The Core Diet Blog

The following content was provided by Registered Dietitian, Stevie Lyn Smith.

As racing season and warmer weather (hopefully) approaches inquiries from athletes about the newest trend, the Ketogenic diet, also known as Keto, keep landing in my inbox.

So what exactly is a Keto diet? Simply put it is a very low carbohydrate, high fat diet where one adheres to eating 20-50 grams of carbohydrate or less per day, with the macros breaking down to 75-80% fat and 15-20% protein. The diet is to exclude fruits, starchy vegetables, and all grains.

What theory is this all based on? The concept stems from the bodies energy systems and how they use fuel during exercise. Whether it’s a sprint or a long endurance event, our body needs fuel to complete what we’re asking it to do. This is where we see differing energy systems come into play. There is the glycolytic pathway that is used for anaerobic exercise such as a thirty second sprint or a resistance exercise. In aerobic exercise (bouts longer than 2-3 minutes) oxygen becomes more available to working muscle and the oxidative pathway is then utilized. This pathway used in endurance activity is where both carbohydrates and fat are both used for fuel.

Those who follow and believe in the Keto diet do so based on claims that by following a low carb, high fat diet that you train your body to adapt and use fat as fuel instead of carbs or what some call ‘metabolic efficiency’. It is impossible to train your body to only use fat for fuel, you may train your body to use fewer carbohydrates but the results don’t indicate a significant performance improvement in endurance athletes. The research that has been done on keto endurance athletes in multiple studies show no improvements in performance in races, impaired exercise economy, and decreased ability of an athlete to perform high intensity work.

Why do we suspect these are the results? The major findings suspect decreased glycogen stores (the stored form of carbohydrate) and reduced economy, meaning that more oxygen is needed to do the same amount of work.  Along with these performance and training limiters you are also adding in another; decreased ability to tolerate and digest race nutrition. As you train your body to utilize less for fuel you are in turn, also training your digestive system to be able digest and handle less race nutrition.  This is especially a concern with long course and endurance athletes as inability to take in and tolerate race nutrition is a major limiter in being able to perform on race day, even in non-Keto athletes. This is why we encourage our athletes to always practice their race nutrition and fuel EVERY workout. We call it ‘training the gut’. This takes out the chance of having your fueling have a negative impact or limit your performance.

Most of the athletic improvements seen in athletes who have followed a keto diet are most likely related to weight loss, as it is well known that weight loss, while maintaining a healthy body weight, can have a significant impact on improving speed.  But these improvements can also be achieved by following a balanced diet to promote weight loss. Due to how restrictive the keto diet is, it can put athletes at risk for nutrient deficiencies and in result have a negative impact on immune health.  Additionally, a recent study has also shown an increase in LDL-cholesterol, or the ‘bad’ cholesterol by 10.7% over six weeks. There is also emerging evidence on the negative effect following a keto diet can have on gut health.

Overall, we don’t know what the long-term effects of this diet are on both athletic performance and health, but the lack of nutrient and fiber rich foods is concerning.  In addition to immune and overall health concerns, although you may train your body to use fewer carbohydrates during exercise the shown effects on performance are minimal to decreased and can lead to race nutrition being a limiter in long course athletes.  At The Core Diet, we instead focus on creating sensible habits, around sound nutrition, that are sustainable for a lifestyle! Fad diets are just that, fads - “an intense and widely shared enthusiasm for something, especially one that is short-lived and without basis in the object's qualities; a craze.”


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