1.1 Introduction to vitamins and why we need them
Before the 19th century, one of the hazards of long sea voyages was a condition called scurvy, whose symptoms were loss of hair and teeth, bleeding gums, very slow healing of wounds, and eventually death. Hundreds of sailors and explorers died from scurvy until a Scottish physician, James Lind, in the 1750s discovered that adding a daily portion of citrus fruit to the rations of those at sea could prevent the condition, whereas adding cider, vinegar or various other substances that he tested, could not. In those days, it was considered that a disease was caused by something bad in the diet, or in the air, but not by the absence of something good, so despite Lind's evidence, his ideas were not accepted by his fellow physicians. Additionally, he was unable to confirm his work by experiments on land since, although he tried to restrict the types of food eaten by a group of volunteers to attempt to produce scurvy in them, he was unable to do so, probably because it can take several months for the condition to develop, and in that time his volunteers occasionally cheated on their diet. However, though he died disillusioned, Lind had actually discovered the importance and source of vitamin C.
Before their detailed chemical structures were known, vitamins were named by being given a letter. They are generally still referred to by that letter, as well as by their chemical name; for example, vitamin C or ascorbic acid. There are two main groups of vitamins: fat-soluble vitamins and water-soluble vitamins. The body can store fat-soluble vitamins, but any excess water-soluble vitamins are easily removed from the body in the urine, so regular intake is necessary. Vitamins are, however, needed in only very small quantities.
The daily requirement of certain vitamins is much less than 1 mg (1mg is one-thousandth of a gram), and so is measured in micrograms per day, written as μg per day, where 1 μg is one-thousandth of a milligram. The values for the daily requirements of vitamins are regularly updated as more information becomes available. The values given in Table 1 are those recommended by the UK Government's Food Standards Agency early in 2005.
Table 1 Vitamins essential for human health, reference nutrient intake (RNI) values taken from the UK Food Standards Agency website, and the main dietary sources of these vitamins.
|Name||RNI values for adults per day||Main dietary sources|
|vitamin A||0.6 mg for women; 0.7 mg for men||liver, cheese, eggs, butter, oily fish (such as mackerel), milk, fortified* margarine, yoghurt|
|vitamin D||0.01 mg (10 μg) for certain groups, e.g. pregnant women, those who rarely go outside, etc.||oily fish, liver, eggs, margarine, some breakfast cereals, bread, powdered milk|
|vitamin E||3 mg for women; 4 mg for men||plant oils (such as soya,corn and olive oil), nuts, seeds, wheat germ, some green leafy vegetables|
|vitamin K||0.07 mg(70 μg), or 1 μg per kg of body weight||green leafy vegetables (such as broccoli and spinach), vegetable oils, cereals; small amounts can also be found in meat (such as pork),and dairy foods (such as cheese)|
|thiamin (vitamin B1 )||0.8 mg for women; 1 mg for men||pork, vegetables, milk, cheese, peas, fresh and dried fruit, eggs, wholegrain breads, some fortified* breakfast cereals|
|riboflavin (vitamin B2 )||1.1 mg for women; 1.3 mg for men||milk, eggs, fortified* breakfast cereals, rice, mushrooms.|
|niacin (vitamin B3 )||13 mg for women; 17 mg for men||beef, pork, chicken, wheat flour, maize flour, eggs, milk|
|vitamin B6 (pyridoxine)||1.2 mg for women; 1.4 mg for men||liver, pork, chicken, turkey, cod, bread, whole cereals (such as oatmeal, wheatgerm and rice), eggs, vegetables, soyabeans, peanuts, milk, potatoes, breakfast cereals|
|folate (folic acid, vitamin B9 )||0.2 mg, but 0.4 mg extra for women who are, or plan to be, pregnant||broccoli, sprouts, spinach, peas, chickpeas, potatoes, yeast extract, brown rice, some fruit (such as oranges and bananas),breakfast cereals, some bread|
|vitamin B12 (cobalamin)||0.0015 mg (1.5 μg)||meat (particularly liver), salmon, cod, milk, cheese, eggs, yeast extract, some breakfast cereals|
|pantothenic acid (vitamin B5 )||none given – should be sufficient in normal diet||chicken, beef, potatoes, porridge, tomatoes, liver, kidneys, eggs, broccoli, wholegrains (such as brown rice and wholemeal bread), some breakfast cereals|
|biotin (vitamin H)||0.01–0.2 mg||meat (such as kidney and liver), eggs and some fruit and vegetables, especially dried mixed fruit|
|vitamin C (ascorbic acid)||40 mg||wide variety of fruit and vegetables, especially peppers, broccoli, sprouts, sweet potatoes, cranberries, citrus fruits, kiwi fruit|
Use Table 1 to answer the following questions.
