Disentangling the Metabolic Effects of the Ketogenic Diet and Ketone Supplements during Exercise
Disentangling the Metabolic Effects of the Ketogenic Diet and Ketone Supplements during Exercise
When it comes to keto, the source matters.
Keto has become mainstream as a way to promote metabolic health, and indeed this is one of it’s most well-known and well-studied benefits. More contentious is whether ketogenic diets have any utility for exercise performance, especially among high-level athletes.
Ketogenic diets are characterized as being very low in carbohydrates (less than 50 grams per day), modest in protein, and high in fat (more than 80% of total energy intake). This dietary approach has been studied for weight loss, metabolic syndrome, type 2 diabetes, epilepsy, Alzheimer’s disease, and cancer.
More recently, the idea of leveraging ketosis for athletic performance has gained traction. Some athletes like ultramarathon runner Zach Bitter have succeeded using a low-carb approach to training and competition, but keto and low-carb have yet to make their way into the hallowed halls of sports nutrition.
In the last decade or so, the development and widespread availability of exogenous ketone supplements have created a way for anyone to achieve ketosis without restricting carbohydrates. You’ve probably seen some of these ketone supplements marketed by companies like H.V.M.N.
I’m particularly interested in the idea of using exogenous ketones to enhance physical performance, but my enthusiasm in recent years has waned.
There’s no question that ketosis is a unique metabolic state with some potential applications to human health and performance. But the theories of how and why ketones/ketosis should improve athletic performance haven’t stood up to the rigor of scientific testing.
In this post, we’ll talk about ketosis, delineate between endogenous and exogenous ketones, and discuss the ramifications of each for performance.
What are ketones?
We can elevate ketone levels in one of two ways: by consuming a low-carbohydrate, high-fat ketogenic diet or by consuming exogenous ketones. However, ketosis achieved via each route is drastically different in terms of how it changes our body’s metabolism of other fuel sources (i.e., fat and carbohydrates).
When we restrict carbs, exercise for a long time, or fast, our body breaks down fat stores and increases the release of free fatty acids into the circulation. These fatty acids are then used by the liver to produce acetyl coenzyme A (acetyl-CoA) which can then kickstart the production of ketone bodies, namely acetoacetate, beta-hydroxybutyrate, and acetone.
Ketones are interesting molecules and are sometimes referred to as the “fourth fuel” because our body can use them as an alternative energy substrate. But they also act as signaling molecules in the body to produce a variety of beneficial adaptations.
Ketones are converted into acetyl-CoA just like carbohydrates and fat, after which they can enter the Krebs cycle to be used in the production of ATP, our body’s “energy currency.” Some have argued that our body prefers ketones over fat or carbohydrates.
When consuming a ketogenic diet, blood ketones will typically increase up to 1 millimolar (mM) within a week and stay around this level until you stop the diet or consume something high in carbohydrates that kicks you out of ketosis.
In contrast, one can increase ketone levels without restricting carbohydrates by consuming one of several exogenous ketone supplements, which can spike blood ketone levels up to 1–5 mM within 30 minutes. Unlike a keto diet, exogenous ketones don’t sustain ketosis unless you keep dosing them — ketosis lasts around 2–4 hours when achieved via exogenous means.
Exogenous ketones can elevate ketones much higher (and much faster) than a ketogenic diet can, but the effects are short-lived. What makes exogenous ketones especially unique is that one can achieve ketosis even when consuming a high-carbohydrate diet. Consuming an exogenous ketone with carbohydrates and your blood ketones still rise.
You can think of exogenous ketones like any other supplement — caffeine, for example. They can be taken for a short time to achieve a desired effect, usually a (hopeful) improvement in cognitive or physical performance. And they don’t require you to change anything else about your diet. That’s why they’re called a supplement.
The keto diet, on the other hand, is a lifestyle change. Given the data that long-term adherence to a ketogenic diet may reduce high-intensity exercise performance (despite the well-known health benefits), athletes are sometimes hesitant to try it out. That’s why exogenous ketones represent such an interesting alternative.
How exogenous and endogenous ketosis alter metabolism
Habitually consuming a high-fat ketogenic diet increases the reliance on fat as an energy source at rest and during exercise. Compared to athletes who eat a mixed macronutrient/non-ketogenic diet, “keto-adapted” athletes metabolize about twice as much fat during exercise — they’re better fat burners.
