Toward a new recommended dietary allowance for vitamin C based on antioxidant and health effects in humans

Comparison of the antioxidant content of fruits, vegetables and teas measured as vitamin C equivalents

Most of the health benefits of black, green and oolong teas made from Camellia sinensis are attributed to their antioxidant content. Many plants and spices have been used to make infusions that are erroneously referred to as 'teas'. The term 'rich in antioxidants' is often used to describe such infusions, often without scientific support. We have used the DPPH method to quantify the total radical scavenging capacity (RSC) of a wide range of 'teas', fruits and vegetables. The results are presented as vitamin C equivalents. These results are compared to the RSC of the recommended portions of fruits and vegetables in the food guide pyramid for a healthy and balanced diet. The EC(50) results show that there are no statistically significant differences in the RSC of black, green and oolong teas. However, the RSC of 'teas' made from other species of plants are significantly lower. Our results show that one or two cups of tea would provide a similar amount of RSC as five potions of fruits and vegetables or 400 mg vitamin C equivalents. This would be comparable to two capsules (200 mg) of vitamin C. Caution is advised in extrapolating these in vitro results to humans due to bioavailability.

Hyperglycemia-induced ascorbic acid deficiency promotes endothelial dysfunction and the development of atherosclerosis

Dehydroascorbic acid, the oxidized form of vitamin C, is transported into mammalian cells via facilitative glucose transporters and hyperglycemia inhibits this process by competitive inhibition. This inhibited transport may promote oxidative stress and contribute to the increase in atherosclerotic cardiovascular disease observed in patients with diabetes mellitus. This review explores the importance of this proposed mechanism in light of current research. For example, recent reports suggest that administration of antioxidants, such as vitamin C, may slow atherogenesis by improving endothelium-dependent vasodilation in individuals with abnormal glucose and lipid metabolism, perhaps by preventing the oxidation of nitric oxide, an important regulator of vasomotor tone. Endothelial dysfunction plays a key role in the development of atherosclerosis and endothelial cells may be particularly affected by hyperglycemia-induced ascorbic acid deficiency as they line the interior of blood vessels. In addition, we discuss evidence of several other mechanisms by which vitamin C status may affect the development of atherosclerotic cardiovascular disease, particularly its inverse relationship to multiple cardiovascular disease risk factors and indicators. Given these factors, vitamin C administration is recommended during periods of both acute and chronic hyperglycemia to help preserve endothelial function.

A vegetable/fruit concentrate with high antioxidant capacity has no effect on biomarkers of antioxidant status in male smokers

The potential benefits of a high fruit and vegetable intake on the antioxidant status and on relevant biomarkers of oxidative damage to lipids, proteins and DNA and on (functional) markers of oxidative stress were evaluated. A randomized, free living, open placebo-controlled cross-over trial of 3 wk, with a 2-wk washout period between treatments, was performed in a group of 22 male smokers with a relatively low vegetable and fruit intake using a vegetable burger and fruit drink. The vegetable burger and fruit drink increased serum levels of vitamin C, alpha-carotene, beta-carotene, beta-cryptoxanthin and zeaxanthin and plasma total antioxidant capacity. However, no effects were demonstrated on any marker of oxidative damage to lipids (malondialdehyde F(2)-isoprostane) proteins (carbonyls) and DNA (Comet assay) and (functional) markers of oxidative stress (reduced/oxidized glutathione ratio, glutathione-S-transferase alpha, glutathione-S-transferase pi and nuclear transcription factor-kappaB). Apparently, these increased levels of antioxidants in serum were not sufficiently high to show beneficial changes with the selected biomarkers. Alternatively, oxidative stress in male smokers with a relatively low fruit and vegetable intake might have been still too low to demonstrate a beneficial effect of antioxidants.

