Summary and interpretation of findings
A series of LCS models were estimated to test lag-leading and reciprocal relationships between weekly number of UK units consumed and mental health. In both minimally adjusted and fully adjusted models, it was found that a model in which mental health was specified as the leading indicator of change gave the best fit (Table 2, Table 3).Plotting the parameters of this model in a vector field (Figure 2) demonstrates the complex relationship between weekly alcohol consumption and mental health, and also helps to visualize the correlation between the mental health intercept and alcohol slope, which is difficult to interpret in isolation. It shows that participants who initially had poor mental health and low alcohol consumption increased their consumption between phases, whereas those with good to adequate mental health who drank at higher levels tended to decrease their consumption between phases while their mental health scores remained relatively stable. Furthermore, it shows that participants with poor mental health and high alcohol consumption on the previous occasion had a shallower decline in their consumption than those with good mental health drinking the same amount. This indicates that in addition to fuelling increases in alcohol consumption among low-level consumers, poor mental health may also be a maintaining factor for heavy alcohol consumption.
Comparison with other work
Our findings contradict the most recently published review on the relationship between alcohol use and depression , which concluded that increasing involvement with alcohol raises the risk of depression by two-fold. This review was, however, met with criticism [64, 65] for primarily being based on previous work by the authors themselves . Furthermore, the review focused on alcohol-use disorders and major depression. Our work is therefore not directly comparable. As outlined earlier, we chose to focus on sub-syndrome symptoms of mental health and alcohol consumption (not problem consumption) as there has been a distinct lack of work exploring actual alcohol consumption (that is, what people drink) in this relationship; previous interest has largely been on the relationship between alcohol-use disorders and major depression. This makes drawing comparisons between our work and others complicated. It may be that there is something about the symptoms of problematic alcohol consumption that increases the risk of having [6, 7, 66–71] or developing [9, 10, 72, 73] depression, independent of the amount of alcohol consumed [46, 74]. Recent work has shown that individuals who self-medicate symptoms of anxiety  or depression  with alcohol have an increased risk of developing (persistent) alcohol dependence. Therefore, it could be that the relationship we observed is part of a larger complex system involving a transition from sub-syndromal symptoms of mental health influencing changes in alcohol consumption (as in our analyses) until a certain threshold is reached, at which symptoms of alcohol dependence take over and increase the risk of developing clinical disorders [9, 10]; that is, there are two separate dynamic systems at play that influence alcohol consumption and mental health pre and post clinical disorder.
Strengths and weaknesses
The approach that we took to modeling the relationship between alcohol use and mental health longitudinally utilized multiple measurement occasions to model change in both variables over time, which is known to improve the accuracy of estimated change [58, 76]. Previous work has also shown that it is important to consider variability in alcohol consumption , and the LCS model methodology directly incorporated individual change both in the total weekly alcohol consumption and in mental health. Furthermore, the method we used allowed for the effect of alcohol consumption on mental health and vice versa to be estimated simultaneously in the same model.
There are, however, several limitations of our study. First, data from phase 5 were used as the starting point, and it is possible that selective attrition may have occurred between the 'true' baseline (phase 1) and the baseline used in these analyses. This would result in a healthier cohort of participants being used to estimate the final model parameters, reducing the generalizability of the findings [78, 79]. Similarly, we used data from the Whitehall II cohort of British civil servants, which is not a representative sample of the general population. Work published using Whitehall II data has been highly influential in epidemiology and public health, shaping research agendas on social inequalities in health  and improving the understanding of the etiology of disease  but this limitation should be noted when considering the generalizability of our findings.
Second, one of the major concerns in alcohol epidemiology is measurement error in self-reported alcohol consumption . It is acknowledged that self-reported measures of consumption are likely to be biased [82–88], and therefore effect estimates obtained may actually be underestimates of the true association of interest. The use of latent variables (upon which LCS models are based; see Additional file 1) has been advocated in the field of alcohol epidemiology  to account for this known measurement error. Additionally, the MCS scale of the SF-36 is not solely concerned with psychiatric symptoms but also with mental health-related quality of life (although evidence exists to suggest that high MCS scores are associated with clinical depression [40, 90, 91]). It is possible that the relationship between alcohol intake and mental health might differ if other psychiatric questionnaires were used to define symptoms of mental health, or if the distinction was made between symptoms of depression and anxiety. However, it is argued that in practice it is difficult to effectively determine specific characteristics of depression from symptoms of, for example, anxiety using self-report measures of symptoms because of the considerable heterogeneity of symptoms between disorders (that is, self-reported symptoms often reflect a comorbidity between depression and other mood/stress-related disorders [92–95]). This has led some investigators to conclude that self-report measures of mental health symptoms at a population level merely reflect a single underlying latent construct of psychological distress [96–99].
