How fat happens

Conventional wisdom is that weight gain is a mathematical phenomenon. It requires 3,500 excess calories to produce a pound of fat. One’s metabolism supposedly is dictated by weight and activity. For example, an active 150-pound man should consume 2,250 calories. If he consumes an average of 1,750 calories a day for a week, he would lose one pound. If he eats a daily average of 2,750 calories for a week, he would gain a pound.

If only it were that easy. Weight gain and loss is highly individual and is the result of a complex set of factors.

One theory is that everyone has a genetically determined weight set point that can fall within a range of 20 to 30 pounds, depending upon nutritional and exercise habits. If a person attempts to go below that range, the body rebels by slowing its rate of metabolism and signaling its hunger.  The familiar lament of the yo-yo dieter – “I gained all my weight back, and then some!” – is a result of the metabolic change.

Some experts speculate the obesity rate is going to increase more slowly because the nation is reaching a sort of obesity saturation point. That implies that nearly everyone who is genetically meant to be obese is now there.

If one parent is obese, the child has a 50 percent likelihood of also being obese. With two obese parents, the risk rises to 80 percent. Obese children almost inevitably become obese adults. The link is likely to be cultural as well as genetic, because family members have similar eating and physical activity habits.

Genes clearly dictate who is the most vulnerable to excessive weight gain, but other factors determine whether weight gain actually happens. For example, the Pima Indians were originally from Mexico, where they were poor farmers without weight problems and the resulting chronic conditions. Those who moved to the United States adopted a different lifestyle. One-half of the Pima Indians in the U.S. now have diabetes and 95 percent of those are overweight.

Despite obesity’s complex causes, researchers continue to attempt to pin the epidemic on one cause.

An Australian researcher declared in 2009 that obesity has doubled in the past 30 years exclusively because of increased calorie consumption. Physical activity had a minor role, he declared, because physical activity levels had not changed much during that time. Children are consuming 350 more calories a day than children of three decades ago, and adults are taking in 500 more.

A Canadian researcher, however, blamed office-based jobs. According to his research, people are eating better and exercising more than they did three decades ago, so sedentary work must be the culprit.

In 1960, about 50 percent of the U.S. jobs required moderate physical activity. That has fallen to 20 percent. That translates to about 120-140 fewer calories expended daily per capita. That lack of activity closely tracks the nation’s weight gain over the past five decades.

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