Obesity is defined as a condition where there is an excessive amount of body fat. The amount of body fat in the body is measured using the Body Mass Index (BMI). This is calculated by dividing the body mass or weight (in kg) by the square of the height (in metres). A BMI of 20-25 = ideal weight, 25-30 = overweight, over 30 = obese (these figures are for adults).

To lose weight you need to take in less energy than the body needs. This can be achieved by increasing the energy expenditure by being more active or reducing the energy intake by consuming less energy. The body then compensates by using up its fats stores to meet its energy requirements which in turn means weight loss.

This is not always so simple as it sounds. When cutting calories, slimmers can sometimes miss out on valuable nutrients, such as vitamins and minerals. In following a weight-reducing diet, therefore, slimmers need an adequate intake of nutrients (fat, protein, carbohydrate, vitamins and minerals) at the same time as controlling calorie intake.

The psychology of complying with any diet is also an important factor for successful weight loss. As it can take several months to achieve a target weight, diets must be palatable and enjoyable enough to be tolerated for long periods. Convenience and ease of use are important.