbA1c levels because a certain degree of residual confounding needs to be considered in the interpretation of the results.
Keywords
- Glycated hemoglobin
- Cardiovascular disease
- Myocardial infarction
- Stroke
- Mortality
- Cohort study
- Meta-analysis
Background
Glycated hemoglobin (HbA1c) is a biomarker for impaired glucose metabolism and indicates the average blood glucose concentration over the previous 2–3 months [1]. Non-diabetic subjects with increased HbA1c, often termed “pre-diabetes”, do not only have a higher risk for the development of manifest diabetes mellitus but also for cardiovascular events and all-cause mortality [2–5]. However, it is unclear how much of the excess risk is contributed by the impaired glucose metabolism and how much by simultaneously increased cardiovascular risk factor levels [6, 7].
- 1)
Underweight (as part of the frailty syndrome in older non-diabetics [19])
- 2)
Inflammation (caused by frequent asymptomatic hypoglycemic episodes [20])
- 3)
Anemia with or without iron deficiency (because of abnormalities of erythrocyte indices [21] and potentially correlated hemoglobin and HbA1c values [22])
- 4)
High alcohol consumption (inhibiting the gluconeogenesis in the liver [23] and shortening the red blood cell lifespan)
- 5)
Liver disease (as a result of high alcohol consumption [17])
- 6)
Chronic renal failure (reduced red blood cell lifespan and increased carbamylated hemoglobin affect the accuracy of HbA1c measurements [24, 25])
- 7)
Hematologic differences according to race/ethnicity (non-Hispanic black people have a special hematologic profile [26] and have more frequently very low HbA1c levels than people of other race/ethnicity [27]).
The objective of this meta-analysis of individual participant data is to investigate whether there is a J-shaped association of HbA1c levels with cardiovascular events, cardiovascular mortality and all-cause mortality in non-diabetic older adults and to explore potential explanations for a potentially increased risk at very low HbA1c levels.
Methods
Study design and study population
This investigation is a meta-analysis of individual participant data of six population-based cohort studies: Tromsø (Norway); ELSA (UK); NHANES (USA); ESTHER (Germany); KORA (Germany); and SHIP (Germany). Details of each study’s acronym, recruitment procedure and data collection are given in Additional file 1. All variables were harmonized in the framework of the Consortium on Health and Ageing: Network of Cohorts in Europe and the United States (CHANCES; www.chancesfp7.eu).
Ethics, consent and permissions
The included studies have been approved by local ethics committees (see Additional file 1). Written informed consent has been obtained from all participants included in the analyzed studies and the studies are being conducted in accordance with the Declaration of Helsinki.
Inclusion and exclusion criteria
Baseline characteristics of participants without diabetes mellitus and number of events during follow-up of included cohort studies
Baseline characteristic | Unit | ESTHER | SHIP | ELSA | Tromsø | KORA | NHANES |
---|---|---|---|---|---|---|---|
Baseline years for this analysis | 2000–2002 | 1997–2001 | 2004–2005 | 1994–1995 | 1999–2001 | 1988–1994 | |
Total sample size | 7,982 | 1,777 | 5,262 | 6,045 | 1,850 | 5,778 | |
Age | Years | 61.8 (6.6) | 63.4 (8.3) | 65.8 (9.7) | 61.6 (7.0) | 61.7 (7.1) | 67.9 (11.0) |
Age ≥65 years | % | 2,481 (31.1) | 733 (41.3) | 2,410 (45.8) | 1,958 (32.4) | 645 (34.9) | 3,157 (54.6) |
Male sex | % | 3,476 (43.6) | 905 (50.9) | 2,334 (44.4) | 2,550 (42.2) | 929 (50.2) | 2,774 (48.0) |
Race/ethnicity | |||||||
Non-Hispanic white | % | N/A | N/A | N/A | N/A | N/A | 3,428 (60.3) |
Non-Hispanic black | % | N/A | N/A | N/A | N/A | N/A | 1,088 (18.8) |
Mexican-American | % | N/A | N/A | N/A | N/A | N/A | 997 (17.3) |
Other | % | N/A | N/A | N/A | N/A | N/A | 211 (3.7) |
School education | |||||||
≤9 years | % | 5,743 (73.5) | 975 (55.2) | 968 (19.0) | 3,228 (53.8) | 322 (17.5) | 2,238 (39.1) |
10–12 years | % | 1,671 (21.4) | 443 (25.1) | 3,137 (61.6) | 1,773 (29.5) | 1,156 (62.7) | 2,147 (37.5) |
≥13 years | % | 398 (5.1) | 348 (19.7) | 992 (19.5) | 1,005 (16.7) | 367 (19.9) | 1,343 (23.5) |
BMI | kg/m2 | 27.3 (4.2) | 28.2 (4.4) | 27.5 (4.6) | 25.9 (3.8) | 28.1 (4.2) | 26.8 (5.2) |
BMI category | |||||||
Underweight | % | 146 (1.8) | 29 (1.6) | 141 (2.8) | 239 (4.0) | 21 (1.1) | 361 (6.3) |
Optimal weight | % | 2,249 (28.2) | 389 (21.9) | 1,356 (26.9) | 2,360 (39.1) | 389 (21.2) | 1,835 (31.9) |
Overweight | % | 3,816 (47.9) | 832 (46.9) | 2,247 (44.6) | 2,603 (43.2) | 907 (49.3) | 2,251 (39.2) |
Obese | % | 1.763 (22.1) | 525 (29.6) | 1,296 (25.7) | 831 (13.8) | 522 (28.4) | 1,303 (22.7) |
