Diets very low in calories can induce rapid weight loss, but much of it could be muscle loss. Italian researchers compared two types of very low calorie diets, one low enough in carbs to induce ketosis (ketogenic diet), and another with enough carbs to prevent high levels of ketosis (non-ketogenic diet).
Both diets contained the same total energy (500-700 kcal/day). The ketogenic diet consisted of 1.2 to 1.5 grams of protein per kilogram of body weight (84 to 105 grams of protein for a 154-pound person) and only 6 grams of total carbohydrate. The nonketogenic diet consisted of 0.9 to 1.1 grams of protein per kilogram of bodyweight (63 to 77 grams of protein for a 154-pound person) and up to 30 grams of total carbohydrate.
After 3 weeks, weight loss was similar between the two groups (about 13 to 15 pounds). However, the non-ketogenic diet that was lower in protein and higher in carbs lost more than 7 pounds of lean body mass, whereas the ketogenic diet that was higher in protein and lower in carbs showed a gain of 4.2 pounds lean body mass.
These findings indicate that very low calorie diets can be implemented for a short period of time and result in substantial weight loss. However, there is significantly better protection from loss of lean mass when the diet has higher levels of protein and is lower in carbohydrate to induce ketosis. Ketones are known to inhibit protein breakdown, and emerging evidence has linked elevated ketones to greater satiety and improvements in several cardiometabolic risk factors.