“Stress accounts for more deaths annually than Alzheimer’s or diabetes,” says Carol Shively, PhD. Shively is a professor of comparative medicine at Wake Forest’s school of medicine.
By now we’ve all heard that stress has huge effects on our physical health. But the scope of the problem may still be surprising. Stress is strongly linked to major illnesses like diabetes, cancer, and heart disease, in addition to other major causes of death, such as accidents and suicide. High rates of stress also help explain why there are such appalling disparities in American health outcomes between socioeconomically secure and disadvantaged communities.
Stress, which is so often due to factors that are entirely out of our hands, is not easy to alleviate. There are some options, but there may be one other modifiable factor that you haven’t thought much about: your diet.
Shively, who shared her work during a presentation at the 2021 American Diabetes Association Scientific Sessions, believes that diet can have a huge impact on how stress can affect our bodies. If stress and diet interact to create real physiological changes, perhaps the negative effects of stress can be ameliorated with dietary change.
Monkeys and Stress
How do you study chronic stress? Shively does it with the help of the cynomolgus monkey, or crab-eating macaque.
These monkeys are good experimental analogues to human beings, because their responses to stress, diet, and aging are fairly similar to our own.
Cynomolgus monkeys form linear and stable hierarchies. It is immediately obvious to researchers which monkeys are dominant, and which are subordinate. And scientists can say with some confidence that the subordinate monkeys are more stressed.
Subordinate monkeys are victims of more aggression, spend more time alone, and spend more time in a state of apparent vigilance than do monkeys higher in the social order. They also receive less grooming, a kind of pampering that relaxes monkeys and lowers their heart rate and blood pressure, just like a nice massage. Physiological indications — such as high cortisol levels — confirm that the subordinate monkeys experience more stress.
Social stress of this sort has an undeniable effect on the physical health of these monkeys. Stress leads to increases in visceral fat and atherosclerosis, just like it does in humans, two significant risk factors for chronic disease and early death.
Two Diets
Shively wondered if different diets might alter the way that stress impacts the monkey’s metabolism.
In a preliminary study, monkeys were assigned to either the Western diet or a Mediterranean diet. They were fed these diets for 31 months — on a human timescale, that would be about 8 years. They were offered the same amount of exercise.
These two diets were matched for macronutrients, so monkeys in each group were given about the same amount of protein, fat, and carbs. The composition of those nutrients differed quite a lot, however. The Mediterranean fats were more often plant-based, with a very high percentage of healthy monounsaturated fats, such as olive oil, resulting in a much healthier ratio of Omega-6 to Omega-3 fatty acids. Western carbs were also more likely to come from refined sources, such as high-fructose corn syrup; the Mediterranean carbs were mostly found in fruits and legumes.
The results weren’t surprising. Monkeys on the Western diet ate more food, gained more weight, had higher insulin and triglyceride levels, and had fattier livers than monkeys on the Mediterranean diet. These are similar to the results seen in humans. The growth of the Western diet, after all, is almost universally seen by experts as a primary cause of the global explosion of obesity and type 2 diabetes.
The Diet-Stress Connection
When Shively subjected the monkeys from the two different groups to stress tests, she found that her hypothesis had been confirmed. The monkeys consuming the Western diet got much more stressed, secreting significantly more cortisol in response to social stress.
“The Western diet exaggerates physiological responses to stress, the Mediterranean diet did not.”
A second study split monkeys into two different groups, not by diet, but by stress level. Actually, the monkeys do it by themselves: in any group of four monkeys, two are always dominant, and two subordinate.
In this study, all monkeys were fed the same Western-style diet for three years. At the end of the study, subordinate monkeys had higher triglycerides, higher fasting glucose, higher levels of circulating insulin, and more insulin resistance than when they began the study. By contrast, the dominant monkeys barely experienced any metabolic change at all, despite eating the same foods.
Nearly 25 percent of the subordinate monkeys had high enough fasting glucose levels to qualify as pre-diabetic; not a single one of the dominant monkeys had the same condition.
This study suggests that the unhealthy diet was not itself enough to cause metabolic dysfunction — both stress and diet had to be present.
Takeaways
Animal studies always have to be taken with a grain of salt — we can’t generally assume that what happens in the body of a crab-eating macaque will happen quite the same way in the body of a human.
Nevertheless, the similarities between monkey and mankind are striking. Both social stress and the Western diet cause some of the very same negative physiological effects in monkeys as they do in humans.
The diet-stress connection is not far-fetched. While it would be almost impossible to prove a causal relationship in studies of humans, other scientists have already explored the interactions between the Western diet, stress, and metabolism. And there is much work to be done on the topic to tease out causation and correlation and test other dietary approaches.
In the meantime, Shively’s work may give readers one more reason to set aside the junk food and reach for more wholesome choices. Stress already causes so much metabolic damage — and that damage may only be compounded by what you eat.