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Vitamins, Minerals and Supplements

Posted on: Wednesday, 9 March 2005, 03:00 CST

DIETARY SUPPLEMENTS

In the last decade there has been a major health care trend in the increased use of vitamin, mineral, herbal and nutritional supplements in the general population, but conflicting claims have been made about the benefits of such supplementation. While deficiency in certain nutrients can be harmful to general health, and in some conditions extra supplements are recommended, high intakes of supplements can also have negative health effects, including toxicity and drug interactions. In the first of an occasional series, part-time health visitor and freelance journalist JUNE THOMPSON looks at the role of these various nutrients and the evidence for and against the use of dietary supplements

Community practitioner 2005; 78, 1: 24-26

Vitamins

Vitamins are micronutrients, which are essential to the normal lunctioning of the body although they are only required in small amounts.

All natural vitamins are organic food substances found only in plants and animals. With a lew exceptions the body cannot manufacture or synthesise vitamins so they must be supplied in the diet or in dietary supplements.

Some vitamins, along with some minerals, also act as antioxidants.

Recommended dietary allowance

The recommended dietary allowance (RDA) - the amount most healthy people need each day to remain healthy - has been determined for most vitamins, and for some, a safe upper limit has been determined.

Intake above this safe level increases the risk of toxicity.

Quantities needed are measured in milligrams (mg) or micrograms (mcg) (1 milligram = 1000 micrograms), or sometimes international units (IU). 1 mg = 1.5 IU. US recommended intakes may vary from those in the UK, as may those of the European Community (EC).

Types of vitamins

There are two types of vitamins: fat-soluble and water-soluble. Disorders that impair the intestine's absorption of fats such as a malabsorption syndrome can reduce the absorption of fat-soluble vitamins and increase the risk of a deficiency.

Fat-soluble vitamins

These are vitamins A, D, E and K.

Vitamin A precursors (carotenoids) and vitamin E are both antioxidants.

Vitamin A (Retinol)

The term 'vitamin A' refers to a group of fat-soluble compounds known as 'the retinoids'.1

Vitamin A is an essential micronutrient throughout the life cycle.2 It consists of preformed vitamin A (retinol and retinylesters) from animal sources and the vitamin A precursors, the carotenoids such as [alpha] and [beta]-carotene (β-carotene) which are largely plant derived.1,3

Function

Vitamin A is needed for the maintenance of epithelial surfaces, for immune competence, for the normal functioning of the retina, and for growth, embryonic development and reproduction.3

Carotenoids have been shown to act as antioxidants.

Sources

Vitamin A is obtained from animals and plant sources: firstly, from fat soluble vitamin A present in milk, eggs, butter and fish liver oils; secondly as provitamin A carotenoids in dark green leafy vegetables, red palm oil, and in red or orange coloured fruits and vegetables such as mango, papaya, sweet potato and carrots.

Most of the ingested retinol is absorbed in the small intestine and transported to the liver, where it is stored. When required it is released into the blood stream and taken up by target cells throughout the body.3

Other sources include single- or multi-vitamin supplements and synthetic forms prescribed for therapeutic purposes, for example certain skin disorders, or certain eye conditions.1

Beneficial effects

Vitamin A has been suggested to be beneficial when used as a chemo-preventative or adjuvant agent in the treatment of some cancers and may improve lung function in chronic obstructive pulmonary disease.1

Vitamin A is used as a treatment for abnormal dark-adaptation and is included in eye drops used to treat blurred vision, cataracts, glaucoma, conjunctivitis and dry eyes.1 It is also used in the treatment of some skin disorders including acne.

β-carotene and other similar compounds have differentiating properties that appear to affect cell growth and maturation.

Toxicity

Acute vitamin A toxicity in humans is rare, but is more likely to occur following ingestion of high close supplements, rather than following high intakes of vitamin A from food.

Vitamin A accumulates in the body and, therefore, individuals who have regular high daily intakes of vitamin A might suffer adverse effects trom chronic hypervitaminosis A.

Symptoms of acute vitamin A toxicity include abdominal pain, anorexia, vomiting, blurred vision, irritability, headache, and in neonates and infants, bulging of fontanelles.1

Epidemiological studies have indicated that exposure to high levels of vitamin A during pregnancy might increase the risk of birth defects and current advice is that women who are pregnant or who wish to become pregnant should not take dietary supplements containing vitamin A except on medical advice.

Isotretinoin, a synthetic retinoid used in the treatment of severe acne, causes congenital foetal anomalies.3

Recent epidemiological data have indicated that postmenopausal women with long-term high intakes of vitamin A have an increased risk of hip-bone fracture.

