Do plant-based milk drinkers have a higher risk of depression?

The study behind the headlines

The study behind the claim investigated the relationship between different types of milk and mental health issues, specifically depression and anxiety.¹ Researchers aimed to understand if drinking certain types of milk could influence the risk of developing these conditions.

The study included 357,568 participants from the UK. The participants were adults aged 37 to 73 years, who reported detailed information about their health, lifestyle, and dietary habits, including their milk consumption. Their mental health was assessed through questionnaires, and they were followed for an average of 13.5 years to see who developed depression or anxiety. This is called an observational study.

The participants provided information on their milk consumption habits through a questionnaire. They were asked, "What type of milk do you mainly use?" and given a list of options: full cream, semi-skimmed, skimmed, soy, other types of milk, never/rarely consume milk, do not know, and prefer not to answer. When the researchers analysed the results, “soy” was included together with the category “other types of milk.”

The majority of participants consumed semi-skimmed milk (65.1%), while smaller proportions reported drinking skimmed milk (20.2%), full-cream milk (6.5%), or other types of milk (4.9%).

To address some of the uncertainties of observational studies, which cannot demonstrate causation, the researchers also performed another type of analysis, called Mendelian Randomization (MR). This analysis was only performed to further test the association between consumption of semi-skimmed milk and mental health. This type of analysis was not performed on the other types of milk. MR uses genetic differences as natural experiments to explore whether milk consumption had a causal influence on health outcomes. MR takes advantage of natural genetic differences among people. These differences can influence behaviors or exposures, such as how much milk someone drinks. By looking at these genetic markers, researchers can infer whether a certain behavior might cause a health outcome.

Key findings:

The study found that drinking full cream milk was associated with a 16% lower risk of anxiety compared to people who drank no milk, whether dairy or plant-based. Semi-skimmed milk was also associated with better mental health, with a 12% lower risk of depression and a 10% lower risk of anxiety compared to those who drank no milk. In contrast, skimmed milk did not show any significant relationship with either conditions.

People who drank other types of milk, including soy milk, had a 14% higher risk of depression compared to those who drank no milk.

The Mendelian Randomization (MR) analysis supported the observational findings for semi-skimmed milk, suggesting it has a protective effect on both depression (17% lower risk) and anxiety (29% lower risk).

Overall, this study suggests that not all types of milk are linked to mental health in the same way.

What to keep in mind when reading the study’s conclusions?

• The study didn’t investigate what specific types of plant-based milk participants consumed or the quantity of milk consumed.

Options like soy, almond, oat, rice, and coconut milk may vary significantly in their nutritional profiles, but the study did not differentiate between the various types of plant-based milks participants consumed. This lack of specificity makes it impossible to draw conclusions about any single plant-based milk type.

Moreover, the researchers didn’t look at quantities of milk consumed. Without data on the quantities consumed by participants in both the semi-skimmed and "other types of milk" groups, it’s impossible to draw meaningful comparisons. For example, if individuals in the "other types of milk" group consumed significantly less milk than those in the semi-skimmed group, the observed differences in mental health outcomes could be influenced by the quantity rather than the type of milk alone.

Further, we don’t know whether the type of plant-based milk that was consumed was calcium-fortified or not. The researchers hypothesise that one of the ways how semi-skimmed milk may protect against depression and anxiety is through its nutrient-rich composition, including unsaturated fatty acids and calcium that supports brain health and mood regulation. If this mechanism is the reason for its protective effect, calcium-fortified plant-based drinks could potentially offer comparable mental health benefits, but further studies would be needed to test this hypothesis.

• The study is observational and, therefore, cannot prove that plant-based milk drinkers have a higher risk of depression, nor that drinking semi-skimmed milk protects against depression and anxiety.

The study suggests a protective effect of semi-skimmed milk against depression and anxiety. However, it’s important to remember that correlation doesn’t imply causation: the study does not conclude that drinking semi-skimmed milk prevents depression or anxiety, or that plant-based milks cause these conditions. Although the researchers did strengthen the association of semi-skimmed milk group and depression and anxiety by performing a MR study, they did not do so for the “other types of milk” group.

Observational studies, such as this one, look for patterns between variables (in this case, between the types of milk people drank and their mental health outcomes) but it cannot determine whether one causes the other. For instance, people who drink plant-based milks may already be at higher risk of depression for reasons unrelated to their choice of milk. Some might choose plant-based milks due to lactose intolerance, ethical concerns, or environmental considerations – factors that could also influence mental health in complex ways.

