A ketogenic diet limits the number of grams of carbohydrate the dieter may eat, which may be anywhere between 0 and 50g per day.
The remainder of the caloric intake must come primarily from fat sources, as well as protein sources, in order to maintain ketosis. (Ketosis is the condition in which the body burns fats and ketones instead of glucose for fuel.)
The Cyclical Ketogenic Diet can be complex, as it requires the dieters to closely watch the number of carbohydrate grams they eat during the intermittent period that they are not maintaining a strictly low carb/moderate protein diet.
When following a low carbohydrate diet, for the first few days, there is an adaptation period during which most people report feeling run-down or tired. Some people report feeling irritable, out of sorts, and unable to make decisions. For most people these feelings disappear after the adaptation period, however, and are replaced with feelings of calm and balance, and more consistent energy.
Not everyone is dedicated enough to wait long enough for the body to adjust, so the potential for burnout on a non-cyclic ketogenic diet (like Atkins) can be high. Although most people report a waning of cravings while in ketosis, some people may crave carbohydrates during ketosis for psychological reasons.
During a hypocaloric ketogenic diet, the carb cravings may combine with hunger pangs, making matters worse. (However, it is noteworthy that most people report having no hunger pangs on a ketogenic diet, due to its higher fat and protein contents, which help to increase a sense of fullness).
A CKD offers a way to combat this. It offers a cyclical "refeed" (sometimes also called a carb-up). During this phase, the diet comprises mostly of complex carbohydrates, with limited fat, sucrose and fructose. Since the glycogen stores in the liver and muscles are depleted, these carbohydrates go straight to refilling them, instead of being added to the body's fat stores. For this reason, the amount of calories consumed during a refeed can be far above an individual's usual dietary intake. While a typical CKD consists of 50g or less carbs per day, the typical refeed consists of 450-600g of carbs. Also a weight gain of 1-2 lbs is usually reported during refeeding.
The goals of a refeed are threefold:
to refill depleted glycogen stores
to upregulate hormones and thyroid activity that are depressed during dieting
to provide a psychological "break" that makes the rest of the diet easier to bear
The refueled glycogen stores can then be used for higher-intensity training to maintain or increase endurance / muscle mass after the refeed. Normally this sort of training would be relatively impossible on a pure ketogenic diet, as glycogen stores in the body are almost constantly depleted.
A refeed is risky, as careless individuals can end up gaining fat, instead of losing it, if they eat more carbs than is needed to refill their glycogen stores. The timing, duration and macronutrient composition of a refeed are crucial to the overall success of the diet.
From Wikipedia: