The 5:2 diet: can it help you lose weight and live longer?

A new eating plan that involves 2 days of dieting each week is being promoted as the key to sustained weight loss and increased longevity. Dr Michael Mosley tried it.

By Dr Michael Mosley

Several months ago I set myself an ambitious goal: to find a way to live longer, stay younger and lose weight. I also wanted to go on enjoying the foods I normally eat and make as few changes to my lifestyle as possible. After talking to several scientific experts, I have spent the past few months trying a controversial diet that challenges conventional views about how and when we should eat. It is called intermittent fasting – reducing your food intake on alternate days.

Although most of the great religions advocate fasting (devout Muslims finish fasting for Ramadan this weekend), I have always been sceptical about the medical benefits and followed the standard advice, namely “never skip a meal and never crash-diet”.

The reasoning behind this is that people who skip meals tend to eat high-fat snacks when they get hungry, while those who crash-diet lose weight fast but what they lose is mainly water, with some fat and muscle thrown in. When crash-dieters give up, as invariably they do, they pile on the pounds, mainly as fat. It is known as yo-yo dieting and does you no good.

So before doing anything that involved fasting, I wanted to find out a lot more about what I was getting myself into.

I started at the London Marathon. At this year’s event there were more than 7,000 runners aged over 50, and seven who were over 80. The most impressive competitor, however, had to be Fauja Singh, also known as “the turbaned tornado”. At 101, he is the world’s oldest marathon-runner.

Fauja is a bit of a mystery. His lifestyle is nothing like that of the average marathon-runner. He is a strict vegetarian, so he is not getting muscle-building protein from fish or meat. He also eats much the same thing every day — he lives mainly on lentils, vegetables garnished with ginger, brown bread, fruit and yogurt — and his portions are tiny (no carb-loading for Fauja). Which explains his weight. He is 5ft 8in tall and weighs just 53kg — about 8 stone.

He attributes his success to his eating habits: “My life is protected because I control my eating.”

Fauja is right to believe that controlled eating is the way to a longer and healthier life. Scientists have known since the Thirties that one of the best ways to extend life is to eat a nutritionally rich but calorie-restricted diet. Mice on such a diet live up to 40 per cent longer than normal mice.

The trouble is, I cannot seriously imagine living on a severely calorie-restricted diet for the rest of my life. Fortunately, there is an alternative that seems to offer many of the same benefits — intermittent fasting.

One scientist who has been studying fasting for many years is Professor Valter Longo, the director of the University of Southern California’s Longevity Institute. When I visited him for BBC Two’s Horizon, he showed me a remarkable little mouse that had been genetically engineered to live longer.

“This,” he proudly told me, “is a dwarf or Laron mouse. These mice hold the record for longevity extension in a mammal.” The average mouse has a lifespan of about two years. Laron mice can live for up to five years.

The mouse I held was the equivalent of an 80-year-old human being. Like his genetically engineered relatives, he should live to the equivalent of 120, maybe even 180.

Laron mice are largely immune to heart disease and cancer, and when they die it is usually of natural causes. Oddly enough, when the mice are examined, scientists are often unable to find a cause of death. The heart just stops.

One of the links between fasting and longevity seems to be a hormone called insulin-like growth factor 1 (IGF-1). As Prof Longo explained, IGF-1 and other growth factors keep our cells constantly active. It’s like driving along with your foot hard on the accelerator pedal.

You need adequate levels of IGF-1 and other growth factors when you are growing, but high levels later in life appear to lead to accelerated ageing. The evidence for this comes not just from animals such as the Laron mice, genetically engineered so that they don’t respond to IGF-1, but also from humans.

Prof Longo has studied certain villagers in Ecuador who have a genetic defect called Laron syndrome. It is incredibly rare: fewer than 350 people worldwide are known to have the condition. Like the mice, people with Laron syndrome don’t respond to IGF-1. They are short, typically less than 4ft tall, with prominent foreheads and underdeveloped jaws.

Most surprisingly, as Prof Longo explained, they appear immune to cancer: “There are no reports, not a single one, of them ever dying of cancer, yet their relatives in the same household, the same age, get cancer like everybody else.”

They are long-lived but, unlike the mice, not exceptionally so. Prof Longo thinks this may be because they have learnt that they are naturally resistant to cancer and diabetes, and tend not to be terribly careful about their diets.

“They smoke, eat a high-calorie diet, then they look at me and say, 'Oh, it doesn’t matter, I’m immune.’ I think they would rather take the 85-year life and do what they want than the 100-year life and have to be restricted.”

Fasting lowers levels of IGF-1 and also appears to switch on a number of DNA repair genes. The reason seems to be that when we run out of food our bodies change from “growth” to “repair” mode. But Prof Longo warned me that fasting is not for the faint-hearted, and is safest done in a specialised centre or under supervision.

“There’s going to be a drop in blood pressure, a drop in glucose levels and metabolic reprogramming,” he said. “Some people faint. It’s not common but it happens.”

I started my fast on a Monday evening, after a final slap-up dinner of steak, and finished it after more blood tests on the Friday. During that time I drank black tea, black coffee and lots of water but consumed no food.

Before I did it I was convinced that hunger would build day by day, getting steadily worse until finally I gave in and raided a local bakery. But what I found was that, after the first 24 hours, things got better. I had hunger pangs, but they passed.

On the Friday morning, I had myself tested again and discovered that I had lost just over 2lb of body fat, my blood glucose levels had fallen dramatically and my IGF-1 levels, which had been towards the top end of the range, had halved.

So I had improved my body chemistry and learnt that I could tolerate hunger better than I imagined. Prof Longo warned me, however, that to maintain these benefits I would have to change what I ate.