Dietary intakes and serum nutrients differ between adults from food-insufficient and food-sufficient families: Third National Health and Nutrition Examination Survey, 1988-1994

Approximately 10.2 million persons in the United States sometimes or often do not have enough food to eat, a condition known as food insufficiency. Using cross-sectional data from the Third National Health and Nutrition Examination Survey (NHANES III), we examined whether dietary intakes and serum nutrients differed between adults from food-insufficient families (FIF) and adults from food-sufficient families (FSF). Results from analyses, stratified by age group and adjusted for family income and other important covariates, revealed several significant findings (P < 0.05). Compared with their food-sufficient counterparts, younger adults (aged 20-59 y) from FIF had lower intakes of calcium and were more likely to have calcium and vitamin E intakes below 50% of the recommended amounts on a given day. Younger adults from FIF also reported lower 1-mo frequency of consumption of milk/milk products, fruits/fruit juices and vegetables. In addition, younger adults from FIF had lower serum concentrations of total cholesterol, vitamin A and three carotenoids (alpha-carotene, beta-cryptoxanthin and lutein/zeaxanthin). Older adults (aged > or =60 y) from FIF had lower intakes of energy, vitamin B-6, magnesium, iron and zinc and were more likely to have iron and zinc intakes below 50% of the recommended amount on a given day. Older adults from FIF also had lower serum concentrations of high-density lipoprotein cholesterol, albumin, vitamin A, beta-cryptoxanthin and vitamin E. Both younger and older adults from FIF were more likely to have very low serum albumin (<35 g/L) than were adults from FSF. Our findings show that adults from FIF have diets that may compromise their health.

Serum concentrations of carotenoids and vitamins A, E, and C in control subjects from five European countries

High intakes of fruits and vegetables, or high circulating levels of their biomarkers (carotenoids, vitamins C and E), have been associated with a relatively low incidence of cardiovascular disease, cataract and cancer. Exposure to a high fruit and vegetable diet increases antioxidant concentrations in blood and body tissues, and potentially protects against oxidative damage to cells and tissues. This paper describes blood concentrations of carotenoids, tocopherols, ascorbic acid and retinol in well-defined groups of healthy, non-smokers, aged 25-45 years, 175 men and 174 women from five European countries (France, UK (Northern Ireland), Republic of Ireland, The Netherlands and Spain). Analysis was centralised and performed within 18 months. Within-gender, vitamin C showed no significant differences between centres. Females in France, Republic of Ireland and Spain had significantly higher plasma vitamin C concentrations than their male counterparts. Serum retinol and alpha-tocopherol levels were similar between centres, but gamma-tocopherol showed a great variability being the lowest in Spain and France, and the highest in The Netherlands. The provitamin A: non-provitamin A carotenoid ratio was similar among countries, whereas the xanthophylls (lutein, zeaxanthin, beta-cryptoxanthin) to carotenes (alpha-carotene, beta-carotene, lycopene) ratio was double in southern (Spain) compared to the northern areas (Northern Ireland and Republic of Ireland). Serum concentrations of lutein and zeaxanthin were highest in France and Spain; beta-cryptoxanthin was highest in Spain and The Netherlands; trans-lycopene tended to be highest in Irish males and lowest in Spanish males; alpha-carotene and beta-carotene were higher in the French volunteers. Due to the study design, the concentrations of carotenoids and vitamins A, C and E represent physiological ranges achievable by dietary means and may be considered as 'reference values' in serum of healthy, non-smoking middle-aged subjects from five European countries. The results suggest that lutein (and zeaxanthin), beta-cryptoxanthin, total xanthophylls and gamma-tocopherol (and alpha- : gamma-tocopherol) may be important markers related to the healthy or protective effects of the Mediterranean-like diet.

Forbidden fruit

Although citrus fruits prevent and cure scurvy, they may not always be as good for you as you thought.

