In the fall of 2015, ultrarunner and blogger Matthew Laye placed himself on a high-fat, low-carbohydrate (HFLC) diet. He got 70 percent of his daily calories from fat and limited his carbohydrate intake to 50 grams per day or less. Laye had read that, by doing this, he would teach his muscles to burn more fat and less carbohydrate when he ran, and his endurance would increase. He embarked upon his HFLC diet journey with high hopes.
Those hopes didn’t last long. Many runners feel lousy for a while as they adapt to a HFLC diet. But Laye never stopped feeling lousy. “I was running nearly a minute [per] mile slower than my normal easy pace,” he reported on his blog. “Fast and hilly running, when I occasionally attempted them, were [nearly] impossible.” Worse, when Laye had blood work done, he discovered that his LDL (“bad”) cholesterol level had skyrocketed. He quit the diet.
Laye’s experience is very common. Many if not most runners who experiment with HFLC diets feel lousy, get slower, and suffer health consequences. Others report positive results, but even for them there are costs, including an unpleasant and disruptive transition period and tremendously restricted food and meal options.
Fortunately, recent science is showing that it’s possible to gain the benefits of HFLC without the consequences through alternative methods that are much less extreme. Specifically, the practice of performing select workouts in a carbohydrate-deprived state has been proven to provide an extra fitness boost—even for athletes who are already fit.
Scientific support for this practice comes from a study conducted by researchers at the French National Institute of Sport (FBIS) and published in the respected journal Medicine & Science in Sports & Exercise. The subjects were 21 well-trained triathletes, who were separated into two groups. Both groups were placed on relatively high-carb diets that supplied six grams of carbohydrate per kilogram of bodyweight daily. But on four days of each week, the members of one group got all of their carbs from their first two meals of the day and ate no carbs at dinner. These carb-free dinners followed an intense interval workout that members of the other group also performed.
The morning after the interval workout, all of the subjects completed an easy one-hour workout before breakfast. This workout was done in a mildly carbohydrate-deprived state by the triathletes who had followed the previous day’s interval session with a normal dinner but was done in a severely carbohydrate-deprived state by those who had followed the same workout with a zero-carb dinner.
These dietary and training patterns were kept up for three weeks. All of the subjects completed performance tests before and after this 21-day period. Members of the group that practiced carb-fasted training saw significant improvements in cycling efficiency, high-intensity cycling performance, and running performance within a triathlon time trial. There were no such improvements among members of the other group.
It’s important to note that high-fat diets and other low-carb diets have never been linked to performance gains in tests like these. In fact, they have repeatedly been linked to performance decrements including loss of anaerobic power, decreased VO2 Max, and reduced time-trial performance. That’s at least partly because the improvement in fat-burning capacity that comes from eating HFLC is counteracted by loss of carbohydrate-burning capacity, which is at least as important. But the subjects of the FNIS study (or half of them, at least) got the best of both worlds: a general diet that was high in carbohydrate to support maximum performance in hard workouts combined with the opportunity to gain the advantages of doing some easier workouts in a carbohydrate-deprived state.
The results of this study should not be taken to indicate that all athletes must follow precisely the same protocol to obtain the benefits of carb-fasted training. In fact, the protocol was a little weird, featuring high-intensity interval workouts on four consecutive days for three straight weeks—something that no successful endurance athlete I’m aware of does in the real world. I suspect that you can benefit just as much from employing the sequence of high-intensity afternoon workout/no-carb dinner/sleep/easy pre-breakfast workout just once or twice a week over a longer time period.
Also, as a nutritionist, I don’t like to see athletes or anyone else eat no-carb meals very often. The only natural foods that contain zero carbohydrate are meat, fish, and eggs. An all-meat/fish/egg dinner is not exactly nutritionally balanced.
An alternative to the protocol used in the FNIS study that does not require zero-carb meals is the early-morning endurance workout. Used more often by elite athletes, this approach entails doing a long, low-intensity workout either before breakfast or after a light, no-carb breakfast (e.g., a hardboiled egg and black coffee) and during which only water is consumed. You might want to consider alternating this type of workout with the FNIS protocol, employing one of the two formats this week, the other next week, and so on.
There’s no need to get too scientific in incorporating carb-fasted training into your race preparations. Just make it a regular, minor element of your routine in a way that fits your lifestyle. Indeed, it’s worth noting that Kenya’s elite runners practice carb-fasted training for reasons that have everything to do with lifestyle and nothing to do with science. The typical elite Kenyan runner does the first of his or her two to three daily runs before breakfast every day, a habit that may be an underappreciated factor in Kenya’s dominance of the sport. It’s also worth noting that elite Kenyan runners eat more carbohydrate overall than do runners of any other nation except their only true rivals, the Ethiopians: 10 grams per kilogram of bodyweight per day, or 76 percent of total calories. Like I said: The best of both worlds.