Why making fat is good for you.

Moderation is usually a good watchword, and new research suggests that this holds true for people worried about fat.

Making new fat from food intake, as opposed to using stored fat, is necessary for health. Scientists haven't understood the molecular mechanisms underlying this benefit, however. To investigate, Clay Semenkovitch of the Washington University School of Medicine in St. Louis and his colleagues deprived mice of both dietary fat and the fat that their own bodies make from sugars. The researchers achieved this by genetically engineering mice to lack an enzyme in their livers called fatty acid synthase, which converts carbohydrates into fat, and then feeding the mice a fat free diet after they reach 4 months old.

The animals' blood sugar concentrations dipped below normal and, ironically, their livers developed fatty deposits. Both conditions were reversed when the researchers restored fat to the animals' diets.

Further study revealed that, in the absence of dietary fat, the mice exhibited a marked decline in the activity of genes critical for the metabolism of glucose, fatty acids, and cholesterol. Those genes are normally activated indirectly by fatty acids.

Although new fat and stored fat are chemically similar, Semenkovitch's team found that only new fat triggered the beneficial metabolic reactions in the mice. The researchers' findings appear in the May Cell Metabolism.