Some say feasting and fasting will keep you younger. (Photo by bowtoo)
I often hear or read that intermittent fasting has all of the same benefits of calorie restriction. The idea is that by not eating every now and then while keeping total calories the same you would enjoy the same health effects as you'd get from simply eating less. Including living longer.
This mantra is repeated even in scientific papers, but is it correct? It's certainly easy to see the appeal: since actual calorie restriction means you're counting every calorie and going hungry for most of the time, intermittent fasting seems like a fantastic choice. Besides, the concept does make sense on the surface. Deprive your body of energy for a while, let autophagy do its work, and live longer. It seems to work for fruit flies and worms, after all, so why not humans?
Because humans are more complex. While simple species like roundworms can be very useful for screening life extension therapies, they are no guarantee that the same therapies work in humans. There are a million ways to extend lifespan in roundworms and fruit flies, but much less in rodents, and even less in humans.
Unfortunately, it looks like intermittent fasting (IF) is one of these cases. For obvious reasons, we don't have lifespan studies on IF in humans, but what we do have is studies on rats and mice. And, despite what the popular belief is, the data is much less promising than one might hope. The rather disappointing conclusion of the studies seems to be that intermittent fasting without caloric restriction does not extend lifespan. When it is accompanied by caloric restriction (CR), it does extend lifespan – and it looks like the degree of life extension is highly dependent on the degree of CR.
The reason you see studies showing increased longevity from IF in the first place is because most rodents eat less when they fast every other day. While humans generally compensate for a fast by eating twice as much the next day, mice and rats generally don't. They do eat more, but not twice as much – which means they are calorie restricted.
Don't believe me? Let's take a look at all the studies on intermittent fasting and longevity in mammals. Note that I've skipped all the alternate-day feeding studies that have not looked at lifespans. There are a lot of papers showing other kinds of health benefits from fasting (some of which are also a result of CR!) , and I'm not saying fasting is not healthy in general, just that it does not seem to extend life.
A critical look at the studies
The first paper I could find on the subject is from 1945 (link), not too long after the positive effect of calorie restriction on lifespan was discovered. This paper briefly mentions an even earlier study from 1934, in which mice fasted for two days in a row each week. The average lifespan of the fasted mice was slightly longer than those of controls, but the difference was not statistically significant. There is no mention of weight and food intake.
In the 1945 study, male and female Wistar rats were put on various versions of intermittent fasting. The rats fasted either one day in four, one day in three, or every other day. Fasting was begun at the age of 42 days and was continued until the rats died.
With the exception of females fasted once every four days, the average lifespans of all fasted rats exceeded that of the controls. The increase in lifespan was slightly greater in males than in females, although females still outlived males in general. In male rats, the most effective method was fasting every other day, while in females fasting once every three days gave the best results on average. However, both the male and female rat that lived the longest (1057 and 1073 days, respectively) were fasted every other day.
Food intake was not measured, but since the intermittently fasted rats weighed less than the control rats, we can assume that they also ate less. No drastic retardation of growth was seen, however. So, the first available study on intermittent fasting shows that when rats are intermittently fasted, they don't compensate for all the missed calories on the ad libitum days, and thus are CR'd, and therefore live longer. No big surprise there.
After this study, there was a gap of four decades before similar experiments were done again. During the 80's and 90's, three papers on intermittent fasting and lifespan were published by the same team. In the first one, male Wistar rats were fed either ad libitum or every other day since weaning (link). The mean lifespan of the fasted rats was 83% greater than that of the control group. And, just like in the 1945 study, fasting resulted in a lower body weight. The abstract doesn't mention the exact weights, but since the fasted rats took 75% longer to become fully grown, it looks like they ended up eating significantly less.
In the second paper, male Wistar rats were again fed ad libitum or every other day since weaning (link). This time they were also allowed voluntary exercise. The fasted rats exercised less in their youth but more when they were older. They also lived longer and weighed less than the control rats. However, in contrast to the first study, their growth duration was the same while growth rate decreased. That is, it looks like in the first study both groups eventually grew to the same size, while in the second study the IF rats remained smaller.
The third paper looked at the longevity effect of intermittent fasting (every other day) on three strains of mice, beginning at various ages (link). In two of the strains, mean and maximum lifespan increased and body weight decreased. The A/J strain, on the other hand, showed different results. When intermittent fasting was begun at 1.5 months, the mice lived longer despite not weighing less. When it was begun at 10 months, they again weighed the same as controls but actually died earlier. The rats that began fasting at 6 months weighed less than controls at some ages but showed no difference in lifespan.
In summary, it looks like intermittent fasting extends lifespan in rats and mice only when it is accompanied by calorie restriction. It does not mean that the animals are also put on CR; rather, they just naturally end up eating less (unlike humans, who tend to be very flexible and good at compensating for calories). And, in the rare cases that the animals actually do eat twice as much the next day, their lifespans are not increased.
For those who are doing IF for other reasons than life extension – such as improving insulin sensitivity or ">weight loss – this is not necessarily a concern. While some of the other health benefits reported in the studies are probably a result of calorie restriction, just like lifespan increases, I suspect IF even without CR still has some benefits in humans. It's just that based on the rodent studies, those benefits won't be enough to make us live longer.
For more information on diet and longevity, see these posts:
Dietary Supplement Increases Lifespan by 11% in Healthy Mice
Slowing Down Aging with Intermittent Protein Restriction
How to Live Forever: My 5 Steps to Immortality
Intermittent Fasting Reduces Mitochondrial Damage and Lymphoma Incidence in Aged Mice