Should Runners Adopt a High-Fat Diet?

Q I’m hoping you can clear up an ongoing nutrition debate among me and a few members of my running club. I believe that in order to be a successful marathon runner, you need to eat quite a bit of carbohydrate. My friends, on the other hand, are trying out a low-carb, high-fat diet, and they tell me that science has shown that it works better than high carb. Is this true? Should I switch? And what exactly is high-fat, low-carb anyway? – JAKE


A Jake, you must have read my mind. I too have been wondering about this, especially since proponents of the diet claim that it will make you thinner and faster. The basis for the argument is this: Carbohydrate stores are limited; whereas, most of us have an abundance of fat stores. We runners can only store so much fuel in our muscles (in the form of glycogen), and while we can load up on this fuel before race day (i.e. carb-loading) because we store limited amounts, we either need to fuel frequently during long, maximal efforts or our tank hits E, and we hit the wall. Fat, on the other hand, is readily available, seemingly limitless (or maybe it just feels that way when gazing in the mirror?), and is more than twice as energy-dense as carbohydrate. So, in theory mobilising and burning fat as a fuel source sounds awesome. But does it work?

Various experts have suggested that athletes “train low” (without a lot of carb on board) to improve future performance, as training with low muscle glycogen stores maximises physiological adaptations to endurance training. There have been a few clinical studies performed on athletes in a “low-carb” state, and some found that this method of training results in modest performance gains. Some experts, such as Timothy Noakes, Jeff Volek, and Stephen Finney, believe that this type of diet for athletes would benefit from a deeper dive.

Other experts, myself included, believe that the scientific (and anecdotal) evidence supporting findings that high-carb diets deliver performance benefits is overwhelmingly strong, and it would be pretty difficult to convince us to change our school of thought. Dr. Steve Hertzler, a registered dietitian and senior scientist with EAS Sports Nutrition, suggests that “if an athlete wants to exercise for long periods at a relatively low intensity, say, 50 per cent of their VO2 max, a low-carb diet may work fine and possibly work better than a high-carb diet. Athletes are primarily reliant on fat oxidation at a lower intensity, and the lower insulin response on a low-carb diet allows better access to adipose triglyceride.”

It’s possible that some of the studies supporting the efficacy of a high-fat diet are shown to work because the athletes in these studies are exercising at low intensities (i.e. not race pace or sprint intervals). Hertzler adds, “if an athlete is to push the intensity to 80 per cent VO2 max or higher, at present there is no evidence for a nutritional strategy that can allow fat oxidation to keep up. So far, high carb still wins at the higher intensity. Some studies of “training low” provide evidence of metabolic adaptations to enhance fat oxidation to some degree, but more importantly, these studies have not demonstrated consistent improvements in performance.”

In summary, if your aim is to train for many, many hours at low intensity and possibly mobilise fats stores and shed some weight, a low-carb, high-fat diet might be for you. But if your goal is to train at a higher intensity and/or a shorter period of time, then your plate should be filled with pasta and fruit. My hesitation in recommending this diet for athletes is based on the applicability of the diet. It’s very difficult to implement and sustain for long periods of time, as many runners’ preferred foods are high in carbohydrate (grains, fruit, veggies) and protein (low-fat dairy, lean meats, legumes) rather than high in fat (butter, plant oils, nuts).

Consider what you’d need to be consuming before jumping on board with this diet. You’d need to trade out your morning bagel and banana, or oats and fresh berries, for eggs doused in heavy oil and cream with a side of bacon. You’d be trading your soup, salad, and sandwich at lunch for some mushrooms sautéed in copious amounts of butter and oil, with a side of nuts, and salad with heavy dressing. For snack, you’ll be enjoying a small portion of berries with some roasted nuts rather than Greek yoghurt and granola. For dinner, instead of a grilled chicken breast with a side of brown rice, steamed broccoli, and a dinner roll, you’ll be dining on grilled ribeye steak topped with seasoned butter and some creamed spinach.

For some athletes, the alternative, high-fat choices sound tasty (because who doesn’t like bacon?), but for others the high-fat diet takes away too many of their favourite foods and replaces them with food choices they usually balk at. So in summary, Jake, the high-fat diet may work for weight loss, if you have the willpower to say no to the bagels, bread, fruit, lean meats, and other perennial favourites and instead fill your plate with low-carb veggies, oils, cheese, and fat. If you’re sprinting toward a new PB, this diet is probably not for you. But if you’re aiming to complete an ultra or a 24-hour race, a high-fat diet might be just what you need. – PAMELA NISEVICH BEDE

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