What Is Fat Adaptation?

The keto diet is based on the principle that your body can burn fat instead of carbs (glucose) for energy.

After a few days, a diet very low in carbs and high in fat puts your body in ketosis, a state in which it breaks down fatty acids to form ketone bodies for energy (1).

“Fat adapted” means that your body has reached a state in which it more effectively burns fat for energy. Keep in mind that this effect needs more research.

Reaching a fat-adapted state

To enter ketosis, you normally eat no more than 50 — and as few as 20 — grams of carbs per day for several days. Ketosis may also occur during periods of starvation, pregnancy, infancy, or fasting (2, 3, 4).

Fat adaptation may start any time between 4 and 12 weeks after you enter ketosis, depending on the individual and how strictly you adhere to the keto diet. Notably, endurance athletes may adapt even sooner (5, 6, 7, 8, 9).

Fat adaptation is thought to be a long-term metabolic transition to burning fat instead of carbs. Among keto adherents, burning carbs for energy is known as “carb adapted.”

Most people following non-keto diets could be considered carb-adapted, although their bodies use a mixture of carbs and fats. The ketogenic diet shifts this balance to favor fat burning.

Fat adaptation has been seen in endurance athletes who follow the keto diet for up to 2 weeks, then immediately restore carb intake before a competition (5, 6).

However, fat adaptation in non-athletes has not yet been studied.

summary

Most people burn a combination of fat and carbs, but those on the keto diet primarily burn fat. Fat adaptation is a long-term metabolic adaptation to ketosis, a state in which your body more efficiently metabolizes fat as its main energy source.

As you enter ketosis, your body begins to draw from its fat stores and dietary fat to convert fatty acids into ketone bodies for energy (1, 3).

At first, this process is often inefficient. When you’re still in the initial stages of the keto diet, a sudden carb increase can easily throw you out of ketosis, as your body prefers burning carbs (1, 3).

In comparison, fat adaptation is a longer-term state of ketosis in which you consistently derive most of your energy from fat given your changes in diet. This state is believed to be more stable, as your body has transitioned to using fat as its main energy source.

However, this effect is mostly limited to anecdotal evidence and has not been studied readily in humans. Therefore, fat adaptation as an efficient and stable metabolic state is not currently supported by scientific evidence.

Theoretically, once you reach a fat-adapted state, you can introduce carbs into your diet for short periods of 7–14 days — which allows your body to easily burn fat for energy once you return to a ketogenic diet.

However, most of this effect is limited to speculation or anecdotal reports.

People who might want to pause the keto diet for short periods include endurance athletes who may need the quick fuel that carbs supply, or those simply wanting a short break to accommodate events like the holidays.

Fat adaptation may be particularly appealing for these individuals, as you can reap keto’s benefits shortly after you transition back to the diet.

However, while keto cycling may provide flexibility, its benefits for athletic performance are disputed. Some reports find that it impairs your body’s ability to metabolize carbs in the short term (10).

Thus, more research is needed on the short- and long-term health effects of this eating pattern.

summary

Fat adaptation is a long-term metabolic state in which your body uses fat as its main source of energy. It’s considered more stable and efficient than the initial state of ketosis you enter upon adopting the keto diet.

Although the signs and symptoms of fat adaptation are primarily based on anecdotal accounts, many people report experiencing fewer cravings and feeling more energized and focused.

The onset of fat adaptation is not well described in scientific literature, though there is some evidence of it in endurance athletes (5, 6).

While a few studies have shown these effects, they’re limited to a timespan of 4–12 months. Thus, comprehensive, long-term studies on fat adaptation are needed (7, 8, 9).

Decreased cravings and hunger

Keto enthusiasts claim that decreased appetite and cravings are one of the signs of being fat adapted.

While the hunger-reducing effects of ketosis are well documented, the duration of this state varies from study to study. As such, there’s insufficient scientific evidence to support the notion that fat adaptation definitively reduces cravings (11, 12).

One study commonly cited by keto enthusiasts involves 20 middle-aged adults with obesity who were placed on a controlled, phased diet for 4 months. It’s worth noting that ketosis in the study resulted from keto combined with a very low calorie diet (13, 14).

