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Vitamin C
Vitamin C (Ascorbic acid) is a water-soluble vitamin, and as
such dissolves in water upon entering the body. The body absorbs
vitamin C easily and just as easily excretes it in the urine (excess
amounts cannot be stored for later use). Vitamin C is an essential nutrient
for humans. Most plants and animal tissues can synthesise vitamin C from
glucose; however, humans do not have the ability to synthetise ascorbic acid, so
dietary intake of the preformed vitamin C is essential to prevent deficiency resulting
in scurvy. Vitamin C acts as an essential cofactor in numerous body enzymatic reactions
and as a potent antioxidant that can help neutralize free radicals. It protects the body
from oxidation by being oxidized itself. Ascorbic acid is also widely used as a food additive, to
prevent food oxidation.
Summary
Chemical data & Metabolism ........................................................................................................................................................... 3
Discovery ............................................................................................................................................................................................................................... 3
Function and health .......................................................................................................................................................................................... 3
Vitamin C and eye health ....................................................................................................................................................................... 4
Recommended daily intakes ........................................................................................................................................................... 4
Dietary sources .......................................................................................................................................................................................................... 5
Useful tips ............................................................................................................................................................................................................................6
Vitamin C
Chemical data & Metabolism
V
itamin C refers both to ascorbic acid and dehydroascorbic acid. It is a six-carbon compound related to glucose. It consists of two inter-converting compounds, L-ascorbic Acid (or ascorbate) and its oxidized
form L-dehydroascorbic Acid, both of which are biologically active (Fig. 1). The biological half-life of vitamin C is
8-40 days(1).
HO
times daily than 1,000 mg taken once daily (4,5). Vitamin C
concentration is higher in leucocytes and platelets than in
erythrocytes and plasma and can be measured by HPLC
method. High levels of vitamin C are also found in the
brain and eye tissues (6).
Studies have reported the excretion and renal tubular
reabsorption of vitamin C with the belief that the excretion
rate rises sharply when plasma concentrations become
high enough (1,7). Vitamin C excess is excreted in the urine
via the kidneys, and can be reabsorbed by renal tubules
as long as the body pool does not exceed 1.5g (8).
HO
HO
O
H
O
HO
O
O
H
OH
O
Discovery
O
O
Fig. 1: Ascorbic acid and Dehydrascorbic acid (Adapted from:
http://commons.wikimedia.org/wiki/File:Dehydroascorbic_acid.
png accessed 8th Oct 2014)
Ascorbic acid is readily absorbed from the gastrointestinal
tract. 70 to 90% of vitamin C usual dietary intake is
absorbed, for doses of 1000mg/day absorption fall
to 50 %(1). It has been shown by Johnston et al that
bioavailability of vitamin C from foods and supplements
are not different(2).
Ascorbic acid is widely distributed in the body tissues.
Plasma concentrations of ascorbic acid rise as the dose
ingested is increased until a plateau is reached with doses
of about 90 to 150 mg daily. Body stores of ascorbic acid
in health are about 1.5g although more may be stored at
intakes above 200mg daily (fig. 2)(3,5).
T
he term ‘scurvy’ for the disease resulting from
prolonged vitamin C deficiency had origins in
‘scorbutus’ (Latin), ‘scorbut’ (French), and ‘Skorbut’
(German). Scurvy was at the origin of the discovery of
vitamin C.
Scurvy was a common problem in the world’s navies in
past centuries and is estimated to have affected 2 million
sailors. The first use of modern scientific methods to
assess disease treatment was established in 1747 when
James Lind conducted a trial for 12 sailors with scurvy. He
identified that only oranges and lemons were effective in
treating scurvy (other test substances included vinegar
and seawater). Scurvy also occurred on land, with a high
number of cases occurring throughout the great potato
famine in Ireland in 1845. Ascorbic acid would however
not be isolated until the early 1933 when Norman Haworth
deduced its chemical structure.
Function and health
100
V
80
60
40
20
0
0
500
1000
1500
2000
2500
Dose, mg/day
Fig. 2: Relationship between vitamin C intake and plasma
ascorbic acid concentrations (5)
Scientists argue that there is a sigmoidal relationship
between intake and plasma concentration of vitamin C.
High plasma concentration levels of vitamin C occur with
a daily dose of vitamin C of between 30 and 100 mg and
complete saturation occurs at 1,000 mg daily. Close to
steady states, plateau concentrations are reached above
200 mg/day. This suggests that there would be better
absorption with 250 mg as supplements taken four
itamin C is a water-soluble antioxidant well distributed in the plasma and tissues of the human body.
Vitamin C is a powerful reducing agent and affects many
oxidation and reduction reactions. It acts and an electron
donor and protects indispensable molecules in the body
such as proteins and lipids from damage by free radicals
like cigarette smoke (9).
