Dear Editor,

In a recent paper by Miyake et al., the researchers studied the association between soy isoflavone intake and the prevalence of depressive symptoms in 1745 pregnant Japanese women [1]. They concluded that higher intake of soy products was associated with lower prevalence of depressive symptoms during pregnancy. Although the study adjusted for saturated fatty acids as a confounder, the results may be more convincing or robust if the researchers controlled for consumption of animal products, such as red meat. It is well known that soy products were the alternative sources of protein for people who consume little or no meat. Previous studies have demonstrated the association between meat consumption and depression. For example, a 12-year prospective cohort study showed that a dietary pattern with high consumption of red meat was related to elevated risk of depression (RR 1.41, 95% CI 1.22, 1.63) [2]. Another cohort study assessed each component of the Mediterranean diet and discovered that meat consumption significantly increased the risk of depression [3]. It is possible that the observed association reflects a “substitution effect”. In other words, participants who consumed high levels of soy products tended to have a dietary pattern with low meat consumption and/or high vegetable/fruit intake. Thus, the likelihood cannot be excluded that the observed inverse association between soy product intake and depression may be partially explained by the reduction in meat consumption.