Chilli heat is a weapon aimed at mammals that birds cannot feel
Capsaicin, the molecule behind chilli burn, grips a pain receptor that mammals have and birds effectively lack. That suits the plant perfectly: birds swallow the seeds whole and spread them, while mammals grind them up with molars. The same receptor later helped win a Nobel Prize for revealing how skin senses heat.
Contrary to kitchen lore, the seeds make none of it. Capsaicin concentrates in the white pith where seeds attach, and in the membranes, with less in the flesh. Only plants of the genus Capsicum produce it, building the molecule in glands of the fruit by joining a compound derived from phenylalanine to a fatty acid chain made from leucine or valine. Beyond discouraging mammals, it also appears to fend off Fusarium, a fungus that damages seeds in wild chillies. A Caribbean tarantula's venom triggers the very same pain pathway, a rare case of plant and animal defences converging.
In the body, capsaicin binds the receptor TRPV1, which normally responds to heat, acid and abrasion. Opening it lets charged particles into the nerve, which fires as if it were being scalded. Explaining this mechanism fed into the 2021 Nobel Prize in Physiology or Medicine, awarded for discovering the skin's sensors for temperature and touch. Christian Friedrich Bucholz first pulled out an impure extract in 1816. Of the natural capsaicinoids, capsaicin itself makes up about 69 per cent, and it and dihydrocapsaicin tie as the most pungent at 16.0 million Scoville heat units.
People exploit it in many ways. Pepper spray uses capsaicinoids to cause pain and breathing difficulty. Farmers in Africa plant chillies as barriers against crop-raiding elephants, and birdseed laced with chilli deters squirrels while birds eat on unbothered. A high-strength skin patch for nerve pain after shingles was approved in the United States and European Union in 2009. Equestrian sport bans it, and four show-jumping horses were disqualified at the 2008 Olympics after testing positive.
Some popular claims hold up poorly. As of 2007 there was no evidence it causes weight loss, and itch-relief trials found too little evidence to judge. It is also genuinely hazardous in quantity: capsaicin capsules have caused heart attacks even in healthy young people, and it vaporises at 115 degrees, so dry-roasting chillies can fill a kitchen with irritant.
Source: Capsaicin
