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Going Keto? Not so Fast...Literally

Posted by Field Work Nutrition Co. on
Going Keto?  Not so Fast...Literally

So you are thinking about "going keto" and experimenting with the latest fad diet that seems to be popping up everywhere.  Heck there are countless "keto" products lining the shelves at stores like Whole Foods and GNC (SAD!  Rest assured, we will not be joining this band wagon).  So what is "keto" anyway, aside from a slightly more scientific sounding, repackaged version of the Atkins diet?  If you are an athlete, or anyone who relies on your body for performance, should you try to implement keto practices into your daily nutrition?  Are there negative effects of excluding fresh fruit and healthy grains from your diet and substantially increasing the amount of fat and protein you consume (well...when you put it like that...).

Read below for a concise summary of the FACTS surrounding the keto diet and how it may affect you as an athlete thanks to our partners over at the Core Diet.  Then decide if it's something you need in your life.



By Stevie Lyn Smith

As racing season and warmer weather (hopefully) approaches inquiries from athletes about one of the newest trends, 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 body’s 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 is the pathway used in endurance activity and is where both carbohydrates and fat are used for fuel.  Exercise below roughly 65% Vo2 max appears to be unaffected by the total percent fueled by carbohydrate or fat, but exercise above 65% is shown to be inhibited when percent of fuel from fat starts to replace available carbohydrate.  Simply put, if you are going about your daily life and not ever pushing your body to work at or above 65% vo2 max, you likely won't notice any effects of a keto diet.  Keep in mind, this is still at the expense of excluding many nutrients from your daily intake.  If you are training or competing above 65% Vo2 max and do not have adequate carbohydrate, you are inhibiting performance.  

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.  There is scientific evidence of this, but not evidence of overall improved performance.  This concept has been dubbed ‘metabolic efficiency.’  It sounds very appealing and positive with a term like ‘efficiency’ attached to it, but there is more to the story. It is impossible to train your body to only use fat for fuel. Though you may train your body to use fewer carbohydrates and preferentially use fat as fuel, the results don’t indicate a significant performance improvement in endurance athletes (Note: greater utilization of fat DOES NOT equal improved performance.  It simply means greater utilization of fat.  Period.). Most research that has been done on keto in endurance athletes in multiple studies show no improvements in performance in races, impaired exercise economy (see also HERE), and decreased ability of an athlete to perform high intensity work.  Best case scenario, there is still a lot to be learned.  Despite some recent anecdotal reports of improved performance (key word "anecdotal"), jumping full-on into this style of eating, at the expense of eating a variety of fresh fruits, veggies, and whole grains over the course of a long period of time poses plenty of risks.  In the case of this study, athletes reported "feeling better," yet nearly all performance indicators dropped.  

Why do we suspect these are the results? The major findings suspect decreased glycogen stores (the stored form of carbohydrate in the body) 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 carbohydrate 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 on your body on race day or limit your performance.

Most of the performance improvements seen in athletes who have followed a keto diet are 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 which can have an overall negative impact on immune health, amongst other things.  Additionally, a recent study has also shown an increase in LDL-cholesterol, or the ‘bad’ cholesterol by 10.7% over six weeks for those adhering to the Keto diet. There is also emerging evidence on the negative effects following a keto diet can have on gut health as the good bacteria that live in your gut and provide so many benefits to you body, from digestive health to immune health and even psychological heath, rely heavily on the foods you eat to maintain an environment suitable for beneficial gut bacteria colonies to thrive. This includes many of the pre-biotic fibers found in fruits and vegetables.

 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 even possibly inhibiting, and can lead to race nutrition being a limiter for long course athletes.  At Field Work Nutrition (and 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.”

If you are an athlete or active person, and are interested in a nice succinct read from the American College of Sports Medicine on how they view fueling and daily dietary intake, not to mention excluding entire food groups...this is a good place to start.

For further reading on Field Work's food philosophy...and why we think diets like this are dumb, check out this article!


Stevie Lyn Smith, a Registered Dietitian with The Core Diet, who received her Bachelors of Science in Nutrition and Dietetics from Buffalo State College while completing her dietetic internship as a part of their coordinated program.

She began competing in endurance sports after a playing soccer and lacrosse competitively from elementary school through college. After her collegiate career as a lacrosse goalie ended she began running and completed her first road race, a marathon just a year and a half later. Since then, she has completed 5 ironman distance races, 17 marathons, a 50 mile ultra marathon, 12 half ironman distance races plus many other types of races, including long distance swimming events. In her 6th year as a triathlete she has qualified and competed in Ironman 70.3 World Championships and has many age group podium finishes. 

Her passion for nutrition began in her collegiate lacrosse career as she began to appreciate how what we eat can help to fuel us not just in workouts but also to be our best selves in our daily lives. This passion has grown throughout her time as an endurance athlete where optimal nutrition and recovery has become key to maintaining her active lifestyle. 


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