Plasma carotenoid levels as biomarkers of dietary carotenoid consumption: a systematic review of the validation studies

Background: Previous research has demonstrated that plasma carotenoids are a reliable biomarker of usual fruit and vegetable intake. The review aims were to synthesize (i) the mean dietary intake and (ii) plasma concentrations of carotenoids reported from validation studies (iii) compare the strength of the relationship between the two, measured using different dietary assessment methods. Methods: Six databases were used to locate studies that included: adult populations, assessment of dietary intake, measurement of plasma carotenoids and reported the comparison between the two measures. Results: One hundred and forty-two studies were included with 95,480 participants, the majority of studies were cross-sectional (n = 86), with randomized controlled trials (RCTs) (n = 18), 14 case-control studies and 13 cohorts. The most common reported dietary carotenoid and plasma carotenoid was lycopene: weighted dietary mean intake (4555.4 ug/day), and plasma concentration 0.62 umol/L (95% CI: 0.61, 0.63, n = 56 studies. The strongest weighted correlation between the two measures was found for cryptoxanthin (r = 0.38, 95% CI 0.34, 0.42) followed by a-carotene (r = 0.34, 95% CI 0.31, 0.37). Conclusion: This review summarizes typical dietary intakes and plasma concentrations and their expected associations based on validation studies conducted to date which provides a benchmark for future validation studies.