Carbs vs. Fats

If you’re a distance runner, you’ve probably experienced “bonking”: when your blood sugar drops, you hit the wall part way through a run and just don’t have any more gas in the tank. It’s not fun. Small wonder, then, that there have been so many studies about alternative fueling options for endurance athletes. Anyone who has spent months training for an event only to lose steam partway through would be within their rights to think there must be a better way. Depending on who you ask, there might be a different, better, approach.


Conventional wisdom has it that carbohydrates are endurance fuel. There are innumerable gels, drinks and bars on the market announcing themselves as “energy food” that are either high in carbohydrates, or caffeine, or both. Millions of marathoners can’t be wrong, right? Well, maybe. Let’s look at the research. While refueling with carbohydrates during endurance events appears to help maintain energy levels in endurance athletes, what would happen if those athletes were using fat, not sugar, as their primary fuel?


A ketogenic diet is basically the opposite of the standard endurance athlete’s diet. Keto looks like about 70 percent of calories coming from fat, another 15–20 percent of calories coming from protein, and only the remaining 10–15 percent of calories coming from carbohydrates. In some studies, a low carbohydrate diet is written off as ineffective because subjects experienced a slight reduction in athletic capacity after a six-week period. One criticism of this type of study is that it can take up to ten weeks for your metabolism to fully switch over to burning fat for fuel. Don’t worry; it’s only the first one to two weeks that you’ll feel the so-called “carb flu” as your body works its way through sugar withdrawal. During this time of adaptation, athletic performance will indeed suffer, as you’ve used up your glycogen stores and your metabolism is flailing about trying to figure out what to use as fuel instead.



The advantages of teaching your body how to use fat as fuel don’t just have to do with performance gains.


People who embark on a ketogenic diet often experience a rather sudden loss of water weight at the beginning of their diet, which can translate into faster race times just by virtue of carrying less physical weight across the finish line. Unfortunately, even seasoned athletes can experience a certain amount of fatigue as they adjust to their new way of eating.


Studies which allow for a longer fat-adaptation period sometimes show no significant change in endurance, or some positive change in trained athletes. One intriguing approach offers the best of both the low-carbohydrate approach and the high-carbohydrate way of eating., forcing the body to burn fat as fuel shifts the metabolism even after loading with carbohydrates for a few days. Following a ketogenic protocol for several weeks, followed by two to three days of carbohydrate replenishment seems to allow the body to continue prioritizing fat as fuel, saving muscle glycogen for later in the endurance event. It makes sense that optimizing all available fuel pathways would result in better performance.


When we’re looking at studies that show clear, conclusive evidence on a topic that has been contentious for decades, it’s important to look at who paid for the study. In the FASTER study on ultra-endurance athletes following a ketogenic diet, athletes were seen to burn significantly more fat as fuel during 3-hour endurance runs, and both the high carbohydrate group and the ketogenic group displayed similar degrees of muscle glycogen depletion. It’s worth noting that in this study the subjects were all seasoned competitive endurance athletes, and those who were part of the ketogenic group had been eating according to the protocol for an average of 20 months. Definitely no concerns that these athletes lacked time to adapt to fat as fuel. The only part that might give pause is that this entire study was jointly funded by Atkins and Quest, both companies who have a vested interest in the low-carb marketing game. It doesn’t mean the results of the study are not valid, but maybe take it as a large grain of salt.



The advantages of teaching your body how to use fat as fuel don’t just have to do with performance gains. People following very low carb diets often report reduced hunger, steadier blood sugar, controlled Type II diabetes, weight loss and better sleep. There’s some evidence showing that a low carb diet can protect your metabolic rate from dropping; research indicates that the body’s total energy expenditure (the number of calories that you burn by just existing plus your daily activities) drops less when subjects lost weight on a very low-carb diet than those who lost weight on a low-fat diet. That being said, a ketogenic way of eating is not for everyone. If a high-carb diet is working well for you, your body composition is where you’d like it to be, and you’re happy … it ain’t broke, so don’t fix it. Likewise, if you embark on a low-carb way of eating and find that even after the adaptation period you’re suffering from low energy, higher perceived effort during normal activities, and feel cold all the time, then by all means ditch it. A ketogenic approach is not for everyone.


Curious? Intrigued? The only way to find out if a ketogenic approach to endurance activities will work for you is to try it out. You might be pleasantly surprised to be free of all those sports beverages and gels.