Role of micronutrients in sport and physical

Many micronutrients play key roles in energy metabolism and, during strenuous

physical activity, the rate of energy turnover in skeletal muscle may be increased

up to 20-100 times the resting rate. Although an adequate vitamin and mineral

status is essential for normal health, marginal deficiency states may only be

apparent when the metabolic rate is high. Prolonged strenuous exercise

performed on a regular basis may also result in increased losses from the body or

in an increased rate of turnover, resulting in the need for an increased dietary

intake. An increased food intake to meet energy requirements will increase

dietary micronutrient intake, but athletes in hard training may need to pay

particular attention to their intake of iron, calcium and the antioxidant vitamins.

Correspondence to:

Prof. R J Maughan,

Department of

Biomedical Sciences.

University Medical

School, Foresterhill,

Aberdeen AB25 2ZD, UK

For normal health to be maintained, a wide range of vitamins, minerals

and trace elements must be present in adequate amounts in the body

tissues, and the dietary intake must be sufficient to meet the requirement.

Many vitamins and minerals play key roles in energy metabolism, and the

adverse effect of deficiencies of these components is well recognised and

easily demonstrated. Marginal deficiency states may have little effect on

the sedentary individual, but small impairments of exercise capacity may

have profound consequences for the serious athlete. Regular intense

exercise training may also increase micronutrient requirements, either by

increasing degradation rates or by increasing losses from the body.

Consequently, there is a great interest shown by athletes in some of these

dietary components because of their role in maintaining or enhancing

physical performance. There is often, however, a failure to appreciate that

it is not inevitably, or indeed even generally, the case that increasing micronutrient

intake to levels above those that are adequate for maintaining

health will improve athletic performance.

Biological functions of vitamins

The use of vitamin supplementation to enhance performance is based on

the known biological actions of these compounds, and some important

biological functions related to physical activity are summarised in Table 1.

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Table 1 Major biological functions of the vitamins in exercise. Vitamin K is not included in

this table, as no specific role for this vitamin in exercise has been identified

vitamin

A

Thiamin (B,)

Riboflavm (Bj)

Niacin (B,)

Pyridoxine (BJ

Folate

Pantothenic acid

Biotin

B]2 (cyanocobalamin)

Ascorbic acid (C)

D

E

Metabolic role

Antioxidant function

Carbohydrate metabolism

Mitochondrial electron transport (as FAD)

Multiple metabolic pathways (as NAD and NADP)

Amino acid synthesis

Red blood cell synthesis

Oxidative metabolism (as CoA)

Biosynthetic reactions

Red blood cell synthesis

Antioxidant, catecholamine synthesis, tissue repair

Calcium homeostasis

Antioxidant prevention of free radical damage

Many vitamins, particularly the water-soluble vitamins, are involved in

mitochondrial energy metabolism: it is, therefore, intuitively attractive

to believe that supplying additional amounts may be beneficial.

Although information on the influence of vitamin supplementation on

mitochondrial metabolism appears to be relatively scarce, athletes often

see dietary supplements as a form of 'insurance policy' - even when

there is no evidence that a deficiency may exist or that intakes above the

normal level may be beneficial, it may be as well to take supplements

'just in case'. This practice is generally harmless, except perhaps in the

financial sense, but there are some concerns over the possible harmful

effects of excessive intakes of the fat soluble vitamins (A, D, E and K)

over long periods. The water-soluble vitamins are simply excreted if

consumed in amounts in excess of the requirement. By definition, all

vitamins must be supplied in the diet if health is to be maintained, but

deficiency states are rare in the industrialised countries, and the classical

symptoms of deficiency are unlikely to be observed in athletes.

Biological functions of minerals

At least 20 different minerals are required in adequate amounts to sustain

normal function of tissues and cells. Many of these are required in only

trace amounts, but others must be supplied in greater quantities. Deficiencies

of all of these elements are theoretically possible, but, in practice,

deficiencies are generally uncommon, with the possible exceptions of iron,

calcium and, in some parts of the world, iodine. A balanced diet consumed

in amounts sufficient to meet energy requirements will normally

supply all the vitamins in the required amounts, but not all athletes have

a high energy intake and many do not eat a varied diet.

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Micronutrients in sport and physical activity

There has been much interest in magnesium homeostasis in athletes.

Magnesium plays a number of vital roles in the regulation of energy

metabolism, acting as a cofactor and activator for a number of enzymes,

and is also involved in calcium metabolism and in the maintenance of

electrical gradients across nerve and muscle cell membranes. Magnesium is

lost in sweat in concentrations that may be higher than those in the blood,

leading to concern about magnesium deficiency in athletes losing large

amounts of sweat. Magnesium deficiency is often proposed as a cause of

exercise-induced muscle cramps, even though there is no experimental

evidence to support this hypothesis1. In some countries, including

Germany, this idea is so fixed that sports drinks intended for athletes

invariably have added magnesium, even though the same products are sold

in other countries without the addition of magnesium salts. Experimental

magnesium deficiency results in a variety of symptoms, but these do not

include muscle cramp.

