Vitamin C
Vitmain C has for many decades been hailed as the ‘wonder pill.’ It has been suggested that it greatly strengthens the immune system, speeds up the healing process – and yes, slows down the process of aging through its anti-oxidant properties. So how does vitamin C operate in the human body? How is it absorved, matabolised and most importantly in this site – does it play any role in retarding the processes associated with aging??? The purpose of the next few pages is to examine those questions in detail and hopefully to provide some enlightening answers.
Definition of Vitamin C
The term vitamin C was coined to collectively apply to any substances that exhibit antiscorbutic properties (antiscorbutic literally means prevention of scurvy). Vitamin C can be divided into two distinct forms, together with their salts. These are
L-ascorbic acid. Usually simply referred to as ascorbic acid
L-dehydroascorbic acid
Note that ascorbic acid is the primary dietary form of Vitamin C.
In many texts, the terms ascorbic acid and ascorbate are used interchangeably. Both forms of the vitamin are basically derived from hexose and are similar in structure to the six-carbon sugar glucose.
The molecular structure of vitamin C
Absorption and Transportation of Vitamin C
One of the things that is unusual about Vitamin C is that only a few species of animals actually require it in their diet. Man, other primates, the red-vented barbul, the guinea pig and Passeriform birds, the Coho salmon, rainbow trout and various insects, together with a few other animals, all need to take this vitamin into their body in its raw form. The majority of other species of animals and plants possess the ability to synthesise this vitamin themselves from glucose or other forms of sugar. This synthesis of vitamin C from glucose is accomplished by using the enzyme L-gulano-gamma-lactone oxidase. This is the final enzyme that takes part in converting glucose to vitamin C and is not present in any vitamin C requiring animals.
In doses of 30 miligrams per day or less, nearly all dietary vitamin C is absorbed through the lumen of the small intestine into the enterocytes (a type of epiphelial cell of the small and large intestine). In doses up to 500 miligrams, this absorption is facilitated via a sodium-dependent active transport mechanism. For doses above 500 miligrams, absorption of vitamin C starts to occur via diffusion.
The primary ascorbic acid intestinal transporter is sodium-dependant vitamin C transporter 1 (abbreviated to SVCT1). SVCT1 also transports ascorbic acid into Liver and Kidney cells. Sodium-dependant vitamin C transporter 2 (SVCT2) seems to help to transport vitamin C into the aqueous humor of the eye.
Dehydroascorbic acid is taken up by the enterocytes via glucose channels. Some ascorbic acid may also be taken up by the enterocytes of the intestinal lumen after first being converted to dehydroascorbic acid by a process of oxidation. Once inside the enterocytes, dehydroascorbic acid (DHA) is reduced back to ascorbic acid by the reduced glutathione enzyme. Once converted back to ascorbic acid, the vitamin C then leaves the enterocytes and enters the portal circulation, then the systemic circulation. From this point the vitamin C is transported by the circulatory system throughout the body.
The highest concentrations of ascorbic acid can be found in the adrenal glands, pituitary gland, the white blood cells and the brain. To enter the tissues of the brain, the ascorbic acid must first cross the blood-brain barrier. Something that it cannot accomplish alone. To cross this barrier, the ascorbic acid is first converted to Dehydroascorbic acid (DHA) by a process of oxidation. Glucose transporter 1 (GLUT1) then comes into play and transports the DHA across the blood-brain barrier by a process of facilitative diffusion. GLUT1 transports the DHA into then out of the endothelial cells that line the surface of the blood-brain barrier. The DHA is then reduced back to ascorbic acid. It then remains in the tissues of the brain as there is no further mechanism to convert the ascorbic acid back into DHA and GLUT1 can only carry the ascorbic acid in its oxidised form.
Note that the GLUT1 mechanism is also used to transport ascorbic acid into the Neutrophils.
Metabolism of ascorbic acid
Large doses of ascorbic acid result in a reduced rate of absorption. This is why the vitamin should be taken in smaller doses throughout the day, rather than in single large doses. Any unabsorbed ascorbic acid will leave the body in the faeces. Once taken into the body, the ascorbic acid can be converted into several metabolites. These include Dehydroascorbic acid, diketogulonate, threonic acid, oxalic acid, ascorbate-2-sulfate and L-xylose. Any access ascorbic acid or its associated metabolites are extracted by the kidneys and are then excreted in the urine.
