´óÏó´«Ã½

Someone with a BMI of below 18.5 is considered underweight. Whilst some people are naturally slim, being underweight from poor nutrition, or as a result of other disease, can have serious health risks. Illness associated with being underweight ranges from simple tiredness due to inadequate energy intake, through to reduced immunity to infections, anaemia, vitamin deficiencies, thinning of the bones, infertility and heart rhythm irregularities. If your BMI is under 18.5 you should discuss it with your doctor. They will want to make sure you don’t have any illness causing the weight loss and then will advise you about how to safely gain weight.

A BMI of 18.5 to 24.9 suggests a normal healthy weight. This means your body is not at risk of weight-related disease. You still need to ensure a balanced diet with plenty of fresh fruit, vegetables and good vitamin and mineral intake, but if your calorific intake is matching your energy output you will remain a normal weight.

With a BMI of 25 to 29.9 many people may only notice they are overweight when their clothes get tighter - by then the excess weight you're carrying will already have silently started to damage your blood vessels, hormone system and joints, raising your risk of diseases such as:

• Heart disease
• High blood pressure
• Diabetes
• Arthritis
• Infertility
• Asthma
• Cancer

Although you may feel no different, you need to take action whilst the effects of being overweight can be reversed more easily - you need to gently reduce your calorific intake and increase your activity level. Crash diets don’t work - you need to change your lifestyle. Even slight changes make a difference - two less chocolate biscuits, or one less glass of wine a day can lose you 5lb in fat during the year. Simply parking the car at the far end of the car park or using the stairs instead of the lift will over time make a noticeable difference.

Visit our health living guide for some more ideas

With a BMI of 30 to 35 you're now medically classified as obese - this means you're at high risk of weight related illness, such as:

• Heart disease
• Diabetes
• Arthritis
• Infertility
• Miscarriage
• Asthma
• Fatty liver disease
• Cancer

But if you can lose even just a little weight your risk will drop - Just a five per cent weight loss (from 80kg to 76kg) has been shown to significantly reduce the health risks associated with obesity, such as reducing your cholesterol levels and blood pressure and giving you better blood sugar control.

To have a BMI over 35 is considered as being very obese and this means that obesity-related illnesses could severely affect the quality of your life and shorten your lifespan by several years. Yet even now it’s not too late to do something about it. Improved fasting insulin and thus glycaemic control in patients with type 2 diabetes, even coming off the medication you may take for diabetes or blood pressure is possible if you can lose some weight. You should see your doctor and talk about getting help.

If your BMI has reached 40 or more you're ‘morbidly obese’ - you will be 80 times more likely to develop type two diabetes than someone with a normal BMI, your lifespan will be shortened by over ten years and your quality of life will be reduced in many ways.

If you're obese you may have lost control of your eating habit, so gaining control is the first step back to a normal healthy lifestyle. Tips to help you, which won’t make you feel hard done by, include:

• Plan your meals and only shop for those meals, even get someone else to do it for you so you aren’t tempted by impulse buys
• Try to watch the clock and keep your meals routine; reduce your snacking
• Reduce your portion size slightly and use a slightly smaller plate so it still looks full
• Change to low cal drinks and alternate with water
• Don’t eat on the go - try to sit down at a table to eat meals
• Choose low fat versions of sauces
• Make sure you get five-a-day of fruit and vegetables to make you fuller
• Read the labels - ‘Low in fat’ can mean high in sugar, whilst ‘low in sugar‘ can mean high fat
• Gently increase your activity to burn off the calories - aim initially for 30 minutes a day of enough activity to make you slightly breathless and gradually increase

The Body Mass Index (or BMI) is a way of seeing if your weight is appropriate for your height. The actual calculation is your weight (in kilograms) divided by your height (in metres) squared but it's also easy to read on the chart. BMI can be divided into several categories and generally the higher your BMI, the greater your risk of a large range of medical problems.

BMI charts are calculated for adults only (separate charts are available for children’s weight and heights). Inaccuracies can also occur if you're an athlete or very muscular as this can give you a higher BMI even if you have a healthy level of body fat and this BMI chart is not appropriate for women who are pregnant or breastfeeding, or people who are very frail.

As BMI is based on weight and height, by losing weight you will reduce your BMI and put yourself into a lower risk group. A healthy diet, including a balance of food groups, vitamins and minerals, is essential for a long and active life. Keeping it simple, body weight and shape are a balance of energy intake (dietary calorific content) against output (calorific burn from activity & exercise).

Many studies have shown that, to slowly and steadily lose weight, any diet which includes a healthy balance will work if you're motivated. Ideally a low fat, high fibre diet is best but low calorie diets, low-carb diets, meal replacement diets or simply reducing portion size will work as long as, at the end of the day, you're not taking in too much energy for your body’s particular needs. Generally, to lose 1lb /week you need to take in 500 calories less every day.