There is a rich literature on human recovery following energy or protein privation. In the Minnesota Experiment, lean healthy males who were fed ‘half-rations’ (1600 kcal and 50 g/d protein) for 6 months lost 1/3 of their lean body mass. Once refeeding with adequate protein and energy was begun, it took them over 4 months to approach their original baseline of strength and function, and in so doing, they added excess body fat. In addition to the Marliss study (Errol Marliss, J Clin Invest, 1978), multiple human studies demonstrate that the maximum rate at which a protein-deprived person can gain back lost lean body mass is ¼ pound per day (ie, 4 g/d positive nitrogen balance). So if a day of total fasting causes the loss of ½ to 1 pound of lean body mass per day (depending upon one’s level of keto-adaptation), then it takes at least 2-4 days of full feeding with protein and energy to recover that one day’s loss.
Given the current popularity of intermittent fasting (IF), one would assume that there would be well-designed studies published to demonstrate that its practice does not impair lean tissue preservation, mineral status, or function in humans. Unfortunately, this is not the case. Yes, there are multiple published studies of ‘intermittent fasting’, defined as not eating from morning until early evening every other day, or not eating any protein until the evening meal. There are also a few studies of people eating 75% less or total fasting for every other day. Weight losses are typically about 5% but plateau quickly. At this amount of weight loss, accurate measurement of body composition are problematic, so good data on lean body mass changes are lacking. But many people who practice IF eat nothing or no protein for more than 24 hrs at a time. And more importantly, some people choose to fast for days at a time, assuming that their body can effectively conserve its functional tissue, or make up for it by rapidly recovering lean tissue in the intervals between these fasts. There are other reasons why people fast besides weight loss. Boosting blood ketone levels is increasingly associated with reduced aging and oxidative stress. There are also many reports of energy restriction and reduced cancer growth, but this benefit is not consistently seen in animal models of intermittent fasting (implying that the benefit is in consistent restriction rather than intermittent restriction).
So what does this really mean? Yes, total fasting can cause rapid weight loss, but typically half or more of this will come from lean tissue. And most importantly, depending upon the amounts of protein and energy one consumes after a period of fasting, the rate of recovery of lean tissue will likely be slower than the rate at which it was lost. To be blunt, in the real world, recovery of lean tissue lost as a result of any period of total fasting takes 2-4 times as long as the period of fasting itself.