Zinc is also involved as a cofactor in many enzyme reactions, and has

many other roles, including promotion of tissue repair processes. Most of

the body zinc content of about 2 g is present in muscle (60%) and bone

(30%). Low concentrations are present in sweat, and exercise may stimulate

urinary loss: this may account for the concern of many athletes, but

there is no evidence that these losses are sufficient to cause concern. Small

amounts of zinc are present in many foods, including both animal and

vegetable products. Copper is another divalent cation with important biological

functions including modulation of enzyme activity and also a role

in the synthesis of haemoglobin, catecholamines and of some peptide hormones.

Once again, deficiencies are rare, as copper is found in a wide range

of foods, including shellfish, liver, whole grain cereals, legumes and nuts.

Selenium has an antioxidant function by virtue of forming an integral

part of the glutathione peroxidase enzyme, helping to protect cells against

the damage that can result from free radicals. There is some evidence to

suggest a role for selenium in protecting against some cancers. In regions

of the world where the soil is low in selenium, vegetables will have a low

selenium content and deficiencies are possible: these are, however, generally

well recognised, and appropriate measures for fortification are in

place, so this should not affect most athletes.

An adequate dietary iodine intake is essential for synthesis of the thyroid

hormones thyroxine (T4) and tri-iodothyronine (T3), and thyroid deficiency

was formerly common in parts of the world where the availability of iodine

is low. Recognition of the role of iodine led to iodination of salt in these

regions. In most European countries, intakes are well in excess of requirements,

and there is no evidence to suggest a greater requirement in

physically active individuals.

A variety of other elements, including cobalt, molybdenum, manganese,

chromium and phosphorus, play important metabolic roles and are

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required in the diet in small amounts. Deficiencies of all of these elements

are sufficiently rare that the possibility of their being encountered in

athletes is negligible. Many of these elements, however, including cobalt,

chromium and phosphate, are used as supplements by athletes, although

there is no evidence either for an increased requirement or for a beneficial

effect of specific supplementation on performance.

A comprehensive review of the role of minerals in exercise performance

and of the requirements of athletes is presented by Clarkson2.

Dietary micronutrient intake in athletes

With regular strenuous training, there must be an increased total food

intake to balance the increased energy expenditure: without this, hard

training cannot be sustained for long. Provided that a reasonably varied

diet is consumed, this will supply more than adequate amounts of protein,

minerals, vitamins and other dietary requirements3. There are of course

always exceptions - as with the general population, not all athletes eat a

varied diet, and some athletes restrict energy intake to maintain low body

weight and low levels of body fat. It must be remembered, however, that

the results of surveys which show intakes of vitamins and minerals below

the RDA in some groups of athletes (most especially female athletes in

sports where a low body fat content is considered essential, including

ballet dancers, gymnasts, and long distance runners) take no account of

the very low body weight of most of these individuals. Indeed the RDAs

are so imprecise that there is generally no attempt to relate the

requirement to body weight. However, it is likely that the increased energy

intake of most athletes will ensure an adequate intake of most essential

dietary components.

There is no good evidence to suggest that specific supplementation with

any of these dietary components is necessary or that it will improve

performance. Deficiencies can only be established by biochemical investigation

or by the identification of specific symptoms as mentioned above.

Where the presence of a specific deficiency is established, this should be

treated wherever possible by directing the individual towards a more

appropriate choice of foods to include those with a high content of the

deficient component. In almost every case, it is possible to meet requirements

from a normal varied diet, and only where clinical signs of an

established deficiency are identified should vitamin or mineral supplementation

be considered. The only exceptions to the generalisation about

the value of dietary supplements in meeting micronutrient needs may be

iron and, in the case of very active women, calcium. There is also some

experimental support for antioxidant supplementation in some situations.

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Micronutrients in sport and physical activity

Iron, haemoglobin and oxygen transport

Iron has a number of functions in the human body, but the principal one

is its role - in the form of haemoglobin - in the transport of oxygen from

the lungs to the tissues where it is required. A fall in the circulating

haemoglobin concentration is associated with a reduction in oxygen

carrying capacity and a decreased exercise performance4. The body stores

some iron in the form of ferritin, and transport around the tissues is

accomplished by another protein, transferrin. The first sign of iron

deficiency is generally a fall in the circulating ferritin concentration.

Anaemia - a low blood haemoglobin concentration - may result from an

inadequate iron intake in the diet, but may also be due to inadequate

absorption of dietary iron, or to a deficiency of vitamins B]2 or folate,

which are both involved in the formation of new red blood cells. The

circulating transferrin level can rise sharply after exposure to any one of a

number of stresses, and cannot be used as an index of iron status.