Imagine observing that people who carry umbrellas are more likely to be wet. This doesn’t mean umbrellas cause wetness, it’s the rain that links the two. Similarly, while this study suggests a link between milk type and mental health, it doesn’t prove that milk choice directly affects depression or anxiety. Understanding this distinction is critical in nutritional science.

• The study provides relative risks, not absolute risks. The baseline risk of depression remains quite low.

The study reports relative risks rather than absolute risks, which is an important distinction as relative risk can make changes seem bigger than they are. Relative risk shows how much more likely depression is in one group compared to another. For example, consumers of “other types of milk” (including soy) compared to non-consumers.

Globally, it’s estimated that about 3.8% of the population develops depression over their lifetime (this is called the baseline risk). For people who reported that they drank mainly “other types of milk”, their risk was 14% higher compared to people who didn’t drink any milk. This means the absolute risk increases from 4 out of 100 to about 4.3 out of 100, a very small relative increase.

So, while the study suggests that there might be a link between “other types of milk” and depression, this added risk is likely much smaller than other leading risk factors for depression such as adverse life events (e.g., unemployment, bereavement, traumatic events).

• The study population may not be representative of the general population.

The participants were mostly middle-aged to older adults of European descent. This means that the results may not be applicable to younger populations and other ethnic backgrounds, limiting the generalisability of the findings. While the long follow-up is a strength, it also introduces potential biases. Participants’ health and dietary habits (including milk consumption) may have changed significantly during this time, which could influence the results.

• The study investigated one specific food and its link to mental health problems, not people’s overall dietary patterns.

Mental health is influenced by a combination of genetic, social and lifestyle factors. While diet plays a role, it is just one piece of a much larger puzzle. This study focused on one specific food group, milk, without accounting for participant’s overall dietary patterns or diet quality.

It’s also important to note that while some headlines suggested that vegans are at higher risk of depression due to plant-based milks, the study did not analyse vegan diets or identify participants as vegan. It merely categorized the milk consumption habits of the participants.

• The study didn’t include information about the participant’s total energy intake (the total amount of calories they consumed).

This could be important because the number of calories someone eats might influence both their choice of milk and their risk of depression or anxiety. By not considering total energy intake, there’s a chance that some of the results could be influence by this missing piece of information. This is called “residual confounding”, meaning other factors might be affecting the findings in ways the researchers couldn’t measure.

The authors of the study acknowledge this limitation and recommend that future research include total energy intake to get a clearer picture of the associations.

What is the current advice on diet, lifestyle and mental health?

• The World Health Organization states that self-care can play an important role in managing symptoms of depression and promoting overall well-being.² These include: trying to keep doing activities you used to enjoy, staying connected to friends and family, exercising regularly, sticking to regular eating and sleeping habits, avoiding or cutting down on alcohol and avoiding illicit drugs, talking to someone you trust about your feelings, and seeking help from a healthcare provider.
• The EU-funded MooDFOOD project, a collaboration of 13 organisations in Europe looking at preventing depression through food, emphasises the role of a healthy dietary pattern, aligned with national dietary guidelines to reduce depressive symptoms in the general population.³ Key components of this diet include eating lots of vegetables, fruits, and fish, which have been identified as particularly beneficial. Dairy products should be eaten in moderation as part of a healthy dietary pattern. Most national dietary guidelines recommend 2-3 servings of dairy, or fortified plant-based alternatives, per day.⁴
• The Royal Australian and New Zealand College of Psychiatrists provide several recommendations for preventing depression through diet and lifestyle choices.⁵ These emphasise the importance of adopting a healthy lifestyle, which include regular psychical activity, maintaining a balanced diet, ensuring good sleep hygiene, and implementing strategies to manage stress.

References

1. Wu, C., Liu, Y., Lai, Y., Wang, Q., Wu, S., Fan, H., ... & Jiang, X. (2024). Association of different types of milk with depression and anxiety: a prospective cohort study and Mendelian randomization analysis. Frontiers in Nutrition, 11, 1435435.
2. World Health Organization. (2023). Depressive disorder (depression). Accessed 9 January 2025.
3. Mood Food. (2018). Preventing Depression Through Food. Accessed 9 January 2025.
4. European Comission. (2024). Food-Based Dietary Guidelines recommendations for milk and dairy products. Accessed 9 January 2025.
5. Malhi, G. S., Outhred, T., Hamilton, A., Boyce, P. M., Bryant, R., Fitzgerald, P. B., ... & Fritz, K. (2018). Royal Australian and New Zealand College of Psychiatrists clinical practice guidelines for mood disorders: major depression summary. Medical Journal of Australia, 208(4), 175-180.