Low calorie diets can do more harm than good

Now that we've outlined the scale of the problem, we’re going to shift our focus to the reasons why a rising number of people seem to be struggling with their weight. Andrew Jenkinson is a bariatric surgeon - he conducts procedures to help patients lose weight when other interventions don’t work. He’s interested, though, in trying to get to the bottom of why it sometimes becomes necessary to take this drastic action at all. He argues that many of the diets his patients go on to try and gain control of their weight have a huge impact on their metabolism, which makes keeping the weight off very difficult. He also points to the changes in our hormones as further evidence as to why so many diets end in failure, and another piece of the puzzle is in our genetics, as James Tytko heard…

Andrew - There are many twin and adoption studies from lots of different countries looking at identical twins, brought up separately in different home environments. It doesn't matter whether they're brought up in a healthy home environment or an unhealthy food and play environment, all of them find around about a 70 to 75% concordance with their body mass index. So they may turn out to be slim, they may turn out to be obese. Just as you would expect them to have the same height and eye colour, they have very similar body mass indexes.

James - That's interesting. So has this genetic predisposition to high risk of obesity always been there and the modern world has just tapped into it?

Andrew - This is normal to the biology of any species. It's called heterogeneity. So this is like the differences between individuals of the same species. There will always be significant differences and that helps the species survive in times of environmental change. So, for instance, if there was a famine, people with genes that select for being able to be metabolically quite efficient and store more fat or energy on board, that section of the population are going to be more likely to survive.

James - So it's this in combination with those environmental factors which account for this epidemic of obesity that we're experiencing at the moment?

Andrew - You need to have a trigger for a chunk of the population. And we see about a quarter to a third of populations living in a western sort of lifestyle environment become morbidly obese. So they really struggle with their weight. Obviously they have a genetic predisposition, but also they have the trigger of the environmental factors that cause their body to want to store weight. You mentioned changes in the environment - we don't do as much activity anymore and we've got really tasty carb heavy, calorie heavy foods that are a bit addictive - that's a very, very simplistic way of looking at what causes obesity. It's not the calories in the food, it's what the food does to you metabolically, what signals it's sending you.

James - What are the factors which control how our metabolism can change, how it can store more energy or release more energy?

Andrew - The regulator of our weight is a hormone called leptin. The more fat cells we have, the higher the level of leptin in our bloodstream and it should act as a message to the weight control centre in the brain called the hypothalamus: "this is how much weight we've got on board." It's a little bit like a petrol gauge in your car. Various different factors in the environment, particularly insulin and some inflammatory factors, cause a blockage of that signal. So if you put on a lot of weight, your leptin levels should go up and your brain should actually sense that and it will then automatically decrease your appetite and increase your metabolism to maintain what it wants your weight to be: a normal weight. So, automatically you won't put even more weight on and you may even lose weight. However, if you are exposed to a Western diet where there's a lot of sugar and refined carbohydrates and also a lot of snacking between meals, your insulin level tends to be quite high throughout the day, much higher than if you ate two meals a day, not massively carb heavy. Leptin is blocked by the hormone insulin. So if we're brought up in an environment where the food gives you high levels of insulin all the time, that blocks that leptin signal. So the brain can't see the fact that you've got far too much fuel or fat on board. In fact, it's getting the opposite signal. And the analogy is, imagine if you're driving along the motorway and your petrol metre is flashing on red, you know you're empty, you'll want to get to the nearest petrol station. When you start filling up, you realise the car's already full. The problem is the petrol metre is broken. This is what leptin resistance is and this is what obesity is.

James - You've touched already on the way people try and control their weight, often unsuccessfully. Most people don't want to live in a way they know is not best for their health, they're going to try and do something about it, so they'll go on a diet. But why do many of the diets people embark on not work? What's the flaw which means they often end up being counterproductive?

Andrew - They go on the premise that obesity is caused by eating too many calories and not exercising enough - that simplified calories in, calories out equation. So they cut calories and they go to the gym, but the body will fight against that. As you lose weight, your brain will automatically go into calorie seeking behaviour. People who go on low calorie diets have extremely high appetites and they're having to use a lot of willpower. They do lose weight at first, but then you've got the other factor: your body can change your metabolism by about 700 kilocalories per day. This is just the energy you normally would expend even before you move. So heating your body, heartbeat, your breathing, your immune system, building or repairing the cells in your body. This is all your metabolism. That can be turned down so not only have you lost a little bit of weight on this low calorie diet and you're absolutely ravenously hungry, but after three or four weeks of being on 1,200 kilocalories a day, for instance, your body's adapted to that low calorie intake and you go on the scales and there's no change. You go to your GP and you say, "look, this diet isn't working anymore." The GP will not understand the fact that your metabolism can significantly decrease. Feeling like that and seeing the scales aren't shifting, they just come off the diet and say it's not working anymore. And then because their metabolism is so low and they're so hungry, they will regain the weight significantly and probably usually more. They end up heavier than when they went on the diet.