VitaminsEffects of Exercise on Requirements

Chapter 21
Vitamins: Effects of Exercise on Requirements
JIDI CHEN
Introduction
Vitamins are a group of organic compounds
required in tiny amounts in the diet of humans
for proper biological functioning and maintenance of health. Vitamins do not supply energy,
but act mainly as regulators of the numerous and
diverse physiological processes in the human
body including vision, skin integrity, bone
ossification, DNA formation, metabolism of
carbohydrate, fat, and proteins, mitochondrial
metabolism, utilization of oxygen in the cells, red
blood cell (RBC) formation and other functions
which are closely related to energy production
and resultant physical performance (see Chapter
20). The human body is not able to synthesize the
majority of the vitamins or the amount synthesized in the body cannot meet the needs. It is
clear that a certain amount of each of the vitamins is essential in the diet and that lack of a
specific vitamin can cause a specific deficiency
disease (Williams 1985; Daniel 1991; Chen & Wu
1996). Due to the many and varied roles of vitamins, they are probably the most widespread
nutrients taken as supplements by both the
general and athletic population. Furthermore,
vitamins meet with great interest in the world of
sports because of their supposed role in enhancing physical performance (Williams 1989; van
der Beek 1991, 1994; Singh et al. 1992b; Weight et
al. 1998a, 1998b).
Exercise enhances energy metabolism and
increases the total energy expenditure which
gives rise to a number of concerns:
• Does exercise training result in increased
needs or deficiencies of vitamins?
• Is the vitamin status of athletes normal?
• Is it necessary for athletes to take vitamin
supplements?
The answers to these questions have varied
through the years and the balance of opinion
continues to change as new evidence appears
(Clarkson 1991, 1995; van der Beek 1991;
Fogelholm 1994; Armstrong & Maresh 1996). The
focal points of the concerns for vitamin nutrition
of athletes are the assumption that athletes
need an increased vitamin intake, the optimum
recommended dietary allowance (RDA) for athletes under different conditions, and the true
effects of vitamin supplementation on physical
performance.
It is accepted that the prevalence of vitamin
deficiency diseases is low in the general population in industrialized societies. Theoretically, the
athlete may have an increased requirement for
dietary intake of vitamins induced by decreased
absorption in the gastrointestinal tract, increased
excretion in sweat, urine and faeces, increased
turnover, as well as the adaptation for the initial
stage of vigorous training and/or acute physical
exercise which may enhance energy metabolism
(van der Beek 1991, 1994). It is generally agreed
that moderate physical activity per se does not
adversely affect vitamin status when recommended amounts of vitamins are consumed in
the diet (Clarkson 1991). Marginal vitamin deficiencies have been observed in athletes, but
many of the published reports of vitamin defi-
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nutrition and exercise
ciencies in athletes are invalid for methodological reasons: a shortfall in recommended intake
relative to published recommended intakes is
not indicative of a deficiency (van der Beek 1991,
1994; Chen & Wu 1996). Athletes with a balanced
diet should receive the RDA provided energy
intake is sufficient to balance expenditure
(Shoorland 1988; Rokitzki et al. 1994a), although
it must be recognized that not all athletes have a
high energy intake, and not all eat a varied diet.
Unfortunately, there are limited and conflicting
data with regard to the micronutrient status of
physically active individuals (van der Beek 1985;
Belko 1987; Fogelholm 1992). Methods for the
assessment of vitamin status are often inadequate, as outlined in the preceding chapter.
Dietary surveys and food records have been used
to assess the vitamin status of athletes, but tables
of vitamin content are inherently unreliable, and
the vitamin loss attributed to vitamin availability, processing, storage, and preparation of the
foods is often not taken into account. Blood or
other tissue levels of vitamins are affected by
several factors including acute exercise and they
may not be entirely accurate as measures of the
nutritional status of athletes; caution should
therefore be taken in interpreting the results
(see Chapter 20). Furthermore, the RDAs are
designed primarily to avoid nutritional deficiencies and do not focus on exercise or stressful
environments. The RDA is determined by
various professional bodies to designate the level
of intake of a micronutrient that will meet the
known nutritional needs of practically all
healthy persons (Armstrong & Maresh 1996),
and is based on an average-sized person with an
average amount of physical activity and an
average physiological requirement. This is then
adjusted by a variable factor to compensate for
incomplete utilization by the body, the variation
in requirements among individuals and the
bioavailability of the nutrients from different
food sources (US National Research Council
1989). The definition of RDA is not identical for
all nations and organizations (van der Beek
1991).
