The Body and Brain's Role in Weight Loss and Gain

Why do we gain weight? Part of the blame falls on our ancestors.

“Because humans evolved in a world where food was available only intermittently, survival required that we have the capacity to store ingested energy for times when none was around,” explains Eleftheria Maratos-Flier, MD, an endocrinologist in BIDMC’s Center for Nutrition Medicine. “Adipose tissue — familiarly known as fat — is the organ that stores this energy.”

 Food was in relatively short supply even as recently as the middle of the last century, and most people did not have enough to eat.

“People were at risk of getting too few calories, leading to illness and starvation,” says Maratos-Flier. “As a result, extra weight was not considered a health risk and was actually associated with increased survival.”

This ability to store fat remains essential to life and can allow a person to survive starvation for several months. Homeostatic feeding — eating to replenish used-up energy stores — is essential, as when we eat in the morning after approximately 14 hours of being without food. But today’s lifestyles and circumstances have made it exceptionally easy to take in more energy than is needed to maintain health.

“Our epidemic levels of obesity are in large part the result of technological progress,” she adds. “Today food is plentiful, inexpensive and widely available. At the same time, our physical activity levels have been drastically reduced not only by technical advances such as the automobile and the escalator, but also by a shift in our work lives being primarily focused on manual labor to being much more focused on sedentary desk jobs.”

The Internal Parts of the Puzzle

The human brain regulates weight by integrating information about the body’s energy needs and the status of its food stores, and then initiating changes in behavior and energy processing in response to these signals. Specialized brain areas coordinate feelings of appetite or satiety (fullness) to cause us to take in more or less energy by eating more or stopping eating. Many molecules provide input to the brain, including the fat hormone leptin, stomach peptides such as ghrelin, and the hormone insulin. Direct connections from the liver to the brain’s vagus nerve may also be important.

It will probably come as no surprise to anyone who has struggled to lose weight that the human body’s regulation mechanisms seem to be slightly biased in favor of preserving fat rather than eliminating it.

“The body doesn’t like net negative calorie balance,” says Maratos-Flier. “When you cut back on calories, your hormones adjust accordingly; for example, leptin levels go down and hunger increases.”

Besides evolutionary factors, the other factor working against us in trying to lose weight is that we are inherently tempted by taste. Known as hedonic feeding, this is the eating we do for pleasure.

“This is when food is no longer just ‘fuel,’ but is a pleasurable activity,” adds Maratos-Flier.

Can Our Energy Balance Be Adjusted?

To offset nature’s physiological obstacles, Maratos-Flier advises that you listen carefully to your own physiology — everybody is different.

“Losing weight is not a one-size-fits-all proposition,” she explains. While many recommendations suggest, for example, that always eating breakfast is key to weight loss, she points out that this may not work for many individuals.

“Some people might do best with just having coffee in the morning, eating a light lunch and reserving their calories for a big dinner,” says Maratos-Flier. “If that’s the routine that feels most comfortable, then that’s what you will be most likely to stick with.”

Straightforward recommendations such as reducing food intake, changing the composition of your diet and increasing physical exercise are always appropriate when it comes to weight loss, says Maratos-Flier. Her other advice: Keep close tabs on your weight.

“I do recommend that people weigh themselves on the scale every day,” she says. “If you are gaining weight, your own internal reporting system isn’t giving you a very good update on your energy balance. The scale substitutes for that and provides an external report on your progress.”

She adds that another physiological point to keep in mind is that as you age, your body’s metabolism slows and the number of calories needed to maintain health and energy reserves declines.

“Even though your idea of what makes for a satisfying meal hasn’t changed over the years, you have to adjust calorie intake accordingly to maintain the same energy balance as when you were younger,” says Maratos-Flier. “And that means fewer calories.”