Other supporting epidemiological data have indicated that this effect may occur in men as well as women.1

β-carotene is not toxic to the liver even in high doses in contrast to vitamin A. Hypercarotenaemia (high plasma β- carotene) has not been associated with adverse effects other than reversible yellowing of the skin (hypercarotenodermia).1

Deficiency

Vitamin A deficiency is a problem largely in developing countries where it is a major cause of morbidity and mortality in childhood.1,3 It can lead to night blindness and xerophthalmia, growth retardation, keratinisation of epithelia, impaired hearing, taste and smell, increased susceptibility to infection that can lead to death from febrile illnesses, notably measles, and gastroenteritis infection and reduced male fertility.3

In pregnancy, vitamin A deficiency can result in congenital malformations.2

In developed countries vitamin A deficiency is rare but may occur when the intake of dairy products and carotene-rich vegetables and fruit is limited and, occasionally, with malabsorption syndrome.

Reference nutrient intake (RNI)

RNI is the amount of a nutrient that is enough for almost every individual, even someone who has high needs for the nutrient.4

Estimations of vitamin A requirement and food content take into account the different activities and variable absorption of vitamin A and provitamin A, and are expressed in terms of 'retinol equivalent' (RE) because β-carotene is converted to retinol in the body.4

For adults the RNI for vitamin A is 700 g RE/day for adult males and 600 g RE/day for women (0.6 and 0.7mg respectively) (6 g β- carotene = 1 g retinol).4

It is not possible to establish a safe upper level for vitamin A.1

Vitamin A in pregnancy

Both lack of and excess of vitamin A during pregnancy can cause problems.

Maternal insufficiency of vitamin A during pregnancy results in foetal death, severe congenital malformations2 and has been shown to increase the risk of maternal mortality.

Deficiency is also associated with premature birth, intrauterinc growth retardation, low birth weight and antepartum haemorrhage due to abruptio placentae.5

Too much vitamin A as retinol increases the risk of foetal malformation, particularly of cranial-neural-crcst origin.6

Unless the mother is advised otherwise, vitamin A supplements which contain retinol, liver, and liver products such as liver pat, which may contain large amounts of retinol, should be avoided during pregnancy. This includes the vitamin A analogue isotretinoin (Accutane) which is used for the treatment of severe recalcitrant cystic acne.

The drug is contraindicated in women who were or might become pregnant during therapy or in the following month, as it has been demonstrated to cause birth defects - the so-called Accutane embryopathy, consisting of craniofacial, heart, and central nervous system defects. The carotene form of vitamin A is safe.6

Both the lack and excess of vitamin A during embryonic development result in congenital malformations.2

Vitamin E (d-alpha-tocopherol)

Vitamin E is a generic term for a group of eight lipid-soluble compounds synthesised by plants. These compounds fall into two classes, tocopherols and tocotrienols.7 Vitamins in both classes are designated by the Greek letters α β, γ and ∂.

The most biologically active antioxidant is d-α- tocopherol.8

Function

Current information suggests that the effects of vitamin E are consistent with an antioxidant role.8 In this regard, vitamin E is thought to have basic functional importance in helping to protect cell membranes in virtually all cells of the body.

It is unclear whether vitamin E functions solely as a lipid antioxidant, or whether it might also be required for the function of some other critical, but unknown metabolic factor.8

Low vitamin E levels are well documented in many forms of liver disease, including alcoholic liver disease and hepatitis C.

Sources

Vitamin E is synthesised only by plants and is, therefore, found primarily in plant products, the richest source being plant oils such as soya, corn and olive oil. Other good sources include nuts and seeds, green vegetables, and wheatgerm (found in cereals and cereal products) with meat, poultry, and dairy products providi\ng only moderate amounts.7-8

Vitamin E is also present in a variety of dietary supplements at doses of up to 268mg/day in multi-constituent products and 670mg/ day in single constituent products. It is also present in licensed medicinal products; the highest doses authorised are 20-100mg.8

Beneficial effects

It has been claimed that vitamin E can prevent free radical damage.

In a cancer prevention study, the incidence of prostate cancer was 32% lower and mortality was 41 % lower in men taking α- tocopherol, than in those not taking the vitamin supplement.

Vitamin E is thought to have a role in the prevention of arteriosclerosis, through inhibition of oxidation of low-density lipoprotein.