In the intestines, vitamin C protects iron from oxidation
and so promotes its absorption. Its ability to convert
the ferric (Fe3+) ion to the more soluble ferrous (Fe2+)
means it assists the absorption of non-haem iron. Vitamin
C is also believed to improve copper absorption(10).
It is an important antioxidant helping to prevent free
radical damage particularly in the aqueous components
of tissues.
Vitamin C has a lower redox potential than other antioxidant vitamins like vitamin E. Vitamin C has been shown
to be an efficient co-antioxidant for the regeneration
of Vitamin E from the tocopheroxyl radical thereby
restoring its antioxidant properties. Increased intake of
vitamin C, therefore, could lead to improved vitamin E
status (Fig. 3)(11,12,13).
3
Vitamin C
a-tocopherol
radical
Ascorbate
Vitamin E Cycle
Vitamin C Cycle
a-tocopherol
Dehydroascorbate
Stabilized
Oxygen
Species
Reactive
Oxygen
Species
Fig. 3: Theorised antioxidant network between vitamin C and
vitamin E (Adapted from http://humannutrition.wikifoundry.
com/page/Antioxidants, accessed 11th Nov 2014)
Vitamin C is needed for the growth and repair of tissues
in all parts of your body. It is required for the biosynthesis
of collagen, L-carnitine, and certain neurotransmitters. It
is also involved in protein metabolism such as collagen,
an essential component of connective tissue, which plays
a vital role in wound healing(14,15,16).
Recent American research, the Age-Related Eye Disease
Studies (27,28) confirmed the value of high doses of vitamin
C (as part of the formulation) in reducing the progression
of AMD. Some studies have also highlighted the lower risk
of cataract formation with high dietary intakes of vitamin
C and high plasma ascorbate concentrations(13,29).
Recommended daily intakes
• General population:
The government sets the Reference Nutrient Intake
(RNI) for Vitamin C in the UK. The RNI represents the
best estimate of Vitamin C that is enough, or more than
enough for about 97% of people in a group (Table 2).
Age
(Years)
Male
(mg/d)
Female
Ongoing research is examining whether vitamin C, by
limiting the damaging effects of free radicals through
its antioxidant activity, might help prevent or delay the
development of certain cancers, cardiovascular disease,
and other diseases in which oxidative stress plays a causal
role like Age-related Macular Degeneration (AMD)(16,17). In
addition to its biosynthetic and antioxidant functions,
some studies suggest that vitamin C plays an important
role in immune function(18,19).
Vitamin C and eye health
A
scorbic acid concentration levels in the aqueous
humour can be up to 20 times higher than in
blood(20,21). The high concentration of ascorbic acid in
the aqueous humour works as a filter that prevents the
penetration of UV light in the lens and thus protects
tissue from oxidative damage, particularly photo-induced
damage(22).
Maintenance of high concentrations of vitamin C in the
ocular structures is carried out by active transport from
plasma through the barrier blood-aqueous humour to
act primarily as a scavenger for free radicals to protect
from oxidative damage. The oxidized form of vitamin C
(dehydroascorbic acid) is reduced to ascorbic acid during
the transport into the humour in the presence of reduced
Glutathione. The reduction of dehydroascorbic acid
contributes to the retention of ascorbic acid in the lens(20).
Cataracts (an age-related clouding of the lens in the eye
that affects vision) and Age-related Macular Degeneration
(AMD, an eye condition which causes damage to the
macula) are two of the leading causes of vision loss
in older individuals (23). Scientists have hypothesized
that Vitamin C may play a role in the treatment and/or
development of AMD and cataracts since oxidative stress
plays a role in the etiology of both conditions(24,25,26).
4
(mg/d)
1-3
4-6
7-10
11-14
15-18
19-50 51-64 65-74
75+
30
30
30
35
40
40
40
40
40
30
30
30
35
30
40
40
40
40
Table 2: Reference Nutrient Intake (RNI) for vitamin C,
by sex and age (35)
Vitamin C requirements are slightly increased (+ 50 mg/
day) in pregnant women(35).
• Specific requirements for
Aging people at risk of AMD:
To reduce the progression of AMD, the AREDS 2
formulation includes 500mg of Vitamin C (Table 3) in the
formulation(28).
Smokers:
Cigarette smoke is responsible for an increase in
the oxidative stress and metabolic turnover, which
deplete the body’s vitamin C levels further. As a result,
recommendations for smokers are significantly higher
(+ 35 to 40 mg/day) to account for the excess oxidative
stress(36).
Smoker
Aging people at risk of
AMD*
Male
80mg
500mg
Female
80mg
500mg
Table 3: Special recommendations for Vitamin C Intake
* Based on AREDS 2 Recommendation (28)
The latest National Diet and Nutrition Survey in the UK (35)
established the mean daily intakes of Vitamin C from food
sources were well above the RNI for all age/sex groups
(table 4). Today, vitamin C deficiency is believed to be
rare and to occur only in case of very restricted diets.
