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National research study Council (US) Subcommittee ~ above the Tenth version of the Recommended diet Allowances. Recommended diet Allowances: 10th Edition. Washington (DC): national Academies push (US); 1989.


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National study Council (US) Subcommittee top top the Tenth execution of the Recommended dietary Allowances.

VITAMIN C

Vitamin C (L-ascorbic acid) is a water-soluble antioxidant that deserve to be synthesized by countless mammals, yet not by humans. In the diet, the is likewise present come some extent in the oxidized form (dehydroascorbic acid), which additionally has vitamin C task (Sabry et al., 1958). Diet deficiency ultimately leads come scurvy, a serious condition characterized by weakening that collagenous frameworks that results in widespread capillary hemorrhaging (Hornig, 1975; Woodruff, 1975). In the united States, scurvy occurs generally in babies fed diet consisting specifically of cow"s milk and also in age persons on limited diets.

The finest defined biochemical home of vitamin C is its function as a cosubstrate in hydroxylations requiring molecule oxygen, as in the hydroxylation that proline and also lysine in the development of collagen (Barnes, 1975; Myllyla et al., 1978), of dopamine come norepinephrine (Levin et al., 1960), and of tryptophan come 5-hydroxytryptophan (Cooper, 1961). It may likewise be connected in reactions entailing a variety of other compounds, consisting of tyrosine (La Du and Zannoni, 1961), folic acid (Stokes et al., 1975), histamine (Clemetson, 1980), corticosteroids (Wilbur and Walter, 1977), neuroendocrine peptides (Glembotski, 1987), and also bile acids (Ginter, 1975). Vitamin C can also influence functions of leukocytes (Anderson and Theron, 1979) and also macrophages (Anderson and Lukey, 1987), immune responses (Leibovitz and Siegel, 1978), wound heal (Levenson et al., 1971), and allergic reaction (Dawson and West, 1965). Ascorbic acid therefore or as present in plant foodstuffs increases the absorb of inorganic iron as soon as the two nutrients space ingested with each other (Hallberg et al., 1987).


Absorption, Transport, Storage, and Excretion

L-ascorbic acid is took in in the intestine by a sodium-dependent transport process (Stevenson, 1974). At low doses, absorption might be nearly complete, but over the range of normal intake in food (30 come 60 mg), 80 come 90% is took in (Kallner et al., 1977). Soaked up ascorbic mountain is current as the anion in blood plasma, unbound to plasma proteins. As the day-to-day intake that ascorbic mountain increases, the plasma concentration rises rapidly and also then reaches a plateau of 1.2 to 1.5 mg/dl at an entry of 90 come 150 mg/day (Garry et al., 1987; Sauberlich et al., 1974).

Body stores of ascorbic mountain in adult men reach a maximum of approximately 3,000 mg at day-to-day intakes exceeding 200 mg. One fifty percent of this level (1,500 mg) is achieved by lot lower day-to-day intakes in the variety of 60 come 100 mg. Lot of the human body stores is normally discovered within cells, in which the concentrations vary widely but are typically severalfold higher than those in blood plasma. In at the very least some tissues these concentrations appear to be achieved by a stereoselective transport process (Moser, 1987).

Ascorbic acid and its miscellaneous metabolites space excreted mainly in the urine. At daily intakes as much as 100 mg, oxalate is the significant product excreted. As soon as larger amounts are ingested, ascorbic mountain is greatly excreted because of this (Jaffe, 1984; Kallner et al., 1979). Little ascorbic mountain is metabolized come carbon dioxide at simple intakes (Baker et al., 1969), yet at big doses, degradation within the intestine might be considerable (Kallner et al., 1985).


Dietary Sources and Usual Intakes

Vegetables and fruits contain reasonably high concentration of vitamin C, e.g., green and also red peppers, collard greens, broccoli, spinach, tomatoes, potatoes, strawberries, and also oranges and also other citrus fruits. Meat, fish, poultry, eggs, and also dairy products contain smaller sized amounts, and also grains save none. Ascorbic mountain in the U.S. Food it is provided is provided almost completely by foods of vegetable origin—38% by citrus fruits, 16% by potatoes, and also 32% from various other vegetables (Marsten and also Raper, 1987). The rest comes from fortified and also enriched products and also from meat, fish, poultry, eggs, and dairy products. The median amount obtainable per capita in the U.S. Food supply enhanced from 98 mg in 1967–1969 come 114 mg in 1985 (Marsten and also Raper, 1987). The median dietary vitamin C entry by adult guys in the United claims in 1985 to be 109 mg (USDA, 1986). The matching intakes because that adult women and children 1 to 5 years of period were 77 mg and 84 mg, respectively (USDA, 1987).

