Taste, Taste Preferences,
and Body Weight
Perhaps the most critical issue in obesity research is whether the current obesity epidemic is caused by genetics or by diet (see Chapters 3 and 78). Given that obesity rates in the United States have doubled over the past two decades, while the genetic pool remained stable, the explanation must lie in reduced physical activity and altered eating habits. Though the mainstream of obesity research continues to focus on genetic, metabolic, and physiological issues, dietary choices and behaviors are more likely to be the answer. The recent Dietary Guidelines for Americans 2000 suggests that the increase in obesity rates may be tied to the wide availability of cheap and palatable foods.
Taste is the key influence on food selection. Infants seek out sweet and reject bitter and sour tastes. In 3-day-old infants, facial expressions in response to sweet involve relaxation, a slight smile, and licking of the upper lip. Young infants prefer sugar solutions to water, select sweeter sugars over less sweet ones, and selectively consume the most concentrated sugar solutions available. Infant response to sweet sucrose leads to a more rapid heartbeat, avid sucking response, and less regular breathing. In contrast, infants presented with bitter quinine or urea stick out their tongue, spit or prepare to vomit, and give every sign of rejection and disgust. Judging from facial expressions and sucking response data, infant preferences for sweet and the rejection of bitter tastes are present at birth.
The pleasure response to sweetness is also wired in. Placing a sweet substance on the tongue of a crying newborn has a remarkable calming effect. This persists for several minutes and can be used to quiet the infant between blood draws and other painful procedures. Studies have linked the sweetness response to the release of endogenous opiate