The Keto Craze: Is It Healthy?

Various health and wellness trends are constantly popping up. Most recently is the “keto craze.” What exactly is the keto diet and if you’re an athlete, is this a good option to try?

The idea behind the diet’s popularity is that it can train your body to burn fat as fuel. This means your metabolism actually changes. You stop burning carbohydrates (your body’s preferred fuel), and start burning fat (and ketones) as fuel.

You become “fat adapted.”

How is this possible? What are the risks and benefits? And how does this apply to athletes?

The whole topic of ketosis is a hotbed of scientific research. More studies are being published regularly, but for now, this is what I’ve uncovered:

Metabolism 101: Carbohydrates vs. Fats (and Ketones)

In order to change your metabolism from being carbohydrate-burning to fat-burning, you need to give it a reason to change. Your body prefers burning carbohydrates as fuel. As a species, we have the ability to use a backup fuel for times of famine (or fasting). Even when fasting you still need to fuel your brain, heart, muscles, and other critical organs in order to survive. So, what does your body do when there aren’t enough carbohydrates to burn?

When your body has exhausted its carbohydrate supply and depleted stored glycogen, it will automatically turn to fat as a backup fuel.

In theory, the ketogenic diet is a diet that mimics the effects of fasting, without actually fasting. When you drastically lower the amount of carbohydrates you consume, your blood sugar and glycogen stores (in your muscles and liver) drop. Your body needs a form of energy to function, so it resorts to “plan B,” and uses fat for fuel.

When your body starts to tap into its fat storage, it naturally produces a type of biochemical called “ketones” (or “ketone bodies”). This is why it’s called “ketosis.”

Normally (when not in ketosis), ketones only account for 5% of the energy provided to muscles. After an overnight fast, the amount of energy your muscles get from ketones goes up to 10%. It can get as high as 20%-50% after 72 hours of fasting.

Another reason why ketosis has drawn such interest lately is that it requires less oxygen to produce the same amount of energy. This makes sense if the state of ketosis is specifically for surviving a famine. Our bodies need to adapt to less food and become more efficient.

Exercise requires a lot of energy. Even very lean athletes have more fat storage in their bodies than carbohydrate/glycogen storage.

Some athletes are becoming “fat adapted,” in the hopes of improved performance. Since our bodies become more efficient in ketosis and fat is our largest reserve of energy, I can see the attraction. However, before I jump on board the ketosis train, the science geek in me needs to completely understand the big picture and the research behind it.

How To Get Into Ketosis

There are two ways to get your body into ketosis.

The first is by eating a ketogenic diet. As you can guess, this is a very high-fat, very low-carbohydrate diet.

The second way to get into ketosis is by using ketogenic supplements.

FUN FACT: The word “ketogenic” means the ability to GENerate KETOnes.

There are your four standard ways to “go keto” with diet. Classic; Medium-Chain Triglyceride Diet (MCTD); Modified Atkins (MAD); and Low Glycemic Index Diet (LGID).

The Classic Ketogenic Diet

The ketogenic diet drastically reduces carbohydrate intake and cranks up fat intake. The fat intake is quite large.

How large?

The classic keto diet requires about 90% of your calorie intake to be fat! This leaves just 10% for both carbohydrates and proteins. Red flag: Many very healthy and nutrient-dense foods (such as fruits and vegetables) contain more than 5 or 10% carbohydrates. If you don’t eat a wide variety of fruits and vegetables, you can become deficient in many vitamins, micronutrients and fiber.

Protein supports muscle growth, repair and recovery. Red Flag: Some athletes may have difficulty consuming enough protein to maintain their muscle mass when protein is as low as 5% of daily calories.

Modified Ketogenic Diets

There are three modifications to the classic ketogenic diet that make it easier to maintain the state of ketosis. By “easier” I mean they require less than 90% of the daily calories to be from fat.

The first is the “Medium-Chain Triglyceride Diet” (MCTD). This modification allows 73% of calories from fat. However, it is mandatory to include a specific kind of fat called medium-chain triglycerides. This is because your liver actually makes ketones directly from these special fats. The MCTD recommends that MCTs make up about half of the fats ingested. This means that if you don’t use pure MCT oil, coconut oil would be your main daily fat source.

The second modification to the ketogenic diet is called the “Modified Atkins Diet” (MAD). This allows 65% of calories from fat as long as the carbohydrate intake stays below 5% of calories.

The third modification is the “Low Glycemic Index Diet” (LGID). This one targets 60% of the calories from fat. Because it allows a higher intake of carbohydrates, the carbohydrates eaten must be low glycemic (which means they have little impact on your blood sugar).

Who Should Definitely Avoid Ketogenic Diets?

Yes the ketogenic diet is strict and can be difficult to maintain. There are also many conditions that can be aggravated by a keto diet. Anyone with the following metabolic conditions should completely avoid the ketogenic diet:

  • primary carnitine deficiency;
  • carnitine palmitoyl transferase deficiencies;
  • carnitine translocase deficiency;
  • beta-oxidation defects;
  • pyruvate carboxylase deficiency;
  • acyl dehydrogenase deficiency;
  • hydroxyacyl-coenzyme A deficiency; and
  • porphyria.                                                                                                                                                         

Even in perfectly healthy individuals, the ketogenic diet can have unwanted side effects. The most common side effect is constipation and it affects between 14%-46% of people who follow a ketogenic diet.

