Does the body have fat-burning zones?

We put this burning question to physiologist Sam Virtue...

Sam - There's a couple of ways of thinking about this question. So, we can think about the organs of our body that maybe use more or less fat. And for example, the heart will use a lot more fat relative to other fuels that it can use for energy, like glucose. But weirdly, I think if we had to pick one place as the fat burning zone in the body, it would be an organ that uses no fat at all. And that's the brain. Because, how heavy we are and how much energy we expend, a huge amount of it is under the control of our brain. And so we can think about just how tightly our body weight is controlled by thinking about how much food we eat not, for example, in a day, but in our lifetime. And that's about eight tonnes of food. And yet our body doesn't change weight a huge amount, even though we will quite regularly completely change the amount of food we eat from day to day. If we have a big Sunday lunch, we may eat many more than the 2000 calories and we won't even consciously think about it. But our body will adjust to reduce our caloric intake and burn off fat.

Chris - That's extraordinary. Eight tonnes in an average human lifetime?

Sam - About that. And if we chuck in water, that gets up to about 38 tonnes. So you can have an articulated lorry, that's about that weight.

Chris - My goodness. And you're saying that basically my body's keeping tabs on how many calories I'm shoving down my throat and it's either gearing up or gearing down my metabolism, and my desire to eat, to compensate to keep things about even stevens?

Sam - Pretty much, yeah, and it's actually one of the biggest puzzles of obesity. The Western world on average is getting heavier by about one kilo a year, and that's explaining the obesity epidemic, but actually it's a really, really subtle defect. It's like half a glass of milk per day. So the question is how have our brains gone wrong but such a tiny amount in order to lead to this? It's not some spectacular decompensation.

Camilla - I read some fascinating work on the origins of obesity about two years ago, and what it showed is that although some kind of very rare genetic differences might make you metabolically susceptible to obesity, actually for the most part the only kinds of genetic associations that you can find with obesity are behavioural, are kind of in the brain, and how satiated you feel after meals or how much you desire food. And those subtle changes only actually manifest as obesity if you grow up in what the authors called a kind of obesogenic environment. So you need both, kind of like addictions, I suppose. You need this trait vulnerability and then you need the state to actually have any effect on the body.

Sam - Absolutely. And so there's been a field of idea about this obesogenic environment, that we would have been evolutionary benefited in times of low nutrient availability by being able to store large quantities of food. But that leads to a problem, because the question then comes, why isn't everyone obese? Because we all have access to huge amounts of caloric dense food. And then there's a counter argument that there's something that actually is acting, and has evolved, to force our weight down. And one of the ideas is predation. The concept is if you are a very fat mouse, you are more likely to be picked off by an eagle. So there may be some advantages to having an upper limit on your body weight as well.