One of the latest dietary trends you’ve probably heard of by now is the ketogenic, or keto diet.

The keto diet is a twist on the age-old low carb, high fat diet - think of it as a more precise version. While estimates differ, most keto diet sources advise keeping total daily carbohydrate intake to 20 or fewer grams to achieve ketosis. This of course leaves you with only fatty and protein-based foods to eat – eggs, dairy, meats, fish, and added fat sources like butter and coconut oil. Most plant foods are out except in small amounts, as this is where our carbs come from. 

Here’s how the keto diet works:

Your body really only has 3 main fuel sources – fats, proteins and carbs. All foods are comprised of these macronutrients to one degree or another.

Of the three, your body preferentially burns carbohydrate – it’s just how we’re wired. Of course, you can also use protein and fat as fuel as well, they just aren’t the first choice.

When you strictly limit your carbohydrate intake (to say, less than 20 grams or so), your body will shift metabolic gears and begin using fat for energy. (In reality, metabolism will chew a bit of protein and fat at all intensities, but in very minute amounts).

When you begin to use fat as an energy source, metabolic byproducts are produced, known as ketone bodies. These can be used as fuel, and this is where the diet gets its name – if you’re burning fat you are said to be ‘in ketosis’.

One important item to note is that ketosis is a far cry from a very serious condition that can affect diabetics whose metabolism has gone off the rails, known as ketoacidosis. This won’t happen to a healthy person eating a decent ketogenic diet!

The keto diet is super popular as of late because it’s touted as the next best weight loss diet (like all of them, right?!). There is of course much debate raging as to whether it’s a viable method to lose weight with academics going back and forth about its merits.

In my clinical experience, we’ve definitely seen it work for weight loss in many people who’ve previously been met with resistance trying to lose weight on other types of diets. Which is to say, there isn’t one single overarching diet that will work for everyone!

What will it do to my performance?

Keto diets are very similar to fat-adapted diets. Chances are if you’re an endurance athlete you’ve heard of this approach as well. Fat adaption simply implies your body is adapted to burning fat, rather than carbohydrate.

The only athletes who thrive (performance wise) on a fat-adapted diet are those who go for long durations at lower intensities.   Think ultra running, Ironman distance-plus triathletes, or Appalachian Trail through-hikers.

Carbohydrate is required for fast-twitch intensities. Not saying you can’t do it, but your chances of bonking are very high if you try to set a 10k PR on a fat-adapted diet!

How long can you be on a keto diet?

There really aren't any limitations to how long you can stay on this diet.  If you've got a weight loss goal, or want to experiment with seeing how fat adaptation works for a season, then go for it.  

My only hesitation with it is that I worry about the lack of phytonutrients (all those healthy compounds found in plants) over the long term.  There's just so many health benefits to be derived from plant-based foods and the science is irrefutable in this area.  

A good solution would be to incorporate something like Greens First, which is essentially a vegetable and fruit powder made of many different plants - it'll deliver those important phytonutrients and boost your nutrition while on a keto diet.  Yes, it's got some carbohydrate in it so you'll need to account for this as your plan your meals.  But it's still a great way to get any missing nutrition in to make sure your body gets everything it needs!

Additionally, when you're on any kind of diet, a good multivitamin is essential. Think of a multi as form of inexpensive health insurance - you'll be sure to get all the vitamins and minerals that can be slightly lacking in a keto diet.  Check out Meta-Fem for women, or Athletic Nutrients for athletes for a couple of great multi's. 

To summarize, the keto diet may be a good diet for you if you’re looking to lose some weight. (There are other benefits to this dietary approach that are beyond the scope of this article). But, unless you’ve spent some time gearing your training to fat-adaptation, a keto diet will probably negatively affect your performance if you do anything other than low intensity exercise.