Another issue concerning the main measures used in this study is that they refer to different time periods; information on alcohol consumption pertained to the previous week whereas information on mental health symptoms referred to the previous 4 weeks. It is possible that this discrepancy in the period of reference may have biased our findings. For example, smaller studies looking at the relationship between mood and alcohol on a daily basis have shown that increased alcohol consumption is associated with decreased happiness on the following day, and that symptoms of sadness are associated with decreased consumption on the next day . These findings contrast with our own, and highlight the importance of the timeframe used in determining the best-fitting temporal sequence between alcohol consumption and mental health.
Furthermore, the competing models we specified allowed only for the previous occasion's alcohol and/or mental health score to influence change in the alternative variable by the next occasion. It is plausible that the relationship might have differed if we had allowed for longer lag specifications, as it may be that the relationship between alcohol intake and mental health takes longer to manifest (that is, the current specification of a single cross-lagged effect may fit the relationship between mental health influencing alcohol intake better than the relationship between alcohol consumption influencing mental health symptoms).
Additionally, there was greater variation in the measure of weekly alcohol intake than in that of mental health. It could be argued that this could also be a possible explanation as to why alcohol consumption was not found to be significantly related to changes in mental health. It may be that within a dynamic system that it is more difficult to effectively predict changes in one variable using a highly erratic alternative exposure. It is conceivable that the reciprocal model might have been of best fit had both measures been relatively stable over time.
A further methodological limitation is that we controlled only for baseline covariate values, and it is possible that their values changed over time. For example, comorbidities may have developed after the first measurement occasion. Health behaviors such as physical activity and smoking could also vary over time, and the changing status of these variables could all be confounders of the subsequent effects of alcohol on mental health and vice versa. However, factoring in changes in the covariate structure over time could be problematic within the current framework, because changes in some values, for example, systolic blood pressure, could be a direct consequence of previous alcohol consumption or mental health status, and thus be considered as intermediate confounders .
Implications and directions for future work
We identified that the dominant process underlying the dynamic relationship between alcohol consumption and mental health at a population level is mental health. Consequently, it could be inferred that targeting interventions to those with poor mental health (as well as introducing measures to ensure that those with normal/good mental health do not deteriorate) would have a beneficial effect in terms of reducing heavy drinking. This may also elicit favorable knock-on effects in terms of improving general physical health and reducing the risk of chronic diseases, as heavy drinking itself is associated with an increased risk of a range of health problems [102, 103] including cardiovascular disease [104–109], cancer, [110, 111] and mortality [47, 112–114]. Furthermore, the finding that mental health affects alcohol consumption may shed some light on the growing literature examining common mental disorders as risk factors for cardiovascular disease [115–119] and all-cause mortality [120–122], because alcohol consumption may be one of many mediators in this relationship.
This work provides further support that on-going efforts to improve mental health at a population level are vital to public health [123, 124]. The proposed implementation strategy  seeks both to tackle he social determinants of mental health  and to target individuals who are at high risk. To do so, a number of avenues will be pursued, including tackling inequalities in access to services (and ensuring equality in the level of service provided). In addition, conscious efforts are being made to tackle the stigma surrounding mental health issues; perhaps if individuals feel more comfortable talking about their mental health problems or seeking treatment for them, then they will not turn to alcohol as a form of self-medication.
Others may, however, be more cynical of our findings and take them to indicate that 1) consuming large amounts of alcohol is acceptable as it does not increase the risk of developing mental health problems, and 2) that it is reasonable to self-medicate with alcohol in response to psychological distress, as it will not worsen symptoms. However, it would be unwise to use our findings as a justification for drinking in a hazardous manner. Although a person’s mental health may not worsen, as highlighted above, increased alcohol consumption would heighten their risk of developing other disorders.
Regarding future work, it is important to examine the role of drinking pattern as well as to provide closer scrutiny of age (e.g. adolescent and elderly populations), sex, socioeconomic, and cultural differences in the dynamic relationship between alcohol consumption and mental health. It is also important that subsequent studies should examine the extent to which time-varying/modified confounding may explain the association observed using appropriate analytic methods . Furthermore, it is also imperative that potential physiological and psychosocial mechanisms, both occurring alongside and precipitating (immediately or earlier in life) the parallel development of both trajectories are studied. This has been acknowledged by others in the field [10, 64, 65]. Understanding the factors that trigger increased alcohol consumption in the presence of poor mental health will allow for more effective interventions to be developed, both in terms of treatment and primary prevention.