Book Description

Sources of vitamin A, an essential nutrient for growth, healthy vision, and proper immune function, include both retinoids and carotenoids. The vitamin A biological activity these compounds contain and the manner in which they are handled by the body can vary considerably. Knowledge of the metabolism of these sources by the different species that consume them can help better estimate daily requirements, identify where they are stored and used, and assess their impact on health. On a global scale, knowledge of the metabolism of the provitamin A carotenoid beta-carotene in biofortified cassava can help determine its ability to alleviate vitamin A deficiency in populations where it is a staple food. The first chapter discusses the food-based interventions to combat vitamin A deficiency that are currently being tested, while the second chapter describes a human feeding study in which we investigated beta-carotene-biofortified cassava. Cassava is currently being crossbred to increase its beta-carotene content by the non-governmental organization HarvestPlus. However, little is known about the bioavailability and bioconversion of the beta-carotene in biofortified cassava in humans. In a randomized, crossover design study, 10 American women consumed a biofortified cassava porridge containing 2 mg beta-carotene with and without added fat, as well as an unfortified white cassava porridge containing a 0.3 mg retinyl palmitate reference dose. Blood was collected six times from -0.5 - 9.5 hours post-feeding and subsequently analyzed. The results suggest the beta-carotene in biofortified cassava is bioavailable since substantial concentrations of beta-carotene and retinyl palmitate were present in the triacylglycerol-rich lipoprotein plasma fraction after consumption of both biofortified cassava meals. The vitamin A equivalence of both biofortified cassava meals was an average of 4.4 [mu]g beta-carotene : 1 ℗æg retinol (mean ± SD), indicating efficient bioconversion. The following two chapters discuss the provitamin A carotenoid beta-cryptoxanthin. Despite its ability to address vitamin A deficiencies as a source of vitamin A, antioxidant activity, and potential anticarcinogenic effects, little is known about the metabolism of this carotenoid. Although carotenoid metabolism can be investigated in human studies, there are limitations in terms of the tissues that can be tested. Animal studies allow for a full body assessment of carotenoids since entire tissues can be extracted and tested. However, results of these studies are not translatable to humans unless an appropriate animal model for human carotenoid metabolism is used, such as the Mongolian gerbil. We investigated the tissue distribution and dose-response effects after the consumption of varying concentrations of beta-cryptoxanthin in the Mongolian gerbil. In addition, we tested its ability to maintain vitamin A stores. Beta-cryptoxanthin was present in the blood and 12 of the 14 tissues tested, with concentrations being significantly different from baseline in all of the tissues in which it was detected. Beta-cryptoxanthin concentrations increased with higher intakes in most tissues but the increases were nonlinear, suggesting beta-cryptoxanthin absorption may be saturable. Whole organ concentrations of vitamin A in the treatment groups were not significantly different from baseline, indicating beta-cryptoxanthin may be able to sustain vitamin A concentrations when provided as the sole source of vitamin A. The final three chapters focus on lesser known retinoids with vitamin A biological activity, such as dehydroretinol and 3-hydroxyretinol, thought to be uniquely produced in freshwater fish. Knowledge of the concentrations of these retinoids in North American freshwater fish tissues and feed can help better quantify their total vitamin A content. Monitoring vitamin A concentrations can help prevent the negative effects of excess intake. In fact, toxic concentrations of vitamin A can lead to health consequences equally as severe as deficiency such as death and skeletal malformations. Prior to this study, a HPLC method capable of identifying all of our retinoids of interest did not exist. Thus, we developed and validated a HPLC method capable of identifying dehydroretinol, 3-hydroxyretinol, and retinol in a single run. Method validation tests indicated the method had good precision, accuracy, linearity, and sensitivity. In a separate study, we measured the concentrations of retinol, dehydroretinol, and 3-hydroxyretinol in the feed, muscle, and liver of several commercially important species of North-American farm-raised freshwater fish. The concentrations of retinoids within and between fish species and tissues varied dramatically. Interestingly, dehydroretinol concentrations (mean ± SD; 22.1 mg/kg ± 11.3) were lower than retinol (54.4 mg/kg ± 25.0) in fish feed, but often higher (237.2 [mu]g/g ± 292.7) than retinol (81.0 [mu]g/g ± 77.1) in fish liver. As supported by past studies of freshwater fish liver, it appears these fish may preferentially convert retinol to dehydroretinol for storage. Muscle results were dissimilar, with retinol and dehydroretinol nearly equal in concentration. Concentrations of retinol in the feed greatly exceeded recommended levels. 3-hydroxyretinol was detected in relatively low concentrations in the fish liver. To our knowledge, this is the first comparison of dehydroretinol and retinol concentrations in North American farm-raised fish versus fish feed, and the first to detect dehydroretinol concentrations in North American farm-raised fish and commercial fish feed.The results of the above-mentioned studies provide insight into the body's response to the consumption of numerous biologically active retinoids and provitamin A carotenoids and raise awareness about their potential impact in humans and animals. With its high beta-carotene content and efficient bioconversion to vitamin A, biofortified cassava may be an effective component of food-based interventions in vitamin A deficient populations where it is a staple food. The whole-body assessment of beta-cryptoxanthin in the Mongolian gerbil indicated it is stored in numerous tissues and may be capable of sustaining vitamin A concentrations. The high amounts of various vitamin A compounds discovered in the tissues and feed of North American farm-raised fish suggest that vitamin A content from multiple sources should be more closely monitored in the future to ensure fish health.