WASHINGTON - US researchers have discovered two genes in which variation affects intake of caffeine, the most widely consumed stimulant in the world, according to a report described Wednesday in the open-access journal PLoS Genetics.

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A team of investigators from the National Cancer Institute, Harvard School of Public Health, Brigham and Women's Hospital, and the University of North Carolina at Chapel Hill examined genetic variation across the entire genome of more than 47,000 individuals from the US.

They found the genes -- CYP1A2, which has previously been implicated in the metabolism of caffeine, and AHR, involved in the regulation of CYP1A2, played roles in regulating intake of caffeine. Individuals with the highest-consumption genotype for either gene consumed 40 mg more caffeine than those with the lowest-consumption genotype, equivalent to the amount of one-third cup of caffeinated coffee, or one can of cola.

Caffeine is implicated in numerous physiological and medical conditions; it affects sleep patterns, energy levels, mood, and mental and physical performance. The identification of genes that have an impact on daily consumption offers opportunities to better understand these conditions. Further exploration of the identified genetic variants may provide insight into the speed of caffeine metabolism, how long caffeine circulates in the blood, or how strong the physiological effects of consuming a given amount of caffeine are.