A more technical version of this discussion is on PubMed.
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 4 figures:
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.
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.
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.
AND FOURTH: 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 3 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 can take at least 3 weeks.
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 studies lasting < 3 weeks, tell them you’re going to await higher quality, long-term research.