The ketogenic diet is a hot topic in nutrition today. Keto recipes are everywhere and everyone knows someone who has at least tried the keto diet. But what exactly are “exogenous ketones,” where are they found, and how do they impact health and performance?
What are Ketones?
Ketones (also referred to as ketone bodies) are metabolites of fat oxidation and are produced in the liver. The foremost circulating ketones in the blood are acetoacetate (AcAc) and 3-beta-hydroxybutyrate (3HB). Their primary function is providing brain fuel because the brain can only utilize glucose and ketone bodies for energy. Additionally, ketones can function as hormone signalers and be oxidized in muscles during exercise.
Nutritional ketosis (i.e., increased ketone levels in the body) can be achieved in multiple ways, including fasting, following a carbohydrate-restricted diet (<50 grams/day), prolonged exercise without carbohydrate intake, or by consuming exogenous ketones. Exogenous means consuming a product (produced outside of the body), whereas endogenous describes breaking down stored fuels in the body (typically carbohydrate, fat and protein).
The terms ketosis and ketogenic have slightly different meanings. Ketosis describes elevated blood ketone levels from either endogenous or exogenous ketones (i.e., produced from the liver or taken as a supplement). Ketogenic describes a state of elevated ketone bodies from following a low-carbohydrate, high-fat diet (such as the ketogenic diet).
Forms of Exogenous Ketones
Exogenous ketones are available in two forms: salts and esters (both mono- and di-esters). Ketone salts are commercially available, relatively affordable (~$4/serving) and palatable (taste similar to sports drinks). They have only a mild impact on ketone levels, usually raising blood ketone levels to around 1 mmol, and carry a high salt load (as the ketone body chemically needs to be attached to sodium, potassium or magnesium).
Ketone esters, conversely, significantly impact blood ketones, potentially raising levels to 3-4 mmol within 20 to 30 minutes of ingestion. However, they are not widely available. At the time of this publication, one product is commercially available and is extraordinarily expensive at ~$30/serving. At this time, ketone esters are largely only accessible for research purposes, which is in its infancy.
Application and Research on Exogenous Ketones
To date, limited human-subject research has examined the effects of exogenous ketones for endurance performance. Interestingly, the two most substantial studies have conflicting results. Cox et al. (2016) and Leckey et al. (2017) demonstrate positive and negative results, respectively. However, it is