Why are people prone to gaining weight?

Being overweight is a problem for millions of people worldwide, and is the cause of many life threatening conditions. But why do people struggle so much with weight loss, and are some techniques, such as calorie counting, misleading?

Giles Yeo, a geneticist from Cambridge University, looks at the genetic and metabolic reasons for weight gain. He talked to Chris Smith about why obesity is a growing problem in many cultures, and started by explaining what fat actually is.

Giles - Fat gets a bad rep, but we really needed to survive because fat is the group of cells that actually, you use to make long term energy storage,and you need it to keep you alive.

Chris - Indeed because if we have too little fat then women don't have any capacity to reproduce. for example, the body uses it for various signals.

Giles - That's correct. So, you need to have enough fat in order to keep your various biological functions going because how much fat you have indicates how long you can last without food.

Chris - How does my body therefore regulate how much fat I've got?

Giles - a fat, although is there and is a storage organ, also releases hormones. What happens is these hormones they call adipokines, but they circulate in the blood and they signal to the brain. the more fat you have, the more of these signals. And so, your brain is exquisitely sensitive to how much fat you actually have sitting in your body.

Chris - Therefore, it interprets how well fed you are based on the levels of those chemicals in the blood.

Giles - Absolutely, how long you can last without food.

Chris - Does that also set appetite then?

Giles - Okay, so your brain regulates food intake, not only from the fat signals which is your long term stores, but also from the gut signals which are your meal to meal variation. Okay, so what I had last time, how many calories did I have last time. it integrates these two signals and then supposedly then controls your feeding behaviour. So, it influences what you then eat the next time around. The problem is, for the vast majority of our human evolution, we have been evolving in a time when there was too little food rather than too much food. So, we are very, very sensitive to having too little food and so, when your brain makes you eat when there's food to make sure that you can survive.

Chris - We are therefore effectively slaves to our genetic evolution because we've evolved not knowing where our next meal is coming from, we are now genetically programmed to try to avail ourselves of all the energy we can.

Giles - Yes. That is actually quite accurate.

Chris - Why is it then that some people can exist in the environment that we have created for ourselves and not gain weight whereas the majority of the population do?

Giles - Okay, so that is the very, very key question. What is clear is that because the prime directive for human beings is to eat enough to reproduce. That's pretty much it. eat enough to survive long enough to actually reproduce. All of the pressure has been to - well, what happens if there's not enough food? Whereas, there has been no selection pressure to say, there's too much food. Well, aside from maybe the sabre-toothed tiger making you too big, but otherwise, there is not a big strong selection pressure for eating too much. So therefore, there is a more natural variation in how one responds to an environment where there's too much food.

Chris - Given that variation potential, does it mean that I might extract different amounts of energy from the food I eat than you do? So, we may eat the same thing. It says on the packet there's 300 calories in this, but the amount of energy you will extract when you eat it may be wildly different than the amount that I get out of it.

Giles - I think there are two questions there. The first is, how do you deal with energy compared to how I deal with energy. So, if we're eating exactly the same type of food, how you or someone else might do, yes, there are going to be subtle variations in what we call energy partitioning. So for example, maybe you may eat a calorie, store half of it and burn half of it. maybe I, store 0.6 of it and burn 0.4 of it. okay, so that's the kind of thing that is there. So, there are definitely differences between how people burn their calories. Your second question has to do with calorie counting so to speak. And in many ways, calorie counting seems to make sense. "Oh! I need to have X hundred calories in this particular meal" because a calorie after all is a unit of energy and it's physics. So, a calorie is a calorie and it should be a calorie whatever you eat. The problem is, how you actually access it when it actually gets down to use. So for example, when you take 100 calories of glucose for example and you eat it, you're likely to get 100 calories of glucose out whereas if you have 100 calories of celery, I think you're likely to take...

Chris - Negative energy.

Giles - Exactly, negative energy and it is negative energy. You're likely to take (I'm making it up now) 110 calories in order to burn those 100 calories. Now, those are the two extremes. But if we then imagine somewhere in the middle where you have a processed mince beef meal as an example, compared to a steak, the energy you need to choose actually break down the steak compared to actually eat the slop that is there are going to be very, very different. So yes, it is the sum total of how much energy you get from a specific food that makes a difference at the end.