The TRPV1 receptor as a heat sensor
Capsaicin, the compound that makes chili peppers feel hot, binds to TRPV1, a receptor that normally detects noxious heat and acidity. Activating TRPV1 produces the burning sensation but also triggers sympathetic nervous system signaling. That signaling can stimulate energy-expending pathways, including activity in brown and beige fat, which is the basis for the modest thermogenic and metabolic effects attributed to capsaicin and its non-pungent relative capsaicinoids. The receptor is found on sensory nerves in the gut and skin, linking a sensory experience to a metabolic response.
Because TRPV1 activation also drives capsaicin's irritant qualities, the same mechanism behind any metabolic effect is what limits tolerable intake.
Putting the metabolic effect in proportion
Studies report small increases in energy expenditure and minor effects on appetite with capsaicin or capsinoids, but the magnitudes are generally modest and short-lived, and people can develop tolerance to the sensory effects. Evidence for meaningful, sustained fat loss in humans is weak, and high intakes can cause gastrointestinal discomfort.
This is general educational information about a mechanism, not medical or weight-loss advice. Capsaicin should not be treated as a reliable fat-loss intervention, and anyone with digestive conditions should consult a qualified professional before high-dose use.