Intermittent fasting is worthless for endurance athletes. There, I said it (donning flak jacket…). You can keep doing it if you want to, just stop doing it because you think it’s going to make you faster.
It’s no better or worse than other forms of caloric restriction
There are three mainstream version of intermittent fasting:
- 5:2 – Eat normally for five days and restrict energy intake to about 600 calories for two days, although the two days don’t need to be sequential.
- 16:8 – Consume all your daily calories within an 8-hour window and fast the other 16. Most commonly, this means extending an overnight fast by skipping breakfast.
- No calories for 1-2 days at a time.
All three can be effective methods of caloric restriction, which will lead to weight loss. However, none of them have been shown to consistently accelerate weight loss or lead to greater weight loss compared to reducing caloric intake every day.
That said, intermittent fasting can be more effective than traditional caloric restriction if it causes a person to focus more on actually eating less. In the short term, almost any caloric restriction strategy will be beneficial if leads people to concentrate and make thoughtful decisions about what they eat. Not only does it lower energy intake, but also people tend to improve the nutritional quality of the foods they eat during these periods of higher focus.
Traditional caloric restriction strategies (“just eat less of what you eat already”) work, but only if people can stick with them. And because reducing portion size is less of a dramatic or attention-grabbing change, it’s sometimes not interesting enough to hold a person’s focus. Long-term compliance is the problem lurking in the background for any dietary strategy aimed a weight control.
Most of you just need to TRAIN
Intermittent fasting is a strategy aimed at tuning your aerobic engine to burn more fat and/or utilize all substrates more efficiently. That’s great, but you’re trying to fine-tune a partially built engine. For athletes who have reached the point of diminishing returns with fundamental aspects of training, optimizing fat oxidation makes more sense. The vast majority of 40- to 60-year old men and women balancing training with work and family priorities, however, have a huge capacity for aerobic development before optimizing fat oxidation will make a substantial difference in performance.
Want to burn more fat? Get MORE FIT.
Your ability to oxidize any fuel for energy increases as you improve your general aerobic fitness and sport-specific fitness. That’s one of the primary adaptations that result from endurance training. Your body adapts to training stress by developing mechanisms to produce more energy more quickly, and that includes developing the mechanisms that convert carbohydrate, fat, and protein into usable energy. Intermittent fasting can help to fine-tune your ability to oxidize fat for fuel, but that improvement pales in comparison to the increased fat oxidation that results from boosting your overall fitness. To put it in financial terms, you’re stepping over dollars to pick up nickels.
There is an in-between step between eating and regular intermittent fasting that can improve performance for some people. The concept of starting select workouts with low carbohydrate availability, called “Train Low”, has been shown to improve endurance performance. Athletes either do two workouts in a day, using the first to deplete glycogen, or they train in the evening and restrict carbohydrate intake through the night. However, Train Low also lowers workout quality, so it has to be balanced with high intensity workout done with high carbohydrate availability. It is also a strategy that is best used when training for generalized fitness, as opposed to during race- or event-focused training.
Weight loss solves a lot of problems
One of the problems with anecdotal stories from athletes who attribute their performance improvements to intermittent fasting is that it’s difficult to discern a causal relationship when the athlete was training and losing weight while also fasting. Many scientific studies suffer from this problem, too. If subjects lose an average of 15 pounds during an 8-week period of intermittent fasting, I’d expect their performance in a time trial or exercise test to exhaustion to improve – no matter the weight loss strategy. Even studies that compare performance improvements following different weight loss strategies typically suffer from small subject pools and issues of individual variability.
Significant weight loss isn’t just beneficial for going uphill. It reduces thermal discomfort and thermal stress, which reduces perceived exertion. It reduces the energy cost of basic daily functions and leads to various hormonal changes. In other words, weight loss leads to a cascade of changes that can improve athletic performance. And the training people did while losing weight helps improve performance, too.
Every workout counts for Time-Crunched Athletes
When you have less time to train, quality takes priority, and high quality training sessions require energy. If you have 3-4 workout opportunities per week, for a total of 6-8 hours per week, and you have fewer than 20 training sessions per month. With schedule conflicts, travel, and work, I find a lot of athletes average around 15 sessions and 25-30 total hours on a good month. My coaches and I have been very successful at improving performance for athletes within this timeframe, but we’ve also realized athletes who want to make significant progress have to be highly protective of that time. A handful of crappy or missed workouts can make a real difference, and a handful of great workouts can, too. Caloric restriction strategies increase the likelihood of low-quality training sessions, and lower power outputs and slower paces during interval workouts.
Eat Less, Just Make Sure It’s Enough
Most people in the US eat more calories than necessary. Athletes fall along the entire spectrum – some eat too much and some are chronically energy deficient. Every athlete has a sweet spot; enough energy from the right combination of nutrients to support high quality workouts and recovery, but without excess energy that leads to gain weight. Finding that sweet spot is a challenge and requires work, but in my view that’s the work you should be doing instead of fasting as a substitution for changing your food choices.
Generally speaking, eating less may be a good long-term strategy. A chronically lower-calorie lifestyle may be associated with longer lifespan and – as long as the food choices are good – reductions in disease risk. This is particularly true for diseases with a strong link to nutrition habits, like Type II diabetes. If fasting fits your long-term lifestyle, it reduces your overall calorie intake, and it makes you feel good, then go for it.
Just, please, stop doing it for performance.
CEO/Head Coach of CTS