vitamins

vitamins In present day Western society, the small print on packets of cereals and of citrus drinks is a ready source of information on the vitamins they provide, the quantity contained therein, and the fraction which that content provides of the recommended daily allowance (RDA). In many homes there is also a stock of packaged tablets or bottled syrups, swallowed mostly on the principle that if a little of these stuffs is necessary for health, then more must be better. In some instances this may possibly be so, but by no means to the extent that vitamin supplements are sold and bought. A vitamin is an organic substance that is necessary only in very small amounts (RDAs all less than 200 mg) for normal growth and health and which must be obtained from the diet because it cannot be synthesized by the body. But, as with many original definitions of this sort, exceptions have emerged with the progress of research.

There were three diseases that during the eighteenth and nineteenth centuries began to be linked in some way to the diet. The cure of scurvy in a ship's company by provision of citrus fruit dates back to 1754, to a historic, but initially neglected experimental observation: men with the disease were divided into pairs, and each pair given different additions to their rations; those, and only those, who received oranges and lemons recovered. The scurvy was successfully avoided with the issue of fruit and vegetables on Captain Cook's voyages in the 1770s, and official introduction of lemon juice into the Royal Navy diet followed before the end of the century. Understanding of the reason for this was to take more than a century. The occurrence of pellagra started to be associated from the mid 1700s with a poor diet of maize-meal without meat or milk in different parts of the world, and this link was still occurring in the US well into the twentieth century. The condition of beri-beri was prevalent in nineteenth-century Asian workers who lived mainly on rice that had been ‘improved’ by new machinery which removed the husks; also chickens who ate rice without its husks developed a peripheral nerve abnormality like that in the human disease. It was at first assumed that some toxin had been introduced by the cooking or polishing. But husks — or an extract made from them — were later shown to cure the condition in both people and chickens. Thus the notion gradually arose that diseases might be caused by an absence of something, rather than the presence of a poison or infection.

The table shows the main sources of each vitamin, and main features of its deficiency. Most of the names refer to a group of substances with similar action.

Substance(s)	Source	Deficiency effects	SEE ALSO
Fat-soluble vitamins
A retinol	Carotenes from	Night blindness.	vision
	plants. Animal/fish	Thickened cornea.
	fats and liver.
D calciferols	Animal fats.	Bone softening: rickets	bone, calcium,
	Fish liver oils.	in childhood,	skin, steroids,
	Skin and sun.	osteomalacia in adult.	sun, teeth
E tocopherols	Most foods.	Wide range proposed;
	Vegetable oils.	no clear evidence of
		specific effect.
K quinones	Plants. Also made	Slow blood clotting,	blood
	by gut flora.	tendency to bleed.
Water-soluble vitamins
B group
B1 thiamin	Grains and pulses.	Beri-beri = ‘extreme	disease
	Pork and offal.	weakness’; peripheral
		neuritis, oedema.
B2 riboflavin	Most foods.	Sore tongue and lips;
		visual impairment.
B6 pyridoxin	Most foods.	Skin lesions, convulsions.
B12 cobalamin	Animal products	Pernicious anaemia;	blood
	only. Requires	neurological disorder.	anaemia
	intrinsic factor for		stomach
	absorption.
Biotin	Liver, yeast, etc.	No specific syndrome.
Folic acid	Liver, greens,	Anaemia.	anaemia
	yeast.	Defective growth.	antenatal development
		Congenital CNS defects.
Niacin	Meat, liver, grains,	Pellagra = ‘rough skin’;
	pulses.	nerve, bowel, and mental
		abnormalities.
Pantothenic acid	Most foods	General illness;
		no specific syndrome.
C ascorbic acid	Fruit and	Scurvy: bruising and	collagen
	vegetables.	bleeding, impaired	wound healing
		healing.

