Swimsuit season is shockingly near, tempting many of us to sample from the diverse buffet of contemporary diets that promise to melt away lingering belly fat. Countless diets make sweeping claims—from flushing away toxins to curbing appetite. But can dieting actually shrink the size of your stomach, making you want to eat less? And for that matter, does overeating stretch your stomach and encourage gluttony? These questions have divided scientists for decades, but recent research is beginning to bring new insights to the controversial subject.
Even before dinner is served, the digestion process has already begun. In anticipation of a meal, saliva builds up in your mouth, while acid and enzymes accumulate in the stomach to help break down food. The stomach muscles also relax to prepare for the impending bombardment in a process called gastric accommodation. For the average adult, this natural “stretchiness” is far more important than baseline volume in determining how full someone gets from a given intake of food. As you eat more, your stomach volume can increase more than five-fold to house all the chow. After the food passes through the digestive tract, however, the stomach returns to its original size.
“The normal volume of the fasting stomach is about 200 milliliters,” says Gianrico Farrugia, a gastroenterologist and CEO of the Mayo Clinic in Florida. “But once the signal comes to relax to accommodate food, it can easily increase in size to hold a liter, and some people can stretch it even further.” On the flip side, some individuals suffer from functional dyspepsia, when the stomach lining is much stiffer and is unable to fully relax. This produces a feeling of discomfort from relatively small amounts of food—even though your stomach is not technically any smaller.
The latest science suggests that chronic food restriction can actually affect how much you need to eat to feel full—with caveats. An upcoming study of fasting mice, conducted by Farrugia and Tamas Ordog at the Mayo Clinic, shows that reducing food intake by 20 percent over four weeks results in a reduction of several important cellular stomach wall factors, reducing the amount of food the stomach can accommodate.
“When you analyze the stomach, you find that the number of nerves, the number of pacemaker cells [which produce coordinated muscle contraction during digestion] and smooth muscle are found in significantly lesser number,” says Farrugia. “So the stomach capacity to relax does actually shrink when there is dietary restriction.” These mice also show delayed gastric emptying, which measures the time it takes for food to move through the stomach and into the small intestine.
The Mayo Clinic team has also performed studies in humans who have lost at least 20 percent of their body weight. Their results, which have yet to be published, show similar cellular reductions from severe weight loss. And though food accommodation has not been directly measured in these subjects, experiments have shown a reduction in the number of neurons that release nitric oxide, a chemical that signals the stomach muscles to relax so it can store more food. This is supported by previous animal studies, which demonstrated that loss of nitric oxide is associated with appetite-dampening effects.
While these findings may seem encouraging for the average dieter, researchers are unsure whether patients who lose less than 20 percent of their body weight will see similar cellular changes and declines in appetite. “We don't know if this is a linear thing —if when you diet a little, your stomach relaxes less, or when you diet a lot and your stomach relaxes a lot less,” says Farrugia.
It is also highly unlikely that short-term fasting is enough to produce long-term changes in the stomach. “Based on our data from animals, we don't have any evidence that a day is enough to do this,” says Farrugia. However, the hunger pangs that often accompany fasting are a result of strong contractions of the stomach, which presumably could temporarily minimize its volume, says David Levinthal, an assistant professor of gastroenterology, hepatology and nutrition at the University of Pittsburgh.
“It is also possible that the tension of the walls may be just a little bit higher,” which could produce stunted appetite after a 12- to 24-hour period of not eating, says Levinthal. Studies in mice have indicated that reintroducing unlimited food after a prolonged period of restriction fails to return the stomach to its original size, “suggesting that at some point these changes became partially irreversible,” adds Farrugia. These findings have not yet been verified in humans.
However, decreasing the ability of the stomach to relax through food restriction can be taken to the extreme, such as in people suffering from anorexia. “What this suggests is that patients with anorexia are not only having a hard time eating mentally, but also physically, because their stomach is actually unable to accommodate food,” says Farrugia. And in some instances of prolonged starvation, patients can even suffer from re-feeding syndrome, in which the sudden delivery of food overwhelms the depleted body with nutrients, leading to downstream effects like heart arrhythmia. Soldiers saw the effects of this kind of extreme depravation after World War II, for example. “This is actually what killed a lot of people when the G.I.s liberated captives from concentration camps and gave them candy bars out of empathy,” says Levinthal.
Do these dynamic stomach properties work in reverse, allowing, say, competitive eaters to permanently increase their stomach size? Numerous imaging studies have shown that the stomachs of obese people are really not that different from those of the rest of the population, indicating that there is little relationship between body size and baseline stomach size, says Levinthal. The same is true for competitive eaters, many of whom are relatively slim. They are instead very adept at psychologically pushing past the sensation of being full. And like athletes, many competitors are genetically gifted, displaying an ability to relax their stomachs far more efficiently than the average person.
Of course, stomach size and stretchiness are not the only factors influencing satiety, adds Levinthal. Other influencers include the sensitivity of neurons lining the stomach walls (signaling distension), hormones such as ghrelin and leptin (communicating hunger and satiety) plus the psychological factors that can curb a desire to eat or compel you to indulge in that extra piece of cake. All of these components vary greatly from person to person based on age, gender, natural metabolism and activity levels. So there are still lots of factors to sift through before doctors can figure out how best to control weight gain across a broad swath of people.
In the meantime, the standard prescription of healthy eating and exercise may be your best bet for fitting into that neglected bathing suit hanging in the closet.