1.7 Vitamin C (ascorbic acid)
What is the condition that results from vitamin C deficiency and what are its symptoms?
You will probably remember from the start of this course that scurvy is the deficiency disease associated with lack of vitamin C and that its symptoms are loss of hair and teeth, bleeding gums, very slow healing of wounds and eventually death.
Vitamin C deficiency causes these symptoms because of its two important roles in the body. Firstly, it is used in the production of collagen, which is found in large quantities in bone, tendons, cartilage and skin, and in smaller amounts in other tissues. And secondly, it is important in enhancing the absorption of iron (which is needed for red blood cells) from vegetable sources.
Relate each of these roles of vitamin C to the symptoms of scurvy.
The symptoms of scurvy can all be largely explained by failure of collagen production, including loss of teeth, which are held in place by connective tissue in the gums. Bleeding gums and very slow healing of other wounds are also caused by lack of collagen, which is needed to hold cells together, including the cells forming the blood vessels, but the poor healing may also be due to the lack of oxygen reaching the tissues due to shortage of iron in the blood (anaemia).
Although we tend to think of scurvy as a disease of sailors long ago, it does still occur in refugee camps where the diet contains insufficient vitamin C, and among homeless people in the UK. Vitamin C is found in vegetables and fruit, especially blackcurrants and oranges. Potatoes are a good source too, though in the modern diet, more is probably obtained from fresh fruit and from fruit juices. Most mammals can synthesise their own vitamin C from glucose and so do not need it in their diet. However, along with humans and most other primates, guinea pigs lack this synthetic capacity. So some of the early research into vitamin C requirements was done on them, which is how ‘guinea pig’ came to mean a test subject in popular speech.
Vitamin C is very soluble in water. It is amongst the least stable of the vitamins and is rapidly destroyed by exposure to light and to air and by heating.
Suggest three ways in which you could maximise the amount of vitamin C retained in cooked vegetables.
You might have thought of some of the following: prepare the vegetables immediately before cooking, so exposing them to the air for the minimum time; cook them in the minimum amount of water (or steam them) for the minimum amount of time; keep the lid on the saucepan to prevent too much contact with the air; serve them immediately; use the cooking water in the meal if possible, since that may contain some of the vitamin C that has been leached out; and eat the vegetables immediately after cooking.
Vitamin C is readily absorbed from the digestive system and the total amount present in the body is typically about 2–3 g. Scurvy results when total body reserves fall below 300 mg. Vitamin C takes part in many of the body's metabolic processes and acts as an antioxidant (destroying free radicals). Vitamin C also appears to be beneficial to the immune system and there is evidence that moderate doses alleviate the symptoms of colds, though there is no convincing evidence that large doses can actually prevent colds. Many studies indicate that higher intake of vitamin C is linked with a lower risk of disease in general.
Studies show that smokers have less vitamin C in their bodies than non-smokers. Why might this be?
It may be because: (i) there is a lower intake – maybe the smoker has a cigarette after a meal, rather than a piece of fruit; (ii) there is poorer absorption in the gut; (iii) smoking generates more free radicals and the vitamin C is used up in destroying them.
Use Table 1, from the beginning of the course, to identify how much vitamin C is needed each day. If a person's diet contained no vitamin C, about how many days would it take for the level to fall low enough for symptoms of scurvy to appear?
Table 1 indicates that 40 mg of vitamin C are needed each day. Assume that the person has an average amount of 2.5 g, or 2500 mg of vitamin C in their body. For scurvy to develop, vitamin C reserves would need to fall below 300 mg, so 2500 mg - 300 mg = 2200 mg would need to be used and not replaced from the diet. At the rate of 40 mg per day, scurvy would appear after 2200 divided by 40 days = 55 days, i.e. in less than 2 months (assuming no vitamin C is available from the diet).
The following activities relate to the whole of this section about vitamins.
Use Table 1 to identify which vitamins are likely to be lost (a) when a piece of frozen beef is defrosted and the water, some of which comes from inside the meat, is thrown away and (b) when fat drips from meat during the roasting process.
(a) Any of the water-soluble vitamins could be lost in the water that drips from the thawed beef. The water-soluble vitamins found in beef are niacin (vitamin B3 ) and some vitamin B12 and pantothenic acid (vitamin B5 ).
(b) The fat-soluble vitamins could be lost in the fat that drips from the meat during cooking, but beef contains very few of the fat-soluble vitamins, just perhaps a small amount of vitamin K.
Which vitamins can be synthesised by cells in the body?
Which vitamins are groups of compounds?
Which three vitamins act as antioxidants? Are any of these synthesised in the body?
Vitamin D can be made in the skin, provided that it is exposed to sufficient sunlight. Vitamin A can be made from β-carotene in the diet, obtained from carrots and from dark green leafy vegetables. Niacin (vitamin B3 ) can be made from the amino acid tryptophan.
Vitamins A, E and K are groups of compounds, as is the vitamin B complex, through the members of this group are given separate names.
Vitamins A, C and E are the vitamins that act as antioxidants. Vitamin A can be made from β-carotene (see above), but the others must be obtained through the diet.
Devise a table to show the similarities between the following pairs of vitamins: (a) riboflavin and niacin; (b) folate and vitamin B12; (c) vitamins E and C.
Consider their sources, functions and signs of deficiency.
Table 3 The similarities between some pairs of vitamins
|Vitamins||Sources||Functions||Signs of deficiency|
|(a) riboflavin and niacin||Both found in milk and eggs.||Both involved in cell metabolism.||Both affect the skin. Riboflavin deficiency causes cracks around the mouth, etc. and niacin deficiency causes dermatitis (one of the symptoms of pellagra).|
|(b) folate and vitamin B12||Both found in yeast extract and breakfast cereals.||Both important in cell division.||Both deficiencies result in anaemia. Folate is linked to megaloblastic anaemia and vitamin B12 to pernicious anaemia.|
|(c) vitamins E and C12||Both found in some vegetables.||Both act as antioxidants by destroying free radicals.||No clear symptoms related to vitamin E. Lack of Vitamin C causes scurvy.|
A mother of three children under 5 is concerned that she is not giving them a balanced diet, since they mostly eat prepared convenience foods. What foods would you suggest that she introduces to their diet to boost their intake of vitamins?
A bowl of cereal with milk at breakfast time, together with eggs sometimes and fresh fruit regularly eaten at other meals would provide a good source of almost all the vitamins. Additionally, introducing some fresh, easy to prepare vegetables, such as broccoli, into their diet would be beneficial.
Have a look at labels on food packets, tins, etc., to see which have information about vitamins. Can you tell whether the food contains the vitamins naturally or whether they have been added by the manufacturer (fortified)?