Vitamin C transport systems of mammalian cells

Vitamin C is essential for many enzymatic reactions and also acts as a free radical scavenger. Specific non-overlapping transport proteins mediate the transport of the oxidized form of vitamin C, dehydroascorbic acid, and the reduced form, L-ascorbic acid, across biological membranes. Dehydroascorbic acid uptake is via the facilitated-diffusion glucose transporters, GLUT 1, 3 and 4, but under physiological conditions these transporters are unlikely to play a major role in the uptake of vitamin C due to the high concentrations of glucose that will effectively block influx. L-ascorbic acid enters cells via Na+-dependent systems, and two isoforms of these transporters (SVCT1 and SVCT2) have recently been cloned from humans and rats. Transport by both isoforms is stereospecific, with a pH optimum of approximately 7.5 and a Na+:ascorbic acid stoichiometry of 2:1. SVCT2 may exhibit a higher affinity for ascorbic acid than SVCT1 but with a lower maximum velocity. SVCT1 and SVCT2 are predicted to have 12 transmembrane domains, but they share no structural homology with other Na+ co-transporters. Potential sites for phosphorylation by protein kinase C exist on the cytoplasmic surface of both proteins, with an additional protein kinase A site in SVCT1. The two isoforms also differ in their tissue distribution: SVCT1 is present in epithelial tissues, whereas SVCT2 is present in most tissues with the exception of lung and skeletal muscle.

Nutritional contribution of eggs to American diets

OBJECTIVES

The main purposes of this study were (1) to assess the nutritional significance of eggs in the American diet and (2) to estimate the degree of association between egg consumption and serum cholesterol concentration.

METHODS

Data from the most recent National Health and Nutritional Examination Survey (NHANES III, 1988-94) were utilized to compare the nutritional quality indicators of diets that contained eggs (USDA food grouping system) with those that did not. Nutrient intake (from 24-hour dietary recall), egg intake (from food frequency questionnaire), sociodemographic data and blood cholesterol levels of subjects who met inclusion criteria (n = 27,378) were grouped according to the occurrence and frequency of egg consumption and were analyzed using SUDAAN.

RESULTS

Daily nutrient intake of egg consumers (EC) was significantly greater than that of nonconsumers (NC) for all nutrients studied (except dietary fiber and vitamin B6). Eggs contributed < 10% of daily intake of energy and vitamin B6, 10% to 20% of folate and total, saturated and polyunsaturated fat, and 20% to 30% of vitamins A, E and B12 in EC. Compared to EC, NC had higher rates of inadequate intake (defined by Estimated Average Requirements (EAR) or < 70% Recommended Dietary Allowance (RDA)) for vitamin B12 (10% vs. 21%), vitamin A (16% vs. 21%), vitamin E (14% vs. 22%) and vitamin C (15% vs. 20%). After adjusting for demographic (age, gender and ethnicity) and lifestyle variables (smoking and physical activity), dietary cholesterol was not related to serum cholesterol concentration. People who reported eating > or = 4 eggs/wk had a significantly lower mean serum cholesterol concentration than those who reported eating < or = 1 egg/wk (193 mg/dL vs. 197 mg/dL, p < 0.01). More frequent egg consumption was negatively associated with serum cholesterol concentration (beta = -6.45, p < 0.01).

CONCLUSIONS

In this cross-sectional and population-based study, egg consumption made important nutritional contributions to the American diet and was not associated with high serum cholesterol concentrations.

Use of vitamin-mineral supplements by female physicians in the United States

BACKGROUND

Rates of vitamin-mineral supplement use by US female physicians are unknown but are of particular interest for several epidemiologic and clinical reasons.

OBJECTIVE

The objective was to determine rates of and variations in vitamin-mineral supplement use among US female physicians.

DESIGN

We used data from the Women Physicians' Health Study, a large (n = 4501) national, randomly sampled mail survey of female physicians aged 30-70 y.

RESULTS

Half of the physicians took a multivitamin-mineral supplement; 35.5% of these did so regularly. However, </=33% took any supplement other than calcium and <20% did so regularly. Regular vitamin-mineral supplement use increased with age, and antioxidant intake was higher in those at high risk of heart disease. Those with a history of osteoporosis were nearly 3 times as likely as those with no history to take supplemental calcium regularly. Those who took any supplement regularly also consumed more fruit and vegetables daily than did occasional users or nonusers (P: < 0.0001). Regular users of any supplement also consumed less fat than did occasional users or nonusers (P: < 0.01). Additionally, vegetarians were more likely than were nonvegetarians to regularly consume any supplement (59.9% compared with 46.3%; P: < 0.001) and those who regularly consumed any supplement were more likely to comply with US Preventive Services Task Force guidelines than were those who were occasional users or nonusers (72.4% compared with 66.5% and 60.2%; P: < 0.0001).

