Carbohydrate taste, appetite, and obesity: an overview.
This paper reviews previous research on sugar appetite and sugar-induced overeating and obesity in the rat, and previews new findings reported in this issue on the rat's taste and appetite for starch and starch-derived polysaccharides. A variety of behavioral methods demonstrate that rats are very attracted to the taste of sugars. At low molar concentrations their order of preference is maltose greater than sucrose greater than glucose = fructose, while at high concentrations it is sucrose greater than maltose greater than glucose = fructose. New findings indicate that rats are also very attracted to starch-derived polysaccharides, such as Polycose. In fact, Polycose is preferred to sucrose and maltose at low concentrations, and is second only to sucrose in preference at high concentrations. Furthermore, rats taste Polycose as qualitatively different from sucrose. These and other findings suggest that rats as well as other rodents have different taste receptors for sugar and starch. The role of these two taste systems in carbohydrate-induced overeating and obesity is discussed. Rats tend to overeat and become obese when fed sugar or polysaccharide diets, but this response depends critically on the form of the diet. Presenting the carbohydrate as a solution or gel supplement to the diet is much more effective in promoting hyperphagia and obesity than is presenting it as a powder supplement or as part of a composite diet. The differential response to hydrated and dehydrated foods may occur because carbohydrates are absorbed at a faster rate in hydrated forms than in dehydrated forms. The postingestive actions of carbohydrates also modulate the rat's preference for carbohydrate tastes and flavors. Other factors that influence carbohydrate appetite and intake include the availability of the carbohydrate source and the macronutrient composition of the diet. Thus, several factors in addition to taste determine the response of rats to carbohydrate foods.