MEDICAL MATTERS:The year's first edition of the Journal of the American Medical Association brought some unexpected good news for those of us who struggle with our weight. Researchers from the Centres of Disease Control and Prevention (CDC) reported that people who are overweight and mildly obese have lower mortality rates than individuals who have a normal weight.
It sounds almost too good to be true, doesn’t it?
In fact this is not the first research to point to the so called “obesity paradox”, whereby those who are moderately overweight appear to be “healthier” – in the broadest context of the term – than people who are defined by body mass index (BMI) as having a normal weight. The studies showing this benefit appear to contradict equally valid research which links a greater chance of suffering with heart disease, stroke or cancer with being obese.
The latest meta-analysis by the CDC’s Katherine Flegal looked at some 97 studies involving up to 2.9 million participants. All of them investigated the relationship between BMI and all-cause mortality. People with Grade 1 obesity (BMI of 30 to 35 kg/m squared) had a lower death rate from all causes compared with participants of normal weight. However those with grade 2 and 3 obesity (BMI greater than 35) had a significantly greater all-cause mortality rate. And people classified as overweight, with a BMI of 25 to 30, had significantly lower death rates.
An accompanying editorial cautions against relying on studies that measure BMI alone and suggest the addition of waist circumference to help quantify risk. But they say there appears to be a protective effect of the overweight or low obesity categories (as measured by BMI) for people with chronic conditions such as heart disease and diabetes.
“Even in the absence of chronic disease, small excess amounts of adipose tissue (fat) may provide needed energy reserves during acute catabolic illnesses, have beneficial mechanical effects with some types of traumatic injuries, and convey other salutary effects that need to be investigated,” they add.
Coming on top of other research, possible explanations for an obesity paradox include the likelihood that heavier patients present for medical care earlier; an increased likelihood they will receive aggressive risk factor treatment; the cardioprotective effects of increased body fat; and a beneficial effect of higher metabolic reserves.
But not everyone in the scientific and medical communities is happy. Some have suggested the interpretation of the Flegal paper has been faulty, especially as it assesses BMI alone.
They contend that she neglects to caution the reader that BMI is not itself decisively established as a reliable measure of unhealthy “fatness” since it fails to account for other known disease and mortality factors such as differing fat levels and differing fat distribution. Nor does it take into account muscularity or nutritional balance. For example a highly muscular individual can have a high BMI and be categorized as overweight, while not necessarily carrying significant excess fat. Critics also argue that abdominal obesity is more harmful than general obesity, when measured independently of BMI.
There is a palpable fear among preventive health experts that the Flegal mega-study will lull people into a false sense of security and possibly even sabotage the efforts of those who are positively motivated to lose weight.
With obesity rates of greater than 20 per cent in developed countries, rising obesity in developing nations and an undeniable link between obesity and greater levels of diabetes and other diseases, encouraging people to be overweight is clearly not an option.
But this latest research cannot be entirely dismissed and may be especially relevant to older people and those with certain chronic diseases whose health is being regularly monitored.
Perhaps, if the “uber” health police will permit me to use the phrase, the Flegal study needs to be taken with a pinch of salt, but not dismissed entirely.
mhouston@irishtimes.com
