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Obesity is associated with such health problems as diabetes, gallstones, hypertension, and heart disease. Obesity is also linked to colorectal cancer and to breast, uterine, and ovarian cancer in women and prostate cancer in men. But how many extra pounds does it take before a person crosses the line from overweight to obese? It depends on whom you ask: The definition of obesity is currently in a state of flux. Traditionally, obesity was defined as 20 percent or more above an optimal weight for height derived from actuarial statistics that correlated with lowest death rates. Now, some health experts say that the weight-for-height yardstick is both imprecise and overly restrictive.

Recent research suggests that more important than the amount of extra weight a person carries is where it is located. Waist-to-hip ratio can be calculated by dividing the number of inches around the waistline by the circumference of the hips. For example, someone who has a 27-inch waist and 38-inch hips has a ratio of 0.71.

Numerous studies show that fat in the hips and thighs is less health threatening than abdominal fat. While other fat cells empty directly into general circulation, the fatty acid contents of abdominal fat cells go straight to the liver, by way of the portal vein, before being circulated to the muscles. This porcess interferes with the liver's ability to clear insulin from the bloodstream. As blood levels of insulin increase, muscles and other cells become insulin-resistant, and blood glucose levels rise as a result. In response, the pancreas cranks out more insulin, prompting the autonomic nervous system (which controls heart rate, blood pressure, and other vital signs) to produce norepinephrine, an adrenalin-like chemical that raises blood pressure. This sets the stage for the development of diabetes, hypertension, and heart problems.

Weight tables do not take age-related weight gain into account (as people age, fat cells become less metabolically active, so one can weigh more and still be healthy) and "arbitrarily" assign lower weights to women at a given height than to men. Some experts advocate broadening the definition of obesity to meet three criteria: weight for age and height rather than for gender and height, waist-to-hip ratio, and presence of such weight-related health problems as hypertension.

As researchers try to figure out why some people get fat and others don't, it is becoming increasingly apparent that obesity has a variety of causes: heredity, environment, metabolism, and level of physical activity. Therefore, no single "cure" exists. Adipose tissue (fat cells) stores energy in the form of fat to meet the body's energy needs when other sources, such as glucose, are unavailable or depleted.

The body has an almost limitless capacity to store fat. Not only can each fat cell balloon to more than 10 times its original size, but should the available cells get filled to the brim, new ones will propagate. As the body stores more fat, weight and girth increase.

A number of studies show that genetics may be the most important determinant of how much you weigh. Some people are more prone to weight gain than others are even when caloric intake is the same. Research indicates that obesity may be linked to the proportion of fat in the diet rather than to the amount of calories consumed.

It is likely that a number of genetic mechanisms exert influence on weight, among them genes that dictate metabolism and appetite. One that is being investigated actively is the gene that codes for lipoprotein lipase (LPL), an enzyme produced by fat cells to help store calories as fat. If too much LPL is produced, the body will be especially efficient at storing calories.

LPL is partly controlled by reproductive hormones (estrogen in women, testosterone in men), so gender-based differences in the activity of the enzyme also factor into obesity. In women, fat cells in the hips, thighs, and breasts secrete LPL, while in men the enzyme is produced by fat cells in the midriff region. Fat cells in the abdominal area release their contents for quick energy, while fat in the thighs and buttocks is used for long-term energy storage. Thus, a man can often pare his paunch more readily than a woman can shed her saddlebags.

Aside from the action of LPL, the body uses other adaptive mechanisms when food intake is reduced. To site just two of them: dieting depresses the metabolic rate so that calories are burned more slowly, and as fat cells shrink, they become more responsive to the action of insulin and do not release their contents readily. The body is very good at defending itself from the danger of underweight, but it is not really equipped to handle overweight.

Obesity is not yet well understood. However, while genetic predisposition towards obesity can be mitigated by exercise and sensible eating habits, some people will have to work a lot harder at keeping weight at optimal levels than others.