Putting on pounds helped us to survive

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The human body is highly adapted to accumulating fat that acts as a vital energy store in lean times. Because of the energy needed for pregnancy and weaning, women are more susceptible to building up fat deposits than men.

Nutritionists have devised an internationally agreed scheme for measuring fatness called the body mass index, which is calculated by dividing a person's weight in kilograms by the square of his or her height in metres. This produces a figure that acts as a measure of weight for a person's height.

Everyone with a body mass index between 25 and 30 is classified as overweight and those over 30 are defined as clinically obese.

Obesity increases considerably the risk of sometimes fatal disorders, such as heart disease, cancer, and type 2 diabetes, which often strikes in middle to later life and is caused by a failure to respond to the production of the hormone insulin, which helps to control levels of sugar in the blood.

The global obesity epidemic has gone hand in hand with a rise in this form of diabetes, and more than 300 million people are expected to suffer from it by 2025.

One theory is that certain ethnic groups have ancestors who in the past had to endure extended periods of starvation. Only those with "thrifty genes" survived. When a high-calorie diet was introduced, obesity and diabetes were the results.

This may explain part of the rise in global obesity but it cannot account for it all. The most important factors are eating too much and exercising too little.