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“This gut brain helps to control muscular contractions and gut secretions. It also balances the body’s hunger and satiety, or feelings of fullness, and communicates those states to the big brain.” Gautam Naik (The Wall Street Journal, January 25, 2011)
Exercise Curbs Appetite: Mechanism Uncovered
How Your Gut Says I’m Full
I wrote in Lean For Life that regular exercise helps our appetite control mechanism work better. People who exercise have an easier time balancing energy expenditures and food intake. One reason is that active people have more leeway in how much they can eat; exercise burns more calories, allowing them to eat more without getting fat. On the other hand, people who don’t exercise are more likely to fall victim to creeping obesity; they eat a little more than they need, and the fat slowly piles up on their body.
People who perform hard physically for prolonged periods (lumberjacks, farm workers, endurance athletes), can eat a huge amount without putting on fat. Interestingly, people who train hard for short periods of time often don’t feel the need to eat more. In both cases, regular exercise helps people know when they’ve had enough to eat.
We’ve long known that exercise helps control eating and fat gain, but we are only now learning precisely how it works. Brazilian researchers at the University of Campinas in Sao Paulo are helping us understand the underlying mechanism. (Their study was reported in the August, 2010, PLoS Biology.)
While the report is long and complicated (with an amazing number of charts and graphs), the bottom line is straightforward: Exercise makes the brain more sensitive to key chemicals that signal when animals have had enough to eat.
I’ll summarize the breakthrough study. I’ll also relate some interesting facts from The Wall Street Journal about sensors in the gut that balance hunger and fullness. We have two appetite control brains, one in our gut and the other in our head. Knowing how they communicate can help us control how much we eat and weigh.
Exercising Rodents Eat Less
The researchers began with this background information:
In mammals, food intake and energy expenditure are tightly regulated by specific neurons localized in [a region of the brain called] the hypothalamus. The hypothalamus can gather information on the body’s nutritional status by integrating multiple signals including potent hormonal signals such as insulin and leptin. The impairment of hypothalamic insulin and leptin signaling pathways is sufficient to promote hyperphagia [overeating], and obesity in rodent models.... We hypothesized that exercise could exert its effects in the central nervous system by modulating the specific hypothalamic neurons responsible for control of food consumption.
Jose B. C. Carvalheira and his medical sciences colleagues at Campinas University demonstrated that fat rodents that began exercising exhibit positive chemical changes in hypothalamic neurons that control appetite—and eat less. “In obese animals, exercise increased interleukin-6 and interleukin-10 protein levels in the hypothalamus, and these molecules were crucial for increasing the sensitivity of the most important hormones, insulin and leptin, which control appetite,” Carvalheira explained.
Simply put, exercise made their hypothalamus better able to interpret hormonal signals of fullness. (Leptin signals satisfaction, while insulin signals that excess calories are available for storage.)
This next part is important to a full understanding of how exercise affects appetite.
“Our study shows that acute exercise per se did not evoke any meaningful effect, in terms of food intake in lean animals,” Carvalheira’s group reported, “but interestingly, it was crucial for suppressing hyperphagia [overeating] mediated by overnutrition” in obese animals with impaired hypothalamic insulin and leptin signaling.
In short, if it ain’t broke exercise won’t fix it. For fat animals—and perhaps humans—who need it, however, exercise can be a Godsend.
Exercise has long been considered a mainstay of weight control, but only now are studies such as this uncovering the full benefits. Exercise not only increases calorie burn, it also regulates and, if necessary, improves communications between the gut and the brain.
That brings us to the appetite control cops in our digestive system.
The Gut Brain
“Your Stomach Really Does Have a Mind of Its Own,” The Wall Street Journal headlined a “Personal Journal” feature by Gautam Naik (January 25, 2011).
Millions of nerve cells in our esophagus, stomach, small intestine, and colon operate as an appetite-control second brain. These digestive track neurons talk together and with brain cells in the cranium—about fullness. “This gut brain helps to control muscular contractions and gut secretions,” Naik reports. “It also balances the body’s hunger and satiety, or feelings of fullness, and communicates those states to the big brain.”
“The body is in a state of continual hunger—its default position,” Naik continues. “But several factors work to curtail the hunger instinct, such as the presence of food in the digestive track, or the flow of nutrients in the blood. When these satiety factors dissipate, the body again demands food.”
Naik describes three key systems in our gut brain that tell our big brain when it’s time to stop eating. Understanding how they work allows us to help them do their job more effectively.
Stomach Stretching: As food fills the stomach, it stretches, and the gut brain sends a neural message to the big brain: Getting crowded down here—stop eating. That’s why it’s important to avoid refined and sugary foods, which are dense with calories and don’t take up much room. Whole foods—especially fruits, vegetables and whole grains—fill the stomach without overshooting calorie needs.
Peptide Release: The gut brain also senses when there are nutrients in the gastrointestinal tract and releases peptides into the blood, sending another fullness signal to the big brain. Peptides are usually formed when proteins are digested in the gut, binding amino acids together. This presents another opportunity to make the system work in our favor. Including high quality protein in meals (eggs, fish and chicken are good examples) helps us feel full with fewer calories.
Ileal Brake: This mechanism also involved peptide release. The ileum is the lower part of the small intestine. The ileal brake sends an I’m full message to the big brain when fat reaches the ileum. That’s why it’s a good idea to include some fat in each meal. (Olive oil, nuts, and fish are good sources.) Fat slows digestion and puts the brake on appetite.
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We are born to be active—and lean. The gut brain reflects millions of years of evolution. Obesity had no place in a world where we had to overcome our dinner and move with the seasons to survive. Exercising regularly and eating wisely maximizes our chances of realizing our birthright. Help your body and it will help you.
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