Capsaicin
Specific molecules
The molecule that burns, and which acts on a heat receptor — not on a taste bud.
Paprika and sweet pepper contain none: no burn, no capsaicin.
Effects and benefits
The molecule that burns does not touch taste: it fools the heat receptor. The body responds by spending a little more energy, about fifty kilocalories a day in meta-analyses, and the receptor wears out: that is why chili becomes easier and easier to take.
In 1997, David Julius's team identified the receptor capsaicin activates, TRPV1, the one that signals dangerous heat above 43 °C (Caterina et al., 1997); the discovery earned the 2021 Nobel Prize. Chili heats nothing: it makes the brain believe the mouth is burning.
Meta-analyses of trials measured a slight rise in energy expenditure and satiety, around fifty kilocalories a day (Ludy et al., 2012; Whiting et al., 2014).
Repeated, the stimulation exhausts the nerve endings: that is the tolerance of chili lovers, and the principle of the capsaicin patches used in medicine against certain kinds of pain.
Sources: Caterina et al., Nature, 1997 · Ludy et al., Chem Senses, 2012 · Whiting et al., Appetite, 2014