Under the Microscope: Obesity is perceived as a massive public health problem in the US, and dire predictions are made as to its future consequences.
Obesity is also rising dramatically in Ireland, and we have excitedly heralded it as our looming number one public health problem. However, there is growing evidence that the negative consequences of obesity have been exaggerated. This evidence is reviewed by W Wayt Gibbs in Scientific American (June 2005).
WHO guidelines assign weight categories in terms of body mass index (BMI), according to the formula BMI equals weight in kilograms divided by the square of your height in metres. BMI less than 18.5 is categorised as underweight, 18.5 to 24.9 is healthy weight, 25 to 29.9 is overweight, 30 to 34.9 is mild obesity, 35 to 39.9 is moderate obesity, and 40 or over is severe obesity. You will all be delighted to hear that my BMI is 24.
Six out of 10 American adults are overweight or obese, and the obese fraction of the population has doubled in America and in parts of Europe since 1980. This is considered to be very bad news.
Official US Department of Health and Human Services guidelines confidently assert that "excess body fat leads to a higher risk for premature death, type two diabetes, hypertension, high blood cholesterol, cardiovascular disease, stroke, gall bladder disease, respiratory dysfunction, gout, osteo-arthritis and certain kinds of cancer".
Julie L Gerberding, director of the Centres for Disease Control and Prevention, has alarmingly predicted: "If you looked at any epidemic - whether it's influenza or plague from the Middle Ages - they are not as serious as the epidemic of obesity in terms of the health impact on our country."
However, a growing number of academic researchers outside the medical community, while acknowledging that extreme obesity is a factor in some illnesses and premature deaths, disputes the assertions that obesity is causing a huge and worsening health crisis. In the words of one, J Eric Oliver, a University of Chicago political scientist: "A relatively small group of scientists and doctors, many directly funded by the weight-loss industry, have created an arbitrary and unscientific definition of overweight and obesity.
"They have inflated claims and distorted statistics on the consequences of our growing weights, and they have largely ignored the complicated health realities associated with being fat."
Another critic, Paul F Campos, University of Colorado professor of law, points out that genetic differences account for 50 to 80 per cent of the variation of fatness within a population. He claims that no safe and practical methods of weight reduction result in long-term weight loss greater than 5 per cent of body weight. Therefore advice to maintain a BMI in the "healthy weight" range is impossible for many people to follow.
In fact, he claims exaggerating the risk of fat and the feasibility of weight loss only encourages unbalanced diets and possibly exacerbates weight gain.
Although adult obesity in America has doubled since 1980, and trebled in children, predicted increases in deaths from heart disease and stroke have not materialised. However, there has been a small increase in deaths caused by diabetes.
An analysis of three large American surveys (Journal of the American Medical Association, April 20th, 2005) has shown that adults who fall in the overweight category have a lower risk of premature death than those in the healthy weight category. The majority of Americans who weigh too much are in this overweight category. In fact, people in the underweight category are more at risk of premature death than those in the mildly obese category.
Everyone agrees that severe obesity increases the risk of numerous diseases but the severely obese account for only 8 per cent of those whose BMIs exceed the healthy weight range. The big question is - are the mild to moderate obese categories piling up the overall health burden of heart disease, cancer and diabetes? For heart disease the statistics say no, or at least not yet. For cancer the statistics show a complex picture. Death rates are significantly increased for several kinds of tumours among overweight and mildly obese subjects, but most obesity-related cancers are rare. On the other hand, being overweight or obese seems to confer significant protection against lung cancer, the commonest lethal malignancy.
The body converts ingested carbohydrate into glucose that enters our blood to be carried to all our tissues. The hormone insulin facilitates the entry of blood glucose into tissue cells where it is "burned" to release energy. In type one diabetes, the body produces insufficient insulin and glucose cannot get into the tissue cells. In type two diabetes, insulin is produced but the cells do not respond to it because of a defect.
Obesity seems to pose its greatest threat to public health through type two diabetes. It is estimated that 55 per cent of adult diabetics in the US are obese, compared to the 31 per cent incidence of obesity in the general public. The worst case scenario is that up to 4 per cent of US adults might have diabetes because of their obesity.
The picture is complex because weight gain is a common side effect of many diabetic drugs. There is evidence, however, that exercise (at least 2.5 hours a week of vigorous walking) lessens risk of diabetes by almost 70 per cent, regardless of BMI.
So what's the take-home message. Obesity is certainly not recommended, but if you are slightly overweight there seems little reason to worry, particularly if you take plenty of aerobic exercise.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC -http://understandingscience.ucc.ie