Does BMI matter?

Today more than 2 in 3 adults are overweight or obese. But how do doctors decide what category you fall into, and what does it mean for your health?

Categories for weight in the UK are mostly measured using a calculator for your body mass index (BMI). This looks at your weight and height and comes up with a number. It divides the result into several categories depending on the number - a BMI of 18.5-24.9 is in the 'ideal' range; under 18.5 is underweight; 25-29.9 is overweight; 30+ is obese. Doctors also use a category called 'morbid obesity' for patients with a BMI over 40 - by the time you reach this category, it's highly likely your weight is having a significant impact on your health.

The health risks of obesity

The rationale for calling out the weight of people who aren't classified as obese is that even being overweight can affect your health. For instance, type 2 diabetes affects at least 1 in 20 adults in the UK and the number affected is rising all the time. If not well controlled, it can damage your eyes, kidneys, nerves and heart. Being in the overweight compared to the ideal category of BMI significantly increases your risk of type 2 diabetes. The higher your BMI, the more likely you are to develop it, especially if it runs in your family.

Being overweight also puts strain on your joints. Osteoarthritis, the most common kind of joint problem in the UK, affects 8.5 million adults. The big weight-bearing joints (hips and knees) as well as the spine are the most common sites. The higher your weight, the more pressure you put them under and the more likely you are to develop osteoarthritis. If you end up needing a joint replacement, success rates are lower and complication rates are higher if you're overweight or obese.

Other health problems linked with being overweight include high blood pressure; heart attack and stroke; obstructive sleep apnoea (where your airways close down when you're asleep, leading to poor sleep quality and tiredness); heartburn; gout and stress incontinence.

BMI isn't accurate in every situation. What we're really trying to measure is body fat - too much is the major cause of health problems where weight is concerned. The BMI calculator uses an approximation of the amount of muscle compared to fat in your body. Because muscle is more dense than fat, people who are extremely fit may have a BMI in the overweight range despite being very healthy. As you get older, your muscle mass tends to drop and the proportion of fat in your body rises - that means it may underestimate the BMI of over-65s.

People of South Asian origin have been dealt a bad hand where body fat is concerned. Fat deep inside your tummy is more dangerous for your health than fat under the skin on your hips, thighs etc. That means even if two people have the same BMI, the one with an 'apple' rather than a 'pear' shape is more unhealthy. People of South Asian origin tend to accumulate weight in their tummies, so for them even a BMI of 23-25 counts as being overweight from a health perspective.

Your practice nurse will be happy to check your BMI and can give useful advice on losing weight. They may be able to refer you to a local NHS service - these vary around the country and range from individual sessions with a dietician to group classes.

In some areas you can access 'exercise on prescription', such as free entry to local authority gym classes. Exercise is great for toning muscle, improving the ratio of muscle to fat in your body and strengthening your heart and bones, protecting against osteoporosis. However, to lose significant amounts of weight, you'll need to adjust your diet as well.

Weight loss surgery is reserved for people with a BMI over 35 or 40, depending where you live. It's not an easy option.

The good news is that you don't need to shed several stone for your health to benefit. In fact, losing just a few pounds can cut your risk of heart attack, stroke and type 2 diabetes. For most people, the most effective way to lose weight long-term is to take a slow and steady approach, aiming to take off 1-3 pounds a week.

With thanks to My Weekly magazine, where this article was originally published.