CONCLUSION

Female physicians, particularly those who were especially health conscious or at higher risk of heart disease or osteoporosis, used supplements at rates at least equal to those of women in the general population.

Potential antiatherogenic mechanisms of ascorbate (vitamin C) and alpha-tocopherol (vitamin E)

The premise that oxidative stress, among several other factors, plays an important role in atherogenesis implies that the development and progression of atherosclerosis can be inhibited by antioxidants. In this minireview we discuss several mechanisms by which the antioxidants ascorbate (vitamin C) and alpha-tocopherol (vitamin E) may protect against atherosclerosis. These mechanisms include inhibition of LDL oxidation and inhibition of leukocyte adhesion to the endothelium and vascular endothelial dysfunction. Overall, ascorbate appears to be more effective than alpha-tocopherol in mitigating these pathophysiological processes, most likely as a result of its abilities to effectively scavenge a wide range of reactive oxygen and nitrogen species and to regenerate alpha-tocopherol, and possibly tetrahydrobiopterin, from its radical species. In contrast, alpha-tocopherol can act either as an antioxidant or a pro-oxidant to inhibit or facilitate, respectively, lipid peroxidation in LDL. However, this pro-oxidant activity of alpha-tocopherol is prevented by ascorbate acting as a coantioxidant. Therefore, an optimum vitamin C intake or body status may help protect against atherosclerosis and its clinical sequelae, whereas vitamin E may only be effective in combination with vitamin C.

The accumulation of ascorbic acid by squamous cell carcinomas of the lung and larynx is associated with global methylation of DNA

BACKGROUND

Cigarette smokers are known to have lower concentrations of circulating ascorbic acid than nonsmokers. In contrast, there is evidence that the extracellular fluid lining of the alveolus, which comes in close contact with cigarette smoke, and the alveolar macrophages of smokers are enriched with ascorbic acid. The clinical significance of these observations is unknown.

METHODS

The authors measured the ascorbic acid concentrations and radiolabeled methyl incorporation (which is inversely related to the degree of DNA methylation in vivo) of paired samples of squamous cell carcinoma (SCC) and adjacent uninvolved mucosa of the lung and larynx (n = 22).

RESULTS

Cancerous tissues had significantly higher ascorbic acid concentrations (mean +/- standard deviation [SD, 485 +/- 77; median, 483 ng/mg protein) compared with their matched uninvolved tissues (mean +/- SD, 151 +/- 52; median, 72 ng/mg protein; P < 0.0001). The radiolabeled methyl incorporation was significantly higher in cancerous tissues (mean +/- SD, 31,419 +/- 2629; median, 31,416 counts per minute [CPM]/microg DNA) compared with their matched uninvolved tissues (mean +/- SD, 11,883 +/- 1567; median, 11,444 CPM/microg DNA; P < 0.0001). The Spearman correlation between ascorbic acid concentrations and radiolabeled methyl incorporation by DNA in SCCs was inverse and statistically significant (r = -0.58, P = 0.008), indicating a beneficial effect of accumulated ascorbic acid in global methylation of DNA. In the uninvolved tissues, this correlation was inverse but statistically not significant (r = -0.20, P =0.35).

CONCLUSIONS

Cancerous tissues of the lung and larynx demonstrated their ability to accumulate ascorbic acid. The accumulation of ascorbic acid by these tissues seemed to facilitate global methylation of DNA.

Vitamin C status and mortality in US adults

BACKGROUND

Low vitamin C status may increase the risk of mortality from cancer and cardiovascular disease.

OBJECTIVE

The objective was to test whether an association existed between serum ascorbate concentrations and mortality and whether the association was modified by cigarette smoking status or sex.

