Abstract: We have proposed that normal food intake is geared toward optimizing the internal milieu for reproduction, despite some components of this milieu being detrimental to health. In dietary restriction, the animal is prevented from eating enough to attain or maintain reproductive capacity and this particular milieu does not materialize. Life extension occurs as a by-product. This idea provides a framework for exploring biomolecular changes in dietary restriction and their relevance to aging. Leptin is a case in point: here, a decrease in leptin level in dietary restriction is explored in the light of leptin's role in the complex signaling system of reproductive switch.