Does the 5:2 diet work?

We hear a lot in the media about fad diets, and one of the more famous ones is the so-called 5:2 diet, also known as intermittent fasting, which involves drastically cutting down calories on two days a week. But does the science behind it stack up? A new review of the evidence suggests that it might. One of the researchers who's been putting this diet to the test in laboratory mice and in humans is Mark Mattson, from America's National Institute on Ageing, as he explained to Kat Arney...

Mark -  Our work started in animals and we found in others that if you have the animals fast intermittently, for example in our lab, we have them fast every other day, so 24 hours no food then they can eat the next day and so on, they can live up to 30% longer.

Kat -  The animals that you're looking at in the lab presumably, you can control their access to food.  What's the evidence that this kind of restricted diet may have benefits in humans?

Mark -  So far, there's been only a few controlled studies.  Dr. Michelle Harvie at the University of Manchester, and she works with women at risk for breast cancer because of their family history and also they're overweight.  There are over 100 women.  They were divided into two groups.  One group, we had them reduce their daily calorie intake by 25%, so counting calories each day.  The other group, we had them 2 days a week eat only 1 modest meal of about 500 calories.  We followed them for 6 months and we found that both groups lost weight but the women on what's now called the 5:2 diet lost more belly fat and their ability to regulate glucose was improved more than the women who counted calories every day.

Kat -  What's going on at a molecular level that might explain this?

Mark -  So, glucose normally, if you're eating regular meals, is the main source of energy for cells and it's stored in the liver.  When that's depleted, which usually occurs in around 12 hours, then what happens is your body starts mobilising energy from fat.  So, that's one major change that happens.  Probably explains in part in a loss of belly fat in the study we did.  Much more work needs to be done in humans to determine if and which types of intermittent fasting diets would be optimal for health.  But let's say that some particular diet was clearly established, for example, the 5:2 diet, to be consistently beneficial in many groups of people, then the issue is how to work this type of eating pattern into the daily and weekly routine.

Kat -  Saying is one thing, but doing is quite another.  Whilst the 5:2 approach may have its weight loss benefits, is it actually feasible to periodically fast for months on end?  Whilst making a BBC Horizon documentary, Dr. Michael Mosley tried fasting for 2 days a week and he's written a book about his experience.

Michael -  On a Monday, I get up and I have some scrambled eggs for breakfast because the protein keeps you fuller.  That's about 180 calories, then I skip lunch and in the evening, I have a pile of vegetables and say, a bit of fish.  That's probably about another 300 calories.  So, the whole lot adds up to maybe 500 calories.  So, I do that Monday, Tuesday - eat normally, Wednesday normally, and on Thursday, I kind of do the same thing again.  I did that initially for 12 weeks and in the course of that, I lost around 9 kilos.  What was really good is, almost all of it was fat.  So, my body fat went down from 28% to 21%.

Kat -  How did you feel particularly on the days where you were restricting your calories?  Did you feel lightheaded or did you just get on with things?

Michael -  I just got on with things.  Actually, I found that I became more energetic.  The evolutionary perspective on that one is that our remote ancestors they had feast and famine.  If they didn't have food, they couldn't just kind of lie on the floor and wait for food to come.  They had to get out there and become more active.  And that's what drives you.  I think there are lots and lots of myths about food and one of them is this thing called 'starvation mode'.  If you don't eat regularly then your blood glucose will fall to the floor and you feel faint.  It is completely an utter nonsense.  Along with the idea that if you stop eating then your metabolic rate slows down, because the studies I've looked at where they have taken volunteers, kept them without food for 6 days,  metabolic rate actually goes up.  It's only in periods of prolonged starvation and when you lose a lot of weight that your metabolic rate goes down.  So, there's a lot of myths out there which need to be hit over the head.

Kat -  As a human being, I go, "Wow!  That sounds amazing!" but as a scientist, I go, "You're just one person.  That's not exactly scientific."  What does the science say about this kind of approach?

Michael -  Well, the science is very strong certainly from the animal data.  Dr. Mattson has done a lot of stuff, Krista Varady has done a lot of things, Dr. Michelle Harvie up in Manchester has similarly done some pretty big trials.  For example, they saw greater fat loss, also, significantly greater improvements in things like insulin sensitivity and also in inflammatory markers.

Kat -  How important do you think it is that we bring science and proper research to weight loss?

Michael -  I think it's absolutely vital.  I think we've applied science to pretty well every aspect of our life and I just think that it is hugely important that we look at it on the big studies,  obviously, studies are hugely important.  But also, trying to understand what's happening at the cellular level.  And that's what this paper is all about.  It is both a description of studies which had been done with humans over quite long periods of time,  but it's also about the sort of pathways, and that's what makes it interesting I think.