What is your ideal weight?

More than half the UK population is overweight and the proportion is increasing all the time. Excess weight puts an extra burden on your joints, increasing your risk from joint injuries, back pain and conditions such as osteoarthritis. But while it is healthy to keep your weight below a certain level, it is also important not to go too far the other way.

Losing too much weight too quickly may affect your body’s ability to maintain and repair your bones and muscles and puts you at greater risk of conditions such as osteoporosis. Crash dieting, excluding food groups from your diet or losing too much, too quickly, can starve your body of nutrients. A slimming diet still needs to provide all the nutrients your body requires.

A good way to work out whether you need to lose or gain weight is to calculate your body mass index (BMI). To work this out using imperial units, simply divide your weight in pounds by your height in inches squared, and multiply by 703. For example, someone who weighs 132lb (9st 6lb) and is 65 inches (5ft 5in) tall will calculate their BMI as below:

A BMI between 18.5 and 25 is associated with the lowest health risks, and means you're a healthy weight for your height. A BMI less than 18.5 means you're underweight for your height, and you might not be getting enough nutrients to maintain strong bones and keep your body healthy. 

The higher your BMI lies above 25, the more weight you need to lose: not only will this take stress off your joints, it will also lower your risk from problems including diabetes, heart disease and high blood pressure.

Your weight is a reflection of the balance between your energy intake and energy expenditure. If your intake is equal to your expenditure, your body weight will remain the same. If your intake exceeds expenditure, the excess energy is stored on your body as fat.

To lose weight you simply need to tip the balance in the other direction, by increasing your expenditure, decreasing your intake, or a combination of both. Energy intake is determined by the amount and the type of food you eat. Energy expenditure is determined by your basal metabolic rate (BMR) and the number of calories you burn each day during daily activity and exercise.

Drugs such as steroids can increase appetite and cause weight gain, so if you are prescribed these drugs you need to be extra vigilant with your weight.

Losing weight is not difficult—it is keeping it off that is the real challenge. Most people who lose weight later regain it because they choose the wrong sort of diet. There are no miracle cures: the only way to lose weight and keep it off is to make long-term changes to your eating habits and lifestyle.

The safest and most effective way to lose weight is slowly and steadily. Between 1–2lb a week is a good level of weight loss—if you lose too much too quickly you might lose lean muscle tissue as well as fat. Since the amount of lean muscle you have helps to determine your basal metabolic rate (the number of calories your body burns in order to function normally), it is a good idea to preserve it.

To lose 1lb of fat in a week you need to burn around 3,500 calories—about 500 calories a day. You can do this by restricting the calories you consume, but diet alone is rarely enough. To lose weight and keep it off you also need to increase the amount of calories you burn. The most effective way to lose weight is by combining a healthy diet with plenty of cardiovascular exercise.

If you are underweight (with a BMI below 18.5), you might not be getting enough nutrients to keep your bones and muscles healthy. In particular, low weight is linked to a much greater risk of osteoporosis. If you have a high metabolism or small appetite, gaining weight can be hard.

The best way to increase your energy intake is to eat three healthy meals a day and plenty of nutritious snacks such as nuts and seeds, wholemeal toast or yoghurt. Avoid drinking with meals or for an hour before a meal as this can reduce your appetite.