Q&A: the science of dieting

Michael Mosley, Giles Yeo and Lucy Cheke return to answer some audience questions sent in from social media about various aspects of dieting. Chris Smith started by talking to Michael about the sedentary world we seem to live in...

Michael -   The environment that we have created is one where all the inducements are to sit down, not to be active, basically, not to move very much. This is not only true of the offices we are in, but also, just getting to the office. If you want to get up to the 7th floor at the BBC where I work, the great incentive is to take the lift because the lift is right there in front of you. If you want to go up the stairs, you have to kind of work your way around the back. I think what we should be doing is making it much harder to take lifts. I think we should do things like slow them down or perhaps put the smell of urine in the lift to discourage people from taking it. I was recently in a hotel in Sweden and there, they had this lovely stairs beckoning you and right around the back was the horrible little lift whereas in most hotels, it's the other way around. That when you actually try and find the stairs, either they are locked or they just smell really horrible.

Chris - Yes, so we need to do the sort of the multi-storey effect and make the lift unappetising. Now, one diet that's been very popular in certainly recent months is this so-called 5-2 diet. We've had some questions in about this. What actually is the 5-2 diet? How does that work?

Michael - So, the 5-2 diet is intermittent fasting. The idea is that instead of being on a low calorie diet for weeks, months on end, that what you do is you eat normally 5 days a week and then 2 days a week, you cut your calories quite dramatically, down to about quarter of the normal level. So, that would be about 500 calories for a woman, 600 for a man. And there appeared to be health benefits which go beyond losing weight, because by doing so it generates a level of stress rather like when you go for a run, you get stressed. Some forms of stress are rather good for you.

Chris - A note here from Carol. She said, "Is it normal to get headaches on those fasting days?" I must admit that on days when I've gone without food and I'm accustomed to eating usually quite a lot she says she gets symptoms and I certainly concur with that.

Michael - I think the main thing you need to do to avoid headaches is drink lots of water because we get a lot of water via the food we eat. And therefore, if you're cutting down on the amount of food you're eating then you just need more water. More water is great from all sorts of perspectives. You'll burn a few calories, just to warm it up and also, it will encourage you to go to the loo a bit more and that means getting up, getting active and things like that. But there's no intrinsic reason why it should lead to headaches. Indeed, if it happens, it should be transitory, it should pass, but headache is normally associated with dehydration.

Chris - Question here for you here Lucy. Why is it so hard to keep those resolutions that we resolutely make every year and then resolutely break?

Lucy - It's very hard to empathise with your future self. Your future self is a very different person from your current self and it's quite easy to think just as I would think, "Oh well! It's perfectly fine for Chris to give up all his food but I'm not sure if I'm willing to do that." it's quite easy to make those kinds of decisions for our future selves as well. So, we tend to - on the first hand - make New Year's resolutions that are quite unrealistic. We are quite harsh on our future selves.

Chris - Not just because we're drunk at that time.

Lucy - Well, obviously partly because we're drunk at that time, but we tend to be quite kind to our present selves and quite harsh to our future selves. So, we make resolutions that are quite difficult to keep. On the other hand, it's also this tendency to think that we're going to be better in the future. We tend to have an idea of our future selves that they are going to fulfil all the things that we currently don't think that we can do. And therefore, we put restrictions on them and say, "Okay, I decide to do it now, then I definitely will do it in the future." But what's needed to be done is to realistically say, "Future me is not actually that different to current me. Current me is kind of lazy, so future me is going to be going to be quite lazy too."

Chris - Giles, we've got this message here from Stella who says, "How is weight loss affected by how big you already are?"

Giles - That's a very interesting question. I think the largest thing about how big one is is that the bigger you are, oddly enough, the more energy you need - that's not odd. But therefore, the more energy you burn. The smaller you are, the less energy you eat, that you need and the less energy you actually burn as well. So, your size will determine actually your metabolic rate. So, that obviously then has a big role to play in how you would then react to cutting specific amounts of calories.

Chris -   Michael, we've got this Tweet here from Iqra who says, "Is eating late at night bad for you or does it just matter what the aggregate number of calories across the day that you consume is?"

Michael - I think broadly speaking, the most important thing is the amount of calories you eat across the day, but there is some evidence that eating late at night is not a good thing to do because actually, what you need is a period of time when you're not eating. That's when a lot of repair goes on. so, if you eat late at night and then get up bright and early and start eating again, you're not giving your body some rest from food. The old idea that we should eat lots, small meals regularly is probably a profoundly bad idea.

Chris - So, this whole question of breakfast like a king, lunch like a prince, dine like a pauper, that's based on sound principles then.

Michael - It's not a bad idea at all and there is some evidence, I've seen a couple of trials which suggest that eating calories in the morning is probably better for you than eating a lot later in the day.

Chris - Giles, Alex Johnston has tweeted @nakedscientists, what is your view of rapid versus slow weight loss? Which is probably more effective do you think?

Giles - I think the whole point about weight loss is not the actual weight loss. I think most of us can actually lose the weight. It's keeping the weight off. So, losing it in a quick way, maybe it's more unsustainable compared to actually doing it in a slow methodical fashion that is then possibly more easy to maintain because it's keeping the weight off that is actually the key challenge rather than losing the weight itself.