Fat, Fasting and the Keto Diet

The keto diet has become popular with both athletes looking to boost performance and people who are trying to lose weight. But does it make sense?

Every year it seems there is some new diet that takes the world by storm. Now, Atkins, Paleo and Whole30 can step aside because America has found a new dietary sweetheart: The ketogenic diet, also known as “keto.”

Social media is rife with posts of successful weight loss stories from people after “going keto.” What you may not know is that keto is no longer seen as just a weight loss panacea and many are turning to a ketogenic diet to address health concerns such as diabetes and heart disease. Athletes too have taken notice, wondering if this high-fat, low-carb diet could give them the edge they’ve been looking for.  But is keto really ideal for athletes or is it just another passing fad?

The Theory Behind Going Keto

Before determining if a keto diet can help your performance, it’s necessary to understand the basic principles.  Despite its recent popularity, the ketogenic diet is far from a recent phenomenon.  In the 1920s a doctor treating epilepsy noticed that patients’ seizures subsided after fasting for two to three days.  This shift was attributed to the body metabolizing fat as fuel by necessity when the body’s stores of carbohydrates were no longer available. Further research revealed this same metabolic shift could be attained without fasting, but instead by providing patients with a diet extremely limited in carbohydrates and very high in fat.  Thus, the ketogenic diet was born.

Normally our body is fueled by a combination of carbohydrates and fat. Severely restricting carbohydrates depletes the body’s glycogen stores so that carbohydrates are no longer an available fuel source. In order to supply needed energy, the body breaks down more fat and produces ketones, which provide fuel for the brain. This state of “ketosis” can be accomplished by fasting for a period of two to three days or by limiting carbohydrates to no more than 20 to 30 grams per day.

Unlike Paleo or similar low-carb diets, protein intake remains moderate and instead fat intake is increased to 70 to 80 percent of one’s total energy intake for the day. Since protein and fat are satiating, people on the keto diet tend not to feel as hungry as those on other diets.

With the number of people claiming success with the keto diet you might assume it is an easy diet to follow.  You would be wrong. First off, most people claiming to be “keto” are far from it. If you’ve never followed a low-carbohydrate diet then the concept of limiting yourself to 20 to 30 grams of carbs per day might not strike you as hard, but it is just that.

Sticking to a keto diet means basically cutting out grains, beans, legumes, fruits, milk and higher-carb veggies. This leaves dieters largely with a menu of meat, seafood, poultry, eggs, cheese, butter, oil, avocado, nuts and seeds. It also limits healthy foods such as fruits, which are high in carbs and provide many of the vitamins the body needs.

Going “keto” requires extreme attention to counting carbohydrates and doing so day in and day out.  Going over 20 (or 30, if you are larger) grams in one day, even a little bit, shifts the metabolism out of a state of ketosis. That’s not easy to stick to when you consider that one banana would cap out your carb count for the day.

Why Athletes Turn to Keto

Low-carb diets are nothing new for athletes.  Many adopt low-carb diets to become “fat-adapted.”  The goal is to train the body to become more efficient at burning fat for fuel. It makes sense when you think about it. The body can store enough glycogen to sustain about two hours of moderate-intensity activity. Once these stores run out, lack of available energy causes athletes to “hit the wall.” Fat, on the other hand, is plentiful in the body and even the leanest athlete has enough fat stores to power them for days of continuous activity.

So, wouldn’t it be great if we could just burn fat for energy instead of worrying about that dreaded moment our legs refuse to take another step?  If only it were so easy.

To be clear, the body never exclusively fuels itself off of carbohydrates or fats.  What fuel your body prefers comes down to how hard you’re working. During lower- to moderate-intensity activities such as a walk or slow jog, fat serves as the primary fuel source with carbohydrates contributing a small amount of energy.  As intensity increases, fat utilization decreases and carbohydrates become the preferred fuel source for the body.  This is known as the “crossover effect.” Those training for endurance events are not only training their muscles to be strong enough to tackle a new distance, but also training their bodies to maximize this crossover effect by adapting the body to use fat as a fuel source more efficiently.

So, if we can teach our body to push this crossover effect through training, why not our diet?
The keto diet may do just that.  Recent research has focused on whether a low-carb diet can delay this conversion, thereby allowing an athlete to exercise longer without the need for constant replenishment from sources like sports drinks or gels. One study found that athletes consuming a ketogenic diet were able to burn more fat during exercise than their carb-consuming counterparts. BUT…and this is a big “but,” the ketogenic athletes were not able to exercise at higher intensities. This means athletes adhering to a ketogenic diet can sustain continuous exercise longer, but are limited when it comes to speed.

That is not to say the keto diet won’t potentially yield a performance benefit for those seeking to hit a P.R.  In sports where strength-to-weight ratio is key, the ketogenic diet can provide weight loss during an athlete’s off-season.  Short-term studies have shown individuals following a ketogenic diet can lose a significant amount of weight.  However, just as with any diet, any weight lost will likely be regained if individuals return to their old eating habits.  Keto is likely no more effective than the plethora of diets marketed to those seeking to lose weight.  Another caveat is we do not yet know the long-term health effects of a ketogenic diet and many physicians worry that such a high-fat diet could increase risk for heart disease or other chronic conditions.

The Take Away

Despite all the hype, the keto diet is not a magic bullet for weight loss or athletic performance. Ultimately a keto diet may allow you to exercise for longer periods of time, but you will only be able to do so at low to moderate intensity. Translation: if your goal is to complete an ultra endurance event without concern for a time goal then keto may mean you’ll need to stress less about eating while on the go during the event.  However, if your goal is to push yourself in an event of any duration then the keto diet could be detrimental to performance.

As the Director of Nutrition at The Edge in Burlington, Jamie Sheahan, M.S., R.D. works closely with athletes to develop customized fueling plans to optimize their health and performance. Sheahan is also an adjunct professor of sports nutrition at the University of Vermont. An avid runner, she has completed more than 20 marathons.

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