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Background
Vitamin C (L-ascorbic acid or ascorbate) is the generic descriptor for compounds having antiscorbutic exercise. Most animals can synthesise vitamin C from D-glucose however people and different primates, along with guinea pigs, fruit bats, some passeriform birds, some fish and a few bugs, are exceptions. People and primates lack a key enzyme, L-3 gulonolactone oxidase, mandatory for the biosynthesis of vitamin C (Nishikimi et al 1994).
Vitamin C is a lowering agent (antioxidant) and it’s possible that each one of its biochemical and molecular features relate to this property. In people, vitamin C acts as an electron donor for eight enzymes, of which three are concerned in collagen hydroxylation (together with features of norepinephrine, peptide hormone and tyrosine metabolism) and two are concerned in carnitine biosynthesis (Dunn et al 1984, Eipper et al 1993, 1992, Kaufmann 1974, Kirirkko & Myllyla 1985, Levine et al 1991, Procop & Kiviikko 1995, Peterkovsky 1991, Rebouche 1991). Vitamin C is present in excessive concentrations in gastric juices (Schorah et al 1991) the place it could forestall the formation of N-nitroso-compounds, that are potential mutagens (Correa 1992).
Vitamin C has been proven to guard lipids in human plasma and low density lipoprotein in ex vivo experiments in opposition to oxidative harm (Frei 1991). However there is no such thing as a proof of in vivo safety. Vitamin C additionally interacts with different vitamins. It aids within the absorption of iron and copper (Hallberg 1985, Harris & Perceval 1991), the upkeep of glutathione within the diminished type (Henning et al 1991, Johnston et al 1993), the regeneration, or sparing, of alpha-tocopherol (Halpner et al 1998) and the stabilisation of folate (Stokes et al 1975).
Ascorbate is discovered broadly in vegetables and fruit. Fruits corresponding to blackcurrants, guava, citrus, and kiwi fruit and greens corresponding to broccoli and sprouts are good sources. The Australian bush meals terminalia ferdinandiana is the richest supply (Model et al 1982). Nonetheless, due to their longer durations of availability, greens typically contribute extra ascorbate to the weight loss plan than fruits. In Australia, some 40% of the vitamin C comes from greens and 19% from fruits and an additional 27% from fruit and vegetable juices(ABS 1998). Vitamin C could be very labile and its content material in meals varies. Vitamin C content material will be affected by season, transport, shelf life, storage time, cooking practices and chlorination of water. Reducing, bruising, heating and publicity to copper, iron or mildly alkaline circumstances can destroy ascorbate. It can be leached into water throughout cooking.
Intestinal absorption of vitamin C happens by way of a sodium-dependent lively transport course of that’s saturable and dose dependent (Rumsey & Levine 1998, Tsao 1997). Kallner et al (1979) confirmed that some 70-90% of traditional consumption is absorbed and that absorption fell to 50% or much less with growing doses above 1 g/day. Dose-dependent absorption and renal regulation of ascorbate permit conservation of vitamin C within the physique in periods of low consumption and regulation of plasma ranges at excessive intakes.
There’s a sigmoidal relationship between consumption and plasma focus of vitamin C (Levine et al 1996, Newton et al 1983). Newton et al (1983) confirmed that for intakes as much as 30 mg/day, plasma concentrations are about 11 µmol/L (or 0.2 mg/dL). Above this consumption, plasma concentrations improve steeply to 60 µmol/L and plateau at 80 µmol/L, the renal threshold. Levine et al (1996) discovered that the steep portion of the plasma focus curve occurred with a each day dose of vitamin C of between 30 and 100 mg and that full saturation occurred at 1,000 mg each day. Near regular states, plateau concentrations are reached above 200 mg/day. Absorption can also be to some extent depending on the dosing routine of vitamin C. For instance, there could be higher absorption with 250 mg as dietary supplements taken 4 instances each day than 1,000 mg taken as soon as each day.
Excessive ranges of vitamin C are discovered within the pituitary and adrenal lands, leukocytes, eye tissues and fluids and the mind (Horning et al 1975). The biologic half-life of vitamin C is 8-40 days (Kallner et al 1979) and catabolic turnover varies broadly, averaging 2.9% over a variety of intakes (Baker et al 1971). A physique pool of lower than 300-400 mg is related to the signs of scurvy (Baker et al 1969). At saturation, the entire physique content material in males is about 20 mg/kg or 1,500 mg (Baker et al 1969, Kallner et al 1979).
Plasma vitamin C concentrations are diminished by 40% in male people who smoke. This can be partly because of people who smoke tending to eat much less vegetables and fruit, however after correcting for intakes of fruit and greens, people who smoke nonetheless present decrease plasma ascorbate than non-smokers (Lykkesfeldt et al 2000). The metabolic turnover of ascorbate is markedly accelerated in people who smoke (Kallner et al 1981).
Vitamin C deficiency causes scurvy, signs of which embrace skeletal and vascular lesions with gingival adjustments, ache within the extremities, haemorrhage, oedema, ulcerations and loss of life. In adults, medical indicators happen at intakes of 7-8 mg/day or much less (Goldsmith 1961, Rajajalakshmi et al 1965, van Eekelen 1953). In childish scurvy, the adjustments are primarily on the websites of lively bone progress and embrace a pseudoparalysis of the limbs (McLaren 1992).