Smoking | |||||||
Never | % | 3,974 (51.1) | 942 (54.6) | 2,390 (45.8) | 1,949 (32.3) | 969 (53.4) | 2,583 (44.7) |
Former | % | 2,479 (31.9) | 486 (28.2) | 1,922 (36.8) | 2,184 (36.2) | 568 (31.3) | 1,956 (33.9) |
Current | % | 1,319 (17.0) | 298 (17.3) | 905 (17.4) | 1,907 (31.6) | 279 (15.4) | 1,239 (21.4) |
Relative alcohol consumption | |||||||
Abstainer or low | % | 3,851 (53.2) | 848 (50.5) | 2,537 (50.1) | 2,394 (51.7) | 1,005 (54.6) | 3,410 (64.2) |
Moderate | % | 2,650 (36.6) | 655 (39.0) | 2,024 (39.9) | 1,745 (37.7) | 638 (34.6) | 1,325 (24.9) |
High | % | 741 (10.2) | 177 (10.5) | 508 (10.0) | 490 (10.6) | 199 (10.8) | 580 (10.9) |
Vigorous physical activity | % | 3,452 (43.4) | 905 (51.5) | 2,232 (42.4) | 2,043 (34.2) | 819 (44.5) | 2,204 (38.2) |
Total cholesterol | mmol/L | 5.7 (1.3) | 6.1 (1.2) | 6.0 (1.2) | 6.8 (1.3) | 6.3 (1.1) | 5.7 (1.1) |
HDL cholesterol | mmol/L | 1.4 (0.4) | 1.5 (0.4) | 1.5 (0.4) | 1.6 (0.4) | 1.5 (0.4) | 1.4 (0.4) |
Subclinical inflammation | % | 2,786 (35.5) | 492 (28.8) | 1,827 (35.0) | 1,079 (20.8) | 520 (28.4) | 2,187 (38.9) |
Serum creatinine | nmol/L | 79.8 (28.1) | 87.1 (18.5) | N/A | 67.6 (16.0) | 76.5 (21.3) | 101.6 (29.4) |
Albuminuria | % | 760 (9.6) | 331 (21.0) | N/A | 675 (12.8) | N/A | 1,166 (22.0) |
Hemoglobin | g/dL | N/A | 13.6 (1.2) | 14.3 (1.4) | 14.1 (1.1) | 14.4 (1.2) | 13.9 (1.4) |
Biomarkers of iron deficiency | |||||||
Ferritin | μg/L | N/A | N/A | N/A | N/A | N/A | 157 (168) |
Transferrin saturation | % | N/A | N/A | N/A | N/A | N/A | 25.2 (10.6) |
Erythrocyte protoporphyrin | μmol/L | N/A | N/A | N/A | N/A | N/A | 0.95 (0.56) |
Hypertension | |||||||
No hypertension | % | 3,270 (41.0) | 807 (45.5) | 2,526 (48.0) | 2,584 (42.8) | 849 (46.1) | 2,553 (44.2) |
Known hypertension or systolic blood pressure ≥140 to <160 mmHg | % | 3,915 (49.1) | 593 (33.4) | 2,313 (44.0) | 2,044 (33.8) | 778 (42.2) | 2,545 (44.1) |
Systolic blood pressure ≥160 mmHg | % | 796 (10.0) | 375 (21.1) | 423 (8.0) | 1,417 (23.4) | 216 (11.7) | 679 (11.8) |
History of MI or stroke | % | 511 (6.5) | 140 (7.9) | 398 (7.6) | 438 (7.3) | 97 (5.2) | 738 (12.8) |
Biomarkers of liver function | |||||||
GGT | U/L | N/A | N/A | N/A | N/A | N/A | 31.6 (45.5) |
AST | U/L | N/A | N/A | N/A | N/A | N/A | 21.9 (11.8) |
ALT | U/L | N/A | N/A | N/A | N/A | N/A | 14.7 (10.6) |
HbA1c | % | 5.5 (0.4) | 5.4 (0.5) | 5.4 (0.3) | 5.4 (0.4) | 5.6 (0.3) | 5.5 (0.5) |
Very low (<5.0 % (<31 mmol/mol)) | % | 476 (6.0) | 336 (18.9) | 345 (6.6) | 626 (10.4) | 63 (3.4) | 678 (11.7) |
Low (5.0 to <5.5 % (31 to <37 mmol/mol)) | % | 2,747 (34.4) | 546 (30.7) | 2,344 (44.6) | 2,719 (45.0) | 603 (32.6) | 1,994 (34.5) |
Intermediate (5.5 to <6.0 % (37 to <42 mmol/mol)) | % | 3,726 (46.7) | 583 (32.8) | 2,201 (41.8) | 2,323 (38.4) | 937 (50.7) | 2,272 (39.3) |
Increased (6.0 to <6.5 % (42 to <48 mmol/mol)) | % | 1,033 (12.9) | 312 (17.6) | 372 (7.1) | 377 (6.2) | 247 (13.4) | 834 (14.4) |
Follow-up | |||||||
All-cause mortality | |||||||
Total sample size | 7,981 | 1,777 | 5,253 | 6,045 | 1,850 | 5,775 | |
Cases (%) | 1,069 (13.4) | 298 (16.8) | 645 (12.3) | 1,704 (28.2) | 180 (9.7) | 2,873 (49.8) | |
Mean FUP (SD) | Years | 11.1 (2.1) | 9.4 (2.1) | 7.0 (1.3) | 14.0 (3.8) | 8.5 (1.3) | 11.4 (4.9) |
Cardiovascular mortality | |||||||
Total sample size | 7,943 | 1,761 | 5,113 | 5,987 | 1,843 | 5,721 | |
Cases (%) | 263 (3.3) | 90 (5.1) | 174 (3.0) | 707 (11.8) | 69 (3.7) | 1,345 (23.5) | |
Mean FUP (SD) | Years | 11.1 (2.0) | 9.4 (2.1) | 7.0 (1.3) | 14.1 (3.8) | 8.6 (1.3) | 11.4 (4.9) |
Cardiovascular events | |||||||
Total sample size | 7,233 | 1,637 | 4,462 | 5,602 | 1,556 | N/A | |
Cases (%) | 595 (8.2) | 65 (4.0) | 315 (7.1) | 1,386 (24.7) | 132 (8.5) | N/A | |
Mean FUP (SD) | Years | 7.1 (2.3) | 9.5 (2.0) | 5.3 (1.4) | 12.6 (4.8) | 8.2 (1.7) | N/A |
Outcome ascertainment
We assessed three outcomes: all-cause mortality; cardiovascular mortality; and primary cardiovascular events. The latter was defined by non-fatal MI, non-fatal stroke or cardiovascular death during follow-up, while subjects with a history of MI or stroke before baseline were excluded. Details about the assessment of the outcomes are outlined in the cohort descriptions (Additional file 1). In brief, all cohorts ascertained deaths by region- or country-wide registries. Data for incident non-fatal MI or stroke cases were available from all cohorts except NHANES. Diagnoses were based on medical records in Tromsø, ESTHER and KORA and on participant-reported physician diagnoses in ELSA and SHIP. If assessed, ICD-10 codes were used to ascertain cardiovascular mortality (I00–I99), MI (I21–I23) and stroke (I60–I69).
