Most of us think of fat as a sign of overindulgence and lethargy. Indeed, it can be, but long-time observers of trends in obesity (weight watchers of a different sort!) know that advice to ‘eat less’ is simplistic and completely ignores the evidence, such as the Department for Environment, Food & Rural Affairs’ (DEFRA) National Food Survey 2000, showing that overall calorie intakes have gone down substantially throughout the 20th century.

But if we’re eating less, why are we putting on so much weight? The answer appears to lie in what, rather than how much, we are consuming.

I’ve been particularly struck by recent reports showing that, far from being superfluous and totally unhealthy, body fat protects bones and organs, regulates hormones and the immune system, and ‘feeds’ vital organs, such as the heart and kidneys. It also produces an important hormone called ‘leptin’ that communicates information about the body’s energy levels to the brain.

British researcher Caroline Pond has made the study of fat her life’s work (see The Fats of Life, Cambridge University Press, 1998). She suggests that mammalian fat is meticulously organised around a dozen specific sites of the body. At each site, the fat’s function is linked to its location, be it on the heart or in muscle. So complex and diverse are the functions of fat that some suggest it should be considered an organ.

Against this backdrop comes the recent concern that chronic exposure to small amounts of environmental toxins, such as organochlorines, may be increasing our risk of excess body weight. Largely ignored by the press, who prefer quick-fix stories, the evidence is nevertheless so compelling that the US National Institutes of Health has recently called for more and better research into this area.

Research in the 1980s showed that the widely used organocholorine pesticide lindane causes a significant increase in food intake and weight gain in animals. In humans, blood levels of organochlorines are generally higher in people with higher body mass. Once in the body, they can interfere with weight controlling hormones such as catecholamines, thyroid hormones, oestrogen, testosterone, corticosteroids, insulin, growth hormone and leptin.

Our first line of defence against this toxic onslaught is body fat. As levels of organochlorines increase, numbers of fat cells also increase to dilute their concentration. This may help to explain some of the great and frustrating mysteries of the dieter’s life – the weight-loss plateau and the often swift rebound weight gain.

As we lose weight, more toxins are released back into the body. In its infinite wisdom, the body responds the same way it did the first time around – it produces more fat cells. The more rapid the weight loss, the more toxins are released and the more strongly the body responds.

Nutritionists have long advised that slow and steady wins the weight-loss race. For those carrying a load of organochlorines (from diets high in conventionally reared meats and dairy products), it may be only way to cross the finish line at all.