BACKGROUND: Addition of capsaicin (CAPS) to the diet has been shown to energy expenditure; therefore capsaicin is an interesting target for therapy. AIM: We investigated the 24 h effects of CAPS on energy substrate oxidation and blood pressure during 25% negative energy METHODS: Subjects underwent four 36 h sessions in a respiration chamber measurements of energy expenditure, substrate oxidation and blood received 100% or 75% of their daily energy requirements in the '100%CAPS', '100%Control', '75%CAPS' and '75%Control'. CAPS was given at of 2.56 mg (1.03 g of red chili pepper, 39,050 Scoville heat units every meal. RESULTS: An induced negative energy balance of 25% was 20.5% negative energy balance due to adapting mechanisms. Diet-induced thermogenesis (DIT) and resting energy expenditure (REE) at 75%CAPS did differ from DIT and REE at 100%Control, while at 75%Control these tended were lower than at 100%Control (p = 0.05 and p = 0.02 respectively). metabolic rate (SMR) at 75%CAPS did not differ from SMR at 100%CAPS, 75%Control was lower than at 100%CAPS (p = 0.04). Fat oxidation at higher than at 100%Control (p = 0.03), while with 75%Control it did not from 100%Control. Respiratory quotient (RQ) was more decreased at 0.04) than at 75%Control (p = 0.05) when compared with 100%Control. pressure did not differ between the four conditions. CONCLUSION: In an effectively 20.5% negative energy balance, consumption of 2.56 mg meal supports negative energy balance by counteracting the unfavorable energy balance effect of decrease in components of energy expenditure. consumption of 2.56 mg capsaicin per meal promotes fat oxidation in energy balance and does not increase blood pressure significantly. TRIAL REGISTRATION: Nederlands Trial Register; registration number NTR2944.