Today's research is a comparison of alternate day fasting and calorie restriction in human subjects. Or rather, I think, one might look on it as an examination of alternate day fasting as an alternative approach to achieving calorie restriction. The type of alternate day fasting here is the better form, in which 36 hours are spent fasting, only eating in a 12 hour window every other day. In practice that means eat normally one day, then fast until the morning two days later. This tends to reduce average calorie intake down to something very similar to a straight calorie restricted diet. That calorie restricted diet might be 1500 kcal/day for an averagely sized human being, and I can assure you that it is very, very hard to eat more than 3000 kal in a 12 hour period, at least not without resorting to heavy duty junk food.
So is alternate day fasting just calorie restriction? In animal studies there are significant differences in gene expression profiles between these two approaches, which is enough to suspect that perhaps fasting and feeding versus a consistent low calorie intake are two different beasts. The effects on metabolism are sweeping in either case, which makes analysis challenging, but the important mechanisms, the upregulation of cellular stress response systems such as autophagy, appear the same. More recent research into fasting mimicking diets has attempted to find the point at which low calorie intake triggers benefits, and quantify how long the low calorie diet must be sustained. The results there suggest that additional benefits emerge after three to four days, in terms of a culling of immune cells. That work also suggests that the process of refeeding after a fast is necessary in order to obtain the full benefits.
So it is possible that neither alternate day nor straight calorie restriction are strictly optimal, and something more intermittent would be better. Still, either alternate day or calorie restriction are such a huge improvement over the dietary choices adopted by most people that it seems almost foolish to spend much time on further optimization. This is particularly true when that time and energy could be put towards advancing the development of rejuvenation therapies capable of turning back aging in ways that no amount of fasting can achieve.
New research outlines a novel way to intermittently restrict calorie intake, a method that achieves the same health benefits while possibly being more manageable than constantly restricting calories. An international team of researchers presented the results of a clinical trial in which "alternate day fasting" resulted in reduced calorie intake, reduced body mass index, and improved torso fat composition. Known as "ADF," it is a diet regimen in which adherents avoid all food and caloric beverages for 36 hours, then eating whatever they want for 12 hours - donuts, cookies, dumpster pizza, whatever.
In this randomized controlled trial, 30 non-obese volunteers who had done ADF for at least six months were compared over a 4-week period to 60 healthy control subjects. While the results of this clinical trial show that ADF had similar health benefits to caloric restriction, even though the "feast days" could include a lot of unhealthy calories. The researchers also write that ADF has some distinct advantages over CR. Mainly, they say it may be easier to maintain the habit.
Previous work on intermittent fasting has shown that restricting an animal's calories - without depriving them of adequate nutrition, of course - can increase their lifespan, though much of the work has been limited to monkeys and other non-human animals. This latest study builds on that existing research by following a mid-sized human cohort for enough time to show not just significant benefits but also no negative side effects.
Caloric restriction and intermittent fasting are known to prolong life- and healthspan in model organisms, while their effects on humans are less well studied. In a randomized controlled trial study, we show that 4 weeks of strict alternate day fasting (ADF) improved markers of general health in healthy, middle-aged humans while causing a 37% calorie reduction on average. No adverse effects occurred even after more than 6 months.
ADF improved cardiovascular markers, reduced fat mass (particularly the trunk fat), improving the fat-to-lean ratio, and increased β-hydroxybutyrate, even on non-fasting days. On fasting days, the pro-aging amino-acid methionine, among others, was periodically depleted, while polyunsaturated fatty acids were elevated. We found reduced levels sICAM-1 (an age-associated inflammatory marker), low-density lipoprotein, and the metabolic regulator triiodothyronine after long-term ADF. These results shed light on the physiological impact of ADF and supports its safety. ADF could eventually become a clinically relevant intervention.