Vitamin E has been reported to relieve the symptoms of fibrocystic breast disease. It has also been claimed that menopausal symptoms may be relieved by vitamin E.8

Vitamin E has been used to treat scleroderma and it has been reported that it may prevent rctrolental fibroplasia and intracranial haemorrhage in premature infants, in animal studies, vitamin E has been found to reduce UV-induced acute and chronic skin damage.8 Vitamin E also has anti-inflammatory properties.

Use of vitamin E and C (ascorbic acid) supplements in combination of high-dose supplements of vitamins E (containing more than 400 IU) and C is associated with a reduction in the prevalence and incidence of Alzheimers disease (AD) in a select elderly population.9

No apparent protective effect was found with the use of vitamin C alone or vitamin E alone.9 Vitamin E intake and carotcnoids are also associated with a reduced risk of type 2 diabetes.10

Until recently, vitamin E has been considered a cardioprotective agent. Currently available evidence from large-scale clinical trials, however, indicates that vitamin E supplementation may confer no material benefit in the primary prevention of cardiovascular disease (CVD) among comparatively well-nourished Western populations.7

Toxicity

Very high doses of vitamin E have been reported to cause a few sporadic adverse effects. These include headache, fatigue, gastrointestinal distress, double vision, muscle weakness and mild creatinuria.

High levels of vitamin E may also antagonise the effects of the other fat-soluble vitamins. Vitamin E also has an anti-platelet and anti-coagulant effect.8

Despite promising animal and early observational studies suggesting a protective effect for development of coronary heart disease and death from coronary disease, large-scale studies have reported disappointing results for vitamin E supplementation.11

Recommended daily amounts

Foods containing large amounts of polyunsaturated fatty acids (PUFAs) will generally contain large amounts of vitamin E. In addition, the requirement for vitamin E increases with the amount of dietary PUFAs consumed.

PUFA intake varies considerably and it has been concluded that daily intakes of 4mg and 3mg of α-tocopherol equivalents could be adequate for men and women respectively,8 but safe intakes have been set at more than these.4

Safe intakes are based on the vitamin E content of breastmilk. Infant formulae should provide not less than 0.3mg/100ml and not less than 0.4mg/g PUFA.4

Intakes of 3.8-6.2mg/day appear to be satisfactory for pregnant and lactating women.8

Vitamin E in pregnancy

Vitamin E deficiency is rare except in malabsorption syndrome, but low concentrations have been associated with abruptio placentae in normal pregnancies.5

The next update in this series will look at other fat-soluable vitamins and the role vitamin D plays in the prevention of rickets.

With a few exceptions the body cannot manufacture or synthesise vitamins so they must be supplied in the diet or in dietary supplements

* There is no evidence for recommending vitamin E and β- carotene supplements for prevention or treatment of cardiovascular disease, including stroke.12

References

1 Food Standards Agency. Safe upper levels for vitamins and minerals: report of the expert group on vitamins and minerals. Part 2. Fat soluble vitamins: vitamin A (Retinol). London: Food Standards Agency, 2003.

2 Zile Maija H. Vitamin A and embryonic development: an overview. Journal of Nutrition 1998; 128: 2, 455S-458S.

3 Potter Andrew R. Reducing vitamin A deficiency. British Medical Journal 1997; 314: 317.

4 Department of Health. Dietary reference values; a guide. London: HMSO, 1991.

5 Ladipo Oladapo A. Nutrition in pregnancy: mineral and vitamin supplements. American Journal of Clinical Nutrition 2000; 72, 1: 280S-290s.

6 Rothman KJ et al. Teratogenicity of high vitamin A intake. New England Journal of Medicine 1995; 333: 1369-1373.

7 Manson JE et al. Does vitamin E supplementation prevent cardiovascular events? Journal of Womens Health 2003; 12: 2, 123- 136.

8 Food Standards Agency. Safe upper levels for vitamins and minerals: report of the expert group on vitamins and minerals. Part 2. Fat soluble vitamins: vitamin E. London: Food Standards Agency, 2003.

9 Zandi PP et al. Reduced risk of Alzheimer disease in users of antioxidant vitamin supplements: the Cache County Study. Archives of Neurology 2004; 61: 1, 82-88.

10 Montonen Jukka et al. Dietary antioxidant intake and risk of type 2 diabetes. Diabetes Care 2004; 27: 2, 362-366.

11 Chung Mina K. Vitamins, supplements, herbal medicines, and Arrhythmias. Cardiology in Review 2004; 12: 2, 73-84.

12 Johnsen Sren P. Intake of fruit and vegetables and risk of stroke: an overview. Current Opinion in Clinical Nutrition & Metabolic Care 2004; 7: 6, 665-670.

Copyright TG Scott & Son Ltd. Jan 2005

Source: Community Practitioner

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