In their recent book The Art and Science of Low Carbohydrate Living, Volek and Phinney describe two stages of keto-adaptation. In the first few days of a keto diet, your body is still running on glycogen stores. This is the toughest part of the process, because in order to break the vicious cycle of glucose-based metabolism, you have to avoid eating carbohydrates, even though your glycogen stores are dwindling. Fat metabolism is still not optimized, and ketone production hasn't become significant.

Another noticeable effect in the first days is water loss. One of the inefficiencies of glycogen storage is that it needs to be stored with water. It takes about 3 or 4 grams of water to store a gram of glycogen [1] . This means that as you deplete your glycogen stores you could lose up to 2 kg of water! Not only that, but high circulating insulin levels cause water retention by inhibiting sodium excretion (see e.g. [2]). The keto diet lowers insulin levels and increases insulin sensitivity, allowing excess fluid to be released. These combined effects are the origin of the claim that the weight lost on keto diets is due to water loss. In the very beginning, this is true, but subsequently, of course, it is not.

When glycogen runs out, you start producing ketones, and some are excreted in the urine. This is easy to measure, and some keto dieters use it to know if they are hitting a low enough level of carbohydrate restriction. This also marks the beginning of the second stage of keto-adaptation. Ketones are now becoming available for fuel, but they haven't yet risen to their stable adapted level. There is an interesting interplay between ketone use in the muscles and the brain. When ketone levels are low, the muscles tend to use them directly for fuel, but as levels increase, the muscles use them less, turning to fat for fuel instead. The brain, on the other hand, uses ketones proportionally to their concentration in the blood. This means that at low levels of ketones, the brain's supply is not much affected, because the muscles intercede, but above some threshold, the brain's supply rapidly becomes much higher. At this point, the brain can rely on ketones, and since it is no longer susceptible to running out of fuel, the need to eat frequently throughout the day to maintain mental function disappears. The muscles in turn now rely on fat: they finally have access to a virtually unlimited supply of energy, which is particularly valuable for athletes.

Much confusion has been generated by scientists not recognizing one or both stages of keto-adaptation. A few studies have been publicized claiming that low carbohydrate diets worsen mental or physical performance (e.g. [3], [4]). On reading the details, it turns out that the testing was done in the first few days of carbohydrate restriction. Obviously, these studies are not valid criticisms of the keto diet, except as measurements of the initial adaptation cost. They do not reflect the longer-term outcome.

Can you check that nsca.com link? I can't get it to open. Thanks!

Hmm... It looks like it is cut off. Hopefully this one will work -

http://www.nsca.com/uploadedFiles/NSCA/Resources/Documents/PowerPoint/Events/
July%2014%2010am%20Volek%20and%20Phinney.pdf

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