The concept of necessary substances in the diet — in addition to the major energy-providers and minerals — whose absence led to disease, entered several minds, and led to many clinical and experimental studies in the early twentieth century. An early pioneer was the British biochemist Frederick Gowland Hopkins, later a Nobel prize winner. He, among others, found that a combination of all known nutrients was not sufficient to allow young animals to thrive. The word ‘vitamine’ was invented in 1911 by Funk, a biochemist also working in London, who believed, rightly, that he had isolated a vital dietary constituent from rice husks — and, wrongly, that it was by chemical nature an amine. Such extracts became known as vitamin B, and the original name without the ‘e’ came to be applied to the family of substances that emerged over the following decades as necessary for health, but required only in very small quantities. In the 1920s ‘vitamin B’ proved to be more than one substance; B₁ was identified as the anti-beri-beri factor, and others with comparable properties, equally essential, were later added to the group. Knowledge emerged, thanks to a young American physician, William Castle, of a vitamin (B₁₂) necessary for blood cell production, which needed a gastric secretion to allow its absorption — explaining the empirical observation that very large amounts of raw liver were needed to cure pernicious anaemia. Vitamin C — which prevented scurvy — was finally identified in lemon juice in 1932, was found to be readily destroyed by heat, was given the name of ascorbic acid, and became the first vitamin to be artificially synthesized. Meanwhile, some factor in fish oils and egg yolks became recognized as being necessary for growth and health, and was initially called vitamin A. Extracts proved effective against rickets, and this component was separately labelled vitamin D. Thus by the time of World War II the main vitamins had been identified, and their importance recognized; national programmes such as vitamin additions to staple foods and provision of enriched juices for infants and children supported wartime public health; vitamins had also become a major commercial proposition.

Each vitamin is a requisite for some essential metabolic function, and most cannot be synthesized in the body. The exceptions are vitamin D, which can be made in the skin in the presence of sunlight; vitamin A, which need not as such be in the diet, since it can be made from carotenes present in vegetables; and vitamin K, which can be made by bacteria in the gut.

The naming of the vitamins does not appear very logical, for various historical reasons. They are described in two categories — water-soluble and fat-soluble. The significance of this distinction relates to their sources, their absorption, storage, and excretion, and to the manner of their action.

The water-soluble vitamins comprise vitamin C and the ‘B group’ (see table). Vitamin C is necessary for collagen synthesis, the basis of connective tissue growth and maintenance. Those of the B group act by forming co-enzymes, essential for a range of vital, enzyme-dependent cellular processes. These include DNA synthesis, different components of the utilization of nutrients, and the respiratory and energy-releasing processes. Folic acid and B₁₂ specifically are essential for the formation of normal red blood cells. B₁₂ is unique in requiring the intrinsic factor secreted by glands in the stomach to enable it to be absorbed from the intestine, and also in being obtainable only from animal sources.

The fat-soluble vitamins are A, D, E, and K. Fat-solubility means not only that their sources are in fats in the diet, but also that they can be stored in adipose tissue and can have actions on cells by dissolving in the lipid cell membranes. Deficiency may occur not only in malnutrition, but also in any condition that interferes with fat absorption, including liver disease because of the role of bile. Unlike water-soluble vitamins, their storage means that signs of deprivation are delayed — and also that over-dosage can become toxic.

For many of the vitamins there are distinctive abnormalities related to their deficiency, which can be reversed by an adequate intake. Deficiencies occur in undernourished or malnourished communities or individuals; in alcoholism, poverty, and neglect in the aged in any society; and in strict vegetarians. Apart from the treatment of deficiency conditions, the question remains open, and the subject of research, as to whether there is any advantage to be gained from supplements in excess of the minimal adequate amounts. Since the relatively recent understanding of the damaging effect of free radicals, the counteracting antioxidant action of vitamins A, C, and E has been recognized; the fat-soluble and water-soluble vitamins may combine to protect against free radical damage particularly to cell membranes, and thus to diminish processes such as arteriosclerosis and others associated with ageing. Some of the B vitamins (folic acid and B12) may have a role in protecting from heart disease. There has been contradictory evidence concerning protection from the common cold or alleviation of its symptoms by large doses of vitamin C. On the other hand, there can be serious ill-effects from overdose of the fat-soluble vitamins and also of vitamin C.

Sheila Jennett

Bibliography

Carpenter, K. J. (1986). The history of scurvy and vitamin C. Cambridge University Press.
Roe, D. A. (1973). A plague of corn: the social history of pellagra. Cornell University Press, Ithaca NY.

See also biochemistry; enzymes.