Vitamin B₁ (thiamin)

Thiamin pyrophosphate is a cofactor in the metabolism of pyruvate to acetyl coenzyme A (CoA), 2-oxoglutarate to succinyl CoA, and in a reaction of the pentose shunt pathway catalysed by the enzyme transketolase. The body contains only about 30 times the daily requirement of this vitamin. Diets high in carbohydrate require more thiamin for their assimilation than diets high in fat, and so, for example, subclinical thiamin deficiency may be unmasked in malnourished patients given glucose intravenously.

Deficiency of vitamin B₁ causes a primarily sensory polyneuropathy (dry beriberi), cardiac failure (wet beriberi), Wernicke’s encephalopathy, characterized by ophthalmoplegia and ataxia and which may progress rapidly to stupor and death, and Korsakoff’s psychosis, of which memory loss is usually the most obvious feature. These can occur alone or in combination. In the UK, the most frequent manifestation is encephalopathy, seen chiefly in chronic alcoholics whose diet is poor.

Wernicke’s encephalopathy responds rapidly to thiamin and, because the vitamin is cheap and non-toxic, this therapeutic response can be used to make the diagnosis. Laboratory tests for deficiency are seldom necessary.

It may, however, be necessary formally to document deficiency in nutritional research. Thiamin concentration can be measured directly in whole blood. Alternatively, a functional assay may be used in which red cell transketolase activity is measured both with and without the addition of thiamin pyrophosphate to the reaction mixture. Enzyme activity may be normal in subclinical deficiency but is increased by the addition of the coenzyme. If the deficiency is clinically obvious, the basal enzyme activity will be low.

An analogous technique can be used to assess riboflavin (vitamin B₂) status by measurement of the red cell enzyme glutathione reductase with and without the vitamin, and pyridoxine (vitamin B₆) using red cell alanine or aspartate aminotransferases. Direct whole blood assays of the vitamins are now also available and are generally easier and cheaper to perform. Deficiency of each of these vitamins (manifest in both cases principally by angular stomatitis, cheilosis and dermatitis) is uncommon in developed countries but may sometimes be seen in alcoholics and grossly malnourished individuals.

Nicotinic acid

Nicotinic acid is the precursor of nicotinamide. This is a constituent of the coenzymes nicotinamide adenine dinucleotide (NAD) and its phosphate (NADP), which are essential to glycolysis and oxidative phosphorylation.

Part of the body’s nicotinic acid requirement is met by endogenous synthesis from tryptophan. The deficiency syndrome, pellagra (comprising an erythematous skin rash that leads to desquamation, gastrointestinal disturbance, particularly diarrhoea, and dementia) can result from either an inadequate dietary intake of nicotinic acid or decreased synthesis. The latter may be a feature of the carcinoid syndrome (see p. 303), in which there is increased metabolism of tryptophan to hydroxyindoles, with consequently less available for nicotinic acid synthesis, and of Hartnup disease, a rare inherited disorder of the epithelial transport of neutral amino acids, due to decreased intestinal absorption of tryptophan from the gut.

Nicotinic acid status can be assessed by measurement of its urinary metabolites, although this is rarely necessary.

Folic acid

A derivative of folic acid is vital to purine and pyrimidine (and hence nucleic acid) synthesis. Folic acid deficiency is relatively common: its most usual manifestation is as a macrocytic anaemia with megaloblastic marrow changes. Folic acid is usually measured by immunoassay, although microbiological assays were widely used in the past. The concentration in red cells reflects the body’s folate reserves, while plasma concentrations reflect recent dietary intake.

It is essential to diagnose the cause of megaloblastic anaemia before it is treated. Giving folate alone to patients with vitamin B₁₂ deficiency risks precipitating or exacerbating the neurological manifestations of vitamin B₁₂ deficiency.

The use of folate supplements in pregnancy to reduce the risk of neural tube defects is discussed in a later section.

Vitamin B₁₂

Vitamin B₁₂ comprises a number of closely related substances called cobalamins, which are essential to nucleic acid synthesis. Deficiency can cause megaloblastic anaemia and neurological manifestations, either alone or together. The neurological features, which may be caused by demyelination, include peripheral neuropathy, subacute combined degeneration of the spinal cord, dementia and optic atrophy.

Dietary deficiency of this vitamin is rare except in strict vegetarians (vegans): considerable amounts are stored in the liver, with the result that deficiency is not common even with severe malabsorption (unless of very longstanding). Vitamin B₁₂ deficiency is most frequently seen in pernicious anaemia. This is an autoimmune disease, in most cases of which there is lack of production in the stomach of intrinsic factor, essential for the absorption of the vitamin from the terminal ileum. Autoantibodies to intrinsic factor are present in 50% of patients with pernicious anaemia and are specific for the condition, whereas parietal cell antibodies, although present in 90% of patients, also occur in many older people with gastric atrophy.

Vitamin B₁₂ is measured in plasma by immunoassay. Subclinical deficiencies of both folate and vitamin B₁₂ increase plasma concentrations of homocysteine which is a risk factor for cardiovascular disease. Decreased availability of vitamin B₁₂ also increases the plasma concentration of methylmalonic acid, measurement of which may be helpful in borderline deficiency of the vitamin.

