These statistics prompted the British Nutrition Foundation (BNF) to commission a report on obesity, published today. It contains the most up-to-date research by a task force made up of the country's leading experts, chaired by John Garrow, formally professor of human nutrition at St Bartholomew's Medical College, London.
The report dispels a number of "fat myths". Many of us are aware that the more you eat, and the less energy you expend, the fatter you become. As the report says: "One of the few statements about obesity that can be made with absolute certainty is that obesity can only occur when energy intake remains higher than energy expenditure, for an extended period of time."
At the root of the problem is the fact that we have grown increasingly inactive; although we eat about 750 fewer calories a day than 20 years ago, we expend about 800 fewer calories. However, the trouble with fat, as anyone who has tried to diet knows, is that it's difficult to shift. It's not always that easy to tell exactly how much excess fat we are carrying. Many gyms and health centres do offer to measure fat content, but their methods are somewhat inaccurate, and the best (using infrared scanners) can assess our body fat only to the nearest kilo.
The report suggests using the body mass index (BMI) to check whether an individual is clinically obese. The one drawback is that it doesn't assess the proportions of lean tissue and fat, and may therefore misrepresent athletes' BMI since they have such a high muscle-to-fat ratio.
BMI is calculated by weight in kilos divided by height in metres squared. A BMI between 20 and 25 is normal, between 25 and 30 is overweight, and above 30 is clinically obese. Recent research indicates that an individual's waist measurement is a good indicator: statistically there is a health risk in a waist size greater than 94cm (34in) in men, and 80cm (32in) in women.
There is evidence to suggest that obesity levels are genetically inherited. Identical twins who are brought up apart tend to weigh a similar amount; adopted children also have similar fat levels to their biological parents, but not to their adoptive ones. The likelihood of inheriting the same position of body fat as your parents - big thighs or a pot belly - is between 18 and 50 per cent. However, a number of other factors, including environment, lifestyle and mental outlook, can interact with a person's genetic propensity to put on weight.
It is a myth that most thin people have a shrew-like metabolic rate and can eat enormous meals. Not all slim people spend their lives in the gym, but they could burn fat by constant fidgeting. Research conducted on the Pima Indians showed that fidgeting ran in the family, and a lack of it was correlated with a predisposition to obesity in later life.
In fact, overweight people have higher metabolic rates than people of normal weight. This is partly because of bigger mass - someone who weighs 40kg too much is lugging around the equivalent of two suitcases - and partly because weight gain is not entirely fat. About a quarter is extra musculature to support the bulk, and intestinal and liver tissue also increase; all these are more calorie-hungry than fat.
Most adults possess about half a billion mature fat cells. The number that are full of fat at any one time changes both minute by minute, and on a more long-term basis. Brown fat cells were once all the rage; any one who was Twiggy-thin was presumed to have a lot of brown fat. We now know that these cells (brown adipose tissue, or BAT) are largely found in animals where they are involved in regulating heat, and can influence obesity in rats, but the report suggests that they play only a minor role in human physiology.
The deposition of normal, or white fat cells, is influenced by gender and genetics. Women tend to have more fat deposits on their hips and thighs, for example, which are believed to provide some of the energy for breast- feeding. It used to be thought that the number of fat cells laid down in infancy would be the amount you had for life. According to the report, this is untrue. In one study conducted more than 20 years ago, 28 obese women lost 13 (out of a total of 38) kg of fat over 26 weeks. Their fat cells shrank, but only 2 per cent of them disappeared.
Recent research confirms this: once the number of fat cells a person has been born with are full, more are produced, and although in theory these fat cells can be killed, in practice it is rare. The purpose of a fat cell is to store fat, thus if someone has a large number of fat cells (even if some are empty), they will be predisposed to gain weight. This may be one reason why people who have initially lost weight put it back on. Even removing fat cells by drastic surgical measures, such as liposuction, does not help in the long term as immature fat cells can subsequently develop into mature cells to replace them.
The report claims that anti-obesity drugs may help: "Drugs will be useful to the extent that they make lifestyle changes easier to achieve, provided that they are cost-effective and safe." According to Professor Peter Kopelman, from St Bartholomew's and the Royal London School of Medicine and Dentistry, Sibutramine can be effective and has, as yet, not proved to be as harmful as other drugs on the market. It acts on the nervous system by preventing hormones - serotonin and noradrenaline - from being taken up the brain, and combats hunger pangs.
Sibutramine can lead to a 10 per cent reduction in body weight in three months. However, drug use is not a miracle cure and Professor Kopelman recommends this strategy only for those with a BMI greater than 30, who have already attempted to alter their lifestyle and diet. The side-effects of Sibutramine include nausea, insomnia, dry mouth, constipation and an increase in blood pressure and heart rate.
The stark truth is that the only way to maintain weight is to eat sensibly and exercise regularly (at least half an hour of brisk walking five times a week); losing a kilo of fat a week means eating 1,000 fewer calories every day. It may be tough, but in the end a healthy lifestyle is the only effective way to keep trim.
'Obesity: Report of the British Nutrition Foundation's Task Force', pounds 29.99, is available from the British Nutrition Foundation, 52-54 High Holborn, London, WC1V 6RQ www.nutrition.org.uk