Which vitamin is needed in the greatest daily amount and how much of that vitamin is needed?
Which other vitamins are needed by men in amounts of more than 1 mg per day?
Which vitamins are needed in amounts of 100 μg or less per day?
Vitamin C is needed in the greatest amount, namely 40 mg per day.
The other vitamins needed by men in quantities of more than 1 mg per day are vitamin E and the B vitamins, i.e. thiamin, riboflavin, niacin and vitamin B6
100 μg is 0.1mg, so vitamins needed in less than this quantity are vitamin D (10 μg for certain groups of people), vitamin K (about 70 μg), vitamin B12 (1.5 μg) and biotin (between 10 μg and 200 μg neeeded).
On a breakfast cereal packet, the nutrition information states that a 40 g serving of the cereal, with 125 g semi-skimmed milk, provides the following vitamins:
|Vitamins||Amount per serving||% RDA|
|thiamin (B1 )||0.4 mg||30|
|riboflavin (B2 )||0.6 mg||40|
|vitamin B6||0.6 mg||30|
|folic acid||110 μg||55|
|vitamin B12||0.75 μg||75|
What is meant by % RDA in the heading of the third column here?
What is the RDA of thiamin in mg? How does this value compare with the information in Table 1 ? Suggest possible reasons for any difference.
Based on the information in Table 1, which of these vitamins are likely to be present only in the cereal, which only in the milk and which could be present in both?
RDA is the recommended daily allowance, so the values in the third column are the percentage of that amount that would be obtained from a bowl of this cereal with milk.
Knowing that 0.4 mg of thiamin represents 30% of the RDA, then the task is to calculate 100% of the RDA.
If 30% RDA = 0.4 mg, then 1% is 0.4 divided by 30 mg and 100% is 0.4 divided by 30 x 100 mg = 1.3 mg. So the RDA of thiamin is 1.3 mg.
Table 1 gives an RNI value for thiamin of 0.8 mg for women and 1 mg for men, so the value quoted on the cereal packet is significantly greater. The reasons for the discrepancies are unclear. It could be that the recommended daily amount of thiamin has been reduced recently, and the cereal packet information has not been updated. It may be that the cereal manufacturer is using USA rather than UK values, which may be different. Or it may be a simple error in a calculation somewhere.
If you did similar calculations for the other vitamins, you would discover similar discrepancies, though sometimes the RDA values on the cereal packet are greater and sometimes smaller than the values in Table 1. This example illustrates the problems with numerical, and other, information in the nutrition area. It is often very difficult to know how much reliance to place on a particular set of figures, or on particular information.
Thiamin, riboflavin, vitamin B6 and vitamin B12 are likely to be present in both the cereal and the milk. Folic acid is probably present only in the cereal. Niacin appears to be present only in the milk.
Looking down the main dietary sources of the vitamins in Table 1, which one food contains the most vitamins? Which vitamins does this food not contain? Suggest an explanation for why this food is so rich in vitamins.
Eggs appear as a source of 9 of the 13 vitamins. Only vitamins E, K, folate (vitamin B9 ) and C are absent. Eggs are the food source for the developing chick and, as such, must contain all the substances needed to build its body, amongst which are the vitamins. For similar reasons, milk is also rich in vitamins.
The following sections consider in turn the fat-soluble vitamins and the water-soluble ones. Vitamins have complex chemical structures, and so, apart from a brief look at vitamin A, we will not be dealing with their chemistry here.