Is BMI Accurate? Waist Beats Big Bones

Chapter summary

What BMI is: weight in kilograms divided by height in metres squared. Under 18.5 is underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight, and 30 or more obese (classes I to III).
Weight has four parts: bone mass (more is better), muscle (more is better), external fat (some is useful, and brown fat helps metabolism) and visceral fat around organs (more is worse).
Age: People get heavier with age. BMI becomes a real issue from about 40. Most people think they are the exception. They are not.
Population data: In Canada, 67% of men and 54% of women aged 18 to 79 were overweight or obese. Rates are similar in the UK, US and Australia.

Men with a 120cm (47.2″) waist face twice the risk of illness compared with 80cm. For women the comparison is 100cm versus 65cm.
Each extra 5cm (2″) of waist raises the risk of premature death by 17% in men and 13% in women.
Abdominal fat releases chemical messengers that can drive chronic disease, according to Tobias Pischon.
Quick test: Measure at the belly button. If your waist is under half your height, you are probably fine. If not, plan to act.

BMI is useful: It is simple, cheap and a good rule of thumb when nothing else is available. Health agencies rely on it.
BMI is flawed: It ignores fat distribution. Athletes with heavy muscle read as overweight. Asian populations face higher diabetes and heart risk at lower BMI, so the WHO suggested extra action points at 23, 27.5, 32.5 and 37.5.
The “obesity paradox”: Flegal’s 2013 JAMA review of mortality data found overweight people did not die earlier than normal-weight people, and some did better. The author notes about 20% of overweight people have few health issues.
The counter-view: About 20% of normal-BMI people carry metabolic syndrome. A Mayo Clinic study of 14,000 people found normal BMI with a high waist-to-hip ratio carried the highest cardiovascular death risk. This is “lean on the outside, fat on the inside”.
The author’s reading: The two views are not in conflict. BMI misses where fat sits, so use waist as well.

Average waist was 95.1cm for men and 87.3cm for women.
29% of men and over 41% of women had a waist size linked to higher risk.
Men often underestimate risk because they measure at the trouser line and not the belly button.

Health Canada (2003) Canadian Guidelines for Body Weight Classification in Adults.
Orpana, H. et al. (2010) BMI and mortality: results from a national longitudinal study of Canadian adults. Obesity 18(1):214-8.
Shiwaku, K. et al. (2004) Appropriate BMI for Asian populations. The Lancet 363:1077.
Pischon, T. et al. (2008) General and abdominal adiposity and risk of death in Europe. N Engl J Med 359:2105-21.
Flegal, K. et al. (2013) Association of all-cause mortality with overweight and obesity using standard BMI categories. JAMA 309(1):71-82.