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You're referring to results of two studies published in the August 21, 2008, issue of Nature that explored how one type of fat can help the body burn another. We all have two kinds of fat cells. We're most familiar with "white fat" cells that store energy. These are the ones that expand when we gain weight - they store calories from food we consume that we don't burn off. "Brown fat" cells work differently. They burn calories steadily and generate body heat. Everyone is born with plenty of brown fat - it keeps body temperature stable in infants. Although humans lose most of these fat-burning cells early in life, mice retain theirs into adulthood.

In one of the two studies, Harvard researchers identified a protein that promotes development of brown fat. When the investigators gave this protein to mice, the animals produced more brown fat, while their white fat was unchanged.

The other study, from the Dana-Farber Cancer Institute in Massachusetts, showed that brown fat cells are recruited from tissue that normally produces muscle cells, and that this process is regulated by a molecular switch called PRDM16. If this switch is turned off, brown fat in mice turns into muscle cells.

These two discoveries could lead to a new approach to fighting obesity - if we can figure out how to prompt the body to make more brown fat cells, or better, how to turn white fat cells into brown ones.

The Harvard team triggered formation of more brown fat in mice by attaching a growth protein to a disabled common cold virus and letting the virus infect the animals. The infected mice became leaner than mice that didn't get the protein. The same group showed that when immature fat cells are transfused into infected mice, the cells produce lots of the protein and develop into brown, rather than white, fat.

The Dana Farber researchers found a close relationship between brown fat and muscle by identifying a protein that determines whether an immature cell will grow into one or the other. High levels of this "switching" protein cause cells to mature into brown fat. With lower-than-normal levels of the protein, the cells become muscle. Earlier studies at Dana-Farber showed that high levels of the switching protein can also reprogram white fat cells, turning them brown.

The next steps are to find out whether increasing brown fat in mice triggers calorie burning, and to make sure that the process doesn't cause any adverse effects. All this will take time. For the present, your best bet for fat cell control (and weight) remains eating right and maintaining physical activity.

Andrew Weil, M.D.