Perceptions of food vary as a function of an individual's genetic factors, such as the set of alleles coding for their taste, irritation, and olfaction receptor proteins. We established a direct link between individual differences in sensitivity to the glucosinolates, a family of bitter compounds in vegetables and roots, and genetic variations in the bitter taste receptor (hTAS2R38) for these compounds. These individual differences in the perception of nutrients likely evolved to influence ingestion. Bitterness and pungency are both believed to signal potentially harmful compounds in our foods, but consumption of many compounds eliciting these sensations is also linked to decreased risks of cancer and degenerative and cardiovascular diseases, implicating the medicinal values of these compounds as well. Since almost all medicines are toxic at high doses, the phytonutrients may be considered both toxins and medicines, depending on the individual's metabolic sensitivities. The conflicting harmful and healthful effects of bitter and pungent compounds might explain why human populations maintain heterozygosity in sensory receptor genes underlying these sensations.