We've invited Professor Roy Taylor to offer his thoughts on diet, healthy eating and the Hairy Dieters' progress throughout the series. Here's his first update, on body fat and some of the instruments he uses to measure it...

How did the Hairy Bikers' starting-point rate amongst those you typically see?

Well, the situation that Si and Dave were in when they came along was really quite similar to the situation that many people with Type 2 Diabetes find themselves in; the big problem is that there’s too much fat in the wrong place.

We now know that different people can tolerate different amounts of fat before glucose problems start, but carrying around more fat than is needed is a bad thing for everyone in terms of day-to-day life and long term problems such as heart disease and stroke.

Where are the dangerous areas that fat gathers?

For men and women it’s the same area: around the stomach and in the internal organs, but women typically tend to store fat under the skin, around the hips and legs.

This is based upon differences in gender and hormones, but there is an important difference in that men tend to store their excess fat inside the belly and inside the organs at an earlier stage of acquiring obesity.

So we see that men get diabetes at a lower BMI than women. It’s important for men to remain slim throughout their life and provided they eat the appropriate number of calories for their exercise and activity then they should remain at the same weight as they were in their early 20s.

In women there’s a bit more tolerance which is likely to have evolved to allow pregnancy to happen. But unfortunately both men and women in today’s society are fat and getting fatter.

The fat Dave and Si had was especially inside their bellies and probably already inside their important organs, although we didn’t measure that directly.

Their waist circumference showed one of the biggest changes we observed. The area around your belly can be changed from a beer belly to something much more like a six pack.

What was the ‘BodPod’ that we saw in the first episode?

The importance of the BodPod is that it allows us to get a very precise measurement of the portions of fat and other tissues in the body. It can do this because the density of fat is very different to the density of the water which makes up the majority of muscle tissue for example.

Firstly, the BodPod tells us the density. Then it uses a very simple formula that says density = weight/volume.

So essentially, the BodPod measures the volume of a person in a very precise manner, and this will tell us how much of that person is water and how much of them is fat.

And what about the Dexa scanner?

This allows us to estimate where the fat is in the body. And it does this by using a very low dose of x-rays, far lower than even a chest x-ray.

So it can produce a picture of approximately where the fat is in the body - and it’s a rapid, simple means of doing this.

How many people have an illness directly related to their weight?

If we look at the strict definition of overweight and obese - with a BMI over 25 - we’re looking at over half the population [cite].

But there are many people who have a normal BMI who would benefit very much from losing quite a bit of weight.  If a person’s body is genetically set up to run best at a BMI of, say, 21, then it will be suffering if that person has a BMI of 23. So we know that the risks of heart trouble decrease steadily as BMI decreases.

One of the factors that can push up blood pressure is carrying around too much weight. If people lose weight then the blood pressure will come down to some extent.

This in itself is one of the huge benefits of losing weight because a person might be able to stop taking tablets for their blood pressure. Of course, this is a decision to be taken in discussion with a person’s GP.

When people realise what their weight is, and the composition of their body, it must be quite an emotional moment. How do people often respond to these tests?

People are usually very surprised to discover how much of them is made up of just fat, and it often conflicts with their own image. Using the Dexa scanner we can show people the image of their body, and the outline of the tummy can be seen to change quite dramatically from before to after weight loss.

Why do we need fat on our bodies and what is the healthy amount of body fat for men and women?

Fat is really very important for long-term energy stores. So if there is any natural disaster and you were marooned without food then the body can survive for a long time, for weeks, without eating. Clearly, that has been very important over the centuries of our evolution.

And then there are the physical properties of fat; so for instance fat cushions the organs, and also insulates the body keeping out the cold. For a man the healthy percentage is usually said to be 10-19%, and for women it’s 20-30%.

Why is it so much more for women?

There’s a biological need for fat stores in women to achieve successful pregnancy and breast feeding even if food is in plentiful supply. It’s also important as fat gives you curves and that is attractive to men - the species must survive!

In the programme, we see a direct correlation between the percentage of fat in the Bikers’ diets and the percentage of fat in their body? Is this often the case?

Yes it is, because fatty foods are often the yummy ones, but they’re also those with the highest number of calories. So for instance one gram of fat contains nine calories, and one gram of carbohydrate contains four calories.

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