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Is Fat Adaptation a Path to Improved Performance and Fewer Gut Problems?

A high-fat diet may help reduce stomach distress for some athletes. Here’s how it works and when to consider trying it.

Photo: Getty Images

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Fueling during exercise is a delicate dance. Eat too little, and you risk heading straight for a bonk. Eat too much, and your gut may revolt, sending you to the roadside (or trailside) privy. Indeed, countless runners—from recreational to elite—have at one time or another pushed their fueling just a bit too hard and paid the price for it in the form of gut distress.

The whole goal of fueling during prolonged exercise is to maintain a steady supply of carbohydrate to burn. The fuel tanks that house carbohydrate in your body are rather small and can become depleted within a few hours, depending on the intensity of exercise. This is precisely why most dietitians tell their athletes to ingest carbohydrate during maximal effort exercise that lasts more than 60 to 90 minutes.

In contrast to carbohydrate stores (called glycogen), fat reserves are comparatively unlimited. Even a runner who weights 125 pounds with 10% body fat has roughly 50,000 kilocalories’ worth of fat energy squirreled away. To put that in context, consider that most runners expend 2,000 to 3,000 kilocalories during a standard 26.2-mile marathon. In other words, there is virtually no risk of your fat fuel tanks running dry during endurance races, including single-stage ultras.

Because fat stores offer an essentially limitless supply of energy, there is a belief among some athletes and practitioners that training the body to burn more fat and less carbohydrate is a win-win scenario. Naturally relying more on fat for fuel would help preserve glycogen and also reduce the need to chow down carbohydrate during exercise, which, in turn, could lessen the odds of stomach troubles. The goal of this article is to examine this fat adaptation theory from a scientific perspective and discuss some of its implications for fueling, performance, and gut function.

Fuel Use during Exercise

The body can oxidize a variety of organic compounds to produce energy, but it has a predilection for carbohydrate (mainly glucose) and fat. When lounging on the couch, the majority of energy produced (say two-thirds) comes from the breakdown of fat, although there is quite a bit of variability between athletes. As people go from resting to jogging to full-out running, the percentage of energy derived from burning fat steadily declines. What this means is that the body inherently prefers to use carbohydrate at higher exercise intensities and that fat burning has a maximal limit, especially during vigorous exercise (>75% of VO2 max).

The human machine is quite adaptable, though, and it turns out that eating loads of fat increases fat utilization during exercise. In one illustrative study, a high-fat diet (60% of energy intake) eaten for just two days caused nearly a doubling in the rate of fat oxidation at a moderate exercise intensity in comparison to a diet that was only 22% fat. More recently, a study of elite ultrarunners who had been following high-fat, ketogenic diets for at least a half year found that fat burning was roughly 60% higher while running at 64% of VO2 max in comparison to similar-caliber ultrarunners who had been following high-carbohydrate diets. These studies are just two of the many that show that by eating lots of fat, the body can adapt to burn more fat during moderate-intensity exercise.

Dietary Fat and Performance

So, given that eating fat profoundly shifts the body’s ability to burn fat, why isn’t every endurance runner on the planet eating a high-fat diet? Although there are several reasons (e.g., taste, food preferences, tradition, etc.), an important one to consider is how high-fat diets impact your capacity to burn carbohydrate. Years ago, the observation that eating a high-fat diet accelerates fat burning during exercise was thought of as a “carbohydrate sparing effect.” This was a positive adaptation in the minds of many, as it meant that an athlete would have more carbohydrate stores remaining near the end of a prolonged exercise bout.

Over the past decade and a half, though, scientists have uncovered something interesting, which is that high-fat diets aren’t really carbohydrate sparing. Instead, they actually interfere with an athlete’s ability to use carbohydrate, particularly during intense exercise. In basic terms, this means that when a runner wants to kick it into high gear, their engine may sputter like a car with only three of its four cylinders working. This is because eating lots of fat dampens the activity of key enzymes needed to metabolize carbohydrate into useable energy during exercise. Indeed, two recent experiments (here and here) led by renowned Australian-based sports scientist Louise Burke confirm that high-fat diets impair performance during races that last about 40–50 minutes. These observations may help explain why elite distance runners often gravitate toward eating high-carbohydrate diets; in one survey of 10 Ethiopian runners, the average carbohydrate content of the diet was 64%.

While high-fat diets probably aren’t an optimal choice for competitions lasting up to a few hours, there is less clarity when it comes to ultras. Ultrarunners just simply can’t sustain high intensities during their races, and most of them lumber away somewhere between 40% and 60% of their VO2 max during single-stage events. Consequently, the contribution of fat to energy production is higher during these races than those lasting less than a few hours. To date, research on whether high-fat diets enhance ultraendurance performance has been mixed, though the studies typically don’t show any detriments to eating more fat. There are anecdotes of athletes successfully following high-fat diets as well as high-carb diets in the realm of ultramarathoning, and when you combine these anecdotes with the equivocal scientific evidence, the take-home point seems to be that performance on fat-laden diets will vary between athletes. Ultimately, a trial-and-error approach is needed to figure out whether a high-fat diet is a good option for an individual athlete.