Measurement of HbA1c
All cohorts measured HbA1c with assays certified by the National Glycohemoglobin Standardization Program (NGSP), which are traceable to the assay of the Diabetes Control and Complications Trial (DCCT). Details about the assays are given in the cohort descriptions (Additional file 1).
Covariate assessment
Socio-demographic, lifestyle, anthropometric and history of disease data were assessed by self-administered questionnaires or in interviews. In addition to self-reported information, some studies measured weight and height and validated the history of MI or stroke by consulting medical records or registries (Additional file 1: Table S1). If measured anthropometric data or validated diagnoses were available, they were used in the analysis and self-reported information was only used to fill missing information. With a modification of the underweight category, BMI categories of the World Health Organization (WHO) were applied to define underweight (<20 kg/m2), optimal BMI (20 to <25 kg/m2), overweight (25 to <30 kg/m2) and obesity (≥30 kg/m2). The underweight category was extended for our population of older adults because it has been previously shown that mortality is already increased at BMI <20 kg/m2 in individuals aged ≥65 years [29]. Total cholesterol, high-density lipoprotein (HDL) cholesterol, C-reactive protein (CRP), serum creatinine, urinary albumin and blood hemoglobin were measured for the total cohorts by routine methods in central laboratories cooperating with the study centers. Serum creatinine was not assessed in ELSA, blood hemoglobin was not measured in ESTHER and urinary albumin was not determined in ELSA and KORA. Subclinical inflammation was defined by CRP ≥3 mg/L [30] and albuminuria by urinary albumin ≥20 mg/L [31]. Biomarkers of liver function (alanine transferase (ALT), aspartate transferase (AST) and gamma-glutamyl transferase (GGT)) and iron deficiency (ferritin, transferrin saturation and erythrocyte protoporphyrin [32]) were only utilized from NHANES because this was the only study to assess all such indices. The analytical methods have been described elsewhere [32, 33]. Race/ethnicity was recorded in NHANES by four categories: non-Hispanic white; non-Hispanic black; Mexican-American; and other. The European studies included almost exclusively Caucasians and further differentiation of race/ethnicity in these cohorts was waived.
The different school-leaving qualifications of the countries were translated into the number of school years attended and three categories of education were devised (≤9 years, 10–12 years and ≥13 years). Reported average amounts of consumed wine, beer and spirits were converted into grams of pure ethanol per day and summed. Although the WHO reports comparable figures of consumed alcohol volumes per inhabitant in the European Union and United States [34], the calculated numbers from the cohorts diverged. In order to further standardize alcohol consumption, cohort and sex-specific percentiles (pct.) were calculated according to the average daily ethanol consumption and the following three categories of relative alcohol consumption were built: abstainer or low alcohol consumption (≤50th pct.); moderate alcohol consumption (>50th to <90th pct.); and high alcohol consumption (≥90th pct.). Vigorous physical activity was harmonized as a dichotomous variable (Yes or No) from questions regarding whether study participants perform physical activity that causes sweating (e.g. sports).
Statistical analyses
Baseline characteristics of subjects without diabetes mellitus by very low and low HbA1c levels and multivariable adjusted odds ratios for associations of characteristics with very low HbA1c levels. Pooled data from six cohort studies
Characteristic | Unit | Weighted mean or proportion (%) (95 % CI) | Pooled odds ratio (95 % CI) | |
---|---|---|---|---|
Very low HbA1c | Low HbA1c | |||
(<5.0 %) | (5.0 to <5.5 %) | |||
(<31 mmol/mol) | (31 to <37 mmol/mol) (reference group) | |||
Age | Years | 61.1 (60.8–61.4) | 62.2 (62.1–62.4) | 0.77 (0.72; 0.81) per 10 years |
Male sex | % | 49.4 (47.4–51.3) | 45.0 (44.1–46.0) | 1.15 (0.93; 1.42)a |
Race/ethnicityb | ||||
Non-Hispanic white | % | 63.3 | 70.0 | Ref |
Non-Hispanic black | % | 20.2 | 11.8 | 1.60 (1.21; 2.12) |
Mexican-American | % | 13.7 | 15.1 | 0.88 (0.65; 1.17) |
Other | % | 2.8 | 3.0 | 0.88 (0.51; 1.53) |
School education | ||||
≤9 years | % | 45.5 (43.4–47.6) | 45.9 (44.9–46.9) | Ref |
10–12 years | % | 36.3 (34.3–38.2) | 39.1 (38.1–40.0) | 0.98 (0.91; 1.05) |
≥13 years | % | 21.4 (19.8–23.1) | 19.0 (18.3–19.8) | 1.00 (0.92; 1.09) |
BMI category | ||||
Underweight | % | 4.5 (3.7–5.4) | 3.9 (3.5–4.3) | 1.13 (0.94; 1.35) |
Optimal weight | % | 36.2 (34.3–38.1) | 34.6 (33.7–35.5) | Ref |
Overweight | % | 44.6 (42.7–46.6) | 43.6 (42.7–44.6) | 1.02 (0.93; 1.11) |
Obese | % | 16.0 (14.6–17.5) | 19.2 (18.5–20.0) | 0.82 (0.73; 0.92) |
Smoking | ||||
Never | % | 46.4 (44.4–48.4) | 45.8 (44.8–46.7) | Ref |
Former | % | 35.3 (33.5–37.2) | 34.8 (33.9–35.7) | 0.94 (0.85; 1.05) |
Current | % | 19.6 (18.0–21.3) | 20.0 (19.5–21.1) | 0.70 (0.61; 0.81) |
Relative alcohol consumption | ||||
Abstainer or low | % | 48.0 (45.9–50.0) | 50.9 (49.9–51.9) | Ref |
Moderate | % | 36.8 (34.8–38.8) | 37.4 (36.4–38.3) | 0.93 (0.87; 1.01) |
High | % | 15.7 (14.3–17.3) | 12.0 (11.4–12.7) | 1.21 (1.10; 1.33) |
Vigorous physical activity | % | 43.6 (41.6–45.5) | 42.3 (41.4–43.3) | 0.95 (0.86; 1.05) |
Total cholesterol | mmol/L | 5.81 (5.76–5.86) | 6.03 (6.01–6.05) | 0.85 (0.81; 0.88) per 1 mmol/L |
HDL cholesterol | mmol/L | 1.51 (1.49–1.53) | 1.52 (1.51–1.53) | 1.13 (1.00; 1.27) per 1 mmol/L |
Subclinical inflammation | % | 27.1 (25.3–28.9) | 28.1 (27.2–29.0) | 1.02 (0.92; 1.14) |
Serum creatinine | nmol/L | 76.2 (75.5–77.0) | 76.4 (76.0–76.8) | 1.00 (0.98; 1.03) per 10 nmol/L |
Albuminuria | % | 15.2 (13.6–17.0) | 13.3 (12.5–14.1) | 0.93 (0.79; 1.10) |
Hemoglobin | g/dL | 14.13 (14.07–14.18) | 14.16 (14.14–14.19) | 1.06 (0.89; 1.25)a per 1 g/dL |
Biomarkers of iron deficiencyb | ||||
