Q.Why do we get bad breath after eating garlic or raw onion?

All living cells run a continuous set of chemical reactions — metabolism, made up of catabolic (breaking-down) and anabolic (building-up) processes — and these reactions unavoidably generate by-products the body must dispose of. Elimination of these metabolic waste products is called excretion, and it is a genuinely different process from simple digestive elimination: undigested food residue leaving as faeces never actually entered a living cell, whereas metabolic waste is generated inside cells and must be transported out of them before it can leave the body.

Human metabolic waste is chemically very varied, and different organs specialise in removing different kinds: water and excess mineral ions (Na+, K+, Ca2+ and others) leave mainly via urine, sweat and faeces; carbon dioxide, a gaseous respiratory waste, leaves via the lungs; nitrogenous wastes (ammonia, urea, uric acid, creatinine) leave mainly via urine; breakdown pigments of haemoglobin (bilirubin via faeces, urochrome via urine) and other pigments, excess vitamins, hormones and drugs are also excreted; and volatile compounds absorbed from strongly flavoured food (spices, garlic, onion) are excreted through the lungs …