General functions of Vitamin C
Effects of reduced levels of Vitamin C
The major syndrome that lack of vitamin C in the diet will result in is Scurvy. This is a very encompassing syndrome, resulting in the sufferer presenting a number of different symptoms. These include:
- Petechiae – Small multiple hemorrhages of the blood vessels beneath the surface layer of the skin.
- Follicular hyperkeratosis – Increase in the concentration of Keratin in the follicles.
- Perifollicular hemorrhages
- Impaired wound Healing
- Inflamed and bleeding gums
- Coiled hairs
- Ecchymosis – A bleeding into the skin or bruising. This differs from Petechiae only in size.
- Arthralgia – Pain in the joints.
- Muscle weakness
- Fatigue
- Myalgia – Muscular pain
- Anemia
- Depression
- Increased susceptibility to infections
- Pulmonary and Kidney problems
- Joint effusions
- Diarrhea
- Anorexia
- Dry eyes and mouth (Sjögren's syndrome)
- Coma and Death!!!!!
Effects of too much Vitamin C
A lot of talk is made about overdosing on vitamins and whilst keeping a firm eye on ones dietary intake is very important for a variety of foodstuffs (including many of the fat-soluble vitamins) concerning yourself about overdosing on Vitamin C should not overly occupy your mind. Ascorbic acid is basically non-toxic. It is a water-soluble vitamin and therefore any access amounts that enter the system are simply excreted.
You should be careful however not to go ridiculously overboard with your Vitamin C intake. A daily dosage of 200 milligrams is just about enough to saturate plasma and lymphocyte levels. The recommended daily intake of Vitamin C in adult non-smokers is 90 mg for men and 75 mg for woman. For smoking adults the RDA's are a little higher - 125 mg per day for a man and 110 mg for a woman. If an individual takes in more than about 200 mg of Vitamin C per day, then temporary conditions such as stomach aches and diarrhoea may result.
Anti-aging effects of Vitamin C
A study was conducted by the London School of Hygiene and Tropical Medicine on the affects of differing levels of Vitamin C in older people. The study involved 1214 individuals aged between 75 and 84 and was followed up over a 4 year period. Samples of vitamin C (ascorbate), vitamin E, vitamin A (retinol) and beta-carotene were taken and detailed questionnaires of dietary habits were also completed by the participants of the study.
The research showed that ascorbate concentrations in the subjects decreased markedly over the course of the study. This indicates that Vitamin C levels naturally reduce as we age. More significantly, the subjects with the highest concentrations of ascorbate (greater than 66 micromol/L) had on average half the risk of dying over the 4 year period compared with those with the lowest levels (less than 17 micromol/L). Also significant here was the fact that the levels of the other tested antioxidants did not seem to correlate with mortality.
For further information on how vitamin C levels in the elderly affect longevity visit http://www.medscape.com/viewarticle/464048
Vitamin C and Cancer
There is a famous study conducted by the researchers Linus Pauling and Ewan Cameron that was published in the early 1970’s. This study compared 100 advanced cancer patients who were treated daily with 10, 000 milligrams of vitamin C with 1000 patients treated by other doctors, whose records were taken from the same hospital. This study showed that the patients treated with the vitamin C survived three to four times longer than those who did not receive this treatment.
This sounds very definite, which is why even today this study is often quoted when trying to push the virtues of vitamin C. Unfortunately however, the story does not end there. The study was not performed in the conventional manner. Usually in this type of study two groups are chosen randomly and treated using pre-defined protocols to give statistically accurate results. In this study however guesses where made on how the control group was treated based on other doctors records. In fact other studies conducted in later years showed no correlation between vitamin C intake and the survival rates of cancer patients. For example the 1978 Mayo Clinic study that was conducted using double-blind methodology and using the same doctors to monitor patients who were given vitamin C and those given a placebo showed that Vitamin C intake had no affect on the survival rate of cancer patients.
In conclusion, vitamin C as a treatment for cancer has so far not been proven.
Vitamin C as an antioxidant
There has recently been some debate as to the status of vitamin C as an antioxidant. One study reported in Nature showed that in some situations this vitamin can function as a pro-oxidant. However most research currently shows vitamin C as a substance that benefits health generally whilst functioning as an antioxidant. It has been found that vitamin C as an aqueous antioxidant can function to scavenge aqueous peroxyl radicals before they have a change to damage lipids (such as those present in cellular membranes and the membranes of Mitochondria).
To comment on vitamin C as as antioxidant or it's role in health and aging, click on this link Discussion
Mark S D'Arcy