BACKGROUND The nut of the Areca catechu palm has long been attributed effects on hunger and the digestive process. OBJECTIVES The objectives were to assess experimentally effects of areca nut on fasting and postprandial energy metabolism, substrate utilization and hunger. SUBJECTS AND METHODS Two randomized, placebo-controlled, double-blind studies were undertaken. In study 1, eight Indian men received bioadhesive gels delivering 0, 5, 10 or 20 mg arecoline to the buccal sulcus after an overnight fast. Resting energy expenditure and substrate utilization were determined by ventilated hood calorimetry over 6 h during which hunger was rated on five occasions. In study 2, 15 Indian men received gels delivering 0 or 10 mg arecoline after consuming a 2.5 MJ meal, and the same protocol was then applied as in study 1. RESULTS Fasting resting energy expenditures exceeded basal metabolic rate (BMR) by 5.4+/-0.8% (Mean+/-SE) after placebo, and 5.1+/-0.7% after 20 mg arecoline, but by 0.9+/-0.8% and 0.7+/-0.5% following 5 mg and 10 mg arecoline, respectively. Carbohydrate (CHO) utilization rates rose after areca nut compared to placebo (F(3,252)= 7.3, p< 0.001). Hunger varied across doses (chi(2) = 10.5, p < 0.02), being lowest after 10 mg and highest after 20 mg, and was influenced by interaction of dose with delta resting energy expenditure. In study 2, areca dose interacted with fat-free mass (FFM) to lower by 5.4+/-11.2% the thermic effect of a meal (F(1,28) = 4.9, p = 0.05), and retarded peak 'digestive-phase' thermogenesis by 60 min (F(1,58) = 5.7, p = 0.02). Postprandial delta CHO utilization was greater (F(1,28) = 4.5, p = 0.05), and hunger was lower (chi:(2) = 3.8, p = 0.05), after areca nut. The areca nut altered relationships of hunger to thermic effects of the meal, and to delta substrate utilization, in ways consistent with appetite suppression. CONCLUSION Areca nut constituents modulate metabolic signals regulating appetite in man. This concurs with customary belief.