I did my masters thesis on the relationship between fasting and aging.


I was browsing your interesting and useful (and generally, in my opinion, well balanced and informative) website on nutritional Q&A, but I was troubled by one item that you may have missed the target on:

In the Q&A item where the questioner asked about the possible benefits of fasting,
you said, basically, that there are no health benefits.

I can’t resist responding. I’m sure that there are many promoters of fasting who have no evidence whatever of its value and that there may indeed be situations in which a person can harm themselves by doing so.

But, as it happens I did my master’s thesis on the relationship between fasting and aging. (There had been historical claims that fasting can slow aging.) I did an extensive search of the research literature on fasting.

As you may know, fasting has been rather extensively studied with human subjects under controlled conditions for periods of up to several months. During World War II the U.S. Army, in particular, took a strong interest in it and had volunteers doing fasts of up to, if I remember correctly, 90 days. So there’s no shortage of data on it. Sadly, I can’t find my old thesis, which would have all the references.

So, from memory, I’ll just mention the extensive classic studies of McKay (or maybe “MacKay”) in the late thirties on rats — which were replicated many times since then. Simply put he found that both constant caloric restriction (below what were previously considered normal healthy ad lib. (as desired) levels) and periods of alternate fasting and feeding cause:

  1. A significant increase in the life spans of the test populations (about 1/3rd more if I remember right)
  2. A significant reduction in a number of pathologies that they were monitoring for.

As I remember he found the optimal “life extension effect” with a schedule of periods of fasting followed by a period 3 times as long not fasting.

If you want to see an overall review of much of the literature on the known effects (many of them known to be or at least suspected to be, beneficial I’d refer you to a 1989 article on “Overview of the effects of food restriction”.

And, although strictly speaking they are not studies of effects of fasting and certainly aren’t controlled studies, the epidemiological records in England showed that the generation of children that grew up during the World War II years with caloric intake well below what we recommend these days and consider normal had better health records than the generations that grew up just before and just after those war years of restricted intake. To give you some idea of what research is on there on this topic (and which you may not have been aware of) I did a quick search on Medline.

You also said that “no cleansing of the body occurs.” Although “cleansing of the body” isn’t exactly a well defined agreed on and quantifiable physiological concept, there is some reason to believe that something like that can take place. It’s documented that pesticide residues and other fat-soluble toxins can be stored in lipid tissue and are released when fat in that tissue is reduced, those toxins are released into the serum sometimes with undesirable effects and do show up in the urine. That could arguably be called “cleansing.

My question to you, then, is this: Based on the references, would you now say it would be fair and correct to say “there is some research both in humans and animals studies suggesting some benefits to fasting, but whether it’s overall effect is beneficial or detrimental in any particular individual and what the optimal fasting schedule would be, are not questions that can be fully answered yet?”

New research on people who eat fewer calories than they need on a daily basis indicates that it can extend lifespan. As to how many fewer calories has not yet been determined. I would ask each individual person to examine how eating fewer calories than they need to maintain a healthy weight would affect their quality of life.

Reducing calories to live longer, however, is not the same as fasting which is not eating any food except water for 1 or more days depending on the individual person’s interpretation of fasting. This practice is not recommended. There is no cleansing effect to fasting. Toxins and heavy metals stored in the body increase in the blood during fasting as fat reserves are burned for energy to fuel the body. These heavy metals would have more detrimental effects in the blood.

I am not referring to people who fast for a day for religious reasons.

People with chronic diseases like diabetes should never fast and should continue taking their medication while eating food as prescribed by their doctor and dietitian. I would also be very hesitant to recommend fasting which would be interpreted as a stamp of my approval by people with anorexia eating disorder that fasting is more healthy than eating sufficient food to maintain a healthy weight which fasting is not.

Lastly, animal research is not always transferable to humans and that is why human research trials are performed to see if the effect seen in animal research holds up in humans before we make nutrition recommendations to the general public. We are not at that point in regards to recommending everyone fast for x number of days or reduce calorie intake by y%.

Published research, in general, can be frustrating for the public when a conclusion such as more research on this xyz topic is needed. People want to know now what they should weight and what to eat to stay healthy. It is confusing enough to the public when research first recommends abc and then turns around to recommend xyz.

BTW it was Ancel Keys who studied conscientious objectors to World War II military draft under the bleachers at the University of Minnesota.  His work focused on the side effects of people who are starving obsess over food among other factors.

Thanks for your thesis information.