Over the past 30 years, rates have increased considerably in adults, as well as in children and adolescents, not only in the United States but worldwide. There are now more people who are obese than go hungry, and malnutrition coexists with obesity in certain parts of the world.

Rockefeller University researcher Jeffrey Friedman, whose work in the 1990s led to the discovery of the fat-regulating hormone leptin, explains that obesity is a "threshold." In other words, while not minimizing the scope of the problem, Friedman notes that any small change in its definition can have profound consequences in the number of people who are considered obese. Currently, we use the imperfect (and even somewhat arbitrary) measurement of body mass index (BMI), that is, the ratio of weight in kilograms to height in meters 2 as the means by which we classify levels of overweight and obesity: from 25.0 to 29.9 kg/m2 for overweight to class 4, or supermorbid obesity, with a BMI of greater than or equal to 50 kg/m2.

No one really knows exactly why rates of obesity have increased so dramatically in recent years. There are many theories, including toxins or even viruses in the environment, increased light exposure (for example, computer screens, television), chronic sleep deprivation, and energy-saving devices that severely curtail our daily physical activity.

Not everyone, of course, develops a weight problem, and given our environment, perhaps as Friedman has said, we should not be asking why so many people are overweight or obese, but rather why anyone remains thin at all. Simplistically, though, it is the First Law of Thermodynamics: calories in, calories expended. What we do believe, though, is that it is more than about self-discipline and more about our biological makeup.

Clearly, there is a genetic component to weight control since up to 70 percent of our weight is determined by hundreds of gene loci. But our genes have certainly not changed in the past 30 years. What has changed, though, according to Boyd Swinburn and his colleagues in a series of articles in the August 27 Lancet, is the availability of much more affordable and processed (that is, "energy-dense") food that leads to an overconsumption of calories and even "portion distortion"—our inability to judge what is an appropriate serving size. These researchers believe that obesity is, for most people, the "result of people responding normally to the obesogenic environments they find themselves in." By 2008 alone, worldwide, there were 1.46 billion overweight adults and 502 million obese adults. Furthermore, many predict these numbers will continue to escalate dramatically over the next 20 years.

As psychiatrists, we should care because these statistics translate potentially into considerable morbidity and mortality for our patients. I would recommend that psychiatrists not only discuss, but also monitor, their patients' weight as part of every psychiatric evaluation and treatment. Many of our psychotropic medications cause not only weight gain, but also severe metabolic disturbances such as abdominal fat accumulation, abnormal lipid and glucose levels, insulin resistance, and even overt type 2 diabetes. Patients are less apt to adhere to treatment recommendations when substantial weight gain occurs with medication. Anxiety and depression are also more common in patients with obesity, though it is not often clear which is cause and which is consequence. Further, the obese may easily suffer from lowered self-esteem, as they become victims of prejudice and overt discrimination in such areas as marriage, education, and employment compensation. In other words, psychiatrists must be sensitive to the possibility that their obese patients may experience the potential double stigma of obesity and mental illness.

Sylvia R. Karasu, M.D., is a clinical associate professor of psychiatry at Weill Cornell Medical College in New York City. She is the senior author of The Gravity of Weight: A Clinical Guide to Weight Loss and Maintenance (American Psychiatric Publishing). The book may be ordered at < www.appi.org/SearchCenter/Pages/SearchDetail.aspx?ItemId=62360>. APA members can purchase the book at a discount.>