This initial keto phase, which allowed only 600–800 calories per day, continued until each participant lost a target amount of weight. Peak ketosis lasted 60–90 days, after which the participants were placed on diets that incorporated balanced macronutrient ratios (13, 14).

Food cravings dropped significantly over the course of the study. What’s more, during the 60–90-day ketogenic phase, the participants didn’t report the typical symptoms of severe calorie restriction, which include sadness, bad mood, and increased hunger (13, 14).

The reason for this is unknown, but researchers believe it could be linked to ketosis. These findings are compelling and warrant further study in larger groups of people (11).

However, you should keep in mind that extreme calorie restriction can damage your health.

Increased focus

The ketogenic diet was initially devised to treat children with drug-resistant epilepsy. Interestingly, children have a greater capacity to effectively use ketone bodies for energy than adults (15).

Ketone bodies, particularly one molecule called beta-hydroxybutyrate (BHB), have been shown to protect your brain. While not entirely clear, the effects of BHB on the brain could help explain the increased focus that long-term ketogenic dieters report (15).

All the same, more research is needed into this effect and its relationship to fat adaptation.

Improved sleep

Some people also claim that fat adaptation improves your sleep.

However, studies suggest that these effects are limited to specific populations like children and teens with morbid obesity or those with sleep disorders (16, 17, 18, 19).

One study in 14 healthy men found that those on a ketogenic diet experienced increased deeper sleep but reduced rapid eye movement (REM) sleep. REM sleep is important because it activates brain regions associated with learning (20).

As such, overall sleep may not have improved.

A different study in 20 adults found no significant correlation between ketosis and improved sleep quality or duration (13, 14).

Thus, further research is necessary.

summary

Though advocates claim that fat adaptation improves sleep, increases focus, and decreases cravings, research is mixed. It’s also worth noting that fat adaptation isn’t well defined in scientific literature. Therefore, more studies are needed.

Due to a lack of comprehensive research, the long-term health implications of the keto diet aren’t well understood.

One 12-month study in 377 people in Italy found some benefits, but fat adaptation was not described. Furthermore, the participants didn’t experience significant changes in weight or fat mass (21).

What’s more, a study in over 13,000 adults linked long-term carb restriction to an increased risk of atrial fibrillation — an irregular heart rhythm that can lead to serious complications like stroke, heart attack, and death (22).

Yet, those who developed the condition reported a much higher carb intake than what keto allows (22).

On the other hand, a 24-week study in 83 people with obesity revealed that the keto diet improved cholesterol levels (23).

Overall, more comprehensive long-term research is necessary.

Precautions and side effects

The keto diet can be difficult to maintain. Short-term effects include a cluster of symptoms known as the keto flu, which includes fatigue, brain fog, and bad breath (15).

Plus, some reports indicate the diet could be associated with liver and bone damage (15).

Over the long term, its restrictions may trigger vitamin and mineral deficiencies. It may also impair the gut microbiome — the collection of healthy bacteria living in your gut — and cause unpleasant side effects like constipation (24, 25).

Additionally, given that very low carb diets are associated with an increased risk of atrial fibrillation, those with heart conditions should consult their physician before implementing keto (22).

What’s more, one case study in a 60-year-old man cautioned against the keto diet for those with type 2 diabetes, as he developed a dangerous condition called diabetic ketoacidosis — though the man also incorporated periods of fasting after a year on the diet (26).

Finally, people with gallbladder disease should not adopt this diet unless directed to do so by a healthcare provider, as increased fat intake can exacerbate symptoms like gallbladder stones. Prolonged intake of high fat foods can also increase your risk of developing this illness (27).

summary

Although more research is needed on the effects of fat adaptation, long-term keto dieting may be unsafe for those with heart conditions, type 2 diabetes, or gallbladder disease.

Fat adaptation is a long-term metabolic adjustment to ketosis, a state in which your body burns fat for fuel instead of carbs. It’s commonly claimed as one of the benefits of the keto diet.

Fat adaptation is said to result in decreased cravings, increased energy levels, and improved sleep. It may also be more stable and efficient than initial ketosis.

Nonetheless, more research is needed to not only determine the long-term effects of the keto diet but also how fat adaptation works.