High carbohydrate diets to achieve peak performance in running have been standard nutrition advice for years. Recently, a number of high profile endurance runners have been challenging conventional wisdom by eating ultra-low carbohydrate diets, also known as the ketogenic diet. This low carbohydrate eating pattern challenges traditional advice and begs the question: has everything we've known about sports nutrition been wrong?

What is the ketogenic diet?

Though ketogenic is the newest fad diet, it's actually not new. It has been around since the 1920s, although until recently, its primary use was to treat epilepsy in children.

The ketogenic diet is a low carbohydrate, high fat diet (LCHF). What distinguishes it from other low carbohydrate diets such as Atkins or Paleo is the degree to which carbohydrates are restricted and the additional restriction of protein. A typical ketogenic diet is made up of 5 to 10% carbohydrates (up to a maximum of 50 grams per day), 15 to 20% protein and 75 to 80% fat. It should be noted that in practice and research there are varieties of LCHF diets that allow for slightly more modest carbohydrate and protein intake.

Both carbohydrates and protein can be converted into glucose and used rapidly as a fuel source by the body. In the absence of glucose, from dietary restriction, the body breaks down fat into ketones to use as an energy source. Hence the name, ketogenic diet.

What is the theory behind it?

The body has limited storage capacity for carbohydrates. Even when consuming a high carbohydrate diet regularly and sufficient carbohydrate loading leading up to an event, the average person only has the carbohydrate energy storage to last approximately 90 minutes of moderate to intense physical activity. Beyond this point, athletes need to ingest carbohydrates, such as gels or a sports drink, to provide fuel to the muscles.

In contrast, the body has limitless ability to store fat and even the leanest athlete has adequate fat storage to fuel an endurance race. The theory of the ketogenic diet is to adapt our bodies to using fat as the primary fuel source and therefore eliminating the need to rely on carbohydrate refuelling during endurance events.

Does the ketogenic diet work for runners?

The theory behind using the ketogenic diet for endurance athletes makes logical sense, but does it actually work? It is true that the body can adapt relatively quickly to using fat as a primary fuel source during exercise; however, there is no proven performance benefit. To put the research findings into perspective, if two equal runners are running a race, one on a ketogenic diet and one on a high carbohydrate diet, at sub-maximal effort (60 to 70% of maximal aerobic capacity), the two runners would be able to run the same distances and speed. But once the runners encountered a hill or wished to sprint to the finish, the ketogenic runner's performance would be compromised because they lack the glucose required to fuel anaerobic exercise.

The ketogenic diet may have a slight advantage over a high carbohydrate one in the recovery phase. Carbohydrate storage (glycogen) can take days to fully recover whereas fat storage does not require a recovery period. This would be useful to runners competing in multi-day endurance events, though a high carbohydrate runner could compensate by taking in additional carbohydrates during the event.

Interestingly, even athletes who eat a ketogenic diet during training and then a high carbohydrate diet leading up to an endurance event, still under-perform in anaerobic activities.

What effect will it have on my overall health?

Beyond the scope of performance, one must also consider the diet's practicality, sustainability and effects on overall health. To meet the dietary restrictions of the ketogenic diet, many foods are eliminated or severely restricted including fruit, starchy vegetables, dairy products (except cheese), legumes (beans and lentils) and all sweets. With such strict dietary restraints, a person undertaking the ketogenic diet will be highly involved in time-consuming carbohydrate counting, meal planning and food preparation. Additionally, attending social functions or eating out can be very challenging and lead to social isolation.

Because of the drastic nature of this diet, there is an adaptation phase of two to three weeks. During this time, flu-like symptoms such as lethargy, light-headedness, intense thirst, and nausea are expected. Once adapted, the symptoms should disappear, although the side effect of bad breath (sometimes known as "keto breath") will remain.

Of course, one must question how a person's health is affected by receiving 80% of their diet from fat compared to the recommended dietary guidelines of 20 to 35%. Pro-ketogenic advocates will point to research where waist circumference, blood pressure and cholesterol have decreased in participants on a ketogenic diet. However, this research was done in obese individuals with established heart disease and/or diabetes, not an active athletic population.

How do I decide if it's right for me?

Because of the endorsement from high profile athletes, the ketogenic diet has been gaining popularity in the endurance running community despite the lack of evidence to support an increase in performance. When determining whether or not to undertake a ketogenic diet, an athlete should ask themselves several questions:

• What is my goal—improvement in performance or decreased reliance on carbohydrate fuelling during endurance events?
• Will my training and/or event include anaerobic activity such as hills or sprints? Am I okay with sub-par performance in these activities?
• Do I have the time to commit to the meal planning and preparation on top of the demands of my training program?
• Am I willing to make sacrifices to my social life to adhere to a strict dietary protocol?
• Have I fully researched the possible impact on my long term health?

The ketogenic diet is not something I would recommend as a short term or long term weight loss strategy because while it may be effective for weight loss in the short term, in the long term the weight will creep back on and likely the person will gain back more than they lost. The success rate of this type of unconventional eating pattern depends on individual tolerance and the activities the athlete is training for. For people wanting to try a diet like this, my recommendation would be to meet with a registered dietitian to receive individualized advice that is tailored to their training and ensures that they are receiving adequate nutrients.

The ability to complete endurance activities without carbohydrate fuelling certainly calls into question the traditional high carbohydrate diet advice. Current sports nutrition is shifting away from the model of eating high carbohydrate at all times and instead focusing periods of high and low carbohydrate eating based on training demands.

Sample Ketogenic Diet

In case you're curious, here is an example of "a day in the life" on the ketogenic diet. If you're considering this approach, I recommend consulting a registered dietitian before making any changes to your eating patterns.

Breakfast
• 2 large eggs scrambled with ½ cup spinach, cooked in 1 tbsp. coconut oil
  2 strips bacon
  Coffee with 2 tbsp. heavy cream

Snack
• 1 avocado with salt and pepper

Lunch
• 1 chicken drumstick
  2 cups romaine lettuce with ½ cup chopped cucumbers
  1 tbsp. chia seeds
  Dressing: 1 tbsp. olive oil and½ tbsp. balsamic vinegar

Snack
• 1 oz. (16 slices) pepperoni
  100 g cheddar cheese

Supper
• 4 oz. salmon roasted in 1 tbsp. coconut oil
  2 cups zucchini roasted in 1 tbsp. olive oil

Snack
• 2 tbsp. almond butter with 1 cup sliced celery

Total: 2100 calories
45 g carbohydrate (8%)
99 g protein (18%)
173 g fat (74%)