Socioeconomic status and the health benefits of moderate drinking among older people

A common finding in studies of the relationship between alcohol use and health is a J-curve relationship - moderate drinkers have better health than abstainers, and better health than heavy drinkers. However, there are a lot of confounding factors in the observed relationship between drinking and health. So much so that it is difficult (perhaps impossible) to establish whether there is a causal relationship between them.

One confounding factor is socioeconomic status, and that is the focus of this 2018 article by Andy Towers, Michael Philipp (both Massey University), Patrick Dulin (University of Alaska), and Joanne Allen (Massey University), published in the Journals of Gerontology: Social Sciences (open access). They used data from the New Zealand Health, Work and Retirement Study 2012 wave, which included nearly 3000 participants aged 52-86 years, and look at how the relationship between daily average alcohol consumption and self-reported health are related, after controlling for socioeconomic status. Their measure of health status is the widely-used SF-12 physical health score, and their measure of socioeconomic status is the ELSI-SF, used by the Ministry of Social Development, as well as income and education.

Figure 1 in the paper illustrates why controlling for socioeconomic status is probably pretty important when looking at the relationship between alcohol consumption and physical health (for women; there is a similar figure in the article that shows the relationship for men):

Notice that the measure of physical health varies over different levels of drinking (LA is lifetime abstainers; and CND is current non-drinkers) in a very similar way to the measure of socioeconomic status (see also this earlier post on the relationship between drinking and earnings). When Towers et al. include both drinking and socioeconomic status in a regression model of physical health, they find that:

For older men, the quadratic relationship between alcohol and health is evident when controlling for SES proxies but disappears when SES is controlled for. For women, the direct linear relationship between alcohol and health exists after controlling for SES proxies and is substantially reduced - though still evident - when controlling for SES.

This probably overstates the case (although not to the same extent as the title of the article does). While drinking becomes statistically insignificant in the model of physical health for men, the point estimates on the coefficients for alcohol fall by roughly two-thirds. So, it would be better to say that socioeconomic status explains about two-thirds of the relationship between drinking and physical health for men. For women, socioeconomic status explains about forty percent of the relationship, which remains statistically significant after socioeconomic status is included.

Of course, none of this is to say that the problem of causality is solved. The models that Towers et al. use are still cross-sectional and only show correlations, and they are up-front about that in the conclusion to their article:

However, while our study does not provide evidence for causal relationships among the variables it is logical to suggest that, rather than alcohol having a beneficial influence on the health of older adults, higher SES levels may simply facilitate lifestyles that enable both regular moderate drinking and the capacity for better health.

There are other limitations as well, not least of which is that current drinking is being linked to current physical health, but current health probably depends on many years of drinking behaviour rather than just current drinking (which is why we need longitudinal studies, and is why it is appropriate that Towers et al. drop current non-drinkers from their analysis sample). However, this study does illustrate the confounding effect that socioeconomic status exerts when applying simple methods to compare physical health between people at different levels of drinking. If you didn't fully consider socioeconomic status, you could easily overstate the positive effects of moderate drinking.