Vitamin C (ascorbic acid)

Ascorbic acid is essential for the hydroxylation of proline residues in collagen and thus for the normal structure and function of this protein. It is a powerful antioxidant and acts by maintaining iron in the hydroxylating enzyme in the reduced (Fe²⁺) state. Vitamin C also facilitates the intestinal absorption of dietary non-haem iron by keeping it in the Fe²⁺ state. Subclinical deficiency of ascorbic acid is quite often present in elderly housebound people. The concentration of ascorbate in plasma reflects recent dietary intake and is a poor index of tissue stores of the vitamin. These are better assessed by determination of ascorbate concentration in leukocytes. In practice, this is seldom necessary, because ascorbic acid is cheap and non-toxic, so a therapeutic trial of vitamin supplementation is the simplest procedure to confirm suspected vitamin C deficiency. (See also Case history 20.2.)

Vitamin A

This vitamin is a constituent of the retinal pigment rhodopsin. It is also essential for the normal synthesis of mucopolysaccharides and growth of epithelial tissue. Mild deficiency causes night blindness, while in more severe cases degenerative changes in the eye may lead to complete loss of vision. The normal liver contains considerable stores of the vitamin, and deficiency is rarely seen in affluent societies. It is, however, an important cause of blindness in many areas of the world.

Vitamin A is present in the diet and can also be synthesized from dietary carotenes. It can be measured in plasma, in which it is transported bound to prealbumin and a specific retinol-binding globulin. A low binding protein concentration can cause the plasma concentration of vitamin A to be low and impair its delivery to tissues even when hepatic stores of the vitamin are adequate. Measurements of vitamin A status are rarely required in practice, because deficiency is rare in the western world, but may be useful (together with vitamin E) for monitoring adequacy of pancreatic enzyme replacement in children with cystic fibrosis (see Chapter 16). In areas where deficiency is endemic the facilities required to provide laboratory confirmation of the diagnosis are often not available, although the diagnosis is usually obvious clinically.

Vitamin D

Vitamin D is obtained from endogenous synthesis, by the action of ultraviolet light on 7-dehydrocholesterol in the skin to form cholecalciferol (vitamin D₃), and from the diet. Dietary vitamin D is largely ergocalciferol (vitamin D₂), the predominant form in plants. The only important dietary sources are fish and some margarines, which are artificially fortified with vitamin D. Vitamins D₂ and D₃ undergo the same metabolic changes in the body and have identical physiological actions. For this reason, the terms ‘cholecalciferol’ and ‘vitamin D’ are frequently used to refer to both forms of the vitamin.

In most individuals, endogenous synthesis is the major source of vitamin D. Privational (dietary) vitamin D deficiency is seen most frequently in people who also have decreased endogenous synthesis, such as the elderly housebound. It is also seen in the UK in people of south Asian origin, particularly women, in whom the effects of low intake may be exacerbated by decreased production by sunlight due to their traditional clothing and darker skin colour. Binding of calcium in the gut by dietary phytates may also contribute to the osteomalacia to which they are prone. Breast milk contains relatively little vitamin D and infants are at risk of vitamin D deficiency particularly if premature (the vitamin is transported across the placenta mainly in the last trimester of pregnancy) or if the mother is vitamin D deficient. Vitamin D insufficiency (Figure 20.1) is increasingly recognized as a public health concern in Europe and North America. At the end of winter, up to one-third of white adults and two-thirds of dark skinned adults may have suboptimal vitamin D concentrations, especially at more northern latitudes.

Figure 20.1 Typical ranges for the interpretation of vitamin D concentration reported by a UK laboratory.

Cholecalciferol itself has little physiological activity. It is hydroxylated first in the liver to 25-hydroxycholecalciferol (25-HCC, calcidiol) and then in the kidney to 1,25-dihydroxycholecalciferol (1,25-DHCC, calcitriol). These metabolites are transported in the circulation by a specific binding protein. Calcitriol is a hormone of vital importance in calcium homoeostasis; its actions and the control of its production are discussed in Chapter 12.

Vitamin D status can be assessed in the laboratory by measurement of the plasma concentration of calcidiol, the major circulating metabolite. This undergoes seasonal variation, being higher in the summer than in the winter. The definition of vitamin D sufficiency has been much debated, but concentrations at which there is no secondary rise in circulating parathyroid hormone are often taken to be optimal. Figure 20.1 shows typical reporting ranges from a UK laboratory.

Decreased synthesis or dietary deficiency of vitamin D causes rickets in children and osteomalacia in adults. Other causes include disordered metabolism of cholecalciferol and malabsorption. The clinical biochemistry of rickets and osteomalacia is considered in more detail in Chapter 15. Vitamin D insufficiency is strongly associated with decreased bone density and an increased risk of fractures, especially in women.

The role of vitamin D is not confined to calcium and bone metabolism: it is also involved in cellular differentiation, particularly of immunocompetent cells. Vitamin D insufficiency has been linked with an increased risk of developing cardiovascular disease, diabetes mellitus, breast and gastrointestinal cancers, infections such as tuberculosis and influenza, autoimmune disorders such as rheumatoid arthritis, and possibly multiple sclerosis. Whether vitamin D insufficiency is a direct cause of these disorders is debatable, but there is a definite inverse association of vitamin D concentration and all-cause mortality in the general population.