Canadian race walker Evan Dunfee taking part in study on high-fat diets

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The social media accounts of Olympic athletes tend to highlight the struggle: vividly inspirational images of gut-busting workouts in gruelling conditions.

Canadian race walker Evan Dunfee, on the other hand, posted a picture on Twitter earlier this month, taken under sunny Australian skies, of the giant cup of straight whipped cream that he was about to dutifully quaff. There was already some stuck to his beard. (To be fair, his Instagram account also features some gruelling workout shots.)

Dunfee is most famous for his fourth-place finish in the 50-kilometre race walk in Rio: he was briefly bumped up to the bronze medal position when a rival was disqualified for bumping him in the final stages of the nearly four-hour race, then bumped back down on appeal, and won widespread acclaim for his sportsmanship in declining to counter-appeal.

But he's also playing a key role in a groundbreaking scientific study of the effects of low-carbohydrate, high-fat diets on endurance athletes – a topic that is, if anything, even more controversial than his race in Rio. The results of the study's first experiments, which took place before last summer's Olympics, have now been published, and a follow-up study – including big cups of whipped cream to rectify unintended weight loss – is now under way.

While low-carbohydrate, high-fat – often referred to as LCHF – diets first gained popularity as a weight-loss tactic, they've more recently acquired a following among ultra-endurance athletes.

The basic reason is simple: Even the skinniest among us carries enough fat stores to fuel days and days of exercise, while our carbohydrate stores are strictly limited and have to be replenished in any activity lasting longer than a few hours. Adhering to a strict LCHF diet forces the body to get better at burning fat; in theory, this might allow you to run for hours without needing to deal with the messy and stomach-upsetting process of consuming food or sports drinks on the go.

In practice, though, studies have found that adapting to burning more fat comes with a cost: You also become less efficient at burning carbohydrates, which are the ideal fuel for high-intensity bursts of exercise.

That's bad news for most Olympic-distance competitive athletes, but may still be a worthwhile tradeoff in longer ultra-endurance events, and for athletes whose primary goal is to finish rather than compete – and perhaps also in race walking, where the rules of the sport forbid breaking into an all-out sprint.

Last winter, scientists at the Australian Institute of Sport assembled a group of 21 elite race walkers from around the world, many of them bound for the Olympics. Each athlete completed one or two three-week blocks of intense training while following either an LCHF diet or a conventional diet with 60-to-65-per-cent carbohydrate, 15-to-20-per-cent protein and 20-per-cent fat.

In order to trigger the desired adaptations, the LCHF diet has to be considerably more extreme than simply adding a few glasses of whipped cream. The diet, which was strictly supervised and individually prepared for each athlete, was 75-to-80-per-cent fat, 15-to-20-per-cent protein and fewer than 50 grams per day of carbohydrate – the equivalent of two small bananas.

The diet was a major adjustment, Dunfee recalls. He had to learn to fuel up for workouts with boiled eggs and "nutballs" (nuts and cocoa, basically), and down hunks of cake and cheese during long workouts. Sticking to such an extreme diet on his own would have been nearly impossible given his "meagre culinary skills," he says.

Before and after each training block, the athletes completed a series of physiological and performance tests. The results, which were recently published in the Journal of Physiology, were telling.

On the plus side, the three weeks of LCHF had turned the walkers into fat-burning machines. During exercise, they were able to reach fat-burning rates of 1.6 grams a minute, among the highest yet reported, and two-and-a-half times greater than their pre-LCHF values.

The problem was that they became less efficient while walking at their desired race pace. To produce a given amount of energy while burning fat instead of carbohydrate, they had to use more oxygen. In automotive terms, it was as if their fuel economy had taken a hit – and that translated into slower times in a series of 10-kilometre time trials compared with the athletes eating the standard diet.

So will this study end the debate once and for all? Not quite. For one thing, LCHF may have other advantages in events lasting tens of hours rather than just four, where efficiency is less important and refuelling is a greater challenge.

There was also an intriguing postscript to the study that didn't show up in the official results. After a few weeks back on carbs after completing his LCHF block last winter, Dunfee went on to set a national record for the 50-kilometre race walk; several other athletes in the study also seemed to have similar positive but delayed responses.

Coincidence? Dunfee wasn't willing to risk it in his Rio build-up, during which he stuck to a standard omnivorous diet. But now he and 27 other international race walkers are back in Canberra, at the Australian Institute of Sport, for an even bigger follow-up study.

This time, the athletes will be monitored and tested for several weeks after the dietary intervention ends, to see if there's a sweet spot where the benefits of the diet are maximized while the costs are minimized.

Dunfee is just finishing his LCHF block, and while he won't prejudge the results, he says one thing is clear: "If we see the results that we think we might, I'm going to need to learn how to cook for this diet."

Alex Hutchinson's latest book is Which Comes First, Cardio or Weights? Follow him on Twitter @sweatscience