It was initially thought that athletes on a ketogenic diet were somehow “sparing” muscle glycogen during exercise due to a greater reliance on fat. Rather, it seems as if they have a reduced or impaired glycolytic capacity (the ability to break down glucose) and an elevated fatty acid oxidation capacity. It’s true that during endurance exercise, athletes consuming a keto diet will rely less on carbohydrates, and this may be partially due to lower glycogen stores from chronic carbohydrate restriction.
However, at rest and during exercise, glycogenolysis (the breakdown of stored glycogen) and endogenous glucose production are lower in ketogenic athletes. This reduced rate of glucose production is offset by greater fatty acid oxidation to support the metabolic demands of exercise, but as previously mentioned, this may not be sufficient to sustain high-intensity performance. At a certain exercise intensity, our body will need rely on glycolysis for energy production, and if fuel stores are low, performance will suffer.
Nevertheless, keto diets do appear to have a “glycogen sparing” effect during exercise, especially if it lasts more than 90 minutes. One of the speculations as to how ketogenic diets may improve (ultra)endurance performance is by leaving enough glycogen for the “finishing kick” after hours and hours of lower intensity racing. This notion isn’t well supported, however.
Another potentially deleterious metabolic adaptation for athletes is the increase in a protein known as uncoupling protein in skeletal muscle. As the name suggests, uncoupling proteins “uncouple” the process of aerobic metabolism from ATP production — consumption of oxygen occurs in the absence of energy production, generating heat instead. Uncoupling proteins are great if you’re trying to lose weight, but less so if you’re looking to optimize your energy efficiency.
This is one explanation for the performance decrements that have been observed in athletes consuming a ketogenic diet. A greater oxygen uptake during exercise at the same intensity means a lower exercise economy and an earlier onset of fatigue. When studies have shown a performance decrease during exercise, it’s usually at an intensity greater than 70% — lower-intensity exercise performance is usually maintained due to a greater reliance on fat/ketones at this intensity.
In the study, it was reported that the low-carb athletes had rates of peak fat oxidation that were over 2-fold higher than the high-carb athletes. They also achieved their peak fat oxidation rate at around 70% of VO2 max, compared to 55% in the high-carb group. Despite consuming less than 50 g of carbohydrates per day, the low-carb athletes also had similar levels of muscle glycogen at rest and after a 3-hour endurance run when compared to the high-carb athletes, suggesting that habitually consuming a low-carb diet does not impair muscle glycogen content as previously thought.
All of the evidence we have available up to now indicates a few things about keto diets and athletic performance: you do become a better fat burner, but that might come at the expense of your glycolytic capacity and may even reduce your exercise economy, especially during higher-intensity exercise.
Let’s transition to discussing how exogenous ketones impact metabolism and fuel use during exercise.
Consuming an exogenous ketone will elevate blood ketone levels in about 30 minutes. However, this occurs independent of what you’re eating and without a concomitant increase in fatty acid availability. When you’re in ketosis after consuming a ketone supplement, you are not burning body fat. In fact, ketone supplements temporarily reduce fat breakdown by around 35–50% or more and inhibit the body’s natural production of ketone bodies.
Ketone supplements don’t appear to influence carbohydrate oxidation during exercise — as muscle glycogen breakdown and post-exercise muscle glycogen levels are similar after consuming a ketone supplement, carbohydrates, or a placebo. The lack of a glycogen-sparing effect when using exogenous ketones is probably explained by the fact that exogenous ketones contribute very little to energy production during exercise — only about 3% of fuel use comes from ketone body oxidation as suggested by some studies.
What this means is that even though blood ketone levels increase drastically following ketone supplementation, we might not use these ketones during exercise. But unlike with ketogenic diets, using a ketone supplement doesn’t require you to change your diet and restrict carbohydrates, and thus, you can begin exercise with higher glycogen levels. More glycogen plus the minimal contribution of ketones to energy production could translate to small but potentially significant improvements in performance, but it doesn’t seem that ketones are providing the “fourth fuel” that we assume they might.
Do exogenous ketones improve performance?
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