Revised Energy Conversion Factor for Inulin-Type Fructans and Implications for Resource Ranking in Optimal Foraging Theory

Current conversion factors for calculating the contribution of carbohydrates to food energy are reviewed in light of recent advances in food science. The undigested carbohydrate inulin, which is present in many economically important plants, resists digestion by alimentary enzymes in the upper gastrointestinal system of humans, and reaches the colon chemically intact where it undergoes partial or complete fermentation by endogenous bacterial flora. Current internationally accepted conversion factors for estimating energy (calories) specify that all carbohydrates provide 4 kcal per gram. However, inulin-type fructans fulfill all necessary criteria as a dietary fiber and during fermentation produce short-chain fatty acids that are utilized by the host for energy and should be assigned the energy conversion factor of 1.5 kcal per gram. The significance of assigning 1.5 kcal per gram vs. 4 kcal per gram to this carbohydrate when ranking resources in optimal foraging theory is illustrated in the caloric return rate of two commonly pit-baked desert plants from semi-arid North America.