In my diabetes clinic I’m often asked by patients: "Why have I got type 2 diabetes? My friends are all fatter than me and they don’t have this disease." The answer is this: we all have a different degree of susceptibility to fat.

Type 2 diabetes is caused by a person acquiring more fat than they personally can cope with, something I demonstrated in my work as a diabetes researcher a decade ago. When it comes to this illness, there’s no ‘one size fits all’ approach to the question ‘How fat is too fat?’

Recently, a study by researchers at the University of Michigan found that the number of people around the world living with diabetes had more than quadrupled since 1980. Today, almost half a billion people have the disease, and 90 per cent of them have the type 2 version. The frightening part is most don’t even realise it.

Who’s at risk?

As someone who has studied type 2 diabetes for many years, these findings don’t surprise me. There’s no mystery surrounding the reason for such an explosion in the condition, which causes the level of sugar in the blood to become too high and can lead to complications such as heart disease and strokes, vision loss and kidney problems. It’s a sickness related to modern lifestyles, in which we cook less food from whole ingredients and buy more of it ready prepared.

But the correlation between type 2 diabetes and obesity is less straightforward than you might think (hence those puzzled patients of mine). Almost three quarters of those with a BMI of more than 45 (i.e., those who are morbidly obese) actually don’t have type 2 diabetes. Meanwhile, 10 per cent of those with the disease have a BMI of under 25, meaning their weight is theoretically ‘healthy’. But in the case of these people, crucially, their weight isn’t healthy for them. Our genes determine how much weight each of us can carry before we enter the danger zone, and for some it’s far less than for others.

Our diets have changed dramatically in recent decades. As a result, both men and women are approximately 10kg heavier today than they were 30 years ago. The problem is not the hugely obese people; it’s that most people are walking around with a few extra kilograms they shouldn’t have.

The finger of blame often points towards ultra-processed foods, and rightly so: these foods are designed to fill us up less and leave us craving more. Thirty to 40 years ago, most people were cooking meals from scratch, and our shift to prepared food has had a disastrous effect.

Exercise plays a part, too, of course, but to a lesser degree. If a large man does a good workout, he still may regain all the calories burnt by eating some chocolate. Exercise is important for keeping your weight healthy in the long term, but nothing beats diet when it comes to shedding kilos fast.

So how do you know if you’re one of the unlucky many walking around without knowing you are diabetic? The short answer is you don’t. This is a disease that creeps up on you silently, not making itself known until it’s well on its way. If you’re passing more urine than usual, feeling thirsty, tired or succumbing to more skin infections, then, yes you should get yourself checked. But, by the time these red flags appear, it’s likely the disease has been present for a while.

It is, however, possible to know if you are at risk before the symptoms appear. If you come from a family with a history of the disease, you stand a fair chance of developing it if your weight goes above a certain level.

Waist size matters

As a rule of thumb, if your waist size is much larger than when you were 21, you are carrying too much fat. Even women who have given birth should be able to get their waist size back to what it was (if not their hip size). And, no, you don’t have to subsist on a lettuce leaf a day. But you do have to watch your portion size, as standard servings today far exceed what the average woman should really consume.

Men, for their part, get type 2 diabetes at a lower BMI than women. Even if a man is quite slim, an extra layer of belly fat in midlife should be viewed as a warning sign. So-called middle-aged spread should be tackled and not laughed off.

The risk increases steadily through life and rises sharply after 60. This is partly because the average 60-year-old is larger than the average 20-year-old. But also, as we age, we lose muscle mass. Even a 60-year-old who can proudly say they’re the same weight now as they were at 40 may still be unwittingly at risk, as they’ve probably lost muscle and gained fat.

So, what can we do? We must understand that any weight gain after we’ve stopped growing is entirely fat. We can find out what percentage of our body weight is fat by using one of the machines many gyms and pharmacies have, which give you an instant reading. If you’re outside the healthy range (between nine and 20 per cent for men, and between 20 and 32 per cent for women) it’s time to take action.

But if you do turn out to be one of those walking around without knowing you have type 2 diabetes, take heart in the knowledge it can be reversed. The science behind this, which is my area of special interest, is still evolving, but we do know that losing the excess fat you have gained can reduce the fat inside the liver and pancreas that builds up to cause diabetes. According to research, the sooner you do that the better.

The Daily Telegraph

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