#SfN13 Stressed out mice turn to carbs for comfort food

This is part of a series of blog posts for the 2013 Society for Neuroscience Annual Conference. 

Poster ZZ3 Ghrelin protects against stress by promoting the consumption of carbohydrates.T. Rodrigues. Z. Patterson. A. Abizaid. Carleton University, Ottawa, ON, Canada

In this world nothing can be said to be certain, except death and taxes.– Benjamin Franklin

Personally, I’d add stress to that.

There’s no question that chronic stress is a killer. Handled badly, stress can lead to anxiety, memory impairments, cardiovascular disease and sleep disorders. We all have our own strategies for coping with stress, some healthier than others. Me? I turn to food.

Apparently, so do bullied mice. Mice are social creatures; when housed together, larger and meaner ones will quickly assert dominance. The little guys have it rough, usually showing signs of anxiety, depression and increased body weight within weeks.

The reason for their weight gain can be traced back to an increase in ghrelin, a hunger-causing (orexigenic) hormone produced in the stomach. Once released, ghrelin travels to the brain and binds to its receptors to increase calorie consumption. But not all foods are equal; new research from Carleton University suggests that ghrelin promotes the intake of comfort foods – specifically, carbohydrates- because they decrease the level of circulating stress hormones such as corticosterone.

In the study, researchers first measured the amount of chow that mice ate per day for 21 days. They then chronically stressed out one group of mice by putting a dominant bully into every cage; the two mice were separated by a see-through glass wall to reduce violence. Every day, the mice had 24hr access to a standard, high-carb chow and a 4hr-window to a fattier alternative. Compared to non-stressed controls, the bullied mice drastically increased their total calorie intake, paralleled by an increase in ghrelin levels but surprisingly normal corticosterone levels.

When researchers broke down in the increase in calories by the type of food, they uncovered an unexpected result: stressed-out mice did not eat more fat, but instead opted for more high-carb chow. In fact, this high-carb binge almost entirely accounted for the increase in total calorie consumption.

However, mice chow does contain ~50% of protein and fat. To rule out a preference towards these two macronutrients in combination, researchers repeated the experiment, but with sucrose solution as the alternative to high-carb chow. As before, stressed-out mice increased their intake of chow, but this time, they also doubled their intake of sugar water compared to their unstressed peers. At the same time, their corticosterone levels were normal, suggesting that they were coping fairly well in the face of daily terror.

Why is ghrelin triggering a preference for carbs? The answer might be internal stress management. When researchers feed both bullied and control groups the same standard chow (~50% carbs), effectively restricting access to stress eating, the bullied mice suffered numerous negative health effects. Their ghrelin and corticosterone levels shot through the roof. They had abnormally low blood sugar levels, signalling the onset of metabolic problems. They even showed signs of depression, refusing to swim when dropped into a deep container filled with water.

These data suggest that under stress, ghrelin levels rise and tip food preference towards high-carb rather than high-fat foods. To see if this is indeed the case, researchers turned to a strain of mice genetically engineered to lack ghrelin receptors. Normally, compared to wild-types, these mutants show similar patterns of eating and hormone regulation, although they tend to be slightly smaller. Once stressed, however, they didn’t respond by switching to the high-carb comfort chow, instead increasing their nibbling of fatty foods. Behaviourally, these mice could not cope – in the swimming task, they spent most of their time immobile, succumbing to their fates.

Researchers aren’t yet sure why ghrelin-induced carb - but not fat - intake helps to manage stress. One reason could be bioenergetics: stress alerts the brain that more energy is needed (and soon!) through ghrelin, which in turn increases the preference for glucose - a fast and efficient energy source. Or it could just be a matter of comfort. These mice grew up on standard mice chow, which just happens to be high in carbs. Perhaps, just like you and me, mice simply prefer familiar and comforting foods after a long, stressful day.