Objective Dietary macronutrient proportions affect substrate utilization, but in practice people consume foods. We hypothesized that in overweight adults, a calorie controlled diet based on core foods and including walnuts may be advantageous in promoting greater use of fat stores. Methods This crossover study tested the effects of diet-related energy expenditure and fat oxidation in 16 overweight individuals over an 8-hour period. The 2 diets included breakfast and lunch meals during the measurement period and an evening meal the night before. They comprised core foods of bread/cereals, fruit, vegetables, milk/yogurt, and meat, and either walnuts (walnut diet) or olive oil (control diet). There was no difference in the energy and macronutrient composition of the diets in the measurement period. Energy expenditure, respiratory quotient (RQ), and macronutrient oxidation were assessed during two 8-hour stays in a room calorimeter facility. Results During the 8-hour measurement period, no difference in energy expenditure was noted between the diets, but a significant difference in RQ was observed between diets (control 0.908 ± 0.046 vs. walnut 0.855 ± 0.036, p = 0.029). Carbohydrate oxidation was lower and fat oxidation was higher during the walnut period than during the control period. Conclusions A calorie controlled diet of core foods including walnuts may be advantageous in promoting the use of body fat stores, at least under acute conditions.