It is not clear that the RDA established for
the general population may apply to athletes,
labourers, or soldiers in heavy training
(Shoorland 1988). There have been few reports
on the setting of separate RDAs for athletes
(Yakovlev 1957; Sports Science Committee of
Japanese Association 1977; Grandjean 1989;
Chen et al. 1992). The majority of dietary surveys
conducted on athletic population clearly indicated that the vitamin intakes of all but a small
minority of athletes exceed the RDA levels if a
well-balanced diet is typically consumed (Sobal
& Marquart 1994). Although vitamin intakes of
less than the RDA do not indicate vitamin deficiencies per se, the further the intake falls below
the RDA, the greater is the risk of developing a
deficiency state.
Athletes have been targeted as a significant
group for vitamin supplements, and dietary
surveys and questionnaires of athletes confirm
the widespread use of the vitamin supplements.
Questionnaires completed by 2977 college and
high school athletes have found that 44% of those
surveyed took one or more vitamin supplements
(Parr et al. 1984). In other, smaller surveys, 31% of
80 Australian athletes and 29% of 347 non-elite
runners (Nieman et al. 1989) and 42–43% of football players, gymnasts and runners (Sobal &
Marquart 1994) took vitamin supplements. Some
studies have documented even higher percentages of athletes taking supplements, including
71% of female runners (Clark et al. 1988) and
100% of female bodybuilders (LamarHildebrand et al. 1989). Supplementation is purported to enhance performance, delay fatigue,
and speed up recovery by some ill-defined
ergogenic mechanism. Despite the lack of evidence that large intakes of vitamin have positive
effects on performance, many athletes are still
taking vitamin supplements, because of a lack of
nutritional knowledge and lack of familiarity
with the dietary guidelines, and quite a number
take very large doses. The concern is that not
only the amounts of supplementation can be
financially costly because of the large doses of up
to 5000 times the recommended levels, but there
vitamins: effects of exercise on requirements
is also the possibility that these excessive
amounts may be harmful to health (see Chapter
20).
In the following sections, for each of the
vitamin, there will be a brief description of
exercise-induced changes in vitamin status and
requirements, the effects of vitamin supplements, harmful effects of overdoses of vitamin
intakes and the main food resources of vitamins
(see Chapter 20). Vitamins are commonly classified into two groups, the fat soluble and the
water soluble. Vitamins A, D, E and K are fat
soluble. Vitamin C and members of the vitamin B
complex are water soluble. Fat-soluble vitamins
can be stored in appreciable amounts in the body,
and their function is largely independent of
energy metabolism. Water-soluble vitamins are
not stored in large quantities in the body and
must be ingested on a regular basis. Clinical
symptoms can be developed in individuals with
a diet deficient in B vitamins. Vitamin B12 can be
stored in the liver for a year or longer.
Thiamin (vitamin B1)
Vitamin B1 as thiaminpyrophosphate (TPP,
cocarboxylase), plays an important role in the
oxidative decarboxylation of pyruvate to acetylcoenzyme A (CoA) for entry into the Krebs cycle
and subsequent oxidation to provide for adenosine triphosphate resynthesis. Thus, there is a
possibility that the increased demand for acetylCoA during exercise would not be met in athletes
with a thiamin deficiency. If this occurred, more
pyruvate would be accumulated and converted
to lactate, with the possibility that fatigue would
develop more rapidly and aerobic performance
could be impaired. Thiamin deficiency could
also result in a reduced availability of succinate,
a coingredient of haeme, leading to inadequate
haemoglobin formation, another factor that
could influence aerobic exercise capacity. However, little evidence has shown that ingestion
of a vitamin B1 supplement by athletes consuming a well-balanced diet has any effect on
performance.
283
It has been noted that there is a good linear
relationship between thiamin intake and energy
intake (van der Beek 1994). It is generally
accepted that the vitamin B1 requirement is
dependent on the total energy expenditure and is
influenced by carbohydrate intake because
vitamin B1 is essential for the intermediary
metabolism of carbohydrate. The vitamin B1 necessary to meet the body’s requirement intake
may vary according to energy intake (Clarkson
1991), and 0.5 mg thiamin · 4.2 MJ–1 (1000 kcal–1) is
recommended for adults in most countries (US
National Research Council 1989). Any increased
requirement induced by exercise should be met
by increased energy intake and well-balanced
diet. However, reports from the Soviet Union
in the early days indicated that the output of
urinary vitamin B1 of athletes decreased as the
training load increased; it was reported that
blood pyruvate levels increased by 30–40% as
compared with sedentary individuals when
vitamin B1 intake was 2–3 mg · day–1 (Yakovlev
1957). In order to keep pyruvate at normal levels,
it was recommended that the vitamin B1 intake
should be 3–5 mg · day–1 in the general population and 5–10 mg · day–1 for athletes undergoing
endurance training (Yakovlev 1957). Vytchikova
(1958) indicated that the usual content of thiamin
1.5–2.0 mg · day–1 in food rations of athletes is
considered insufficient and that medical observation recommend approximately 10–20 mg
daily supplementation.
