Our research group recently published an article that observed the inverse association between dietary protein and depressive symptoms in American adults^{(Reference Li, Zhang and Li1)}. Meanwhile, another article by our research group found that the relationship between different types of dairy products and depression was inconsistent. Skimmed milk and moderate milk desserts were negatively associated with depressive symptoms, while whole milk was positively associated with depressive symptoms^{(Reference Sun, Wang and Zhang2)}. Kawada provided some concerns about the association between milk intake and depressive symptoms^{(Reference Kawada3)}.

Both of our studies were investigating factors affecting depressive symptoms. The difference is Li et al. focused on studying the relationship between dietary protein intake and depression and detected an inverse relationship between protein derived from milk and depression^{(Reference Li, Zhang and Li1)}. While Sun et al. focused on the association between diary and depression symptoms and found that the relationship between different types of milk and depression was inconsistent^{(Reference Sun, Wang and Zhang2)}. Based on these two studies, different types of milk might have different effects on depressive symptoms. The mechanism may be due to the different nutrients contained in different types of milk.

Previous studies have inconsistent conclusions on evaluating the association between dietary protein and depressive symptoms. Some of them found that the intake of dietary protein was inversely related to the risk of depression^{(Reference Wolfe, Arroyo and Tedders4–Reference Nanri, Eguchi and Kuwahara6)}, whereas the relationship was not statistically significant in other studies^{(Reference Oishi, Doi and Kawakami7–Reference Aparicio, Robles and Lopez-Sobaler9)}. Milk is an important source of dietary protein. However, few studies have determined the effect of protein from milk on depressive symptoms. To our knowledge, only Li et al. examined the relationship between protein from milk and depressive symptom and found the negative association^{(Reference Li, Zhang and Li1)}. More research is needed to explore and verify the relationship between protein and other nutrients from milk and depression in the future.

Limited studies have examined the impact of milk on depressive symptoms. Hockey et al. reviewed studies on the relationship between dairy consumption and depressive symptoms or disorders in adults^{(Reference Hockey, McGuinness and Marx10)}. Part of the studies were about milk and depressive symptoms or disorders. In a Polish cross-sectional study of 150 men and women, no association of milk and clinically diagnosed depression was observed^{(Reference Stefańska, Wendołowicz and Cwalina11)}. While a cohort study found that lower milk intake was related to increased risk of depressive symptoms in men, whereas these relationship were not statistically significant in women^{(Reference Wolfe, Arroyo and Tedders4)}. Moreover, Pasco et al. conducted a cohort study to evaluate the effect of milk intake on depression in pre- and postmenopausal Australian women and found that milk intake was a risk factor for depression in postmenopausal Australian women^{(Reference Pasco, Williams and Brennan-Olsen12)}. In addition, some studies on the relationship between milk intake and depressive symptoms were limited to specific groups, such as older adults^{(Reference Almeida, Norman and Hankey13)}, pregnant women^{(Reference Miyake, Tanaka and Okubo14)} and overweight adults^{(Reference Grossniklaus, Dunbar and Tohill15)}. Moreover, there was no study to explore the effect of different types of milk on depressive symptoms except the study conducted by Sun et al. ^{(Reference Sun, Wang and Zhang2)}.

In the present limited studies, significant heterogeneity existed between study designs, especially there were widespread variations in measurement and categorisation of milk intake. A small number of studies and high heterogeneity among studies limited the ability to conduct meta-analysis. As Kawada mentioned^{(Reference Kawada3)}, higher-quality studies on the association between different types of milk and depressive symptoms are needed in the future.

The author has indicated no financial support.

There is no conflict of interest in this study.