Fig. 5.

Monkeys’ preferences for fat and sugar shift their daily nutrient balance away from dietary reference points. (A) Schematic of a mixture triangle (49) that plots nutrient rewards, monkeys’ choices, and ecologically relevant reference points in a common space, defined by percentage proportions of fat, sugar, and protein to total energy. (Protein content was constant by design; therefore, the protein axis was similar across stimuli and unlabeled.) Colored circles: offered rewards; red triangle: nutrient balance resulting from monkey’s aggregated choices between LFLS and HFLS options (determined by the relative energy intake from each reward); and black markers: reference points. (B) Nutrient balances from monkeys’ aggregate choices occupied distinct and stable regions in nutrient space, depending on offered rewards. For choices between low-nutrient reward and high-fat or high-sugar options, nutrient balances were dominated by preferences for fat and sugar. Ellipses: 95% CIs. Inset shows how nutrient balance shifts away from LFLS reference toward high-nutrient stimuli. (C) Comparison with reference points: nutritionally optimal (low-fat, high-protein, and intermediate-carbohydrate) diet composition for adult macaques based on dietary guidelines (black diamond) and macaque milk (black star). Inset shows reference points projected onto the line connecting offered rewards (corresponding to the closest achievable approximations of reference points). (D) Monkeys deviated from optimal diet when choosing between high-fat, high-sugar rewards and a low-nutrient option. Histograms of nutrient balances in all three animals deviated from the projected reference point toward higher fat or sugar content (resulting in about 20% more fat or sugar intake than the optimal diet composition).