The dietary vitamin C might be substantially lower 보다 the calculated quantity in the food ingested, largely because of its destruction by heat and oxygen and its lose in food preparation water. ~ above the various other hand, the mean total intake that vitamin C may also be considerably greater because (1) supplements of vitamin C are ingested through 35% of a representative U.S. Adult population (Stewart et al., 1985), (2) food ingredient tables supplied in the U.S. Room of agriculture surveys administer the L-ascorbic acid content only and also do not encompass the biologically energetic dehydroascorbate, and (3) ascorbic acid is added to part processed foods because of that is antioxidant or various other properties (NRC, 1982).


Criteria for Assessing Nutritional Status

Vitamin C condition is generally evaluated from indications of clinical deficiency, plasma (or blood) levels, or leukocyte concentrations. The has also been evaluate from isotopic estimates of body stores.

Clinical indicators of scurvy, including follicular hyperkeratosis, puffy or bleeding gums, petechial hemorrhages, and also joint pain, are connected with plasma (or serum) vitamin C values of much less than 0.2 mg/ dl, leukocyte concentrations of less than 2 µg/108 cells, and also a body pool dimension of much less than 300 mg (Hodges et al., 1969, 1971; Sauberlich, 1981). To get rid of clinical indicators of scurvy in several teams of masculine subjects, vitamin C intakes varying from 6.5 come 10 mg/day were compelled (Baker et al., 1971; Bartley et al., 1953; Hodges et al., 1969, 1971).


Recommended Allowances

Adults The diet allowances because that vitamin C have to be set, rather arbitrarily, in between the amount necessary to prevent overt symptom of scurvy (approximately 10 mg/day in adults) and also the amount beyond which the bulk of vitamin C is not kept in the body, however rather is excreted therefore in the to pee (approximately 200 mg/day). In between these limits, human body stores vary directly with intake, albeit no linearly. Because vitamin C is poorly kept in the body in the lack of continuous intake, the RDA has actually traditionally been set at a level the will stop scorbutic symptom for number of weeks on a diet lacking vitamin C. It was observed depletion rates in a little group that well-nourished adult men with a body pool of roughly 1,500 mg to be exponential and averaged 3.2% everyday (range, 2.2 come 4.1% in ripe subjects), which would certainly yield a body swimming pool of vitamin C of 300 mg (the amount below which scorbutic symptoms deserve to occur) in around 30 job (Baker et al., 1971). In 6 of 11 healthy, well-nourished young women fed ascorbate-free diets, scorbutic symptoms arisen within 24 days (bleeding, red, or soft gums) in association v blood levels regular with human body stores listed below 300 mg (Sauberlich et al., in press). By method of steady state evaluation of ascorbate kinetics in men, Kallner et al. (1979) found the turnover time to differ from about 56 days at short intakes (approximately 15 mg/day) to about 14 job at join of approximately 80 mg/day. Above 80 mg/day, urinary excretion of unmetabolized ascorbate enhanced rapidly. Kallner and also colleagues reported the a three-pool design was compelled to to the right the it was observed kinetic data and also postulated that one of these pools reflect ascorbate bound within cells. They likewise failed to watch a clear-cut renal threshold for ascorbate. Because the sales time varied v tissue stores, Kallner (1987) proposed the the depletion prices observed in previously studies might be erroneously low. Saturation of organization binding, and also maximal prices of metabolism and renal tubular absorption, appeared to it is in approached in ~ turnover rates of 60 come 80 mg daily, indistinguishable to body stores of around 1,500 mg.

The subcommittee has collection the RDA because that adult guys at 60 mg/day, the very same as in the vault edition. This amount is based upon (1) the observed variation in depletion rates and also turnover rates; (2) the median depletion rates and the secure state turnover prices at a pool dimension of 1,500 mg; (3) the less than complete absorption the ascorbic acid, approximated at 85% because that usual intakes; and also (4) the change loss that ascorbic acid in food preparation. This level that intake will prevent signs of scurvy because that at least 4 weeks. Provided the breakthrough of early on scorbutic symptoms in adult women considered to be well nourished ~ somewhat much less than 4 weeks of depletion, the subcommittee recommends the same RDA because that adult women as for men.

