People who consume low–fat milk and yoghurt, rather than whole–fat dairy products, are less likely to have depression, according to researchers in Japan and China.

Dairy consumption has long been linked to a wide range of physical health benefits, but its effect on emotional health has remained unclear. Now, a new study published in the journal Social Psychiatry and Psychiatric Epidemiology reveals that people who consume low–fat dairy products may be less prone to depression.

Professor Ryoichi Nagatomi of Tohoku University and colleagues in Japan and China investigated the association between whole and low–fat dairy consumption and depressive symptoms such as exhaustion, sadness, anxiety, helplessness and hopelessness. This is the first study to consider different components of dairy products (whole fat and low fat) and the occurrence of depression.

The study involved 1,159 Japanese adults between the ages of 19 and 83. There were 897 men and 262 women, of which 31.2% and 31.7% respectively, were depressed.

The researchers asked the participants in a questionnaire how often they consumed whole– or low–fat milk or yoghurt. Depressive symptoms were evaluated using the self–rating depression scale, which consists of 20 questions and is a tool to distinguish people with and without depression.

The result showed that people who consumed low–fat dairy products between one and four times a week are less depressed. The correlation remained even after considering other critical factors such as age, sex, health status, nutrition status and life style.

The study found no association between whole–fat milk consumption and depressive symptoms. The researchers speculate that this is because trans–fatty acid contained in whole fat milk, which is associated with depression, cancelled out the anti–depressive effect of another milk component, tryptophan.

The researchers note that since this was a cross–sectional study that analyzed a population at a single point in time, it could not explain what actually caused such outcomes. Other dairy products, such as cheese and butter, were not included in the study. It is also unclear whether milk or yoghurt had a stronger influence. Further studies are necessary to confirm and clarify the causality of the findings.