Even if you have a "normal" BMI, you may still have dangerous amounts of body fat.
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It’s official. We Kiwis are among the fattest in the developed world. A recent study in the Lancet found the number of obese people around the world has nearly doubled since 1980, and among higher-income countries, only the US is worse than us in terms of the average body mass index (BMI). Given one in four New Zealand adults is obese, that’s not too surprising.
Does this mean those of us in the theoretically healthy BMI range of 18.5-24.9 can congratulate ourselves that we’re not part of the obesity epidemic that’s set to strangle healthcare systems worldwide? Possibly not. The World Cancer Research Fund, in its major report published in 2009, advises us to reduce our cancer risk by being as lean as possible within the normal range of body weight and to avoid weight gain and increases in waist circumference throughout adulthood.
But now there’s another reason not to be complacent: normal-weight obesity, a term first coined in Italy in 2005 by Professor Antonino De Lorenzo. He identified participants in his research who were of normal weight but with excessive body fat and metabolic factors associated with obesity. If you’re thinking beer bellies, think again; it seems women are more likely than men to have normal-weight obesity.
Although BMI is useful as a population measure with a high correlation to body fatness, it doesn’t distinguish between fat and muscle mass. This is why many All Blacks fall into the overweight or obese categories, when clearly their extra weight is from lean muscle. If your BMI puts you in the obese category, you can probably guess whether it’s from carrying too much fat or from spending hours in the gym building muscle. But with normal-weight obesity it’s harder to tell the difference.
The reason researchers are interested in this new category of fatness is not just to wipe the smile off those slenderer faces. It’s long been known that obesity is linked to risk factors for cardiovascular disease, such as high blood pressure, high cholesterol and insulin resistance, but now it seems people with normal-weight obesity are also at risk.
It turns out that subcutaneous fat, the stuff just under the skin that we can see and feel, isn’t so bad. Visceral fat is the problem. That’s the fat deeper inside the belly around the organs. Visceral fat secretes adipocytokines, which have been linked to the development of insulin resistance and cardiovascular disease. The Italian women De Lorenzo discovered with normal-weight obesity had an average BMI of 22.9 and 39% body fat, compared with the normal-weight control group with an average BMI of 20.3 and only 20% body fat.
Rozanne Kruger from Massey University presented her findings on normal-weight obesity at the Nutrition Society Conference in Wellington in December. When recruiting women for an iron study, she and her team noticed the BMI measures didn’t seem to correlate with their body-fat measures.
They subsequently launched a pilot project investigating whether normal-weight obesity existed here, and if so how common it might be. Hormonal changes undermine most women’s efforts to stave off gaining some fat in their fifties; Kruger’s pilot study looked at 18- to 44-year-old women. They were surprised to find normal-weight obesity in 25 of the 71 women with a “normal” BMI.
As Kruger says, “This was a group of seemingly healthy young women. Waist measurements would not indicate excess fat.” And they had a tendency to have higher metabolic-risk factors.
The intention now is to do a larger study to confirm the findings. What Kruger can tell us, though, is “there was a significant difference between the commuting habits of the normal-weight obese and the others with normal BMI. [The normal-weight obese] were the ones who would jump in the car to go to work rather than walk. They also spent the least time on any type of recreational activity.” Whatever it is that gets you moving, it would seem just doing it is a very good idea, even if you do have a so-called healthy BMI.