According to the National Research Council Committee on Diet and Health (1989), of the adult population of the United States, more than 25% of women and 31% of men are overweight or obese, based on use of the Body Mass Index (BMI; defined below)! That was in 1989; in the year 2000, the figures are fast approaching 50%.

To judge from the numbers of patients that actually do something about losing weight, most people are unaware that overweight, even if only slight, can kill. Statistics on how many people actually make an effort to lose weight are difficult to obtain, but a Gallup poll in 1985 indicated that anywhere from 2% to 7% of those asked had been told to lose weight by their doctors. The percentage who made a serious effort to follow that advice, in a supervised program, was insignificant! Since that survey, the availability of both commercial weight loss programs and hospital programs has increased substantially, but current estimates are still that less than 4% of those who must lose weight actually enrol in a suitable program, so there is much room for improvement!

The survey mentioned also revealed that a large proportion of the population claimed to follow diets of their own design, and that the main motivation for any sort of diet (even the "wishful thinking" type) was appearance; many of those surveyed who claimed to be on a diet did not even need to lose weight!

The relationship between diet, body weight and health is very complex, and is becoming even more so as science pushes the borders of our knowledge outwards. For example, few are aware that being underweight reduces life span more than being overweight, and that having too little body fat is associated with serious health risks. Again, not many realize that high-fat diets based on plant and fish products are associated with much lower incidences of some "diseases of civilization" than low-fat diets which include red meat products and margarine.

For body weight, the most important, and most general, observation is that mortality increases as body weight increases or decreases away from the mean defined as ideal weight. In other words, if you deviate from the mean, your chance of ending up dead before your time is greater. If you are fat, and want to stay alive, then lose weight. Conversely, if you are too thin, then gain weight.

The risk of death can be quantified, using the concepts of excess mortality or anticipated survival. Excess mortality is a definition of how much likelier death is for the person concerned, while anticipated survival expresses the shortening of the life span, in both cases using standard figures for persons of normal weight and the same age and sex to give a baseline for comparison. These approaches are based on statistics, so a few fat people do survive to ripe old ages. Most do not!