Dietary red pepper suppresses energy intake and modifies macronutrient intake. We have investigated whether a stimulus in the mouth and the sensation of spiciness are necessary for red pepper-induced changes in energy and macronutrient intake in human volunteers. In a preliminary test, sixteen Japanese male volunteers tasted samples of a soup with graded doses of red pepper in order to define a moderate and a maximum tolerable (strong) dose of red pepper. On the day of the experiment, a standardised breakfast was given to the volunteers. At lunchtime, the subjects ingested one of four experimental soups containing either a placebo, a moderate or a strong dose of red pepper plus placebo capsules, or a placebo soup plus capsules delivering a strong dose of red pepper. The rest of the meal was given ad libitum to all subjects. The amount of food, protein and carbohydrate ingested was similar for all conditions. Energy and fat intake were similar after the ingestion of the moderate soup compared with placebo. However, the strong soup significantly lowered fat intake compared with placebo (P=0·043), and ingestion of strong capsules also tended to suppress it (P=0·080). Moreover, energy intake after strong soup and capsules tended to be lower than placebo (P=0·089 and 0·076, respectively). The present results indicate that the maximum tolerable dose is necessary to have a suppressive effect of red pepper on fat intake. The main site of the action of red pepper is not in the mouth.

The effects of red pepper and caffeine ingestion on energy and macronutrient balances were examined in eight Caucasian male subjects. All subjects participated in two randomly assigned conditions: control and experimental (red pepper and caffeine). After ingesting a standardized breakfast, subjects ate three meals ad libitum (lunch, dinner and breakfast) and snacks which were served approximately 2 h after the lunch and dinner over a 24 h period. Two appetizers (2×322 kJ with or without 3 g red pepper) were given before lunch and dinner, and a drink (decaffeinated coffee with or without 200 mg caffeine) was served at all meals and snacks except for the after-dinner snack. It is also important to note that on the experimental day, 8.6 and 7.2 g red pepper were also added to lunch and dinner respectively. Red pepper and caffeine consumption significantly reduced the cumulative ad libitum energy intake and increased energy expenditure. The mean difference in energy balance between both conditions was 4000 kJ/d. Moreover, the power spectral analysis of heart rate suggested that this effect of red pepper was associated with an increase in sympathetic:parasympathetic nervous system activity ratio. These results indicate that the consumption of red pepper and caffeine can induce a considerable change in energy balance when individuals are given free access to foods.

Two studies were conducted to investigate the effects of red pepper (capsaicin) on feeding behaviour and energy intake. In the first study, the effects of dietary red pepper added to high-fat (HF) and high-carbohydrate (HC) meals on subsequent energy and macronutrient intakes were examined in thirteen Japanese female subjects. After the ingestion of a standardized dinner on the previous evening, the subjects ate an experimental breakfast (1883 kJ) of one of the following four types: (1) HF; (2) HF and red pepper (10 g); (3) HC; (4) HC and red pepper. Ad libitum energy and macronutrient intakes were measured at lunch-time. The HC breakfast significantly reduced the desire to eat and hunger after breakfast. The addition of red pepper to the HC breakfast also significantly decreased the desire to eat and hunger before lunch. Differences in diet composition at breakfast time did not affect energy and macronutrient intakes at lunch-time. However, the addition of red pepper to the breakfast significantly decreased protein and fat intakes at lunch-time. In Study 2, the effects of a red-pepper appetizer on subsequent energy and macronutrient intakes were examined in ten Caucasian male subjects. After ingesting a standardized breakfast, the subjects took an experimental appetizer (644 kJ) at lunch-time of one of the following two types: (1) mixed diet and appetizer; (2) mixed diet and red-pepper (6 g) appetizer. The addition of red pepper to the appetizer significantly reduced the cumulative ad libitum energy and carbohydrate intakes during the rest of the lunch and in the snack served several hours later. Moreover, the power spectral analysis of heart rate revealed that this effect of red pepper was associated with an increase in the ratio sympathetic: parasympathetic nervous system activity. These results indicate that the ingestion of red pepper decreases appetite and subsequent protein and fat intakes in Japanese females and energy intake in Caucasian males. Moreover, this effect might be related to an increase in sympathetic nervous system activity in Caucasian males.

The effects of dietary red pepper added to high-fat (HF) and high-carbohydrate (HC) meals on energy metabolism were examined in thirteen Japanese female subjects. After ingesting a standardized dinner on the previous evening, the subjects took an experimental breakfast (1883 kJ) under the following four conditions: HF meal, HF and red-pepper (10 g) meal, HC meal, or HC and red-pepper meal. Palatability of the experimental meals was measured immediately after the meals. Expired air was collected before and for 210 min after the meal to determine energy expenditure and macronutrient oxidation. Diet-induced thermogenesis was significantly higher after the HC meals than after the HF meals. Lipid oxidation was significantly lower and carbohydrate oxidation was significantly higher after the HC meals than after the HF meals. Addition of red pepper to the experimental meals significantly increased diet-induced thermogenesis and lipid oxidation, particularly after the HF meal. On the other hand, carbohydrate oxidation was significantly decreased by the addition of red pepper to the experimental meals. Addition of red pepper to the HC meal increased the perceived oiliness of the meal to the same level as that of the HF meals. These results indicate that red pepper increases diet-induced thermogenesis and lipid oxidation. This increase in lipid oxidation is mainly observed when foods have a HF content whereas the increase in the perceived oiliness of the meal was found under the HC meal conditions.