According to Prof Longo, like many of us on a Western diet, I eat too much protein, and that helps to keep my IGF-1 levels high. Foods such as meat and fish are rich in protein, but so is milk. A skinny latte, which I drink most mornings, comes in at around 12g of protein. Recommended levels are a relatively measly 55g of protein a day.

Now this is clearly bad news for anyone on a high-protein diet such as the Atkins or the Dukan. But it is also tough for someone like me who enjoys their meat. Prof Longo is pretty much a vegan. I was prepared to give up lattes but not to go that far.

I met my next scientist, Dr Krista Varady of the University of Illinois at Chicago, in an old-fashioned American diner. We ate burgers and fries while she told me about a diet that she has been testing on human volunteers. It’s called Alternate Day Fasting (ADF) and is very simple. One day you eat whatever you want. The next day, you fast.

Fasting ADF-style isn’t as drastic as the fasting I’d tried with Prof Longo. On Dr Varady’s fasting days you are allowed about 600 calories a day if you are a man, 500 if you are a woman. The real surprise was that on my “feed days” I could eat exactly what I wanted.

Dr Varady has finished a trial, to be published next month, in which she took two groups of volunteers doing ADF for 10 weeks. One group were put on a low-fat diet on their feed days, while the other were encouraged to eat lasagnes, pizza — a typical American high-fat diet. As Dr Varady explained, the results were unexpected.

“When they signed up for the study, the people randomised into the high-fat group weren’t happy because they assumed that they wouldn’t lose as much weight as those randomised to the low-fat diet. But they did. People on the high-fat diet were losing as much and sometimes more weight, week after week.”

And it wasn’t just weight loss: the groups saw similar falls in LDL cholesterol — “bad cholesterol” — and blood pressure. Dr Varady is running a year-long trial to assess the longer-term effects of ADF on weight loss and health. She is keen to see how much her volunteers lose and how many will still be on the diet after a year.

I thought long and hard about doing ADF, but decided in the end that it sounded a little too much like hard work and would play havoc with my social life.

Instead I opted for a less dramatic variant: the 5:2 diet. With this regimen you eat what you want five days a week, then twice a week you restrict yourself to just 600 calories.

There have been few human trials of the 5:2 diet, so no one is certain whether it is better to eat those 600 calories in one meal or to spread them out through the day. I decided to try different approaches, to see what would work for me.

I tried skipping breakfast and lunch, then eating my 600 calories in one go at supper time. The trouble is, I hate starting the day hungry. So then I tried a large breakfast and nothing else. I got really irritable in the evenings.

Finally, I went for a split: 300 calories for breakfast and 300 for supper. A typical breakfast now means two scrambled eggs and a slice of ham (quite a lot of protein, but well within limits), with plenty of water, green tea and black coffee to see me through a working day. Then, in the evening, I tuck into something like grilled fish and lots of vegetables. It is impressive how few calories there are in vegetables, and when you are hungry they are unexpectedly delicious.

There is no official guide to what a 600-calorie meal looks like — Dr Varady gives her volunteers specially prepared low-calorie meals — so I made mine up by trawling recipe websites.

It doesn’t seem to matter which days of the week you do it; I prefer to do my intermittent fasts on Tuesdays and Thursdays — something, apparently, that the Prophet Mohammed recommended.

I have kept this up for two months and, after a settling-in period, it has become quite easy. It is reassuring, when you are on a restricted-calorie day, to know that the next day you can eat whatever you want. The surprising thing is that you don’t. Dr Varady said that her team expected people to gorge on their feed days, but most people are happy to eat their normal diet. Such is the power of habit.

Six weeks after starting the 5:2 diet, I had another full medical. The results were impressive. I had lost well over a stone, down to less than 12st. My blood glucose, which had been borderline diabetic, was normal and my cholesterol levels, previously high enough to necessitate medication, were also down in the healthy range.

I have swapped lattes for espressos and generally cut back on protein, so my IGF-1 stayed low. I look better and I feel good on it. I still sometimes eat burgers, biscuits and cakes, but on my fasting days I eat healthily.

Intermittent fasting is not something that you will find many doctors recommending because, while there is plenty of animal data, so far there is limited evidence of its efficacy in long-term human trials. It will not suit everyone, nor is it safe for everyone. It worked for me and I will keep doing it. Or I think I will. We shall see.

• Michael Mosley is a medical doctor, journalist and television presenter. To watch his BBC Two Horizon programme on intermittent fasting.

The 5:2 diet: what is it and how does it work?

• With the 5:2 diet, you can eat whatever you like five days a week — so-called feeding days. On the two “fasting days” you eat 500 calories if you are a woman, or 600 calories if you are a man.

• It doesn’t matter which days are spent “feeding” and which “fasting”, as long as the fasting days are non-consecutive and you stick to the 5:2 ratio.

• On fasting days you can consume your calories in one go, or spread them through the day — there is no medical research into whether filling up at breakfast or snacking throughout the day is more effective for weight loss.

• A typical fasting-day breakfast of 300 calories might consist of two scrambled eggs with ham (good sources of protein), plenty of water, green tea or black coffee. For a typical 300-calorie lunch or dinner, try grilled fish or meat with vegetables.

• On feeding days you can eat whatever you like. Most dieters, rather than feeling a need to gorge, found that they were happy to consume around 2,000 calories — the recommended daily intake for women (2,600 for men) — and did not crave high-fat foods.

• Contrary to popular opinion, fasting can be a healthy way to lose weight. It can reduce levels of IGF-1 (insulin-like growth factor 1, which leads to accelerated ageing), switches on DNA repair genes and reduces blood pressure, cholesterol and glucose levels.

• According to current medical opinion, the benefits of fasting are unproven. As a diet, it is not recommended for pregnant women or diabetics on medication. Anyone considering a diet that involves fasting is advised to consult their GP first, and to do it under medical supervision.