Polyunsaturated fats (PUFA) are preferentially utilized as fuel rather than stored. Diets containing PUFA rich foods (and perhaps also suitable food matrices) may be advantageous in promoting fat oxidation. Greater fat oxidation may lead to reduced fat mass, the target for weight loss. The aim of this study was to examine the effects on fat oxidation of a walnut based diet compared to a control with olive oil. In a crossover design testing effects of walnuts vs olive oil, 16 overweight adults consumed isocaloric breakfast and lunch meals based on core foods during 8hr in a room calorimeter facility. There was no difference in energy expenditure between the diet phases but fat oxidation was higher during the walnut period compared to the control (RQ for control 0.908 ± 0.046 vs walnut 0.855 ± 0.036, P = 0.029).Under acute conditions this result indicates that the walnut containing meals produced a more favorable fuel utilization profile for body fat loss. Further research would determine whether the acute fat oxidation effect translates to greater loss of body fat in the long term.