Other side effects include:

  • Lethargy;
  • Metabolic abnormalities (e.g. dehydration, hypoglycemia, excessive ketosis, metabolic acidosis, and electrolyte imbalance);
  • Gastrointestinal side effects (e.g. nausea, vomiting, and rarely hepatitis or pancreatitis);
  • Kidney stones;
  • Growth failure, and vitamin/mineral deficiencies and their outcomes (including low bone density and increased fractures);
  • Heart and blood fat issues (e.g. cholesterol, etc.). Interestingly, these blood fat issues seem to return to normal after people stop the keto diet.

Keto Supplementation

Forget the keto diets; can I just take a supplement?


The ketones your body produces internally are called “endogenous” ketones. These ketones are produced by the liver from the fats in the blood.

Keto supplements are “exogenous” ketones because they’re not made by the body. These exogenous ketones help your body get into the state of metabolic ketosis, without the restrictive diet.

They are in the form of β-hydroxybutyrate salts (β-HB) or ketone esters and they produce ketosis for 30 minutes up to 6 hours. The esters are often buffered with a mineral salt such as sodium, potassium or calcium.

After taking a ketone supplement (usually a powder to be mixed with water), it quickly increases your blood ketone levels. This happens despite levels of blood sugar, glycogen, or insulin. In other words, you don’t have to restrict carbohydrates to increase your blood levels of ketones with these supplements.

These exogenous ketones are broken down and absorbed by the gut. They get into the bloodstream and go to the liver to be metabolized.
Some of the reported side effects of ketone ester supplementation are flatulence, nausea, diarrhea and dizziness.

Ketogenic Diets For Athletes

As an athlete, you use glycogen (stored carbohydrates) as your main fuel. When it runs out while exercising/competing it’s known as “hitting the wall.” Avoiding this is the reason why you use energy drinks and gels – to ensure there are enough carbohydrates to fuel your performance.

Athletes who are fat adapted, burn fat and ketones instead of glycogen. Using fat as fuel has the benefit of improved efficiency because less oxygen is required. It also produces less lactate.

But when your muscles use the fat and ketones as fuel, the energy uptake doesn’t happen as quickly as when you’re burning glucose and carbohydrates.

Ketosis can negatively affect your athletic performance for high intensity exercise, when energy is required very quickly.

There seems to be a lack of performance benefits for most athletes who eat low carb, high fat diets. In fact, at least one study has shown impaired performance during high intensity workouts. At moderate to high intensities, muscle glycogen is the preferred source of energy.

Here is a quote from Chang, et. al, 2017:

“There is emerging evidence that [low carbohydrate high fat] LCHF diets could be beneficial, particularly for performance in ultra-endurance sports. It appears that at least several months of adaptation to a LCHF diet are required for the metabolic changes and restoration of muscle glycogen to occur.”

Basically, what the science says so far is that a ketogenic diet may help with ultra-endurance, but it will take several months to become fat adapted.

When it comes to high intensity workouts, some studies show promise, some show no effect, and some actually show impaired performance.

Ketogenic Supplements For Athletes

There is a small difference between ketosis from a ketogenic diet and ketosis from a supplement. The main difference is that, with the supplements, there would still be sugar and insulin in the blood, and glycogen in the muscles.

Ketosis from supplements does a few things:

  • Allows muscles to keep their glycogen stores;
  • Decreases lactate concentrations;
  • Increases fat oxidation.

In rat studies, ketone supplements show improved energy efficiency (more energy with less oxygen needed). Do ketone supplements have the same effect in humans? More studies are needed. There is not a lot of research just yet on the effects of keto supplements on sports performance.

One recent study looked at two groups of cyclists who supplemented with optimal levels of carbohydrates. Some of them also incorporated a ketone supplement, and others had just the carbohydrates. The group who supplemented with both showed a small improvement in performance. They were able to go 2% farther (400 m) on average than those who only supplemented with carbs.

So, there may be some benefit to aerobic competitions when supplementing with both ketones and carbohydrates, particularly in highly trained endurance athletes. But, this was just one small study, so more research is definitely needed.

“Although ketone body supplementation has been proposed to be beneficial for endurance athletes and ketone esters are speculated to be routinely used by professional cyclists, to the best of our knowledge there is currently limited information on the effects of ketone body supplementation on exercise metabolism and performance in recreational and/or elite athletes…In conclusion, based upon the few available data and our current understanding of ketone body metabolism during exercise in a sports specific setting, we conclude there is currently no evidence to support the use of ketone bodies as an ergogenic aid under conditions where optimal evidence based nutritional strategies are applied.” (Pinckaers, 2017)

The bottom line with keto supplements is that they may help, but we really don’t know for sure. More research is needed.


There are several ways to get into the state of “metabolic ketosis.” One option is the classic keto diet, which consists of 90% fat. There are also three modified versions of the keto diet. And finally, there are ketone supplements that you can take to get in the state of ketosis for up to 6 hours.

The keto diet is not without risks. Because it is very restrictive, it can result in constipation, metabolic symptoms, gastrointestinal issues, as well as nutritional deficiencies, to name a few.

As for athletes, the results are inconclusive.
“Unless you’re an ultra-endurance athlete, becoming fat-adapted or adopting a ketogenic diet probably won’t improve your performance.” (Precision Nutrition)

And when it comes to keto supplementation, there is a lot of speculation, and little evidence.

“Our current understanding of ketone body kinetics during exercise is insufficient to warrant their use as an ergogenic aid in any practical sports setting.” (Pinckaers, 2017)


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Precision Nutrition, The Ketogenic Diet: Does it live up to the hype?

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