If you are [a] zebra running for your life, or [a] lion sprinting for your next meal, you body’s physiological response mechanisms are superbly adapted for dealing with such short-term physical emergencies… When we sit around and worry about stressful things, we turn on the same physiological responses—but they are potentially a disaster when provoked chronically.
– Robert M. Sapolsky in “Why zebras don’t get ulcers”
Funny old humans. The things we perceive in our minds can make our bodies sick. To a zebra it makes little sense—because once a threat is gone, it’s gone. But the worries and cares of Homo sapiens never seem to leave us alone, and their effects on how we function are real.
Health professionals know that chronic stress doesn’t occur in a bubble—it’s closely related to obesity and to mood disorders like anxiety and depression. And scientists have found the gut microbiota is also related to obesity, mood disorders, and stress. Commonly, an individual with obesity might tend to experience a high level of chronic stress and also struggle with low mood; the person may also happen to eat a lot of calorie-dense foods. But it’s hard to know where the cycle begins or how to end it. Would tackling the stress cut down on the need for ‘comfort’ foods and thus help the person lose weight? Or would weight loss improve mood and self-confidence, reducing perceived stress? Can the answer be found in the gut microbiota?
Chronic stress is not an isolated element but is associated with obesity, anxiety and depression
Researchers from China and the US recently decided to try to answer some of these questions. Their findings (in mice) were a reminder that males and females might respond very differently to the same set of stress-related factors.
The researchers fed groups of mice a normal diet, and then put a subset of them on a high-fat diet that made them become obese. They then exposed all mice to several sources of mild stress (including things like damp bedding, cold water swimming, and cage tilting) and watched their behaviour, while keeping tabs on their gut microbiota composition throughout the process.
The male and female mice started with distinct gut microbiota compositions. But all of the mice experienced dramatic gut microbiota shifts the moment they began consuming the high-fat diet. Male and female mice on a high-fat diet had similar weight gain and began to show gut microbiota that looked similar to each other. But when the mice were subjected to stress, the male and female gut microbiota diverged again, showing different trajectories in response to the stress.
The gut microbiota, obesity, eating disorders and stress are all closely related
And what of their behaviours? When stressed and consuming the high-fat diet, male mice showed more anxiety-like behaviours and they showed less activity. But the females on the high-fat diet undergoing stress didn’t show these reactions. One effect of stress was specific to the lean female mice on a normal diet, however: their gut microbiota compositions changed to look like that of the obese females on the high-fat diet. Probably not a good thing.
Researchers are not yet sure how their findings on the triple-whammy of stress, high-fat diet, and obesity apply to humans—but it seems the gut microbiota should not be ignored in trying to untangle the links between diet, stress, and obesity, and furthermore, we can’t assume that males and females experience this trio in the same way. Further research will help us understand which interventions could help which individuals, male or female.
Bridgewater LC, Zhang C, Wu Y, et al. Gender-based differences in host behavior and gut microbiota composition in response to high fat diet and stress in a mouse model. Scientific Reports. 2017; 10776. doi:10.1038/s41598-017-11069-4