An input of 60 mg is easily listed in ordinary mixed diets. In the previous edition of the RDAs, an intake of 45 mg/day because that adult men was taken into consideration to carry out an mean pool size of 1,500 mg, and also an entry of 60 mg/day was recommended to provide a margin of safety. Higher values, in fact, have been said to productivity a pool size of 1,500 mg (Kallner, 1987), which would result in a recommended pin money of about 100 mg/day. It to be the check out of the subcommittee, however, that an allowance of 60 mg/day because that both men and women gives an adequate margin the safety.

Persons 65 years and older ingest an ext vitamin C top top the median (90 to 150 mg/day) than the median intake because that all periods (Garry et al., 1982; USDA, 1984). Short plasma concentrations are, however, observed commonly in some teams of elderly persons (Cheng et al., 1985; Newton et al., 1985). Such short levels are believed to reflect poor intake in the groups examined. Therefore, no increment in the RDA because that the yonsi is recommended.

Cigarette smokers have lower concentration of ascorbic acid in serum (Johnson et al., 1984; Pelletier, 1975; Schectman et al., 1989; Smith and also Hodges, 1987) and leukocytes (Brook and also Grimshaw, 1968). The lower serum level are only partially defined by the decreased vitamin C intakes of smokers (Schectman et al., 1989). The metabolic sales of men who exhilaration 20 or much more cigarettes everyday was uncovered to be boosted to a level 40% greater than the of non-smoking males (Kallner et al., 1981). Native calculations based upon all this observations, the vitamin C requirement of smokers has actually been estimated to be as much as twice that of nonsmokers. The subcommittee recommends that continuous cigarette smokers ingest at least 100 mg the vitamin C daily.

Pregnancy and also Lactation throughout pregnancy, the concentration of vitamin C and also several various other solutes in blood plasma reduce (Morse et al., 1975), more than likely as a an outcome of the hemodilution that accompanies pregnant (Hytten, 1980; Rivers and Devine, 1975). Fetal and infant plasma levels of vitamin C room 50% greater than those the the mother (Khattab et al., 1970; Salmenpera, 1984), however, indicative both of energetic transport across the placenta and also of a higher relative pool size in the fetus and also infant.

If the need for vitamin C per unit body weight is similar to the of nonpregnant adults, the increment in need for the fetus near term would certainly be little (approximately 3 come 4 mg/ day). Needs are likely to it is in somewhat higher because the catabolic rate in the fetus is more than likely greater. To offset losses indigenous the mother"s human body pool during pregnancy, a 10 mg/day increment in the maternal vitamin C RDA is recommended during pregnancy.

The concentration the vitamin C in human milk varies commonly (3 to 10 mg/dl), relying on the dietary intake of the nutrient and also other components (Bates et al., 1983; Byerley and also Kirksey, 1985; Salmenpera, 1984; Sneed et al., 1981; Tarjan et al., 1965). Assuming a concentration the 3 mg/dl, and also average milk volumes of 750 and also 600 ml in the very first and second 6 months, respectively, the subcommittee estimates the mean maternal losses room 22 and also 18 mg/day. Allowing for sports in milk production (2 SDs, or 25%), and an minister absorption performance of 85%, a everyday increment the 35 mg is recommended throughout the first 6 month of lactation and 30 mg thereafter.

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Infants and Children Breastfed babies with vitamin C intakes of 7 to 12 mg/day and also bottle-fed babies with vitamin C join of 7 mg/day have been defended from scurvy (Goldsmith, 1961; Rajalakshmi et al., 1965; valve Eekelan, 1953). There space no various other data on which to basic an RDA. Accordingly, the subcommittee recommends 30 mg/day throughout the an initial 6 month of life top top the communication of the vitamin C contents of milk, i m sorry should provide an adequate margin the safety. Premature babies may exhibition transient tyrosinemia (Irwin and Hutchins, 1976) and also may thus require a larger amount. The RDA beyond 6 months of period is slowly increased to the adult level.