A number of scientific studies suggest that properly-conducted fasting may benefit human health. But what are the health benefits of fasting? Does it even facilitate, prevent and repair DNA damage ? Credit: National Human Genome Research Institute

When it comes to the world of diets and health, the concept of fasting and especially intermittent fasting has been an increasingly popular topic in recent time. The notion is, that instead of eating three meals a day, an eating schedule of “intermittent fasting” could not only help fight obesity but many related diseases of modern life, such as diabetes, heart disease, cancer and Alzheimer’s, and even slow down aging, researchers say.

Intermittent Fasting

With its claimed benefits to overall health, it is claimed to reduce the risks of several diseases, such as cancer and it has even been suggested to slow down aging and increase longevity.

What probably contributed to its popularity lately was the BBC Horizon documentary that aired in 2012 with the aim of evaluating the 5:2 diet and also “alternate day fasting”.

In this documentary, we get to follow BBC journalist Michael Mosley who set about to test the above. To examine fasting and its claimed benefits to health.

Intermittent fasting has however been a relatively popular diet for many years in some circles, though. Among fitness buffs and training forums around the Webb. But then more often with diets such as the so-called 20:4, the 16:8 and the 1:1. These ratios refer to how many hours or days you are to abstain from eating. Eating during a window of 4 hours, and abstaining from 20 hours. Eating during an 8-hour window, and fasting during 16. Eating one day, and fasting the net. All of which effectively implies “intermittent fasting”.

Caloric restriction is something that has been extensively studied for several decades, much more thoroughly than intermittent fasting specifically. But as the concept of intermittent fasting entails caloric restriction for longer or shorter periods – this research can probably be relevant.

But we will however only look at the science on intermittent fasting particularly. Aiming to assess what we actually know and what this implies.

Intermittent Fasting, Mortality, and Aging

There are actually quite a lot of research on the effects of calorie restriction relative to aging. But this has mostly been done on animals. And especially small animals such as rats and mice.

Since the research aims to assess the effects of fasting on aging, it is indeed natural to use animals that live for a short time. Since larger animals or indeed humans would take decades to provide credible results.

We will examine three studies; two of these has been conducted on monkeys and one has been conducted with humans. We will first examine the research done with monkeys, then what the human research establish.

Monkey Research

Although the research was done on small animals also contribute to our overall knowledge. The two studies done with monkeys surely provides more compelling data relevant for us, humans, compared to similar studies on mice, rats etc.

There is a problem, though since the two experiment done on monkeys actually contradict each other. The older primate study “Caloric restriction delays disease onset and mortality in rhesus monkeys” indicates that caloric restriction does decrease mortality.

The newer study Caloric restriction delays disease onset and mortality in rhesus monkeys were done in 2009 and it instead concludes that there was no change between fasting monkeys and nonfasting monkeys on mortality and longevity. It did, however, indicate a slight reduction in diseases and cancer among fasting monkeys. Its data also showed lower blood sugar levels and fewer triglycerides among fasting monkeys. It was published in Nature and we will examine it a little closer and compare it to the older study.

Nature. 2012 Sep 13; 489 (7415) :318-21. Impact of caloric restriction on health and survival in rhesus monkeys from the NIA study.

Our Study Suggests a separation between health effects, morbidity and mortality, and similar to what has been shown in rodents, study design, husbandry and diet composition may strongly Affect the life-prolonging effect of CR in a long-lived nonhuman primate. The effect of calorie restriction on longevity in monkeys. The monkeys that lived during calorie restriction (CR) developed slightly fewer age-related diseases than the control group.

So why then did a previous study on calorie restriction in monkeys indicate a big difference in longevity and mortality when this second study did not. The difference may have an explanation in what diet both groups of monkeys received.

As the older study made sure that both groups received the same level of vitamins and minerals to attain RDI (Recommended Daily Intake), dietary fat in a predetermined amount, carbohydrates, and protein to achieve RDI, the diet also consisted of exactly 28,5 percent sugar.

This newer study instead aimed at providing the groups a “natural diet”, a diet that does not only constitute RDI but also consists of food that the money would eat if living in the wild. With only 3,9 percent sugar. This study also provided all monkeys with a 40 percent extra amount of vitamins and minerals, to be sure they were not deficient.

This is an important difference between the studies. As for the older study, the monkeys were allowed to eat as much as they wanted to. But in this newer study, the money was instead given an amount of food that they need to stay at a normal weight (not more, or less). The two groups of monkeys in the newer study, therefore, consisted of a calorie restriction group and a plus-minus zero diet (ad libitum) that provided just the amount of calories the monkeys needed.

The newer study, therefore, consisted of a normal-weight and an underweight group. The older study instead consisted with an underweight calorie restriction group and an overweight calorie abundant group.

So the question remains if fasting (intermittent fasting) and calorie restriction really did increase lifespan when examining both studies, or if it is simply a protection against poor diets.

The Human Evidence

There has only been one study that involves humans as of today. And it is a very old study indeed, from 1957. But a review study was published in 2006 that examines the experiment that was carried out in 1957.

The 1957 experiment took place in Spain with 120 people at an old people’s home who were split into two groups. The first group ate a normal diet, this was the control group. The second group ate a normal caloric diet one day and then a restricted diet (900 calories (kcal)) the next day.

After three years the results were in. With 13 deaths in the control group and only six deaths in the intermittent fasting group. But since the sample size is so small, the difference in mortality could be a statistical fluke. An experiment of this kind is by today’s standards not acceptable from an ethical perspective, as understandable. As the participants in this experiment from the 50’s had probably had not given their consent.

Longevity and Diseases

Longevity and mortality can probably be thought of from two main perspectives. A decreased mortality induced by a decrease in the risk of disease. And a decreased mortality induced by some cellular or genetic change, possibly slowing down the genetic breakdown.

The major part of published research into the potential preventative effects of intermittent fasting and caloric restrictions looks at specific biological markers associated with chronic disease. Such as insulin-like growth factor-I (IGF-I) which is a known factor associated with cancer. Research into the disease perspective is therefore much more scientifically covered than any other.

A clinical review from 2007 “Alternate-day fasting and chronic disease prevention: a review of  human and animal trials” examines intermittent fasting and alternate day fasting in particular. The researchers looked at disease preemption and concluded that alternative day fasting does have a protective effect against heart disease, type 2 diabetes, and cancer. However, they also concluded that more research is required to establish the definitive effects of alternate day fasting.

Side Effects?

Anecdotal reports of effects include; difficulties sleeping, bad breath, irritability, anxiety, dehydration and daytime sleepiness. But there have been no systematic attempts to study potential side effects.


The evidence of intermittent fasting is limited, but there seem to be some potential benefits to health with intermittent fasting and mainly those related to disease prevention. It may however not work by some magical pathway compared to that of caloric restriction. At least, we can confirm that this has not been established yet. What has been established, is that fasting (inadvertent, enhanced calorie restriction or similar) does improve lifespan in lab mammals and improves various health markers associated with aging and longevity in both humans and animals.

Not everyone can safely fast, so if you knew to having some health issues. It is recommended to first talk to your GP to see if it is suitable for you. If you are considering fasting.

Impact of caloric restriction on health and survival in rhesus monkeys from the NIA study
Caloric restriction delays disease onset and mortality in rhesus monkeys
Intermittent fasting and caloric restriction ameliorate age-related
behavioral deficits in the triple-transgenic mouse model of
Alzheimer’s disease

Alternate-day fasting and chronic disease prevention: a review of
human and animal trials