Ketones Definition

chemical confiuration of a ketone
Chemical configuration of acetone, which is one the ketones our bodies produce. Alain Lacroix

Definition: There are two ways to look at ketones. Here, we are concerned with certain ketones (also called ketone bodies) in the human body, but for the record, a ketone is a compound containing a carbonyl functional group bridging two groups of atoms. 

There are a number of ketones in the human body, but the ones we are concerned with are generated in the liver when we metabolize the fats we eat, or if we use stored fat for energy.

Ketones can be used by most of the cells in the body for energy. Notably, the brain can use ketones for about 70-75% of its energy needs. Ketones are most likely to be generated under under certain conditions:

  • Low-Carbohydrate/Ketogenic Diet where the the body must rely on fats for energy
  • Starvation or fasting (a "low-everything diet")
  • Conditions where carbohydrate is present, but the body is not able to utilize glucose, usually due to a lack of insulin in Type 1 (and, rarely, Type 2) diabetes.

Most of the time, our bodies have mechanisms to prevent ketone levels from getting dangerously high, but the last condition -- "not enough insulin" -- can allow ketone levels to spiral upward into a dangerous condition called "diabetic ketoacidosis". This does not happen when ketones are elevated due to dietary restrictions alone.

What is a Normal Ketone Level?

For a person eating a "regular" mixed diet, ketone levels in the blood will generally be low - around 0.1 mmol/L (minimolars per liter).

After an overnight fast or after vigorous exercise, we might find that the level has gone up to 0.3 mmol/L. 

If there is no access to food, or if we consciously restrict carbohydrate in our diet, we will generate more ketones as we use our fat for energy.  A ketogenic diet will induce what has been dubbed "nutritional ketosis", which is between 0.5 mmol/L and 3 mmol/L, although some people find that they have levels that are somewhat higher.

  At this level, appetite decreases and many people find that fat loss is easier. For more about this, including why some think ketones are bad for us, check out What is Ketosis?

If a person goes on a fast or simply does not have access to food for 3 weeks or so, the ketone level might climb as high as 10 mmol/L.  And the dangerous state of ketoacidosis can send levels up to as 25 mmol/L or so.

Are Ketones Beneficial?

There is no doubt that in times of low access to food, ketones have kept people alive, as the body is not able to store much glucose.  On the other hand, people usually have quite a lot of fat that can be used as energy stores, and tissues in the body that can't use fat directly, such as the brain, can use ketones for energy.

In addition, there is growing evidence that using ketones may have positive effects above and beyond helping to prevent starvation.  In particular, when the brain uses ketones for energy, it may have a protective effect.  For more on the benefits of a ketogenic diet, see What is a Ketogenic Diet?

During low-carbohydrate ketogenic diets, dieters will sometimes monitor their ketone levels using urine or blood tests.

Pronunciation: KEE-tones

Also Known As: ketone bodies (also, colloquially, "ketone buddies")

Common Misspellings: keytones

Examples: acetoacetate, ß-hydroxybutyrate, acetone (these are the three in the human body generated during fat metabolism)


American Diabetes Association. Living with Diabetes: Checking for Ketones from the American Diabetes Association Web site 6/28/13

Gasior, Maciej, Rogawski, Michael, and Hartman, Adam. Neuroprotective and disease-modifying effects of the ketogenic diet. Behavioral Pharmacology (2006) 17(5=6):431-439

Volek, Jeff and Phinney, Stephen. The Art and Science of Low Carbohydrate Living. Beyond Obesity, LLC. 2011. Print.

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