One reason for naming obesity a disease is the fact that being markedly overweight is positively correlated with a variety of health problems. Some of these problems are risk factors for diseases, such as hypertension, abnormal blood lipid levels, and sleep apnea. Others are diseases in their own right, such as heart attack, stroke, gallbladder disease, and osteoarthritis. Obesity is also a risk factor for some cancers, including those of the endometrium, breast, and colon.

Another reason for declaring obesity a disease is financial. It will nudge health care payers, including private insurers and the federal government, to pay for anti-obesity services, including weight loss counseling and programs. Why, proponents ask, should we pay physicians and hospitals tens of thousands of dollars to open blocked arteries yet refuse to spend a fraction of this amount on diet and fitness programs that might prevent the problem in the first place?

Yet everyone who is obese does not get sick, and many normal-weight people do not stay healthy. I have known slim and trim people who took scrupulous care of themselves throughout their lives yet fell ill and died young. Others who exhibited no particular interest in their health and did not watch their weight lived to a ripe old age. In most cases, we simply cannot tell from a person's weight what lies ahead for them in life.

Consider Winston Churchill. Though average in height, Churchill weighed upwards of 250 pounds. He smoked cigars. He drank relatively heavily. He did not jog or work out. Yet he became perhaps the most important statesmen of the 20th century and one of the greatest political orators in history. He served twice as Britain's prime minister, guiding his nation through a particularly perilous chapter in its history, and won the Nobel Prize for literature. He lived to age 90.

Thinner isn't always better. A number of epidemiological studies have concluded that normal-weight people are in fact at higher risk of some diseases, including cardiovascular disease, compared to those whose who are overweight. And there are health conditions for which being overweight is actually protective. For example, heavier women are less likely to develop osteoporosis than thin women. Likewise, among the elderly, being somewhat overweight is often an indicator of good health.

Of even greater concern is the fact that obesity turns out to be very difficult to delineate. It is often defined in terms of body mass index, or BMI. BMI equals body mass divided by the square of height. An adult with a BMI of 18 to 25 is often considered to be normal weight. Between 25 and 30 is overweight. And over 30 is considered obese. Obesity, in turn, can be divided into moderately obese (30 to 35), severely obese (35 to 40), and very severely obese (over 40).