Abstract
Diet and nutrition are critical factors influencing energetics and health. Laboratory studies show that organisms adjust to changes in nutrient intake through flexible metabolic responses such as fuel switching. While the physiological effects of nutrient balance in humans have been studied, data from closely related species living in nature are lacking. We integrate macronutrient regulation and metabolic flexibility to elucidate how wild orangutans (Pongo pygmaeus wurmbii) are buffered against natural fluctuations in nutritional resources. We found that these orangutans regulate protein and regularly switch between exogenous and endogenous nutritional substrates as preferred food resource availability declines. When total caloric, lipid, and carbohydrate intake declined during episodes of fruit scarcity, orangutans drew on fat and endogenous amino acids for energy. This strategy is beneficial only in the context of alternating periods of fruit scarcity and abundance. We discuss our findings in relation to the current global obesity pandemic, which has arisen with transitions in human diets toward low-cost, energy-dense, protein-dilute foods.