The observation that VO2max can be increased by artificial elevation of

the circulating haemoglobin concentration, by use of red cell re-infusion

procedures5*6 and, more recently, recombinant erythropoietin, to enhance

performance has focused attention on the possible limitation to oxygen

transport imposed by the oxygen carrying capacity of the blood. Although

these procedures are, quite properly, banned in athletes, the search for

legitimate means of achieving the same end goes on. This explains in part

the popularity of altitude training among athletes, as well as the

widespread use of iron supplementation. In a controlled study, welltrained

subjects received either a sham saline infusion or an infusion of red

cells in a double-blind crossover design7. Blood volume was unchanged 24

h after re-infusion, but haemoglobin concentration was increased by 9%

from 151 to 165 g/1. This was accompanied by a 5% increase in VO7mav

and a 34% increase in treadmill running endurance time (from 7.2 to 9.7

min).

In view of the apparent importance of the oxygen carrying capacity of

the blood for oxygen transport, it seems odd that one commonly observed

adaptation to endurance exercise is a decrease in the circulating haemoglobin

concentration, commonly referred to as sports anaemia. This is not

a true anaemia, however, and the decrease in haemoglobin concentration

is a consequence of the disproportionate increase in plasma volume. The

total circulating haemoglobin mass is usually increased or at least

maintained in the trained state. This may be considered to be an adaptation

to the trained state, but hard training may result in an increased iron

requirement and exercise tolerance is certainly impaired in the presence of

anaemia. Low serum folate and serum ferritin levels are not associated

with impaired performance, however, and correction of these deficiencies

does not influence indices of fitness in trained athletes.

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Stimulation of erythropoiesis - the formation of new red blood cells -

is apparent within the first day of two of exercise training, and a

similarly rapid response is observed on going to altitude. If the body's

iron stores are inadequate at this time, there will certainly be some

impairment of the process of adaptation. Special attention to dietary

iron intake is, therefore, necessary for the sedentary individual who

embarks on a strenuous training programme or for the individual,

whether sedentary or athletic, who plans to spend some time at an

altitude of more than about 1500-2000 m.

Calcium

Osteoporosis is now widely recognised as a problem for both men and

women, and an increased bone mineral content is one of the benefits of

participation in an exercise programme. Regular exercise results in

increased mineralisation of those bones subjected to stress8 and an

increased peak bone mass may delay the onset of osteoporotic fractures;

exercise may also delay the rate of bone loss. The specificity of this effect

is demonstrated by the unilateral increase in forearm bone density

observed in tennis players9.

In athletes training hard on a regular basis, there is likely to be a

decrease in circulating levels of sex steroids in both men and women.

Oestrogen plays an important role in the maintenance of bone mass in

women, and low oestrogen levels cause bone loss10. Many of these women

also have low body fat and, because of their low body mass, also have low

energy (and calcium) intakes in spite of their high activity levels. All of

these factors are a threat to bone health. The loss of bone in these women

may result in an increased predisposition to stress fractures and other

skeletal injury and must also raise concerns about bone health in later

life11. It should be emphasised, however, that this condition appears to

affect only relatively few athletes. Hard sustained training is a relatively

new phenomenon, particularly among female athletes, and it remains to

be seen whether the long-term effects are clinically significant.

For these athletes, as for all individuals, and especially for women, an

adequate calcium intake should be ensured, although calcium supplements

themselves will not reverse bone loss while oestrogen levels remain

low. It must be emphasised that, although there is little scope for harmful

effects, calcium supplements should only be taken on the advice of a

qualified practitioner after suitable investigative procedures have

indicated an inadequate intake. The recommended dietary calcium intake

varies between countries, but for men the recommended intake is

normally about 800 mg/day, and for women about 1200 mg/day. Intakes

of as much as 2000 mg/day are sometimes recommended. Even then,

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alternatives to supplementation, specifically alterations in the selection

of foods to achieve a higher intake must also be considered, and should

be sufficient to meet needs.

Antioxidant nutrients

Athletes engaged in very hard physical training and sedentary

individuals participating in unaccustomed exercise show signs of muscle

damage in the post-exercise period, and there is evidence of free radicalinduced

damage to muscle membranes and subcellular structures. There

is some evidence for an adaptive increase in antioxidant status in

response to regular exercise12, and this may help protect against further

damage. Supplementation of the diet with antioxidant nutrients has

been proposed as a possible way of further reducing the harmful effects

of exercise. Some studies suggest that the severity of muscle damage —

as assessed by circulating levels of muscle-specific proteins — can be

reduced by supplementation with large doses of vitamins A, C and E,

but the evidence is not entirely convincing, and further information is

required before any specific recommendations can be made.

Key points for clinical practice

1 Regular strenuous exercise increases the demand for energy

intake, and a high dietary energy intake will meet the demand for

all micronutrients, provided that a varied diet is consumed.

2 Not all athletes have a high energy intake, and the effects of a

marginal intake may be more apparent in an active individual

than in a sedentary person.

3 Modest levels of exercise stimulate bone mineralisation, but very

hard exercise may promote calcium loss, especially in women.

4 Vitamin supplementation is generally unnecessary, but further

research into the requirements of athletes for antioxidant

nutrients is required.