What You Should Know About Keto-Adaptation

Are you keto-adapted?

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Keto-adaptation (also sometimes called "fat-adaptation") is the process the body goes through on a ketogenic diet as it changes from using primarily glucose for energy to using primarily fat for energy. The "keto" part refers to ketones, which are water-soluble molecules that the liver makes when metabolizing fats, particularly when carbohydrate intake is low.

Ketones can be used for energy by most tissues in our bodies, including the brain (which cannot use fatty acids directly).

Our bodies are always using a mix of fat and glucose for energy, but in a non-keto-adapted state, the body reaches for glucose first, since only low amounts of ketones are normally generated during fat metabolism and these are preferred by other tissues such as the heart. Since the brain cannot use fat, it is dependent on glucose when we are in a non-keto-adapted state. Because of this, when we go on a low-carb diet we can sometimes experience what I call "carb crash" or is sometimes referred to as "the Atkins flu" when our bodies run out of glycogen stores (this is the main way our bodies store glucose). It is when the glycogen stores get low that the body begins the process of keto-adaptation.

A Brief History

Some of the first rigorous research looking at keto-adaptation was in the 1980s when researcher Dr. Stephen Phinney studied various groups of people on ketogenic diets.

One of the studies was of highly trained bicycle racers. At first, the performance of the cyclists declined on the diet, but soon the decline began to reverse, until by the end (4 weeks) they were able to accomplish the same amount of cycling that they had at the beginning, but with noticeably less fatigue.

This decline and recovery were dubbed "keto-adaptation". In the years since we have learned that many athletes on ketogenic diets for more extended periods can improve their performance substantially above their baseline levels.

How Long Does Keto-Adaptation Take?

There is a fair amount of individual variability, but the process begins after the first few days on a ketogenic diet. Then, after about a week to ten days, many low-carbers suddenly start to feel the positive effects of keto-adaptation. They report improved mental concentration and focus and sometimes more physical energy as well. In insulin resistant people, blood pressure and blood sugar have usually begun to normalize.

By the end of the second week (sometimes up to 3 weeks), the body has usually accomplished the majority of its work in adapting to using fat for energy. By this point, hunger and food cravings are diminished and people often feel they have more physical energy.

After this, the body continues to make more subtle changes. For example, it gradually becomes more conserving of protein, so that people often notice a lessening of a desire to eat lots of protein. Another change that athletes often notice is less lactic acid buildup in their muscles with long training sessions.

Most non-athletes would not notice the more subtle changes, but athletes might. It can take up to 12 weeks for these changes to complete.

Can I Do Anything to Help My Body Adapt?

A lot of the tips on how to get through the first week on a low-carb diet can be helpful in assisting your body through the change from being a glucose-burner to being a fat-burner. Dr. Phinney and other experts also emphasize the importance of getting enough salt in the first two weeks, as the body lets go of a lot of sodium and it can leave a person feeling weak and tired.

Make sure to eat plenty of fat if you get hungry.

What Other Changes Occur With Keto-Adaptation?

We know that ketogenic diets (and low-carb diets in general) can reverse the signs of metabolic syndrome, Type 2 diabetes, and polycystic ovarian syndrome. Ketogenic diets are successfully used to treat some seizure disorders and signs are good that they may help other neurological disorders such as Parkinson's disease.

Truth to tell, the research on this is ramping up, and the more scientists look, the more they find. For example, we now know that people on ketogenic diets have less of the bad type of saturated fat in their blood. It also now seems that using ketones for energy may decrease oxidative stress on the body as well as inflammation, and may even be involved in turning on some genes that may be beneficial to health.

Once a Person Is Keto-Adapted, What Does It Take to Reverse the Process?

Some people find that their ketosis is pretty stable, as long as they eat a low-carb diet under about 50 grams of net carb per day. Some people, especially athletes and heavy exercisers, can eat more than that and still stay in ketosis. Others find they need to eat less carbohydrate to stay in ketosis. Other influences, such as hormonal fluctuations and stress, have been known to throw people out of ketosis.

The best thing to do if you are interested in experimenting with keto-adaptation, is to begin to measure your blood ketones (urine ketones can be used at first, but it's been shown that over time the body gets better at recycling the particular ketone excreted in urine).

Want more information of the science sort (and many more of the scientific citations than I will list here)? I highly recommend two books by the scientists Stephen Phinney and Jeff Volek: The Art and Science of Low Carbohydrate Living and The Art and Science of Low-Carbohydrate Performance.  Also, Jimmy Moore shares his experiences with long-term nutritional ketosis as well as technical information in his book Keto-Clarity, which he co-wrote with Dr. Eric Westman.


Forsythe, CE, Phinney, SD, et al. "Comparison of Low Fat and Low Carbohydrate Diets on Circulating Fatty Acid Composition and Markers of Inflammation". Lipids. January 2008 43:(1):65-77.

Phinney SD, Horton ES et al. "Capacity for Moderate Exercise in Obese Subjects after Adaptation to a Hypocaloric, Ketogenic Diet." Journal of Clinical Investigation 1980 66(5): 1152–1161.

Phinney SD, Bistrian BR et al. "The Human Metabolic Response to Chronic Ketosis Without Caloric Restriction: Preservation of Submaximal Exercise Capability With Reduced Carbohydrate Oxidation." Metabolism. August 1983 32(8):769-776.

Shmiazu T, Hirschey MD et al "Suppression of Oxidative Stress by ß-Hydroxybutyrate, an Endogenous Histone Deacetylase Inhibitor." Science 11 January 2013. 339:6116 pp. 211-214.

Volk BM, Kunces, LJ, et al. Effects of Step-Wise Increases in Dietary Carbohydrate on Circulating Saturated Fatty Acids and Palmitoleic Acid in Adults with Metabolic Syndrome. PLoS One. 2014 9(11).

Westman EC, Feinman RD et al. "Low-Carbohydrate Nutrition and Metabolism." American Journal of Clinical Nutrition August 2007 86(2):276-284.