Babies have lots of brown fat to help keep warm. Until April 2009, biologists believed that the brown fat quickly disappeared and was not found in adults. Dr. Sven Enerback of the University of Goteborg in Sweden and others then reported that some brown fat tissue persisted in adults, raising the possibility that if the cells could be made more active, a person might burn off more fat.

In a parallel line of research that has now converged with the brown fat discovery, Dr. Spiegelman has long been studying the body’s white fat cells and how they are controlled. In 1994 he found the body’s master regulator of white fat cells. Turning to brown fat cells, he followed the general assumption that they were derived from white fat cells.

A key element in making brown fat cells seemed to be a kind of protein called a zinc finger (because it reaches into the spiral of a DNA molecule to switch on particular genes). Dr. Spiegelman figured that if he inactivated all the relevant zinc finger proteins in brown fat cells, they should turn back into their precursors, the white fat cells.

The experiment worked. The brown fat cells did revert, but not into white fat cells. They turned into muscle cells.

“It was the most bizarre experiment my lab ever did,” Dr. Spiegelman said Wednesday.

His discovery that muscle cells are the natural precursors of brown fat cells was made last year. Dr. Spiegelman has now found that the zinc finger protein, in combination with a second protein produced in muscle cells, is the master switch for brown fat cells and will also convert skin cells into brown fat, even though this is not the process nature intended.