Ferritin | μg/L | 210 (268) | 152 (148) | 1.12 (1.06; 1.18) per 100 μg/L |
Transferrin saturation | % | 27.5 (13.1) | 26.0 (10.6) | 1.13 (1.03; 1.23) per 10 % |
Erythrocyte protoporphyrin | μmol/L | 1.09 (1.17) | 0.93 (0.42) | 1.25 (1.14; 1.38) per 0.5 μmol/L |
Hypertension | ||||
No hypertension | % | 47.0 (45.1–49.0) | 47.0 (46.1–48.0) | Ref |
Known hypertension or systolic blood pressure ≥140 to <160 mmHg | % | 39.8 (37.9–41.7) | 40.2 (39.3–41.1) | 1.02 (0.96; 1.10) |
Systolic blood pressure ≥160 mmHg | % | 14.2 (12.9–15.7) | 14.0 (13.4–14.7) | 1.03 (0.94; 1.13) |
History of MI or stroke | % | 7.9 (6.9–9.1) | 7.1 (6.6–7.6) | 1.04 (0.87; 1.24) |
Biomarkers of liver functionb | ||||
GGT | U/L | 44.3 (92.9) | 29.9 (40.3) | 1.02 (1.00; 1.04) per 10 U/L |
AST | U/L | 24.7 (18.7) | 22.0 (11.9) | 1.12 (1.00; 1.26) per 10 U/L |
ALT | U/L | 16.0 (13.4) | 14.6 (11.3) | 0.86 (0.75; 0.996) per 10 U/L |
For being plausible explanations for an association of very low HbA1c levels with mortality or cardiovascular outcomes, known cardiovascular/mortality risk factors should be associated with very low HbA1c levels in the logistic regression model and with mortality outcomes in the same direction (i.e. being a risk factor for both very low HbA1c and mortality). Risk factors of interest were underweight, subclinical inflammation, blood hemoglobin concentration (biomarker of anemia), ferritin, transferrin saturation or erythrocyte protoporphyrin (biomarkers of iron deficiency), alcohol consumption, albuminuria (biomarker of renal function), serum creatinine (biomarker of renal function), AST, ALT or GGT (biomarkers of liver function). Furthermore, adding the variables to an age- and sex-adjusted model for mortality should attenuate the observed association of very low HbA1c with the outcome. The described analyses to identify plausible explanatory variables were only conducted in the NHANES because only this cohort assessed all variables of interest listed above.
For longitudinal analyses, Cox proportional hazards regression models were utilized after the proportional hazards assumption was tested with Schoenfeld residuals (which was fulfilled). We compared HbA1c categories with respect to the outcomes all-cause mortality, cardiovascular mortality and primary cardiovascular events in a “simple”, age- and sex-adjusted model and the “full” model (see list of variables above, except no adjustment for history of MI or stroke for the outcome “primary cardiovascular events” because of exclusions).
We used a two-step approach: we first analyzed the single studies and pooled the results thereafter by meta-analysis. Meta-analyses were conducted with the statistical software Comprehensive Meta-Analysis 2.0 (Biostat, Englewood, NJ, USA). A one-step approach was not possible because UiT The Arctic University of Norway did not consent to send individual data of the Tromsø study to the analyzing center in Heidelberg, Germany. This was also the reason why a dose-response analysis with restricted cubic splines could not be conducted in a pooled data set. Instead, such curves were exemplarily retrieved from the NHANES with five a priori defined knots at HbA1c levels of 4.5 %, 5 %, 5.5 % and 6.25 % and 5.25 % as the reference [35]. Results from the NHANES are a good approximation for the results from the total consortium because the NHANES assessed all potential confounders and dominated the meta-analyses with its high case numbers.
In meta-analyses, statistical heterogeneity among the studies was examined with Cochrane’s Q test and the I2 statistic. Fixed effects models were reported unless significant heterogeneity was present, taking only the sample size of the cohorts into account. In the few occasions of significant heterogeneity, this was indicated and random effects model results were reported, taking the sample size of the cohorts and the between-study variance into account. In the fixed effects model, the weight of the studies was expressed as the inverse of the variance of the logarithm of the estimated hazard ratio (HR). In the random effects model, a variation of the inverse-variance method, invented by DerSimonian and Laird, was applied, which adjusts for the heterogeneity in the meta-analysis and produces less precise pooled effect estimates than the fixed effects model [36].
Subgroup analyses were carried out for both sexes and two age-groups (<65 and ≥65 years). Subgroup analyses were restricted to cohorts that could contribute to subgroups with a sufficient number of events. In sensitivity analyses, cohorts with diagnoses of non-fatal events based on self-reported physician diagnoses (ELSA and SHIP) were excluded from analyses of primary cardiovascular events. If not stated otherwise, analyses were conducted with SAS 9.3 (SAS Institute Inc., Cary, NC, USA).
Multiple imputation was employed to impute the number of missing baseline covariate values shown in Additional file 1: Table S2. The proportion of missing values was below 5 % for most variables, between 5 % and 15 % on seven occasions and higher than 15 % on three occasions (HDL cholesterol in ESTHER (37.9 %), alcohol consumption in Tromsø (23.4 %) and GGT in the NHANES (25.9 %)). To the best of our knowledge, data were missing at random, which was the assumption of the multiple imputation. Separately for the cohorts, 20 complete data sets were imputed with the SAS 9.3 procedure “PROC MI”, using the Markov chain Monte Carlo method. Variables from the “full” model were used for the imputation model but the outcomes were not included. Variables were modelled continuously if possible and the logarithm was taken if they were not normally distributed (employed for BMI, systolic blood pressure, HDL cholesterol, CRP, urinary albumin, serum creatinine and GGT). This log-transformation of variables was applied in the multiple imputation process only. All multivariable analyses were performed in the 20 imputed data sets and results of the individual data sets were combined by the SAS 9.3 procedure “PROC MIANALYZE”, taking the variation between the results of the imputed data sets into account.
