Sign Up To The British Vogue Beauty NewsletterBesides having a slightly dubious detoxification system, the main takeaway from my results was my body’s genetic inability to process caffeine. Right, let’s break that down: rs762551 is a variation of the CYP1A2 enzyme gene. Committing either term to memory is not important, but understanding the role they play in your caffeine tolerance is. AA: The fast metaboliserApproximately 41 per cent of people are AA – fast metabolisers – of caffeine, meaning their CYP1A2 enzyme works efficiently. “Caffeine is cleared relatively quickly, with a typical half-life of around two and a half to four and a half hours,” Ricci explains.