DESIGN

Serum ascorbate concentrations were measured in adults as part of the second National Health and Nutrition Examination Survey (1976-1980). Vital status was ascertained 12-16 y later.

RESULTS

The relative risk (RR) of death, adjusted for potential confounders, was estimated by using Cox proportional hazards models. Men in the lowest (<28.4 micromol/L) compared with the highest (>/=73.8 micromol/L) serum ascorbate quartile had a 57% higher risk of dying from any cause (RR: 1.57; 95% CI: 1.21, 2.03) and a 62% higher risk of dying from cancer (RR: 1.62; 95% CI: 1.01, 2.59). In contrast, there was no increased risk among men in the middle 2 quartiles for these outcomes and no increased risk of cardiovascular disease mortality in any quartile. There was no association between serum ascorbate quartile and mortality among women. These findings were consistent when analyses were limited to nonsmokers or further to adults who never smoked, suggesting that the observed relations were not due to cigarette smoking.

CONCLUSIONS

These data suggest that men with low serum ascorbate concentrations may have an increased risk of mortality, probably because of an increased risk of dying from cancer. In contrast, serum ascorbate concentrations were not related to mortality among women.

The role of natural antioxidants in preserving the biological activity of endothelium-derived nitric oxide

Endothelium-derived nitric oxide (EDNO) is a pivotal molecule in the regulation of vascular tone via the stimulation of vascular smooth muscle cell relaxation and concomitant vasodilation. In addition, EDNO exerts a number of other potent antiatherogenic effects, including inhibition of leukocyte-endothelial interactions, smooth muscle cell proliferation, and platelet aggregation. Endothelial vasodilator dysfunction has been observed in patients with CAD or coronary risk factors such as hypercholesterolemia, hyperhomocysteinemia, essential hypertension, diabetes mellitus, smoking, and aging. Most of these conditions are associated with increased oxidative stress, particularly increased production of superoxide radicals and elevated levels of oxidized LDL, both of which can attenuate the biological activity of EDNO. The levels of superoxide and oxidized LDL can be decreased by administering the small molecule antioxidants vitamins E and C. Vitamin C also spares intracellular thiols, which in turn can stabilize EDNO through the formation of biologically active S-nitrosothiols. Here we review the role that vitamins E and C and thiol compounds play in endothelium-dependent vasodilation. Understanding the mechanisms of the reversal of endothelial dysfunction by natural antioxidants will lead to successful therapeutic interventions of CAD and its clinical sequelae.

Position of the American Dietetic Association: nutrition, aging, and the continuum of care

Scientific evidence increasingly supports that good nutrition is essential to the health, self-sufficiency, and quality of life of older adults. With the population of the United States living longer than ever before, the older adult population will be more diverse and heterogeneous in the 21st century. The oldest-old and minority populations will grow more quickly than the young-old and non-Hispanic white populations, respectively. For the current 34 million adults 65 years of age and older living in the United States, there are about 12 million caregivers who provide formal or informal care. A broad array of culturally appropriate food and nutrition services, physical activities, and health and supportive care customized to accommodate the variations within this expanding population of older adults is needed. With changes and lack of coordination in health care and social-support systems, dietetics professionals need to be proactive and collaborate with aging-services and other health care professionals to improve policies, interventions, and programs that service older adults throughout the continuum of care to ensure nutritional well-being and quality of life. The American Dietetic Association supports both the provision of comprehensive food and nutrition services and the continuation and expansion of research to identify the most effective food and nutrition interventions for older adults over the continuum of care.

Contribution of nutrition to the health transition in developing countries: a framework for research and intervention

The major focus of public health programs in developing populations is alleviating undernutrition. In South Africa, however, as in many other developing countries, the African population is experiencing rapid urbanization characterized by a double burden of disease in which noncommunicable diseases (NCD) become more prevalent and infectious diseases remain undefeated. The possible mechanisms through which nutrition contributes to the additional vulnerability to NCD experienced by developing populations are explored and research priorities in this area are identified.