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Phys Ed Gretchen Reynolds on the science of fitness.

Gluten-free diets are increasingly popular in the fitness community. But a new, carefully designed study of the effects of gluten-free diets on athletic performance suggests that giving up gluten may not provide the benefits that many healthy athletes hope for.

A study last year of almost 1,000 competitive athletes in Australia found that 41 percent currently were following some type of gluten-free diet, many because they thought it was healthier than their previous eating habits. A majority, however, said that they avoided foods containing gluten, a protein found in wheat, rye and barley, because they thought that they were allergic or overly sensitive to it, although only 13 percent had received a formal medical diagnosis of celiac disease, a serious autoimmune condition, or other gluten-related disorders.

Many also told the researchers that they believed that a gluten-free diet would reduce digestive problems, which are distressingly common among athletes. By some estimates, as many as 90 percent of distance runners, cyclists and triathletes experience occasional bloating, cramps, diarrhea and other gastrointestinal symptoms during or immediately after exercise, in part because the exertion diverts blood and fluids from the digestive system to areas of the body where they are needed more pressingly, such as the leg muscles.

In effect, those athletes in the study who were following a gluten-free diet expected that it would make them more physically robust, less prone to gastrointestinal upsets and generally better athletes.

However, no well-designed study had yet determined whether gluten-free diets fulfilled those expectations, as Dana Lis, a Ph.D. candidate in Australia who had conducted the 2015 study, realized.

So, together with colleagues from the University of Tasmania in Australia and the Canadian Sport Institute Pacific in British Columbia, she decided to mount the first randomized, controlled, double-blind trial of gluten-free diets for athletes.

Such an experiment would not be simple, because it is not easy to fool people about the type of foods that they are eating. People notice, for instance, if they are receiving or being denied gluten-rich bread.

So for the new study, which was published last month in Medicine & Science in Sports & Exercise, Ms. Lis and her colleagues hit on an ingenious solution, developing two sports bars that were indistinguishable in taste, although one contained gluten and the other did not. (The scientists served as tasters during the bars’ development, and “if I never eat another similar-tasting bar again, it will be too soon,” Ms. Lis told me.)

They next recruited 13 male and female competitive cyclists with no known gluten sensitivities and invited them to the lab. There the scientists tested their general fitness, as well as their digestive health, based on the levels of certain biochemical markers in their blood that can indicate G.I. damage and inflammation.

Then they had all of the cyclists start a gluten-free diet, using the sports bars and other foods provided by the researchers. The cyclists were to follow this diet for two sessions, each a week long.

During one of those weeks, each cyclist would be fully gluten-free. But during the other, he or she would be eating a fairly hefty amount of gluten, courtesy of the sports bar. Neither the cyclists nor the researchers would know when a cyclist was eating gluten and when he or she was not.

The riders were asked to maintain their normal training throughout the study and to complete daily questionnaires about any gastrointestinal symptoms and their general well-being.

At the end of each weeklong session, the riders returned to the lab and completed a strenuous time trial to test their athletic performance, and the scientists again checked for markers of intestinal inflammation.

Then the researchers compared how each rider had performed and, more generally, how they felt when either ingesting or avoiding gluten.

The results, strikingly, showed no significant differences. Cycling performance had proven to be essentially identical after a week in which a rider ate zero gluten or large amounts of gluten. Inflammatory markers likewise were indistinguishable.

For their part, the riders’ daily reports about the states of their digestive tracts and moods likewise showed little variation whether a rider was eating gluten or not.

Over all, Ms. Lis said, “we did not find a beneficial or negative effect of a gluten-free diet for athletes who had no clinical necessity for the diet.”

Of course, this study was small and short-term, with athletes following each diet for only a week. Whether avoiding gluten for a longer period of time would lead to verifiable improvements in performance or well-being remains unknown, Ms. Lis said.

For now, the takeaway from this study is that a gluten-free diet is not a panacea, she said. If you have frequent gastrointestinal symptoms, whether they are related to exercise or not, you might want to consult a nutritionist or physician and request a test for gluten-related disorders.

“An athlete’s nutrient intake and timing are so critical to performance,” Ms. Lis concluded. “I hope that people learn to be more objective in terms of what they hear and read about gluten-free diets and nutrition in general.”

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