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 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.
Normal Ketone Levels
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.
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.
The Benefits of Ketones
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. During low-carbohydrate ketogenic diets, dieters will sometimes monitor their ketone levels using urine or blood tests.
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.