Intermittent fasting: Trying it out for science

If losing weight is on your January wish-list, you're not alone - it's one of the most common New Year resolutions. But diets can be hard to stick to at any time of year. In the first of a three-part series, the BBC's Peter Bowes steeled himself for the task by joining a clinical trial for an experimental fasting diet.

Knowing which diet advice to follow can be perplexing and frustrating. Hardly a day goes by without news of a new scientific study. Fad diets come and go.

Sales of high-protein shakes have surged in recent years, but the scientific evidence suggests that most people are consuming too much protein. Low-fat diets were once all the rage; now they seem to have fallen out of favour.

"People do get confused," says Dr Lawrence Piro, CEO and President of The Angeles Clinic and Research Institute, a private medical facility in Los Angeles.

People receive mixed messages about what to eat, Piro acknowledges.

"Eat fish and don't eat red meat," he says, listing some of the medical advice he's seen doing the rounds in recent years. "But then don't eat farm-raised fish because it may be too high in various minerals that are toxic. So now don't eat any fish at all and switch to vegetables - be vegan..."

The popularity of intermittent fasting has grown over the past year or so. The 5:2 diet, which involves dramatically reducing your calorific intake on certain days of the week, is one example.

But more clinical data is needed to confirm the benefits of such regimes. Doctors are generally reluctant to recommend them. The UK's National Health Service questions how sustainable intermittent fasting is in the long term.

Curious about the scientific research that goes into devising a new diet, I decided to volunteer as a subject in a five-month clinical trial at the University of Southern California (USC).

As a human guinea pig, I signed up to test a strict diet regime and subject myself to a battery of clinical tests to evaluate its effect on my body.

It involved surviving, for five consecutive days, on a narrow range of foods that contained as little as 500 calories per day - about a quarter of the average person's consumption. There was to be no cheating, no falling off the wagon and no treats. It was an opportunity to be part of study that may help scientists unravel the complex relationship between food and the human body.

The clinical trial, which is still ongoing, is designed to investigate the feasibility, safety, potential benefits and psychological changes associated with a calorie-restricted diet. It is based on previous experiments, at a number of institutions, which have shown that mice live longer and healthier lives if their food intake is cut by up to 30%.

Research at USC's Longevity Institute has also shown, in rodents, that short-term fasting before chemotherapy can prevent some of the toxic side-effects of the treatment. There is a growing body of opinion that fasting has a potent, beneficial effect on organisms and that it is potentially extendable to humans.

But it is still unlikely that a doctor would put a patient on a restricted diet because of the potential risk of nutritional deficiencies. Also, fasting regimes are tough to follow through, for most of us at least.

This is why the current USC diet does not involve a complete fast and is designed to be repeated in short bursts over a number of months.

"I don't think there is a solid data-supported study to show that cycles of low-calorie diet will actually have a beneficial effect, so that's what we are trying to achieve," explains Dr Min Wei, the study's lead investigator.

There was an enthusiastic response from members of the public when the university appealed for volunteers.

"California people are especially conscious of diet, exercise and health," says Wei, "especially in the environment where obesity is a huge problem, as well as diabetes and cancer."

Since fasting can be dangerous, I sought the advice of my family doctor, who confirmed that it was "medically safe" for me to participate.

The food, during the period of the restricted diet, was designed to be highly nutritious. It consisted of plant-based soups, kale chips, a nutty bar, a herbal tea and an energy drink. The total number of calories, in five days, was about 2,500 - a little more than the average person consumes in one day. No additional food was allowed. For the rest of the month we were allowed a normal diet. The regime was repeated three times, followed by a control period, when we could eat anything.

During the test and control periods, blood samples were taken; body weight and composition (bone density and body fat) were measured using a Dexa (dual X-ray absorptiometry) machine. Brain activity was recorded during one-hour MRI (magnetic resonance imaging) sessions, to determine whether the diet had any impact on cognitive abilities.

The diet had unexpected consequences for everyone, including extreme hunger in some cases - and an aversion to the limited amount of food in others. On the positive side, volunteers, including myself, reported a heightened feeling of mental well-being.

Our stomachs may have been grumbling but we experienced a surprising sense of alertness and sharpness of the mind.

This is the first of a three-part series. Tomorrow: Surviving on 500 calories. Friday: How my body changed.

Follow @BBCNewsMagazine on Twitter and on Facebook

On a tablet? Read 10 of the best Magazine stories from 2013 here