A second study at the Phoenix lab, which will involve both slim and obese participants, is more long-range. During a six-week period, the thin people will be fed meals containing 150 percent of their weight-maintaining needs. Some will get a normal amount of protein; for others, the diet will be very low in protein. Obese people will be underfed by "50 percent of their weight-maintaining needs," said Susanne Votruba, a research nutritionist at the lab.
Researchers will analyze every bit of what comes in and out of these participants. "For the long-term study, we measure output (urine and stool) and input (food) . . . to determine the exact calories that are going in and out," Vortuba wrote in an e-mail.
Vortuba hopes to figure out who among the thin volunteers gains more weight over time, and why, and who among the fatter volunteers loses more weight, and why.
In Bethesda, Celi and another investigator, Kong Chen, are taking a slightly different approach. Since humans spend so much time at rest and since "obesity is an imbalance between energy intake and energy expenditure," Celi said, he and Chen are testing what happens to a person's output of energy, stress hormones and thyroid hormone levels when his or her body gets cold.
Some studies suggest that colder temperatures help stimulate brown fat to burn more calories. Brown fat, which runs along our neck, shoulders and spine in small amounts, is like muscle tissue in that it burns calories and helps keep the body's internal temperature stable. Only recently have scientists discovered that brown fat persists in humans beyond infancy.
They have also found that lean people tend to have more brown fat than obese people.
If temperatures can influence brown fat so that the body expends more energy, people with a lot of brown fat may find it easier to lose or maintain their weight, Celi and Chen believe. If that is the case, then "instead of working on hunger, which is deep-seated in the brain," Celi said, "we are working on one tissue [brown fat] that has been proposed as the holy grail of the [human metabolic] system. We bypass the brain."