Results
Baseline characteristics of subjects without diabetes mellitus by increased and low HbA1c levels and multivariable adjusted odds ratios for associations of characteristics with increased HbA1c levels. Pooled data from six cohort studies
Characteristic | Unit | Weighted mean or proportion (%) (95 % CI) | Pooled odds ratio (95 % CI) | |
---|---|---|---|---|
Increased HbA1c | Low HbA1c | |||
(6.0 to <6.5 %) | (5.0 to <5.5 %) | |||
(42 to <48 mmol/mol) | (31 to <37 mmol/mol) (reference group) | |||
Age | Years | 64.5 (64.2–64.7) | 62.2 (62.1–62.4) | 1.41 (1.32; 1.49) per 10 years |
Male sex | % | 47.0 (45.2–48.7) | 45.0 (44.1–46.0) | 0.99 (0.88; 1.11) |
Race/ethnicityb | ||||
Non-Hispanic white | % | 44.0 | 70.0 | Ref |
Non-Hispanic black | % | 33.8 | 11.8 | 5.92 (4.56; 7.68) |
Mexican-American | % | 17.8 | 15.1 | 2.11 (1.62; 2.77) |
Other | % | 4.4 | 3.0 | 2.69 (1.69; 4.28) |
School education | ||||
≤9 years | % | 55.6 (53.7–57.5) | 45.9 (44.9–46.9) | Ref |
10–12 years | % | 32.7 (30.9–34.4) | 39.1 (38.1–40.0) | 0.97 (0.90; 1.04) |
≥13 years | % | 14.6 (13.3–16.0) | 19.0 (18.3–19.8) | 0.99 (0.90; 1.08) |
BMI category | ||||
Underweight or low weight | % | 3.2 (2.5–3.9) | 3.9 (3.5–4.3) | 0.81 (0.66; 0.99) |
Optimal weight | % | 20.3 (18.9–21.8) | 34.6 (33.7–35.5) | Ref |
Overweight | % | 45.3 (43.5–47.0) | 43.6 (42.7–44.6) | 1.02 (0.92; 1.14) |
Obese | % | 32.4 (30.8–34.1) | 19.2 (18.5–20.0) | 1.61 (1.43; 1.81) |
Smoking | ||||
Never | % | 44.5 (42.7–46.3) | 45.8 (44.8–46.7) | Ref |
Former | % | 31.9 (30.2–33.5) | 34.8 (33.9–35.7) | 1.03 (0.92; 1.16) |
Current | % | 24.1 (22.6–25.7) | 20.0 (19.5–21.1) | 1.73 (1.53; 1.96) |
Relative alcohol consumption | ||||
Abstainer or low | % | 62.9 (61.1–64.7) | 50.9 (49.9–51.9) | Ref |
Moderate | % | 30.5 (28.8–32.2) | 37.4 (36.4–38.3) | 1.05 (0.96; 1.13) |
High | % | 6.9 (6.0–7.9) | 12.0 (11.4–12.7) | 0.74 (0.66; 0.83) |
Vigorous physical activity | % | 36.0 (34.3–37.7) | 42.3 (41.4–43.3) | 0.93 (0.85; 1.02) |
Total cholesterol | mmol/L | 6.08 (6.03–6.12) | 6.03 (6.01–6.05) | 1.18 (1.14; 1.22) per 1 mmol/L |
HDL cholesterol | mmol/L | 1.36 (1.34–1.37) | 1.52 (1.51–1.53) | 0.62 (0.54; 0.71) per 1 mmol/L |
Subclinical inflammation | % | 44.7 (42.9–46.4) | 28.1 (27.2–29.0) | 1.47 (1.34; 1.61) |
Serum creatinine | nmol/L | 85.4 (84.5–86.3) | 76.4 (76.0–76.8) | 1.04 (1.00; 1.08) a per 10 nmol/L |
Albuminuria | % | 21.2 (19.6–23.0) | 13.3 (12.5–14.1) | 1.27 (1.11; 1.45) |
Hemoglobin | g/dL | 13.98 (13.92–14.04) | 14.16 (14.14–14.19) | 0.86 (0.77; 0.96) a per 1 g/dL |
Biomarkers of iron deficiencyb | ||||
Ferritin | μg/L | 165 (170) | 152 (148) | 0.97 (0.91; 1.03) per 100 μg/L |
Transferrin saturation | % | 23.8 (10.1) | 26.0 (10.6) | 0.85 (0.77; 0.95) per 10 % |
Erythrocyte protoporphyrin | μmol/L | 0.94 (0.44) | 0.93 (0.42) | 1.02 (0.91; 1.15) per 0.5 μmol/L |
Hypertension | ||||
No hypertension | % | 34.7 (33.1–36.4) | 47.0 (46.1–48.0) | Ref |
Known hypertension or systolic blood pressure ≥140 to <160 mmHg | % | 49.5 (47.8–51.3) | 40.2 (39.3–41.1) | 1.08 (1.01; 1.15) |
Systolic blood pressure ≥160 mmHg | % | 17.1 (15.8–18.5) | 14.0 (13.4–14.7) | 1.10 (1.01; 1.20) |
History of MI or stroke | % | 11.8 (10.7–13.0) | 7.1 (6.6–7.6) | 1.32 (1.03; 1.70) a |
Biomarkers of liver functionb | ||||
GGT | U/L | 33.5 (36.0) | 29.9 (40.3) | 1.02 (0.99; 1.05) per 10 U/L |
AST | U/L | 21.5 (13.1) | 22.0 (11.9) | 0.81 (0.70; 0.94) per 10 U/L |
ALT | U/L | 15.0 (11.2) | 14.6 (11.3) | 1.25 (1.08; 1.45) per 10 U/L |
In contrast to the results for very low HbA1c, increased HbA1c levels were generally significantly associated with characteristics that are associated with adverse cardiovascular outcomes and mortality; i.e. older age, other than non-Hispanic white race/ethnicity, obesity, current smoking, high total cholesterol, low HDL cholesterol, subclinical inflammation, high serum creatinine, albuminuria, low blood hemoglobin concentration, low transferrin saturation, hypertension, a history of MI or stroke and high ALT, with the exceptions of high alcohol consumption, underweight, high GGT and high AST (Table 3). Results for the intermediate group point in the same direction for these variables as outlined for the group with increased HbA1c but with lower effect estimates (Additional file 1: Table S3). Overall, a consistent step-wise increase in the burden of cardiovascular and mortality risk factors was observed with increasing HbA1c levels in subjects without diabetes mellitus. The exceptions were high alcohol consumption associated with a higher proportion in subjects with very low HbA1c levels (15.7 %) than in subjects with low (12.0 %), intermediate (9.0 %) and increased HbA1c (6.9 %), the biomarkers of liver function (GGT, AST, ALT) and ferritin levels with highest values in subjects with very low HbA1c levels and second highest values in subjects with increased HbA1c (U-shaped associations) as well as erythrocyte protoporphyrin levels, which were increased in the very low HbA1c group, only.
