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 500g /week you need to take in 500 calories less every day.