Obesity is one of the most pressing national public health problems [1
]. There are a variety of strategies available for individuals attempting to reduce body weight [2
]. Foods that evoke multiple actions should theoretically hold greater benefit. This can be complimented through modification of foods with various spices and seasonings. Previous studies have reported that capsaicin, the pungent principle in hot red peppers (RP), reduces hunger, stimulates thermogenesis, and alters substrate oxidation in humans [3
]. However, there have been conflicting reports on these outcomes, including a recent study noting no effects on satiety or thermogenesis [9
]. An improved understanding of the basis of these inconsistencies is required to establish the role of dietary oral irritants, such as capsaicin, in weight management.
One methodological issue that may account for much of the inconsistent data on RP concerns the characteristics of study populations. RP test loads in previous studies have been widely divergent, ranging from ~ 1 g [4
] to 10 g [6
]. These doses mirror wide variances in RP consumption. High concentrations are a hallmark of Asian and Latin American cuisines (e.g., average daily consumption among Koreans = ~ 7 g) [11
], while RP is a more variable component of diets in the United States (e.g., daily consumption of peppers of any kind = 10.5% [12
] and mean preference in our study population = ~ 1 g/meal). It is well established that there are individual differences in the sensitivity to the burn of spicy foods and to the affective interpretation of that sensation (e.g., regular spicy food users rate capsaicin’s burn as less intense and more pleasant than non-users) [13
]. This may be attributable to familiarity effects as repeated exposure to high capsaicin concentrations, during a 16-day period, lowers burn intensity ratings [15
]. Further, hedonic effects have been documented with controlled feeding trials that entail chronic adherence to low [18
] or high [19
] sodium and low-fat diets [20
]. Similarly, in a Mexican highland village where chili pepper is a predominant spice, repeated experience with gradually increasing levels of spiciness enhanced preference and tolerance for spicy foods as documented through interviews, direct observations, and sensory measurements [21
]. Another possibility is that the variable palatability of spicy foods is related to consequences of physiological processes, independent of sensory responsiveness. One possibility involves cephalic phase responses which are generally stronger for more palatable sensory stimuli. Cephalic phase responses, including release of satiety hormones [22
] and catecholamines that stimulate thermogenesis [23
], could provide positive metabolic feedback. Although a cephalic phase insulin response to oral irritation has not been explored, exposure to spicy foods leads to increased insulin sensitivity in regular spicy food users versus those with less frequent exposure [25
]. Thus, it is vital that the acceptability of RP and frequency of exposure be controlled in studies of the satiety and thermogenic responses to RP. The present study was designed to document potential variances in thermogenic and appetitive responses to RP between regular spicy food users and non-users. If differential responses are demonstrated, further study will be warranted to document the potential mechanisms (e.g., cephalic phase responses). The hypothesis of the current study was that RP would lead to greater increases in thermogenesis and moderation of orexigenic appetitive sensations than no RP, and that these changes would be more pronounced among non-users than users.
Capsaicin has been documented to increase thermogenesis through stimulation of catecholamine secretion and subsequent sympathetic nervous system (SNS) activation. However, the effects are variable in magnitude and duration [3
]. SNS stimulation preferentially increases fat oxidation [26
], which is consistent with studies reporting that RP enhances fat oxidation [8
]. However, some studies report that RP augments carbohydrate oxidation [6
] or has no effect on substrate oxidation [9
]. A possible explanation for these seemingly contradictory observations is that RP increases the oxidation of available substrate regardless of its nature. To determine if RP exerts substrate-specific effects, the present study used macronutrient-controlled lead-in diets, administered in random order, prior to assessments. It was posited that RP would enhance postprandial energy expenditure, irrespective of lead-in diet, and that fat oxidation would be increased on high fat (HF) diets and carbohydrate oxidation would be increased on high carbohydrate (HC) diets.
An additional question concerns the role played by sensory stimulation in the thermogenic and appetitive responses to capsaicin. The capsaicin receptor, transient receptor potential vanilloid 1 (TRPV1), has unique physiological functions in allowing humans to detect to the oral burn associated with chili pepper consumption, regulate core body temperature, and sense external temperature (≥ 42° C) [28
]. Mixed findings have been reported concerning the necessity of sensory stimulation. One previous study indicated that thermogenesis and appetitive control were greater when RP was ingested orally, compared to in capsule form [4
]. These stronger effects with oral exposure were believed to indicate a sensory effect of RP, given that consumption in capsule form bypasses oral irritation [4
]. However, another study showed no added effect with oral stimulation [5
]. A possible explanation is that maximum effects may be achieved when stimulus concentrations match individual preferences. Again, this may be attributable to activation of cephalic phase responses [22
]. The present trial measured the thermogenic and appetitive responses of regular users and non-users of spicy foods to determined preferred capsaicin concentrations in a food system delivered orally or in capsule form. It was hypothesized that RP would lead to greater increases in thermogenesis and moderation of appetitive sensations when delivered orally rather than gastrically.