For the longitudinal analyses, the mean follow-up time varied by cohort and outcome between 5.3 and 14.1 years (Table 1, bottom). In summary, 6,769 of 28,681 study participants died during a mean follow-up of 10.7 years (standard deviation (SD) 3.6) of whom 2,648 died of cardiovascular disease. Of those 20,490 study participants without a history of MI or stroke at baseline, 2,493 experienced a primary cardiovascular event during a mean follow-up of 8.5 years.
Dose-response relationship of meta-analyzed associations of HbA1c levels with (a) all-cause mortality, (b) cardiovascular mortality and (c) cardiovascular outcomes in subjects without diabetes mellitus with increasing adjustment for potential confounders. Crosses, point estimates of “simple” model; circles with 95 % confidence intervals, effect estimates of “full” model. Reference group: HbA1c 5.0 to <5.5 % (31 to <37 mmol/mol)
Dose-response relationship of HbA1c levels with all-cause mortality in subjects without diabetes mellitus in the NHANES with (a) adjustment for age and sex and (b) adjustment for all potential confounders (“full” model including biomarkers of iron deficiency and liver function). Solid line, estimation; points in solid lines, knots; horizontal dashed line, null effect value (hazard ratio = 1); curved dashed lines, boundaries of 95 % confidence interval band of estimation
Attenuation of strength of the association of very low HbA1c levels (<5 % (<31 mmol/mol)) with all-cause and cardiovascular mortality by adding potential confounders to the “simple” model in the NHANES
Model | HR (95 % CI)a | Attenuation for both outcomes | |
---|---|---|---|
All-cause mortality | CV mortality | ||
Age + sex (“simple” model) | 1.201 (1.057; 1.328) | 1.221 (1.005; 1.409) | N/A |
Age + sex + race/ethnicity | 1.184 (1.042; 1.341) | 1.203 (0.989; 1.453) | Yes |
Age + sex + alcohol consumption | 1.192 (1.051; 1.353) | 1.212 (1.000; 1.469) | Yes |
Age + sex + BMI | 1.172 (1.033; 1.330) | 1.197 (0.988; 1.450) | Yes |
Age + sex + physical activity | 1.208 (1.065; 1.370) | 1.231 (1.016; 1.491) | No |
Age + sex + hemoglobin | 1.166 (1.027; 1.324) | 1.188 (0.980; 1.440) | Yes |
Age + sex + ferritin | 1.193 (1.052; 1.354) | 1.211 (0.999; 1.467) | Yes |
Age + sex + total cholesterol | 1.192 (1.051; 1.353) | 1.226 (1.012; 1.486) | No |
Age + sex + erythrocyte protoporphyrin | 1.177 (1.036; 1.337) | 1.208 (0.995; 1.466) | Yes |
Age + sex + GGT | 1.178 (1.038; 1.337) | 1.206 (0.995; 1.462) | Yes |
Age + sex + AST | 1.199 (1.057; 1.361) | 1.222 (1.009; 1.481) | No |
Age + sex + ALT | 1.199 (1.057; 1.360) | 1.215 (1.003; 1.472) | Yes |
Age + sex + race/ethnicity + alcohol consumption + BMI + ferritin + erythrocyte protoporphyrin + GGT + ALT | 1.081 (0.950; 1.230) | 1.106 (0.909; 1.345) | |
All variables of Table 1 | 1.103 (0.968; 1.256) | 1.120 (0.921; 1.363) |
In the meta-analysis of all cohorts, HRs for the comparison of subjects with increased HbA1c levels and subjects with low levels were strongly attenuated by increasing adjustment for cardiovascular risk factors but remained statistically significant: all-cause mortality (1.14 [1.03; 1.27]; Additional file 1: Table S4); cardiovascular mortality (1.17 [1.00; 1.37]; P <0.05; Additional file 1: Table S5); and cardiovascular events (1.19 [1.04; 1.37]; Additional file 1: Table S6). The covariates that were most responsible for the attenuations were smoking, CRP and the renal function biomarkers serum creatinine and albuminuria (data not shown).
No association of intermediate HbA1c levels with any of the outcomes was observed in the “full” model (HR point estimates between 1.00 and HR 1.03; Additional file 1: Table S4–S6). Statistically significant heterogeneity was only observed in meta-analyses on very low HbA1c levels and mortality outcomes with associations indicating a potentially increased mortality risk at very low HbA1c levels (HR >1.10) in three cohorts, a potential protective effect (HR <0.90) in one cohort and potentially no effect (HR 0.90–1.10) in two cohorts (Additional file 1: Table S4).
