The researchers also compared the dieters' metabolic rates before and after dieting. In general, as people lose weight their metabolism slows, in what seems to be the body's attempt not to lose weight (which makes sense for our evolutionary past but seems rather perverse to anyone trying to drop a few pounds). But in this study, the dieters' metabolisms slowed way down .
Next the researchers correlated the metabolic slowdowns for the dieters with the two main changes in their blood samples, the reduction in leptin and the elevation in organochlorines. Leptin is known to raise metabolic rates, so a decrease in it might well be connected with a decrease in metabolic rates. But as it turned out, the link with an increase in organochlorines was stronger.
The team hypothesized that organochlorines, which are stored in fat cells, get squeezed into the blood as the fat cells shrink during dieting. Then the body, worried about being poisoned, lowers its metabolic rate to keep the cells from letting any more out.
Theoretically, this same process could happen in everybody since everybody is polluted with organochlorines, says Angelo Tremblay of Laval University in Quebec, a study coauthor. But it's more likely to happen in obese people because their fat cells are storing more poisons. Are there more pollutants in the environment now than there used to be? Well, duh.
Our experts weigh in: "I am intrigued," says James Hill of the University of Colorado. Adds Tremblay, "I hesitate to say it's a major determinant of obesity. But you know, there are not only unhealthy eating and sedentariness. There are also real toxins."
The theory: Call it the Goldilocks Syndrome. If people rarely get too cold or too hot, but almost always stay "just right" in their temperature comfort zone -- as Americans do, these days, year-round -- they will gain weight.
The research: Environmental temperature can affect weight. A 2002 study of Dutch men found they burned more calories when in a room that was a chilly 61 degrees than when it was a cozier 72 degrees.
That same year, a study with Dutch women found they consumed fewer calories in a too-toasty 81-degree room than in a 72-degree room, because they chose lower-calorie foods to eat. That fits with other science. Studies of people (and pigs) have found they eat less in hot environments.
If the effects seen in short-term experiments hold over time, you'd expect that people who experience sizable temperature variations would be thinner than people who live in conditions that always suit them.
This could only be implicated in the widespread widening of Americans if they're staying in their comfort zone more than they did 30 years ago -- and it seems that they are. Although most homes were heated in the 1970s, better units and better insulation now help keep people in those homes warmer in winter. But greater progress has probably been made in beating the summer heat. Data from the Air-Conditioning, Heating and Refrigeration Institute show that nearly 90% of new homes are now built with central air conditioning, while only about one-third were in the 1970s. And Americans are even more likely to stay cool on the road. Almost every new car has AC now, whereas only about 60% used to back then.
Our experts weigh in: "I am very skeptical about this one," says James Hill of the University of Colorado, and Susan Roberts of Tufts University calls it "small stuff." But Dr. George Bray of Louisiana State University thinks there may be something to it. He notes that temperature affects energy intake and output -- so the more the temperature varies, the more a person's energy balance may bounce around.
The theory: The low-fat food fad made us fatter. We ate high-carb foods that weren't as satisfying -- and, by gosh, we ate more as a result.
The research: In 1990, federal health officials recommended for the first time that Americans should eat less fat to lower their risk of heart disease. Many people believe that they've gained weight on such diets. Low-fat diets are generally high-carb diets. And, Dr. David Ludwig of Harvard Medical School would say that too often they have a high glycemic load.
When people diet successfully, their metabolism slows down. That slows down any continued weight loss they might hope to have. But research with mice and people has shown that how much your metabolism slows down depends on what you eat.
In one study, Ludwig and his colleagues found that metabolism slowed down about twice as much for those on a low-fat diet compared with those on a diet with a low-glycemic load (meaning foods that are digested and absorbed slowly, producing only gradual rises -- not rushes -- in blood sugar and insulin).
Those on the low-glycemic load diet also felt less hungry, perhaps because of fewer blood sugar crashes.