Plasma carotenoids as biomarkers of vegetable intake: the University of Minnesota Cancer Prevention Research Unit Feeding Studies. 
High vegetable intake has been associated with a decreased risk for various human cancers in epidemiological studies.  Carotenoids are plant compounds that may both possess chemopreventive activity and be useful biomarkers of vegetable and fruit intake.  Nineteen men and women were randomized into a controlled cross-over feeding study to measure the effect of vegetable intake on plasma carotenoid concentrations.  Participants consumed each of 4 experimental diets for 9 days.  The control diet consisted of commonly consumed foods and was essentially carotenoid free.  High vegetable diets (carotenoid, cruciferous, and soy) consisted of the control diet plus carrots and spinach (carotenoid), broccoli and cauliflower (cruciferous), and tofu and FriChik (soy).  Plasma carotenoid concentrations were highest on the carotenoid and cruciferous diets.  When compared to the control, mean plasma alpha-carotene, beta-carotene, and lutein concentrations were 5.2, 3.3 and 2.2 times higher on the carotenoid diet, respectively (P < 0.001).  Mean plasma lutein concentrations were 2.1 times higher on the cruciferous versus the control diet (P < 0.001).  There were no differences between diets in plasma beta-cryptoxanthin and lycopene concentrations.  These data indicate that plasma alpha-carotene, beta-carotene, and lutein may be useful biomarkers of carotenoid-rich food intake and that lutein may act as an intake biomarker of commonly consumed vegetables in the Cruciferae family.  These findings should prove useful in undertaking dietary intervention trials because they suggest the feasibility of monitoring intake of some plant foods and of distinguishing among plant food groups.