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Endurance athletes can experience gastrointestinal symptoms – and races are the worst of all, with a toxic combination of emotional stress and maximum effort.Michelle Siu/The Globe and Mail

After months of early mornings and gruelling workouts, the 25,000 runners in next month's Scotiabank Toronto Waterfront Marathon and its associated races will face one final hurdle before the gun fires: the lineup outside one of the 500 portable toilets deployed near the start.

For many, it will be an urgent mission. Researchers recently estimated that more than 65 per cent of marathoners and other endurance athletes "sometimes or often" experience gastrointestinal symptoms during exercise – and races are the worst of all, with a toxic combination of emotional stress and maximal effort.

The solution? A study published earlier this month suggests that a class of poorly digested carbohydrates collectively known as "FODMAPs" may be the underlying trigger for many athletes. But before anyone gets too excited about a potential new dietary fad, the researchers involved are hoping that runners heed some important caveats.

FODMAP stands for "fermentable oligosaccharide, disaccharide, monosaccharide and polyols." That includes a wide array of foods, including wheat, cow's milk, onions and garlic, stone fruit and many legumes. What they have in common is poor absorption in the small intestine, which in turn leaves them to ferment and produce gas in the colon.

In recent years, FODMAP restriction has emerged as a promising approach to managing irritable bowel syndrome – whose symptoms, including diarrhea, bloating, abdominal pain and flatulence, are strikingly similar to what many endurance athletes report, says Dana Lis, a Canadian sport nutritionist who recently completed a PhD at the University of Tasmania.

In a recent survey of more than 900 endurance athletes, Lis noticed that more than half of them reported avoiding certain foods that, unbeknownst to them, fell into the FODMAP category. Intrigued, she tried restricting FODMAPs for three days leading up to a series of hard runs in a 31-year-old triathlete with persistent gut problems, and noted an improvement in his symptoms.

One problem with this approach – and with athletes trying self-experimentation on their own – is the role of expectation. "If you're told something will work, the belief effect will have a strong impact," Lis says.

To get around that, Lis and her colleagues enrolled 11 recreationally competitive runners for a blinded crossover trial. The volunteers completed two six-day periods on different diets, which they were only told would test different types of carbohydrate, and reported their symptoms each day, including during intense running workouts.

The food was provided in unlabelled packages, with subtle substitutions so that it wouldn't be obvious which diet was "supposed" to help. For example, a high-FODMAP dinner was grilled chicken with sweet potato, beets, garlic and red onion on couscous; the low-FODMAP equivalent was grilled chicken with a smaller portion of sweet potato, red pepper and spinach on quinoa.

The results, which were published in Medicine & Science in Sports & Exercise, showed a consistent reduction in GI symptoms throughout the day in nine of the 11 subjects, with particular improvement in flatulence, urge to defecate, loose stool and diarrhea. Symptoms during running, on the other hand, were unchanged – perhaps because the test workouts weren't long or hard enough.

So is a low-FODMAP diet ready to take off as the answer to prerace toilet lines, and to gut issues more broadly – a new and improved version of gluten-free?

Not so fast, Lis says. The problem is that restricting so many foods comes with a nutritional cost. For example, avoiding wheat and legumes takes away valuable sources of prebiotics, which over time can produce negative changes in your gut bacteria that may weaken your immune system.

"I don't think I would ever suggest an entire FODMAP-free diet," agrees Trent Stellingwerff, a physiologist with the Canadian Sport Institute Pacific who was a co-author of the study. "There are just too many great fruits and veggies in there."

Instead, Stellingwerff says, a targeted reduction of FODMAPs for a few days leading up to an important competition may be a better approach. And a given athlete may only need to reduce a few selected FODMAPs once they figure out which triggers affect them most. Common examples are lactose, garlic, onion, and fruit such as apples and watermelon that contain fructans, he says.

That caveat raises an important question, says Patrick Wilson, a researcher at Old Dominion University in Virginia who studies GI problems in endurance athletes. If it's individual foods that cause problems in different athletes, is it really appropriate to blame FODMAPs as a group?

For example, lactose, a common trigger of GI problems, even in those without diagnosed lactose intolerance, was reduced to less than one gram a day from 28 grams a day in the new study. It's conceivable, Wilson points out, that all of the observed improvements could have resulted from the elimination of lactose.

Lis, too, agrees that more research is needed, and says FODMAP restriction shouldn't be a first response to GI problems – and certainly shouldn't be adopted as a general dietary approach by otherwise healthy people. But for those with persistent problems who have tried other approaches, FODMAPs may offer a new direction.

"My primary advice for athletes with GI syndrome would be to systematically work through possible GI triggers, food triggers, psychological stress, and avoid jumping on a trend and unnecessarily restricting foods," she says. "Athletes need to fuel properly to reach full potential, and a low-FODMAP diet can be very restrictive."

Alex Hutchinson is the author of the forthcoming book Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance, which will be published by HarperCollins in February.

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