We’ve known for a long time eating less (much less) can prolong life in many animals. Researchers have carried out experiments on a variety of species over many decades asking the same basic questions: does eating fewer calories — and eating less often — prolong life? And do these eating habits also protect against disease? Given fasting diets are now all the rage, I decided to investigate the science. Is fasting good for humans?

Calorie restriction

Over the past 80 years, research on everything from worms to rats has shown reducing the overall amount of calories consumed (generally by 30–40%) extends life. Restricting calories, while still maintaining a nutrient-rich diet, is the only way we know to significantly increase lifespan in mammals. For example, mice can live up to 40% longer on a calorie-restricted diet. In many different species, a variety of biological pathways which slow ageing are switched on when only minimal calories are eaten.

But it is important to note these animals all have relatively short lives. Whether the same slowed ageing is true of humans is not yet clear. Even in other primates, results are not conclusive. In one 23-year study of rhesus monkeys published in 2012, the average lifespan of the monkeys was not extended by restricting overall calorie intake.

Compare this to a 2009 study which reported even moderate calorie restriction extended the life of the same monkey species. After 20 years, 50% of the animals fed a normal diet were still around, whereas 80% of the monkeys eating a reduced diet were still alive and kicking. And diabetes, cardiovascular disease and cancer were all lower in the monkeys not eating much.

Could going hungry a couple of times a week prolong your life? Image credit: Nick Kenrick via Flickr.

Who wants to be hungry all the time?

But for the past 70 years, research has also focused on a different strategy: intermittent fasting. This is essentially eating as per normal most of the time, punctuated by short periods of greatly reduced calorie intake. In 1945, experiments showed rats who fed only on alternate days received the same overall benefits in terms of increased lifespan as those eating less all the time.

And long-term studies of monkeys indicate fasting may not necessarily increase the length of life, but instead may protect against a host of diseases. Diseases like cancer, diabetes, cardiovascular disease, and various forms of brain degeneration. In a mouse study, results suggested intermittent fasting may protect the brain against diseases of ageing, such as Alzheimer’s.

We know from animal models that if we start an intermittent fasting diet at what would be the equivalent of middle age in people, we can delay the onset of Alzheimer’s and Parkinson’s. Mark Mattson, National Institute on Aging

Can we have our cake and eat it too?

Hence the school of thought that regular periods of fasting may give the same benefits as constant calorie restriction and have the power to prolong the years of life spent in good health. All this without having to go hungry, or at least not as often.

Fasting has long been an important part of many religions but it is fairly new for Western people to view fasting as a way to stay healthy.

One of the key arguments is that fasting acts as a mild form of stress on our bodies. This means our cells are always in a state of high defence against any form of damage. Research suggests intermittent fasting also makes our bodies more responsive to insulin, which in turn means we regulate blood sugar more effectively.

And by cutting access to food, we put our bodies into ‘repair mode’ rather than ‘growth mode’. This is a good thing for many diseases, for example by slowing the growth of cancer. In some mouse studies, cycles of fasting were found to be as effective as chemotherapy in slowing the growth and spread of cancerous tumours.

A widely publicised study this year found fasting for 3–4 days at a time had the power to essentially reboot the immune system, killing off old or damaged immune cells and generating new healthy ones. Importantly, fasting for three days before undergoing chemotherapy protected a small group of cancer sufferers from some of the toxic effects of the treatment.

In fact some experts argue that we evolved to go without food for intermittent periods. There would have been many times during our history when food was hard to come by and humans had to make do with not much. Others point out eating three meals per day is the most common pattern of eating in industrialised countries but there is no evidence this is the best way to consume nutrition or that it has any evolutionary history.

Enter the 5/2 diet

The 5/2 diet has been steadily gaining popularity since mid-2012. Essentially it involves 5 days a week of eating pretty much anything you want combined with 2 days a week of eating only a quarter of what you would normally eat (the suggested intake is 500 calories for women and 600 calories for men).

Dr Michael Mosely termed it the Fast Diet on the BBC’s Horizon program: Eat, Fast and Live Longer. There are growing numbers of devotees to the 5/2 diet (and many variants such as the alternate day fasting diet).

What’s the attraction?

Committed followers of intermittent fasting report significant weight loss, reduced levels of unhealthy cholesterol, reduced blood pressure, and improvements to diabetes.

One human study suggested intermittent fasting may protect against coronary heart disease. Another showed alternate day fasting improved asthma.

Dedicated intermittent fasters report fasting makes them more alert and more productive. This makes some evolutionary sense – if food is scarce you need your brain to work harder to find some.

Of course, there is also the argument that reducing your food consumption is good environmentally — reducing both food requirements and cost.

Is intermittent fasting the solution?

Does intermittent fasting provide the answer to obesity and various other modern health crises? Hmmm, not so fast!

First: a lot of the evidence comes from animals, not humans. Second: we just don’t have any good long-term studies of fasting in humans yet.

There have been some promising human studies including the one that showed huge improvements for asthma sufferers. Another study suggested that the 5/2 diet might help lower the risk of breast cancer.

In 2007, researchers concluded alternate-day fasting may protect against type 2 diabetes, cancer and heart disease, but they were quick to point out more research is required before we can be sure of the effects of intermittent fasting.

And even in animals, fasting isn’t without problems. In one study, rats on a long-term intermittent fasting diet ended up with increased blood glucose and, in another, rats developed stiffened heart tissue which made it much harder for the heart to successfully pump blood around the body.

Experts warn fasters may unknowingly consume insufficient nutrition and be at risk of diseases like osteoporosis. And of course fasting isn’t safe for pregnant or breastfeeding women, or for people with chronic conditions like diabetes. There is some evidence that fasting is bad for women in general.

Importantly, it been suggested fasting may be followed by periods of binging which could trigger or exacerbate eating disorders like bulimia. Others worry fasting brings about an unhealthy obsession with food in some people, which can also result in eating disorders.

At the end of the day, the benefits of intermittent fasting are unproven. But that doesn’t stop a lot of people reporting feeling fantastic when they follow this pattern of eating.

I must admit I’m yet to decide whether to give it a go. Are you a faster?

Links and stuff

• BBC Horizon: Eat Fast and Live Longer full episode
• 2014 report that prolonged fasting induces immune system regeneration
• BBC Science: How to live longer
• Excellent review of gender differences in responses to fasting – it may not be such a good thing for women