Age-stratified analyses of the associations of HbA1c levels with mortality and cardiovascular outcomes in subjects without diabetes mellitus
Outcome/stratum | Very low HbA1c (<5.0 %) (<31 mmol/mol) | Low HbA1c (5.0 to <5.5 %) (31 to <37 mmol/mol) | Intermediate HbA1c (5.5 to <6.0 %) (37 to <42 mmol/mol) | Increased HbA1c (6.0 to <6.5 %) (42 to <48 mmol/mol) | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
ntotal | ncases | IRa | HR (95 % CI)b | ntotal | ncases | IRa | HR | ntotal | ncases | IRa | HR (95 % CI)b | ntotal | ncases | IRa | HR (95 % CI)b | |
All-cause mortality | ||||||||||||||||
50–64 years | 1,688 | 205 | 10.0 | 0.95 (0.81; 1.12) | 6,563 | 769 | 9.8 | Ref | 6,336 | 761 | 10.1 | 0.91 (0.82; 1.01) | 1,514 | 272 | 15.8 | 1.18 (1.02; 1.37) |
≥65 years | 632 | 314 | 53.3 | 1.13 (1.01; 1.27) | 3,779 | 1,556 | 42.6 | Ref | 4,767 | 1,978 | 43.3 | 1.06 (0.99; 1.13) | 1,413 | 723 | 50.4 | 1.14 (1.04; 1.26) |
Cardiovascular mortality | ||||||||||||||||
50–64 years | 1,218 | 61 | 3.7 | 0.99 (0.74; 1.33) | 4,824 | 210 | 3.2 | Ref | 4,915 | 220 | 3.4 | 0.93 (0.76; 1.13) | 1,194 | 78 | 5.3 | 1.14 (0.87; 1.50) |
≥65 years | 548 | 155 | 28.4 | 1.33 (0.86; 2.04)c | 2,574 | 589 | 20.9 | Ref | 3,343 | 753 | 21.2 | 1.07 (0.82; 1.39)c d | 1,035 | 249 | 24.2 | 1.18 (1.01; 1.38) |
Cardiovascular events | ||||||||||||||||
50–64 years | 1,276 | 77 | 5.1 | 0.76 (0.60; 0.97) | 5,626 | 456 | 8.1 | Ref | 5,470 | 507 | 9.6 | 1.09 (0.96; 1.25) | 1,165 | 114 | 10.6 | 1.18 (0.95; 1.47) |
≥65 years | 409 | 82 | 22.4 | 1.15 (0.90; 1.46) | 2,491 | 488 | 22.8 | Ref | 3,207 | 598 | 23.0 | 0.99 (0.88; 1.12) | 846 | 171 | 26.7 | 1.19 (0.98; 1.45) |
Results also did not change when biomarkers of liver function were modelled in quintiles or dichotomously based on clinical cut-points indicating potential liver disease (data not shown). Finally, excluding cohorts with self-reported MI or stroke information from analyses did not change the overall results (data not shown).
Discussion
In this individual participant data meta-analysis of six prospective cohort studies in subjects without diabetes mellitus, a linear association of HbA1c levels with primary cardiovascular events was observed. The observed effect estimates for increased HbA1c levels (6.0 to <6.5 % (42 to <48 mmol/mol)) were strongly attenuated by adjustment for cardiovascular risk factors (mainly by adjustment for smoking, inflammatory status and renal function) but remained statistically significant for all three outcomes (primary cardiovascular events, all-cause mortality and cardiovascular mortality). At the lower end of HbA1c levels, cohorts of the consortium yielded inconsistent results for the mortality outcomes and the pooled effect estimate was not statistically significant. In one cohort with a pronounced J-shaped association of HbA1c levels with all-cause and cardiovascular mortality (NHANES), the following confounders of the association of very low HbA1c levels with mortality outcomes were identified: race/ethnicity; alcohol consumption; BMI; as well as biomarkers of iron deficiency anemia and liver function. The association of very low HbA1c levels with mortality outcomes also lost statistical significance in this cohort after adjusting for these confounders.
The observed increased cardiovascular risk and increased mortality of subjects without diabetes mellitus but with increased HbA1c (6.0 to <6.5 % (42 to <48 mmol/mol)) is in agreement with results from previous population-based cohort studies [5, 6, 11, 12, 16–18, 37–40]. This consistent finding from observational studies is supported by the fact that coronary atherosclerosis and plaque vulnerability are advanced in subjects with increased HbA1c levels even if they are below the threshold for a diabetes diagnosis [41]. However, we observed a strong attenuation of effect estimates by adjustment for conventional cardiovascular risk factors, which was also observed by others [5–7]. This attenuation could be explained by confounding mostly by smoking, CRP and renal function, factors which were associated with increased HbA1c. The fact that smokers have higher HbA1c levels than non-smokers or ex-smokers was also observed in another consortium of cohort studies [42]. Nevertheless, effect estimates remained statistically significant in comprehensively adjusted models, which could indicate a small independent contribution of impaired glucose metabolism, below the diagnostic threshold for diabetes mellitus, to the development of cardiovascular disease. However, the small effect estimates could also be simply due to residual confounding because it is impossible to perfectly adjust for all cardiovascular risk factors in observational studies. The majority of randomized controlled trials (RCTs) in non-diabetic subjects with increased HbA1c failed to observe significant effects when aiming to reduce the cardiovascular risk and mortality of these individuals [43]. However, the short average follow-up time of 3.75 years was a limitation of previous trials and further RCTs, with larger sample size and longer follow-up are required to explore the efficacy of non-drug and drug-based approaches to reduce the cardiovascular risk of non-diabetic subjects with increased HbA1c [43].
With respect to very low HbA1c in subjects without diabetes mellitus, this meta-analysis of the CHANCES consortium yielded inconsistent results for the outcomes all-cause and cardiovascular mortality and a consistent, albeit statistically non-significant decreased risk for primary cardiovascular events in subjects with an HbA1c <5 % (<31 mmol/mol). The latter contrasts with the meta-analysis of the ERFC that observed a significantly increased cardiovascular risk in subjects with an HbA1c <4.5 % (<26 mmol/mol) (HR 1.23 [1.02; 1.50]) compared with subjects with an HbA1c of 5 to <5.5 % (31 to <37 mmol/mol). However, the result for this HbA1c category was only based on 127 cardiovascular events from 24 studies. Our meta-analysis included 159 cardiovascular events from five studies in the lowest HbA1c category (<5 % (<31 mmol/mol)). Low numbers of events from single studies can affect the model stability and can result in high point estimates with wide confidence intervals. Therefore, despite the overall large sample size of this meta-analysis and the meta-analysis of the ERFC, the results for the lowest HbA1c category of both could be biased by low sample sizes and be random findings.
