A ‘healthy’ body mass index (BMI, defined as weight in kg/height in m squared) does not take into account anything about body composition. Two people with the same BMI can have very different proportions of muscle vs fat. They can also have a very different pattern of fat deposition – one person may have more of the metabolically dangerous central fat (around the waist, abdomen and abdominal organs), whereas another person with the same BMI may have a tendency to deposit fat in the less dangerous fat stores under the skin, for example in thighs and breasts.
Further, some people with a BMI in the ‘healthy’ range (18.5-25) actually do carry excess fat – this is called Normal Weight Obesity, and is more common in women. These people will often have an elevated waist circumference, which reflects excess central fat stores.
A recent study examined whether women with normal weight obesity had any difference in mortality risk compared to women with healthy BMI without normal weight obesity. The study followed over 156,000 women from the Women’s Health Initiative study from enrolment (1993-1998) until 2017 (at which time their average age was 63). They defined normal weight obesity based on a waist circumference of >88cm.
Compared to women with healthy range BMI and no central obesity, death rates were:
- 30% higher for women with obesity by BMI and central obesity
- 16% higher for women with overweight by BMI and central obesity
- 31% higher for women with healthy BMI but central obesity
- 9% lower for women with overweight by BMI but no central obesity
- 7% lower for women with obesity by BMI but no central obesity
Also, women with healthy range BMI and central obesity had a 25% higher risk for cardiovascular death and 20% higher risk for death from cancer, compared to women with healthy range BMI without central obesity.
We are seeing two key points emerge here. The first is that central obesity is the dangerous fat that is associated with adverse health consequences.
The second point is that this data reflects the obesity paradox – carrying a little excess weight seems to be associated with longer life as we get older. All the women with central obesity (top three categories above) had increased mortality risk, but the risk was lower in the overweight category than in the healthy BMI category. Similarly, the women with overweight and obesity by BMI but no central obesity had lower death rates than women with healthy BMI and no central obesity.
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