There are a number of potential indices of vitamin C necessities in people, together with evaluation of medical outcomes, vitamin C turnover and biochemical indices of standing (eg plasma, urine, leukocyte). Some research have raised the query of whether or not vitamin C has helpful results on regular human topics at intakes, and tissue ranges, significantly higher than these wanted to forestall or remedy scurvy. Nonetheless, the proof has been conflicting. There’s potential confounding in meals consumption research associated to the difficulty of concomitant intakes of different protecting vitamins in vegetables and fruit, corresponding to phytochemicals. As well as, research usually don’t present the dose-response knowledge on which common necessities will be ascertained. (COMA 1991, FNB:IOM 2000, FAO:WHO 2002).
Because of this, the estimates of vitamin C necessities on this report are based mostly on prevention of scurvy, vitamin C turnover research and biochemical indices of vitamin C standing in man.
Suggestions by life stage and gender
Infants
Rationale: Breast milk focus varies broadly in accordance with maternal consumption and doesn’t essentially replicate toddler wants (Irwin & Hutchins 1976, Olson & Hodges 1987, van Zoeren-Grobben et al 1987). Human milk usually can differ from 30 mg/L to 80 mg/L or extra, relying on the consumption of the mom (Bates & Prentice 1988, WHO 1998). Scientific scurvy has not been noticed in absolutely breast-fed infants, even in communities the place the vitamin C intakes of the moms are low. Scurvy is seen solely at intakes of about 7-8 mg/day or much less, usually in non-breast-fed infants. The AI for 0-6 months was subsequently calculated by multiplying collectively the typical consumption of breast milk (0.78 L/day) and a breast milk focus of 30 mg/L, and rounding up. The AI for 7-12 months was calculated on a physique weight foundation from that of youthful infants.
Youngsters & adolescents
Rationale: Within the absence of enough knowledge for kids and following the strategy of the FAO:WHO (2002), the EARs had been interpolated from the grownup and toddler suggestions, though these figures are considerably arbitrary. The RDI was set assuming a CV of 20% for the EAR, as for adults.
Adults
Rationale: The EAR for grownup males was set on the idea that the very best indicator of adequacy presently obtainable is the consumption at which physique content material is midway between tissue saturation and the purpose at which medical indicators of scurvy seem. This equates to 900 mg physique content material. Assuming an absorption effectivity of 85%, a catabolic price of two.9%, and rounding, the EAR for adults was set at 30 mg/day (900 x 2.9/100 x 100/85). This EAR supplies sufficient vitamin C for people who smoke. There’s a identified CV for catabolism of 21% (2.9%/day, SD = 0.6%) (Baker et al 1971) which, with rounding, provides an RDI of 45 mg/day. Plasma concentrations of vitamin C fall extra quickly in girls than males (Blanchard 1991), so the male suggestion was retained for ladies though girls have decrease physique sizes.
Being pregnant
Rationale: There’s a average drain on vitamin C throughout being pregnant, notably within the final trimester, most likely because of haemodilution in addition to switch to the foetus. On condition that 7 mg/day will forestall scurvy in younger infants, (Goldsmith 1961, Rajalalakshmi et al 1965, van Eekelen 1953), an additional 10 mg/day in being pregnant ought to allow reserves to build up to fulfill the additional calls for of the rising foetus. The EAR is subsequently set at 40 (or 38) mg/day and the RDI set assuming a CV for the EAR of 20%, and rounding up.
Lactation
Rationale: The EARs for lactation are estimated from the EAR for non-lactating girls plus wants for the toddler. The RDI is about assuming a CV for the EAR of 20%.
Higher Degree of Consumption Vitamin C – “vitamin c upper limit”
It’s not attainable to ascertain a UL for vitamin C, however 1,000 mg/day is a prudent restrict.
Rationale: It’s not attainable to ascertain with any certainty a UL for supplementary vitamin C, as knowledge are too inconclusive. Nonetheless, professional our bodies have advised that intakes of not more than 1,000 mg/day for adults could be prudent (UK Skilled Group on Nutritional vitamins and Minerals 2003, German Diet Society 2002).
The UK Skilled Group on Nutritional vitamins and Minerals (2002) has advised a steerage degree of 1,000 mg based mostly on a LOAEL of three,000-4,000 mg/day from the research of Cameron & Campbell (1974), making use of an UF of three to extrapolate to a NOAEL of 1,000 mg/day. The US Meals and Diet Board used the identical knowledge however utilized an UF of just one.5 to provide a NOAEL of two,000 mg which it adopted because the Tolerable Higher Consumption for adults ranging all the way down to 400 mg in youngsters aged 1-3 years.
Gastrointestinal results are the most typical antagonistic results related to acute, excessive doses of vitamin C given over a brief time frame. Different reported results embrace metabolic acidosis, adjustments in prothrombin exercise and ‘conditioned want’ scurvy (excessive ingestion in being pregnant conditioning the necessity for increased quantities within the toddler). It has additionally been advised that vitamin C consumption might improve oxalate excretion. Nonetheless, research in people haven’t revealed a considerable improve in urinary oxalate stones with excessive intakes of vitamin C. Key research embrace these of Auer et al (1998), Cameron & Campbell (1974), Prepare dinner et al (1984), Gokce et al (1999), Levine et al (1996, 1999), Mai et al (1990), Morton et al (2001), Urivetsky et al (1992), and Wandilak et al (1994). These research counsel that vitamin C isn’t related to vital antagonistic results and there aren’t any apparent particular key poisonous endpoints.
Vitamin C may improve non-haem iron absorption and thus might improve iron-induced tissue harm in people with haemochromatosis (McLaran et al 1982). Haemochromatosis is a situation of glucose-6-phosphate dehydrogenase deficiency that happens in about 1 in 300 folks of northern European descent (George & Powell 1997). Nonetheless, the opportunity of such antagonistic results on this group has not been systematically examined.
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