If the distribution of the nutrient requirement is known to be skewed for a population, other approaches are used to find the ninety-seventh to ninety-eighth percentile to set the RDA. The RDA for a nutrient is a value to be used as a goal for dietary intake for the healthy individual. As discussed in Chapter 9 of this report, the RDA is not intended to be used to assess the diets of either individuals or groups or to plan diets for groups. When we analyzed all antioxidant vitamins together, our findings indicate that among these antioxidants, β-cryptoxanthin and vitamin E have the strongest relation to both FEV1% and FVC%. This finding is somewhat surprising because lycopene and β-carotene have higher blood levels and because lycopene has been considered to be a more powerful antioxidant than β-cryptoxanthin .
The efficiency of conversion of provitamin A carotenoids into retinol is highly variable, depending on factors like food matrix, food preparation, and one’s digestive and absorptive capacities . Vitamin A is essential for normal growth and development, immune system function, and vision . Currently, the only essential function of carotenoids recognized in humans is that of the provitamin A carotenoids, α-carotene, β-carotene, and β-cryptoxanthin, to serve as a source of vitamin A .
Table of Contents
Vitamin E
The process for setting the RDA depends on being able to set an Estimated Average Requirement . Before setting the EAR, a specific criterion of adequacy is selected, based on a careful review of the literature. When selecting the criterion, reduction of disease risk is considered along with many other health parameters. PubMed and Cochrane databases were systematically assessed for studies, including case–control, cross-sectional, and cohort studies, that evaluated the associations of MetS with carotenoids and retinyl esters and retinol . Free and albumin-bound bilirubin are efficient co-antioxidants for alpha-tocopherol, inhibiting plasma and low density lipoprotein lipid peroxidation.
Nevertheless, the strong pro-oxidant nature of the iron-ascorbate complex in vitro raises concern that consumption of vitamin C supplements by individuals with high iron stores may contribute to oxidative damage in vivo. In addition, dietary ascorbic acid can enhance the intestinal absorption of nonheme iron . Another possible adverse effect of high vitamin C intake is enhanced iron absorption leading to iron overload.
Thus, the requirement for women is extrapolated based on body weight differences from those established for men (see Table 1-1). At a vitamin C intake of 90 mg/day, the plasma ascorbate concentration reaches 50 µmol/L which has been shown to inhibit LDL oxidation in vitro in both cellular and cell free systems (Jialal et al., 1990). Although it is not known whether vitamin C prevents LDL oxidation in vivo, if it does this might be relevant in the prevention of heart disease (Jialal et al., 1990).
Again, if the results are positive, indicating that antioxidants do indeed offer protection, it will be important to determine if combinations of antioxidants in various doses can further increase the beneficial effect. Many studies that provided vitamin C supplements to apparently healthy well-nourished populations were investigating pharmacological rather than nutritional effects of the vitamin. This may obscure possible relationships between vitamin C intake and disease risk in the range of dietary intakes.
Health Research
Our findings indicate that in addition to vitamin E, carotenoids and retinol may play a role in respiratory health and that the most important carotenoid may not be β-carotene. Longitudinal and intervention studies are necessary to demonstrate a temporal relation and whether antioxidants can slow the decline in pulmonary function with aging. The evaluation of the association of antioxidant vitamins with pulmonary function is important because reduced pulmonary function is a risk factor for chronic disease mortality and antioxidant vitamins could help to reduce the risk. Many of these studies have been conducted with supplemental intakes that are far above those that can be obtained from food, and some questions and controversy remain regarding the linkage of these antioxidant nutrients with increased risk of chronic disease.
Oxidation of proteins produces protein carbonyls and amino acid derivatives such as methionine sulfoxide and nitrotyrosine, derived from the reaction of peroxynitrite (ONOO−) with tyrosine. Purine and pyrimidine metabolites that are derived from oxidized nucleic acids can be detected in tissues and in urine. Since virtually all tissues are exposed to ROS and RNS, there will always be a baseline production of these biomarker molecules. What is important in evaluating the extent of oxidative stress is the change in the level of the biomarker compared to a baseline or steady-state level. Intake of the major carotenoids and the risk of epithelial ovarian cancer in a pooled analysis of 10 cohort studies.
Clinical Effects Of Inadequate Intake
Circulating vitamin C levels have been shown to be reduced during prolonged administration of oral contraceptives containing Oestrogen, Tetracycline and Aspirin. The decrease in vitamin C level may be due to drug induced impaired absorption or increased utilization of the vitamin for drug metabolism. Within the enterocytes, provitamin A carotenoids may be cleaved by either β-carotene 15,15’-oxygenase 1 or by β-carotene 9’,10’-oxygenase 2 . BCO1 catalyzes the cleavage of provitamin A carotenoids into retinal, which is further reduced to retinol or oxidized to retinoic acid . Β-Apocarotenal derived from the cleavage of β-carotene by BCO2 can be cleaved further by BCO1 to produce retinal. Although provitamin A carotenoids can be converted into apocarotenals by BCO2, the activity of this enzyme is higher toward nonprovitamin A carotenoids.
It is also possible that serum levels of vitamin C may be poorer indicators of tissue antioxidant activity than serum levels of vitamin E, especially in the lung. It has been suggested, in fact, that one possible response to oxidative stress may be the greater transfer of vitamin C than vitamin E from serum into respiratory epithelial lining fluid of the lung ; this localized action in the lung may not be well reflected by serum levels. In addition, the transfer of vitamin C to the respiratory epithelial lining fluid may result in lower serum vitamin C levels .
Because all cells are exposed to oxidants, generated either endogenously from metabolism or exogenously from a variety of environmental insults, the problem arises as to what constitutes an oxidative stress. Furthermore, different cells can be exposed to the same level of oxidants, but depending on the level of antioxidants or protective mechanisms available to the cell, they may or may not experience an oxidative stress. Here, oxidative stress is defined as an imbalance between the production of various reactive species and the ability of the organism’s natural protective mechanisms to cope with these reactive compounds and prevent adverse effects.
Researchers supported by NCCIH and other components of NIH are conducting a variety of studies using antioxidant supplements. If you are considering a dietary supplement, first get information on it from reliable sources. Keep in mind that dietary supplements may interact with medications or other supplements and may contain ingredients not listed on the label. If you are pregnant or nursing a child, or if you are considering giving a child a dietary supplement, it is especially important to consult your (or your child’s) health care provider.
In general, antioxidant systems either prevent these reactive species from being formed, or remove them before they can damage vital components of the cell. However, reactive oxygen species also have useful cellular functions, such as redox signaling. Thus, the function of antioxidant systems is not to remove oxidants entirely, but instead to keep them at an optimum level. Relatively strong reducing acids can have antinutrient effects by binding to dietary minerals such as iron and zinc in the gastrointestinal tract and preventing them from being absorbed.
The metabolic syndrome was defined based on standard criteria, using measured parameters in each study that was included. Shaded studies are studies not included in the quantitative meta-analysis, due to a study design not comparable with cross-sectional studies (eg, cohort, case–control, and randomized controlled trial). In the final model we examined the association of all carotenoids, retinol, and, in addition, vitamins C and E with FEV1% and FVC%. Table 6 shows the results of multiple linear regression after inclusion of these variables in a regression model adjusted for other covariates. The correlation of FEV1% with β-cryptoxanthin was stronger than with vitamin E, and the regression coefficients were little changed when both variables were included in the regression models.
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