Obesity is more than just a few extra pounds.

Obesity is the heavy accumulation of fat in your body to such a degree that it rapidly increases your risk of diseases that can damage your health and knock years off your life, such as heart disease and diabetes.
The fat may be equally distributed around the body or concentrated on the stomach (apple-shaped) or the hips and thighs (pear-shaped).

For medical purposes, the body mass index (BMI) is used to determine if your weight is in the healthy range. Doctors use BMI because it compares your weight against your height.

You are in the normal range if your BMI is between 18.5 and 25 (kg/m2).
You are overweight if your BMI is between 25 and 30.
You are obese if your BMI is 30 or higher.
You are morbidly obese if your BMI is 40 or higher.

What problems can obesity cause?

Psychologically, being overweight can affect your body image and damage self-esteem. In some cases this can cause social anxiety and depression.
Common physical problems include:
difficulties breathing
difficulties walking or running
increased sweating
pain in the knees and back
skin conditions such as acne
gallstones.

The following medical conditions are also more common in obese people than in those of normal weight:

high blood pressure
high cholesterol
diseases related to hardening of the arteries such as heart attack and stroke (cardiovascular disease).
type 2 diabetes
some types of cancer.

These conditions are often known as obesity-related diseases and are some of the most common causes of death before the age of 75. This is why obesity increases your risk of mortality.

What causes obesity?

Obesity can be hereditary, so some people are at increased risk.
Genetic factors can affect appetite, the rate at which you burn energy (metabolic rate) and how the body stores fat. Examples of genetic diseases are polycystic ovary syndrome (PCOS) and hypothyroidism. But even if your genes make weight gain more likely, it is not inevitable that you will be overweight.

Obesity develops from:

overeating
irregular meals
lack of daily physical activity.

This is why obesity has trebled since 1980, when only 6 per cent of men and 8 per cent of women were obese. In this time our lifestyles have changed rapidly, with the ready availability of convenience foods and car journeys replacing walks to work and school. It is lifestyle that determines how the genes develop. Medicines such as antidepressants, corticosteroids and oral contraceptives can also cause weight gain.

When is obesity dangerous?

If you have a BMI of more than 25, you should lose weight. The same is true if you carry too much fat around the middle because this increases your risk of heart disease and diabetes.
Your waist should be no more than 102cm/40" (men) or 88cm/35" (women), with stricter targets for Asians of less than 90cm/35" (men) and 80cm/32" (women).
To reduce your risk of cardiovascular disease further, you should watch your waistline and make sure it's no more than 94cm/37" (men) and 80cm/32" (women).

In the long term

While plenty of diets and slimming products claim to offer quick fixes, obesity is not something that can be cured or brought under clinical control within a few weeks or months.
Treatment such as diet and exercise may need to continue for years.
Weight-loss plans from a GP or dietician are an effective way to lose weight, but a greater challenge is to achieve a way of life that maintains weight and reduces the chances of putting it back on.
This can only be achieved by permanently changing your eating and exercise habits.