The effect of d-fenfluramine on intake of carbohydrate supplements is influenced by the hydration of the test diets.

The effect of d-fenfluramine on carbohydrate intake in the rat has been investigated using a dietary paradigm in which a pure carbohydrate supplement is presented in addition to the normal balanced chow diet. This experimental strategy has been used as an alternative to the macronutrient selection model in an attempt to increase the sensitivity of the measurement of drug effects on carbohydrate intake. This issue is important because of the postulated theoretical relationship between brain serotonin (5-HT) and carbohydrate intake. The effect of d-fenfluramine was dependent upon the texture and density of the supplement (hydrated vs. non-hydrated) and upon the nature of the carbohyhdrate employed - either sucrose (sweet-tasting) or Polycose (bland-tasting). Selective carbohydrate suppression, by d-fenfluramine, was detected (as predicted by the 5-HT/carbohydrate theory) but only under specific environmental conditions, namely hydrated chow supplemented with dry Polycose. Therefore, suppression of carbohydrate may depend, not on any inherent biological link between carbohydrate consumption and brain 5-HT activity, but on a shift in relative preference for the diet choices available.