Body Story

Fat attack: Why we get fat

Whereas older generations can still remember the reality of ration books during and after the Second World War, younger Britons have grown up accustomed to food in plenty, at all times. But even at the height of rationing, when desirable foods such as meat, chocolate and sugar were in short supply, people rarely went hungry. We in the West are fortunate enough to see famine and starvation as the problems of a bygone era, or another continent.

But our bodies have longer memories.

• The legacy of evolution
In our evolutionary history consistent food supplies were far from certain. To eat you had to hunt or gather edible vegetation. With the supply of such meals inevitably being subject to the vagaries of nature and luck, our ancestors lived with the real possibility of starvation. So it is not surprising that our bodies learned to stockpile food when it was abundant, in anticipation of harder times.

This evolutionary legacy remains with us today. Food provides the energy the body needs to carry out the metabolic processes that keep us alive, including the very process of digesting food, as well as breathing and the heartbeat. It also provides the fuel for anything else that we do, such as walking and talking. We measure this energy in units called calories and kilocalories (1,000 calories). Confusingly, when we refer to calories in everyday conversation, what we actually mean is kilocalories.

When we eat more food than we need, the body stores any extra calories, just in case. These are normally stored as fat, because, weight for weight, fat can store almost twice the calories that carbohydrates and proteins can. One kilogram of fat stores approximately 9,000 kilocalories.

• What is fat?
Fat is an umbrella term for a number of chemical compounds that are stable, not soluble in water, and oily or greasy to the touch. Together with oils, they comprise one of the three principal classes of foodstuffs, the others being proteins and carbohydrates. Nearly all plant and animal cells contain these basic substances, though in different quantities. Nut kernels can be up to 70% fat, while a white potato is less than 0.1% fat.

In the body, special cells called adipose cells store large globules of fat. Together, these cells form adipose tissue — large reservoirs of fat cells — mainly beneath the skin, but also around the organs of the body.

Apart from being the principal medium for storing energy in the body, fat has other valuable functions. The fatty adipose tissue around our organs acts as cushioning protection, while the fat under our skin insulates us from the cold. Fat is also vital for storing and transporting important fat-soluble vitamins (A, D, E and K) in the body, and for maintaining healthy skin and hair.

• Calorie counting
Between the car, the office, the sofa, the supermarket and the microwave, we do not expend as much energy as we once did. And we eat far more regularly. Though necessary energy intake changes with age and lifestyle, it is recommended that women eat between 2,000 and 2,500 kilocalories a day, and men between 2,500 and 3,000 kilocalories a day. If we eat more than this, or use less than this, the extra is stored away as fat. And if we regularly store fat away, and rarely plunder our reserves, we end up being overweight. (See Fat risks for definitions of what 'overweight' means.)

• No excuses
Why some people are fatter than others is less of a mystery than many of us would like to believe. That some people put on fat more readily than others is true. But this is more likely to be related to a greater interest in food, or a less energetic lifestyle, than to genetics, hormones, or metabolic rates.

Obesity can run in families, but even this may be the result of early feeding patterns taught by parent to child rather than a genetic predisposition to fat accumulation. Such patterns can apply to nations as well as families. Affluent populations like our own and that of the US have an abundant supply of high-calorie foods, and increasingly sedentary living habits, which can easily lead to patterns of over-eating and consequent obesity.

Of all the factors that contribute to obesity, hormonal and glandular defects are thought to be the least important, being demonstrable in only about 5% of all obese individuals.

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