A study has found that middle-aged monkeys fed a plant-based Mediterranean diet were more resilient to stress than those fed a Western diet containing a lot of animal protein, saturated fat, salt, and sugar.

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According to a survey by the polling organization Gallup in 2019, people living in the United States reported some of the highest levels of psychological stress in the world.

Chronic stress not only increases a person’s risk of depression and anxiety but also their chances of developing diseases including obesity, type 2 diabetes, cardiovascular disease, stroke, and Alzheimer’s.

Reducing stress is not easy at the best of times, however, and it is even more difficult in the face of circumstances such as political turmoil and an ongoing pandemic.

The idea that simply changing our diets could improve how our bodies cope with stress may seem far-fetched. But observational studies have found that people who eat a lot of fruits and vegetables or, specifically, follow a Mediterranean diet, report less stress.

Conversely, researchers have discovered associations between high sugar and saturated fat intake and high blood levels of the stress hormone cortisol.

The trouble with such studies is that they do not prove a causal relationship between the diet and stress. Other factors that might influence people’s diets, such as where they live, their level of education, or their socioeconomic status, are equally likely to determine how much stress they experience daily.

Controlling for all these variables in a longitudinal study involving people is all but impossible.

Instead, researchers at the Wake Forest School of Medicine, in Winston-Salem, NC, compared the long-term effects of a typical Western diet with those of a Mediterranean diet on stress resilience in macaques under controlled experimental conditions.

“Unfortunately, Americans consume a diet rich in animal protein and saturated fat, salt, and sugar, so we wanted to find out if that diet worsened the body’s response to stress, compared to a Mediterranean diet, in which much of the protein and fat come from plant sources,” says Carol A. Shively, a professor of pathology and comparative medicine at the Wake Forest School of Medicine and the study’s principal investigator.

Prof. Shively and colleagues found that monkeys fed a Mediterranean diet were more resilient to the effects of stress and were slower to develop age-related increases in stress sensitivity.

Their study has been published in the journal Neurobiology of Stress.

The researchers compared the effects of two diets on 38 middle-aged female macaques over a period of 31 months, which is roughly equivalent to 9 human years.

They formulated their experimental Western diet to be similar to that consumed by middle-aged American women. It contained protein and fat mainly from animal sources, and it was high in salt and saturated fats and low in monounsaturated and omega-3 fatty acids.

The Mediterranean diet contained protein and fats derived mainly from plants, some lean protein from fish and dairy, and a high monounsaturated fat content, which came principally from extra virgin olive oil. The diet incorporated more complex carbohydrates and fiber and less salt and refined sugars than the Western diet.

The scientists report that the Mediterranean diet’s ratio of omega-6 to omega-3 fatty acids was “similar to a traditional hunter-gatherer-type diet.”

Both of the study’s diets had equivalent contents in terms of calories and cholesterol.

In the course of the experiment, the animals eating the Western diet ate more, accumulated more fat tissue, and had a different profile of gut bacteria, compared with those who received the Mediterranean diet. They also developed greater insulin resistance and fatty liver disease.

To determine the interaction between diet and the effects of chronic stress, the researchers took advantage of the stable social hierarchy that groups of female macaques naturally establish.

They explain that the monkeys with a subordinate status in the group are more likely to be a target of aggression and less likely to be groomed, and they spend more time “fearfully scanning the group.”

The scientists created brief, acute stress by isolating individuals from the rest of the group for 30 minutes at a time.

The macaques on the Mediterranean diet were more physiologically resilient to these stress challenges. Activity in the sympathetic nervous system, which enacts the “fight or flight” response, was lower compared with that of the animals on the Western diet.

In response to acute stress, their heart rate recovered more rapidly and they produced less of the stress hormone cortisol.

This suggests a stronger response from their parasympathetic nervous system, which enacts a relaxation response to restore the body to a restful state after a stressful experience.

Cortisol responses and activity in the sympathetic nervous system increase as an animal ages, but in the animals that ate the Mediterranean diet, these changes were delayed, compared with those on the Western diet.

“Our study showed that the Mediterranean diet shifted the balance toward the parasympathetic nervous system, which is good for health,” says Prof. Shively. “By contrast, the Western diet increased the sympathetic response to stress, which is like having the panic button on all the time — and that isn’t healthy.”

The study’s authors conclude:

“Based on the findings reported here, the Mediterranean diet pattern may serve as a dietary strategy to reduce the deleterious effects of stress on health without the side effects of medications typically prescribed to manage stress responsivity, and [adopting it] may have a significant public health impact.”

It is worth noting, however, that the effects of different diets on stress in monkeys may not closely reflect their effects in humans.

The researchers also acknowledge that the Mediterranean diet that they created for this experiment had not previously been tested in nonhuman primates. In addition, they say, future investigations need to determine the effects of the diet on stress responses in males.