There are various ways of judging how fat someone is. You can, for instance, measure skinfold thickness with callipers or look at waist-to-hip ratios. The body mass index was used in NHANES and is the most popular. Using kilograms and meters, divide weight by height squared. For adults, 25 or more is overweight, 30-plus is obese, and 40 and over is extremely obese (1 in every 21 U.S. adults today). That BMI may not mean the same all over the world is nicely illustrated by a recent World Health Organization meeting (Lancet, 363: 157-63, 2004), but the WHO experts decided against any radical adjustment. As paper #7 suggests, BMI is more reliable than indices based on self-reported weight and height. For example, the Behavioral Risk Factor Surveillance System (A.H. Mokdad, et al., JAMA, 286:1519-22, 1999) yielded obesity rates one-third lower than the NHANES figure for a similar time frame.

Medical concern over obesity relates to its association with cardiovascular disease and diabetes. Dr. Katherine M. Flegal and her colleagues from the National Center for Health Statistics, Hyattsville, Maryland, note that over the years when obesity has increased, cardiovascular mortality has fallen. This may not be as paradoxical as it sounds. Relationships between risk factors are complex (e.g., recent guidelines on blood pressure suggest that systolic blood pressure falls by between 5 and 20 mm Hg for every 10 kg of weight loss). Furthermore, the falls in mortality might have been even more impressive if obesity had not been increasing. The management of heart disease has improved too. The public health importance of the NHANES series lies in the accuracy of the data, as Dr. Flegal emphasizes to Science Watch. Having data based on measured weights and heights "enable us to document clearly the marked and continuing increases in obesity in the U.S. since 1980, which are a matter of considerable concern," she says. They serve as a "starting point" for discussions about the causes of the rise and policies to reverse it. Unfortunately, reversing this trend has become a political hot potato in 2004 with U.S. officials not seeing eye to eye with the World Health Organization’s Global Strategy on Diet, Physical Activity and Health, notably on diet.

One might argue that the starting point of obesity lies largely in children’s eating and exercise habits, and much preventive activity in the U.K. at present is focused on that notion. An accompanying NHANES paper to #7 has fewer total citations (111 versus 227, as of late January 2004) but seems just as important. Dr. Cynthia L. Ogden and her colleagues (JAMA, 288: 1728-32, 2002) found that 15% of U.S. children aged 6-19 years were overweight, this being defined as being at or above the 95^th percentile for the appropriate BMI.