Adapting to Fat on a Low-Carb Diet

How long does it take for your body to become adapted to a low-carbohydrate diet? This question has relevance for everyone trying low-carbohydrate diets for weight loss, and also for a raging scientific controversy.

Some critics have pointed to studies lasting just a few days, as evidence that low-carbohydrate diets are detrimental to metabolism. But these studies have a fatal flaw, as it relates to long-term effects.

On a standard high-carbohydrate diet, the brain is critically dependent on glucose. So if you restrict carbohydrate in your diet (or you fast for more than a day), your body must initially break down protein from muscle for conversion into glucose. However, this response is only temporary because over time, the body converts to a special fuel, called ketones.

Ketones are produced directly from fat and have the critical ability to cross the blood-brain barrier and nourish the brain. (Ketones may also have anti-inflammatory, anti-aging, and anti-cancer actions, but that’s another story.)

How quickly does this process of fat adaptation take? At least several weeks, as shown in these 6 figures:

Owen OE, et al. JCEM 1983, 12:359–379

FIRST: Generally speaking, the most potent stimulus for ketone formation is fasting, since the consumption of anything that could be converted into glucose (carbohydrate and protein) is zero. As this figure shows, the blood levels of each of the three ketones (BOHB, AcAc and acetone) continues to rise for at least 3 weeks. The prolonged nature of adaptation to complete fasting has been known since the classic starvation studies of Cahill 50 years ago. It stands to reason that this process might take even longer on standard low-carbohydrate diets, which inevitably provide some carbohydrate and significant protein.

Yang MU et al. JCI 1976, 58:722–30

SECOND (above): Men with obesity were given a calorie-restricted ketogenic diet. As you can see from the figure, ketones in the urine continued to rise for 10 days through the end of the experiment, and by then had achieved levels only equal to those on day 4 of starvation. Presumably, this process would be even slower with a non-calorie restricted ketogenic diet, because that would inevitably provide more carbohydrate and protein, slowing down the process of converting to ketosis.

Vazquez JA et al. Metabolism 1992, 41:406–14

THIRD (above): Women with obesity were given a calorie-restricted ketogenic diet compared to a non-ketogenic diet, both with the same protein. For 3 weeks, the break-down of lean tissue like muscle (i.e, nitrogen balance, see bottom panel) was greater on the ketogenic diet compared to the non-ketogenic diet, but this difference was completely abolished by week 4.

Phinney SD, et al. The human metabolic response to chronic ketosis without caloric restriction: physical and biochemical adaptation. Metabolism 1983; 32:757–68

FOURTH (to the left): Lean athletic men were first fed a standard high-carbohydrate diet, then changed to a ketogenic diet (same protein). Here again, nitrogen balance initially plummeted, but then returned to normal after about 1 week) more quickly than for the woman with obesity in the study above). So the time required to adapt to a high-fat diet may depend on initial weight and metabolic factors.

Horton TJ et al. Fat and carbohydrate overfeeding in humans: different effects on energy storage. Am J Clin Nutr 1995, 62:19–29.
Hall KD, et al. Energy expenditure and body composition changes after an isocaloric ketogenic diet in overweight and obese men. AJCN 2016; 104:324–33

FIFTH (to the left): In a classic overfeeding study, 16 men were studied for 2 weeks while consuming 50% extra calories from either carbohydrate or fat. During the first few days, the participants stored more body fat on the high fat compared to high carbohydrate diet — an initial difference of about 80 g per day. But this difference quickly diminished, and by 2 weeks, daily fat balance was statistically indistinguishable between the two diets. What would happen after a few months? We can’t say.

AND SIXTH (to the left): In a study by a chief critic, 17 men with high body weight were first given a standard diet, followed by a ketogenic diet. Unfortunately, the study wasn’t randomized, and for many reasons considered elsewhere, was biased against the ketogenic diet. Even so, you can see that for the first 2 weeks on the ketogenic diet (left arrow), the rate of fat loss decreased. But after 2 weeks (right arrow), there was a clear acceleration in fat loss on the ketogenic diet.

Indeed, among the relatively few studies with adequate numbers of participants lasting more than 2.5 weeks, there is some evidence for an advantage of low-carbohydrate diets for metabolism, and possibly body composition (the relative amounts of fat to lean tissue), but we need more research to know for sure.

The bottom line is that fat adaptation to a low-carbohydrate diet takes at least 2 to 3 weeks, and perhaps longer. During that time, you might not feel quite as energetic as you would subsequently. And if someone tells you a low-carbohydrate diet won’t work based on very short studies, tell them you’re going to await higher quality, long-term research.

ADDENDUM APRIL 2019: Late last year, we published one of the largest and longest macronutrient feeding trials, including 164 participants studied for 20 weeks on low-, moderate- or high-carbohydrate diets. The results, linked here, show a metabolic advantage for reduced-carbohydrate diets over a relatively long-term, consistent with the Carbohydrate-Insulin Model of obesity.

ADDENDUM AUGUST 2019: A major review in the premiere nutrition journal AJCN confirms that adapting to a ketogenic diet is a: “prolonged process involving adaptations from multiple organs to generate and use KBs [ketone bodies] as an alternative energy source. It takes weeks for the body to start using KBs [efficiently] and may require months to reach an adequate and steady level.”

ADDENDUM FEBRUARY 2021: We updated and re-analyzed a prior meta-analysis of lower- versus higher-carbohydrate diets and energy expenditure (calorie burn). The shorts trials (less than 2.5 weeks) show a small advantage to the high-carb diet. However, the longer trials show a larger advantage to the low-carb diet. The statistical confidence in this time effect was extremely high. We need more research, but one thing is clear: When interpreting short studies:

Proceed With Caution




Physician, Nutrition Researcher, and Public Health Advocate. #1 NY Times bestselling author ofALWAYS HUNGRY? and ALWAYS DELICIOUS

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Dr. David Ludwig

Dr. David Ludwig

Physician, Nutrition Researcher, and Public Health Advocate. #1 NY Times bestselling author ofALWAYS HUNGRY? and ALWAYS DELICIOUS

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