Recent case reports of scurvy indicate that vitamin C
deficiency may be more prevalent that generally assumed
and poor vitamin C status is relatively common among
adults living on a low income(32).
Vitamin C
very good, or excellent source of dietary vitamin C. In
addition, many vegetables such as Broccoli, cauliflower
or spinach contain significant amounts of vitamin C and
are also considered good dietary sources. To ensure
the coverage of the daily vitamin C requirement, it is
recommended to consume about 5 portions of fruit and
vegetables per day(33,34).
However, low intakes of fruits and vegetables and a poor
appetite overall lead to low vitamin C intakes and are not
uncommon among people aged 65 and older(30,31).
Boys
Age (Years)
Men
Girls
Women
4-10
11-18
19-64
65+
4-10
11-18
19-64
65+
86.9
89.7
91.4
85.7
86.5
79.0
87.6
80.3
290
239
229
214
288
212
219
201
Average daily
intake from food
The same thing that makes vitamin C so important its ability to protect against free radical damage - also
makes it very prone to damage by heat, oxygen, and
storage over time. The vitamin C content of food will
start to decline as soon as it is picked, even though this
decline can be slowed down and minimized by cooling
and retention of the food in its whole form. But a fresh,
vitamin C-rich vegetable like broccoli (if allowed to sit
at room temperature for 6 days) can lose almost 80% of
its vitamin C. That potential vitamin C loss is one of the
reasons it is so important to store with caution fruits and
vegetables rich in vitamin C.
sources (mg)
% of RNI
Table 4: Average daily intake of Vitamin C from food sources in
the UK for all age/sex groups, in mg/day and % of RNI (35).
Dietary sources
T
he best food sources of vitamin C have a single thing
in common: they are all plant foods. Most people
associate citrus fruits with vitamin C. This is not a mythcitrus fruits (such as orange and grapefruit) are excellent
sources of vitamin C. Many non-citrus fruits are also highly
rated sources (Table 5). Papaya, strawberries, pineapple
and cantaloupe are also very good sources of vitamin C.
You should expect almost any fresh fruit to be a good,
Long-term storage of vegetables can cost a significant
amount of vitamin C. Kept frozen for a year; kale can
lose half its vitamin C or more. Canning is even more
detrimental, with 85% of the original vitamin C lost over
the same year. Cooking vegetables and fruits in water
removes as much as two-thirds of its vitamin C content.
Figure 4: Foods sources rich in high Vitamin C(37).
Blackcurrant (80g)
Kiwi fruits (2 medium)
Grapefruit (medium)
Lychee (6-8)
Medium orange
Red pepper (1/2 pepper)
Broccoli (2 spears)
Pineapple (Large slice)
Cantaloupe melon (1 large slice)
Strawberries (6-7)
Lemon (medium)
Papaya (large slice)
Brussels sprouts (8)
Green chili pepper (20g)
Raspberries (1 handful)
Passion fruit (5-6)
Mandarin orange (2 medium)
Mango (2 slices)
Blackberry (1 handful)
Cauliflower (8 florets)
Kale (4 heaped tablespoons)
Red chili pepper (20g)
Spinach (2 heaped tablespoons)
0 mg
70 mg
70 mg
88 mg
72 mg
78 mg
68mg
60 mg
53 mg
50 mg
64 mg
50 mg
160 mg
130 mg
152 mg
30 mg
24 mg
24 mg
22 mg
21 mg
39 mg
33 mg
29 mg
24 mg
50 mg
100 mg
150 mg
5
Vitamin C
Useful tips
Hereafter some tips to help to reach RDA and help to store fruits and vegetables to
minimise nutrient loss from each food and therefore reach the daily recommended
intakes of vitamin C.
Tip 2 :
Instead of boiling vegetables
in water, try steaming them over
a small amount of water. Not only
will steamed vegetables retain their
nutritional value, they’ll also keep their
shape and color. Replace blanching in water
with a quick steam bath for vegetables. Hot
water cooks the water-soluble vitamin out of foods,
so using less liquid means saving more nutrition.
Tip 1 :
Many foods are a goods source of
vitamin C: citries fruits (orange, lemon,
grapefruits), strawberry and pineapple are
also goods sources. Vegetables like brocoli,
spinach contain also significant amount vitamin C.
Tip 3 :
Longer cooking methods
usually require water. Vitamin
C will leave the food but can be
kept in the dish if the water is used.
Waterless cooking methods retain a much
higher percentage of the food’s watersoluble vitamin content. Roasting, sautéing and
stir-frying require no water for cooking.
6
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