People in the intermittent fasting group on Yahoo are trying out many variations of intermittent fasting. The most popular appears to be daily (or frequent) fasts of greater than 18 hours, without caloric restriction. This normally means following the Fast-5 program of eating all food within a five hour window each day. The Fast-5 approach avoids longer fasts, but provides many more of them. Following this approach, one is fasting most of the time. There has been little scientific data on the relative benefits of different variants of intermittent fasting, so many of us are keen to learn which provide the greatest benefit. Indeed, some remain unconvinced that intermittent fasting provides any benefit at all.
The Mattson group has just published a study of the effects of reduced meal frequency on glucose regulation (Carlson et al. 2007, Metabolism 56:1729. "Impact of reduced meal frequency without caloric restriction on glucose regulation in healthy, normal-weight middle-aged men and women"). They found that people on a diet that involves a single meal each day show elevated fasting glucose levels and impaired glucose tolerance. This is in contrast to an earlier study (Johnson et al. 2007, Free Radical Biology and Medicine 42:665) that tested alternate day calorie restriction and found improved diabetes risk profiles (also see the Johnson upday downday diet, which is promoted by the same James B. Johnson). The authors suggest (in the Carlson et al. paper) that the key difference is an overall reduction in energy intake.
That may be true, but there are two other important differences between the Fast-5 approach and alternate day fasting (including my method of fasting three times each week from dinner one day until dinner the next day, which is less rigorous than a full bedtime-to-waking fast of 30+ hours). First, even a dinner-to-dinner fast is longer (roughly 23 hours, on average, as opposed to 19 or 20). If the benefits of fasting kick in after 12 hours or so, this difference could be more significant than one might otherwise think. Second, the consumption of all daily calories within a five hour window is very intense, and ad lib eating over 24 hours need not be. Between fasts I usually eat dinner, a late-night snack, breakfast, lunch and dinner. Less intense eating means less insulin secretion and less food in the stomach at the beginning of a fast, which would further increase the effective length of the fast.
Perhaps the longer fasts are more effective. Further research will tell, and it's being done.
