Energy harvest from the human diet is consistently misestimated because key features of the digestive process are routinely ignored, including the activity of commensal gut microbes and the metabolic cost of food digestion. Data showing that microbial activity and digestive costs vary with food type, the level of food processing, and host energy status reveal specific opportunities to improve our understanding of the net caloric value of the human diet. This symposium brings together anthropologists, nutritionists, microbiologists, physiologists, and biochemists to discuss the importance of these omissions in the estimation of dietary value and their implications for our understanding of human energy budgets in the past and present. With respect to the past, a more complete picture of energy harvest allows us to better evaluate the dietary shifts hypothesized to underlie the evolution of our genus, Homo, and their legacy in modern humans. With respect to the present, this symposium will draw attention to the need to improve the estimates of food calorie value that are communicated to the public via nutrition labels. Given the rising prevalence of obesity and malnutrition worldwide, achieving greater accuracy in food labeling has significant global health and economic consequences.