I have been asked this question often enough to address it here.Whether, and to which extent intermittent fasting (IF) transforms human health still needs to be seen, but there are some indications for beneficial effects.
IF refers to dietary patterns in which individuals (a) have no or very little energy intake for extended periods of time (e.g. 16–48 h), while having normal energy intake between these periods, and (b) follow this interval pattern on a recurring basis.
Naturally it is much easier to test the effects of such dietary patterns in shorter lived animals than in humans. Mice, with an average lifespan of 3 years, can give us much faster insight into intervention effects on health and life expectancy.
Some of the more promising effects of IF in animal models is improved glucose handling (relevant for preventing diabetes), and in aging related diseases such cardiovascular disease, Alzheimer’s dementia, and frailty.
One of the key pathways through which IF seems to work is cellular senescence, the aging of cells, that ultimately leads to functional decline and death. The fasting promotes an upregulation of the cellular repair mechanisms that are essential for keeping cells functional and preventing cell cycle arrest.
There is still a lot to be investigated because outcomes vary by animal model, IF protocol, age at which IF is introduced, and duration of the intervention. The same will probably hold true for humans.
Taken together, what we can safely say is that IF doesn’t harm you, in all likelihood improves glucose handling and potentially has some protective effects against cellular aging.
To which extent these effects prevent disease events (such as heart attacks, stroke, dementia) remains to be seen. We simply have insufficient data to claim either way.
The problem is that trials capable of showing such effects need to be of durations that are longer than what human participants willingly endure.
That’s why IF became a surrogate, and more tolerable, protocol for caloric restriction (CR) in the first place. CR refers to continuous restriction of energy intake to 30-50% below normal, while ensuring adequate nutrient supply.
This protocol has produced substantial expansion of life expectancy in primitive organisms, such as C. elegans (a microscopic round worm whose life is measured in days rather than in years). But the higher up you go on the complexity ladder of species, the lower the returns. Still, in primates, our closest relatives, significant beneficial effects on health and life expectancy have been documented.
Unless you are really prepared to adopt a CR or IF lifestyle for good, my guess is that you may experience some subjective effects on health, but they will dissipate rather quickly, once you return to “normal”.
Let me give you a personal example. My wife and I have made IF a regular affair. Last meal of the day is latest at 16:00, first meal of the following day comes at sometime between 09:00 and 11:00, and always after 60-90 minutes pre-breakfast moderate-to-high intensity workouts.
Let’s call this a n=2 experiment/trial. It gives us a 17-19 hours food-free period. Deduct from this 4 hours for the post-prandial period (the period of nutrient absorption following the day’s last meal) and we have a net fasting period of 13-15 hours.
We have been doing this for approximately 5 years now. With my research focus on healthy aging and cardiovascular functionomics, I have, of course, designed this “trial” in accordance with best evidence, and I have the advantage of being able to monitor physiological functions for the 2 of us regularly, using a medical device that I specifically developed (together with my team) for this purpose.
Cutting a long story short: at 64 my wife has the cardiovascular function and fitness of a 35-years old woman, the body contour many 40-year olds wouldn’t mind to have (Just put a few seconds of, admittedly amateurish, video clips together, to prove my point). At 2 years younger than her, I am not that lucky, but still can compete in the age range of men 20 years younger than I.
I attribute this to the summation of physical exercise, IF and dietary quality. In science speak, this is, however, only anecdotal evidence.
So, if you want to find out for yourself, go with the Nike motto: Just Do It.