Absolute and relative rewarding properties of fructose, glucose, and saccharin mixtures as reflected in anticipatory contrast.

Rats preferred 2% fructose (F) to 2% glucose (G) in daily 5-min two-bottle preference tests, but preferred 8% G to 8% F with the same testing procedure. In four subsequent experiments brief (3 min) once-per-day sequential presentations of two F or two G solutions showed the following results. Anticipatory negative contrast (suppressed intake of the initial solution) was greater with quantitative variations in fructose (2% F followed by either 8, 16, or 32% F) than was the case when the same concentrations of G were paired. No contrast occurred with qualitative pairings of the two sugars--instead G enhanced the intake of F whether it was presented before or after F. A solution of 0.15% saccharin produced more suppression (contrast) of 2% glucose than of 2% fructose. Also, mixtures of 0.15% saccharin with either sugar (2 or 8% F or G) enhanced absolute intake of the sugars but did not substantially increase their contrast-producing properties--suggesting a distinction between absolute and relative rewarding properties of sugar/saccharin mixtures. In summary, anticipatory negative contrast can be produced by either taste or postingestive factors but the relationship between two-bottle preference, absolute reward value (as reflected in consumption in noncontrast conditions), and relative reward value (measured by the capacity to produce contrast) is complex.