Because many previous studies [13–18] have observed an increased mortality of non-diabetics with very low HbA1c levels and this was also found in two out of six of the cohorts included in our meta-analysis (NHANES and ESTHER), we aimed to explore potential explanations in the NHANES because this study assessed all relevant variables that could confound the association of very low HbA1c levels and mortality. The strongest confounder was anemia, which was expected because hemoglobin and HbA1c concentrations are correlated [32]. The biomarkers of iron deficiency ferritin and erythrocyte protoporphyrin were also identified as confounders but whether iron-deficiency anemia, non-iron-deficiency anemia or both are of relevance for very low HbA1c levels needs to be determined by further studies because the underlying biology is complex [21]. Non-Hispanic black race/ethnicity was also an expected confounder because African-Americans, compared with white Americans, have a different hematologic profile including lower hemoglobin values [26]. High alcohol consumption and biomarkers of liver disease were further confounders, which could be explained by an inhibition of the gluconeogenesis in the liver [23] and a shortening the red blood cell lifespan. Very low HbA1c values can simply originate from everything that reduces the red blood cell lifespan because some time is needed for glucose and hemoglobin to interact and form glycosylated hemoglobin [24]. BMI also played a role but not as expected. Underweight was not associated with very low HbA1c levels and obesity was protective for very low HbA1c levels. The confounding for mortality could result from obesity that has been found to be protective for mortality at older age [44]. However, it is yet unclear whether this “obesity paradox” is due to statistical biases or has a plausible underlying biology [44]. In summary, from the hypotheses listed in the introduction, only subclinical inflammation and renal function were not confirmed as confounders for the association of very low HbA1c levels and mortality in the NHANES.
The first and, to our knowledge, only other study that aimed to discover potential mechanisms that could explain an increased risk of mortality in subjects with very low HbA1c levels did not find any attenuation of the strength of the association of very low HbA1c levels (<5.0 % (<31 mmol/mol)) with all-cause mortality after additional adjustment for diseases, weight measures, education, alcohol consumption, physical activity, smoking, hemoglobin, red blood cell mean corpuscular volume, fibrinogen and leukocyte count [17].
In our meta-analysis, a significant interaction was observed between very low HbA1c levels and age for the outcome “primary cardiovascular events”. To our knowledge, this is a novel finding but since it is from a subgroup analysis, further studies are required to corroborate this interaction with age. There is room for doubts, because this interaction was not significant for the other outcomes “all-cause mortality” and “cardiovascular mortality” and stratification by 5-year intervals in the NHANES also showed that there was no age-difference in the association of very low HbA1c levels with fatal outcomes.
When interpreting our results, the following limitations and strengths should be considered. The main limitation of this meta-analysis is the observational nature of the included prospective cohort studies. Despite adjustment for known potential confounders, we cannot rule out the possibility that the observed associations are confounded by other unmeasured factors (e.g. biomarkers of liver function and iron deficiency in cohorts other than NHANES and other unmeasured factors known to affect HbA1c assay test results, such as participation in endurance sport, late pregnancy and major blood loss [45]) or residual confounding by variables that could have been more precisely measured (e.g. physical activity). It can be expected that some residual confounding is present in the data and observed small effect sizes for increased HbA1c levels could be due to residual confounding despite the observed statistical significant associations. Furthermore, it is possible that people with pre-diabetes at baseline developed manifest diabetes mellitus in the first years of follow-up and experienced a cardiovascular outcome or death in later follow-up due to diabetes and not pre-diabetes. However, diabetes incidence information was not collected for this analysis and this could not be further elucidated.
Other limitations include the fact that non-fatal MI and stroke information was solely based on self-reported information in two cohorts but the overall results did not change when these cohorts were excluded in sensitivity analysis. Furthermore, other glucose measures (i.e. fasting glucose or measures based on an oral glucose tolerance test) were not included, which could have yielded different results. Furthermore, HbA1c was measured once whereas measurements at different time-points could have corrected better for intra-individual variation and random measurement errors. In addition, different HbA1c assays were applied in the cohorts but they were all traceable to the assay of the DCCT trial and therefore comparable.
Strengths of our study include the variety of cohorts from all over Europe and the United States, the overall large size enabling subgroup analyses for age and sex, almost complete mortality registry-based follow-ups and the common statistical analysis strategy. A particular advantage over previous studies is the adjustment for a large number of cardiovascular risk factors including biomarkers of inflammation, renal function, lipid metabolism, liver function and anemia.
Conclusions
In this meta-analysis of subjects without diabetes mellitus from six prospective cohort studies a linear association of HbA1c levels with primary cardiovascular events was observed. The observed effect estimates for increased HbA1c levels were strongly attenuated by adjustment for cardiovascular risk factors for all three outcomes (primary cardiovascular events, all-cause mortality and cardiovascular mortality). The cohorts yielded inconsistent results for the associations of very low HbA1c levels with mortality outcomes. For cardiovascular and all-cause mortality, the observed small effect sizes at both the lower and upper end of HbA1c distribution do not support the notion of a J-shaped association of HbA1c levels because a certain degree of residual confounding was likely present in the meta-analyses, which could not be adjusted for iron deficiency anemia and liver function as they were not assessed in most cohorts.
Availability of data and materials
No data will be shared because of data protection regulations.
Declarations
Acknowledgements
This analysis was part of the CHANCES project funded in the FP7 framework programme of DG-RESEARCH in the European Commission (grant no. 242244) and additional funding was obtained from the German Research Foundation (DFG; grant no. 616604). Further funding sources of participating cohorts are listed in the Additional file 1.
Contributors from the CHANCES group were the following: Annette Peters; Christa Meisinger; Andrea Schneider (all Institute of Epidemiology II, Helmholtz Zentrum München, German Research Center for Environmental Health, Neuherberg, Germany); Roberto Lorbeer (University Medicine Greifswald, Institute for Community Medicine, Greifswald, Germany); Bernd Holleczek (Saarland Cancer Registry, Saarbrücken, Germany); Wolfgang Koenig (Department of Internal Medicine II, Cardiology, University of Ulm Medical Center, Ulm, Germany); and Michael Roden (German Diabetes Center, Düsseldorf, Germany).
The authors and collaborators wish to thank the following peer reviewers of this manuscript for their constructive reviews and many suggestions that helped to improve the final version: Marjan Alssema (Unilever Research, Vlaardingen, the Netherlands and EMGO Institute for Health and Care Research, VU Medical Center, Amsterdam, The Netherlands); José María Mostaza (Servicio de Medicina Interna, Hospital Carlos III, Madrid, Spain); and Darren C. Greenwood (Division of Epidemiology and Biostatistics, University of Leeds, Leeds, UK).
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
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