Fat Adaptation as a Gut Remedy

Gut problems are incredibly common during ultrarunning, and on an intuitive level, it makes sense that some digestive trouble is inevitable when your run 50+ miles straight. As one example, 96% of runners at the 2013 Western States 100-mile Endurance Run reported some type of gut symptom, and 6 out of 10 reported nausea. Furthermore, these problems became increasingly prevalent during the latter half of the race, a time when an athlete’s internal carbohydrate stores typically dwindle. For an athlete who naturally burns more carbohydrate and less fat (due to following a carb-rich diet), running out of carbohydrate reserves can be disastrous in this scenario. If they happen to experience gut problems during the race, they will inevitably slow down because they are less capable of relying on their fat stores for energy and they won’t be able to consume much carbohydrate as their gut revolts. In contrast, a fat-adapted runner is naturally less reliant on carbohydrate to begin with and may not need to fret as much about potential stomach troubles from overeating carbohydrate.

These supposed benefits of high-fat diets on gut comfort during ultrarunning are entirely speculative. In other words, there aren’t studies showing that fat-adapted athletes have less gut distress during competition because they avoid consuming large amounts of carbohydrate. Still, for an athlete who has a sensitive stomach, a high-fat diet is one of several approaches they could take to minimize their risk of gut woes. The longer the race, the more likely it is that a high-fat diet may offer some benefits. As an example, it’s more likely that a high-fat diet would enhance performance during a 100-mile race than a 31-mile (50 km) race.

Best of Both Worlds?

So far, I’ve referenced two polar approaches when it comes to pre-competition nutrition: high-fat versus high-carbohydrate diets. What if, however, there were a mixed strategy that offered the best of both worlds? It turns out that several research groups have tried to achieve this sort of balancing act by having athletes follow a high-fat diet for a week or two followed by a day or two of crushing carbohydrate before competition. In this type of scenario, the goal is to upregulate fat burning while simultaneously maximizing muscle stores of carbohydrate. As a bonus, this strategy might also allow the gut to better tolerate carbohydrate ingestion during the race itself, should an athlete consume a good amount of it (say 45–90 grams per hour). The gut is trainable to an extent, and after just a few days on a high-carbohydrate diet, digestion and absorption of carbohydrate can become more efficient.

Studies that have done this sort of high-fat diet followed by carbohydrate restoration have shown interesting—albeit sometimes inconclusive—performance results. In one instance, six days of a high-fat diet followed by one day of eating loads of carbohydrate was compared to a standard high-carbohydrate diet. Ultimately, the high-fat and one-day carbohydrate restoration regimen resulted in a roughly 11% higher power output during a 1-hour cycling test that was carried out after four hours of cycling at 65% of VO2 max. However, these effects didn’t quite reach what’s called statistical significance, meaning it’s possible the difference was due to chance. Another study used a similar protocol and found comparable results.

Unfortunately, it’s incredibly challenging to recruit competitive runners for weeks-long studies where their diets and training are completely controlled. On top of that, the exercise tests often last 3 to 5 hours, and the volunteers repeat these tests a couple of times. (Imagine the fun of staring at a laboratory wall for 5 hours while on a bike or treadmill.) Indeed, the two studies I just outlined only had seven and five subjects, respectively! These tiny samples make it extremely difficult to distinguish between true performance benefits and day-to-day noise in performance measurements.


Fat is a key fuel for sustaining any form of prolonged exercise, and it’s not hard to understand why some runners are drawn to the appeal of high-fat diets. Before making the switch, it’s important to consider what sort of activity you want to excel at. If you’re trying to PR in the half-marathon, for example, eating a fat-laden diet is not your best choice. In contrast, if you’re running trail races that last 3+ hours, then a high-fat diet is a more viable choice. In theory, a high-fat diet could reduce gut issues that arise from trying to ingest loads of gels and sports drinks during competition.

On a practical level, most studies looking at the performance effects of high-fat diets used intakes of about 60%–70% of energy as fat. In the table below you can see a menu that provides roughly 61% of dietary energy as fat. Remember, if you do decide to go high-fat, it might be wise to re-up your carbohydrate intake for 1 to 3 days before competition so that your gut is ready for any carb-rich foods you eat along the way to the finish line.

Breakfast Avocado (1/2 c.)
2 scrambled eggs with cheese
2% milk (12 oz)
Snack Mixed nuts (1/2 c.)
Baby carrots (4 oz)
Lunch Smoked salmon (3 oz)
Mixed greens (1.5 c.)
Balsamic vinaigrette (3 T.)
Blueberries (1 c.)
Snack Brie cheese (2 oz)
French bread (1 small slice)
Dinner Mixed greens (2 c.)
Italian dressing (3 T.)
Fried tater tots (1 c.)
Free-range beef burger patty (5 oz)
Snack Mixed nuts (1/2 c.)

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