Is there fat gain during the carb-up (cheat day)?

During the first 24 hours of the carb-load, caloric intake will be approximately twice maintenance levels. This raises concerns regarding the potential for fat gain during this time period. We will see that fat gain during the carb-up should be minimal as long as a few guidelines are followed.

In a study which looked surprisingly like a CKD (Cyclic Keto Diet), subjects consumed a low-carb, high fat (but non-ketogenic) diet for 5 days and depleted muscle glycogen with exercise.Subjects were then given a total 500 grams of carbohydrate in three divided meals. During the first 24 hours, despite the high calorie (and carb) intake, there was a negative fat balance of 88 grams. This suggests that when muscle glycogen is depleted, incoming carbohydrates are used preferentially to refill glycogen stores, and fat continues to be used for energy production. Additionally, the excess carbohydrates which were not stored as glycogen were used for energy.

In general, the synthesis of fat from glycogen (referred to as de novo lipogenesis) in the short term is fairly small. During carbohydrate overfeeding, there is a decrease in fat use for energy. Most fat gain occurring during high carbohydrate overfeeding is from storage of excessive fat intake. Therefore, as long as fat intake is kept relatively low (below 88 grams) during the carb-up phase of the CKD, there should be minimal fat regain.

In a similar study, individuals consumed a low-carb, high fat diet for 5 days and then consumed very large amounts of carbohydrates (700 to 900 grams per day) over a five day period. During the first 24 hours, with a carbohydrate intake of 700 grams and a fat intake of 60 grams per day, there was a fat gain of only 7 grams. Collectively, these two studies suggest that the body continues to use bodyfat for fuel during the first 24 hours of carb-loading.

In the second 24 hours, with an intake of 800 grams of carbohydrate and a fat intake of 97 grams, there was a fat gain of 127 grams indicating that the body had shifted out of 'fat burning' mode as muscle glycogen stores became full. This is unlike the suggestions being made for the CKD, where the carbohydrate intake during the second 24 hours should be lower than in the first 24 hours. A large fat gain, as seen in this study would not be expected to occur on a CKD.

As long as fat intake is kept low and carbohydrate intake is reduced to approximately 5 gram/kg lean body mass during the second 24 hours, fat regain should be minimal. Once again, individuals are encouraged to keep track of changes in body composition with different amounts and durations of carb-loading to determine what works for them. Those who desire to maximize fat loss may prefer only a 24 hour carb-up. This allows more potential days in ketosis for fat loss to occur as well as making it more difficult to regain significant amounts of body fat.

In summary:

- keep your carb up to 1 day per week only - during carb up try to limit fats

Reference: The Ketogenic Diet: A complete guide for the Dieter and Practitioner by Lyle McDonald