Ketogenic diets or low carbohydrate diets routinely appear in the media with stories ranging from the negative health implications they can have including reduced bone health to the benefits they have on weight loss, diabetes and performance in elite athletes. With such conflicting information out there, let’s turn to the literature to see what the truth is about ketogenic diets and performance.
What Is Keto?
For a diet to be considered ketogenic it needs to contain less than 50g of carbohydrates (CHO) per day, be high in fats 70-80%, with the remaining 15-25% coming from protein. For athletes, especially endurance athletes CHO’s are the preferred fuel source for enhancing performance therefore by restricting CHO intake it forces the body into ketosis. In a ketosis state, ketone bodies (fat molecules) are released from the liver, producing an alternative fuel source to CHO’s.
Why Athletes Choose a Keto Diet
The utlization of fats as fuel becomes one of the primary reasons endurance athletes are attracted to ketogenic diets. This attraction occurs because fats provide a better source of energy than carbohydrates and there is an abundance of fat stores compared to the limited glycogen (CHO) stores in the muscles. Other reasons athletes are attracted to ketogenic diets include, the feeling of enhanced recovery, improvements in body composition and reduced post exercise inflammation.
It is well documented that ketogenic and low CHO diets are beneficial for body composition changes, namely weight reduction through decreases in adipose (fat) tissue. This isn’t surprising considering fat oxidation (fats being burned for fuel) becomes the primary energy source in a ketogenic diet.
Keto Diets and Athletic Performance
In terms of athletic performance the evidence is less clear. Majority of the current studies looking at athletic performance and ketogenic diets routinely involve small sample sizes of elite athletes and over a short time frame (3-10 weeks). As a result, any findings from these studies need to be interpreted and considered with other available literature. With such a specific cohort (study population) transferability of the results to other non-elite athletes becomes difficult.
In terms of endurance performance, there appears to be a reduction in performance during high intensity bouts above 70% of VO2Max when on a short term (3-10 week) ketogenic diet. The postulated causes include; fats require greater oxygen uptake and energy to be broken down compared to CHO, possible impaired glycogen metabolism at higher intensities. However, one study which looked at endurance athletes who were on ketogenic diets for longer than 6 month showed no changes in muscle glycogen stores.
For resistance training and lean muscle mass it appears that ketogenic diets result in reduced body mass, with conflicting evidence on whether it impacts on lean muscle mass. Regardless, it doesn’t appear to negatively impact strength.
Bone health is an interesting area. An Australian study published early this year revealed that bone resorption (break down of bone) increased and bone formation (new bone formation) decreased in elite athletes (race walkers) while on a short term (3 – 3.5 week) ketogenic diet. This study received a lot of press because the implications are potentially serious; ketogenic diets leading to reduced bone mineral density and bone injuries in elite athletes. However, this study needs to be put into context. It looked at elite race walkers, predominantly males who were on short term ketogenic diets. This is a very specific subgroup of people. As the study rightly points out, further research is needed to understand these findings further before any definitive answer is known about ketogenic diets (short and long term) and bone health in elite athletes and in the general population.
The Wash Up
When it comes to athletic performance ketogenic diets improve fat oxidation and reduce body mass, which is important in endurance sports and weight-class based sports such as boxing, weight lifting, martial arts etc. They don’t negatively affect exercise capacity at submaximal workloads below 70% VO2Max but they do appear to reduce exercise capacity at high intensities. They may also reduce endurance capacity but this appears to be individual, with some athletes being affected and others not. At present, finding the right balance of lower CHO levels which are higher enough not to impact performance at higher intensities is the best approach to maximize athletic performance through a ketogenic diet. One final note, even though CHO intake levels are low, this does not mean an athlete will be running a negative energy balance; energy expenditure should always match energy intake.