Athletes do not have a lower intake of vitamin
B1 than the RDA and only very few have any
signs of a biochemical deficiency (Fogelholm
1992), but athletes who are on energy-restricted
diets for weight control are likely to have a less
than adequate intake, and athletes who take
a high percentage of their energy from low
nutrient-density food such as candy, soda, etc.
may be at risk (Clarkson 1991). Nutrition surveys
in Chinese elite athletes indicated that about half
of the athletes investigated had vitamin B1
intakes that were lower than the RDA. The
average dietary intakes of vitamin B1 of the
athletes undergoing vigorous training was 0.37–
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nutrition and exercise
Table 21.1 Vitamin RDA for Chinese athletes. From Chen et al. (1992).
Condition
Vitamin A
(RE)*
Vitamin B1
(mg)
Vitamin B2
(mg)
Niacin
(mg)
Vitamin C
(mg)
Training
1500
3–6
2.5
25
140
Special condition†
2400
5–10
2.5
25
200
* RE, retinol equivalent.
† Special condition refers to intensive vision for vitamin A, endurance training for vitamin B , competition period
1
for vitamin C.
0.48 mg · 4.2 MJ–1 (1000 kcal–1), and 25% of them
have been found to be in a state of vitamin B1
insufficiency as assessed by TPP method (blood
transketolase coefficient) (Chen et al. 1989). In
addition, systematic nutritional investigation
showed that there has been a trend towards a
decrease in vitamin B1 intake because the consumption of cereal, especially whole grains, has
decreased and the intake of animal foods has
increased. The RDA of vitamin B1 for Chinese
athletes has been set at 3–6 mg · day–1, which is
about 1 mg · 4.2 MJ–1 (1000 kcal–1) (Table 21.1)
(Chen et al. 1989, 1992). The US National
Research Council reported that the increased
need of vitamin B1 for athletes should be met by
the larger quantities of food consumed. There
has been no evidence for vitamin B1 toxicity
through oral ingestion. Good food sources of
thiamin are identified in Chapter 20.
Riboflavin (vitamin B2)
Riboflavin functions as a coenzyme for a group
of flavoproteins concerned with cellular oxidation: flavin adenine dinucleotide and flavin
mononucleotide are the most common, and these
act as hydrogen carriers in the mitochondrial
electron transport system, being a component of
oxidative enzymes, and are thus considered
important for aerobic endurance activities. These
coenzymes may also be important for the efficient functioning of glycolytic enzymes, and may
have an effect on anaerobic type performance as
well. The RDA of vitamin B2 for adults is 1.5–
1.7 mg · day–1 for males and 1.2–1.3 mg · day–1 for
females (US National Research Council 1989;
Chinese Nutrition Society 1990). Since riboflavin
is a component of several respiratory enzymes,
the requirement is usually linked to energy
intake, and the level of riboflavin intake recommended by the WHO is 0.5 mg · 4.2 MJ–1
(1000 kcal–1). The RDA of vitamin B2 for Chinese
athletes has been set at 2.5 mg · day–1 (Chen et al.
1992). Exercise training may increase the need of
vitamin B2. Using a RBC enzyme as an indicator
of riboflavin status, it was noted that at an intake
of 0.6 mg · 4.2 MJ–1 (1000 kcal–1), young women
who started a jogging programme developed a
riboflavin deficiency (Belko et al. 1983), although
riboflavin supplements have not been shown to
have an effect on physical performance or
aerobic capacity (Belko 1987). Most athletes have
an adequate or greater than adequate intake of
vitamin B2 (Guilland et al. 1989; Burke & Read
1993), although biochemical insufficiencies were
found for some athletes. The incidence of
vitamin B2 insufficiency has been reported
to be relatively lower than that for thiamin
(Haralambie 1976; Chen et al. 1989, 1992). Overdose problems have not been reported, and there
is no evidence of toxicity. The possibility of
riboflavin deficiency should be a concern for the
vegetarian athlete if all dairy foods and other
animal protein sources are omitted. Good
sources include wheat germ, yeast, green leafy
vegetables, and enriched cereals (see Chapter
20).
Niacin (nicotinamide, nicotinic acid)
Niacin is a component of two important cofactors: nicotinamide adenine dinucleotide and
vitamins: effects of exercise on requirements
nicotinamide adenine dinucleotide phosphate
which serve as hydrogen acceptors and donors
in glycolysis, fatty acid oxidation, and in the electron transport system. Niacin deficiency may
possibly impair glycolysis and/or the oxidation
processes of the citric acid cycle, so both anaerobic and aerobic type performances may be
affected adversely. On the contrary, niacin supplementation in high doses may suppress free
fatty acid release through decreased lipolysis
which would result in a decreased availability of
a major fuel source, forcing the muscle to rely
more on its glycogen stores, and this may also
adversely affect prolonged exercise performance
(Williams 1985; Clarkson 1991).
The RDA for niacin is expressed as niacin
equivalent because niacin may be synthesized
in the body from tryptophan (1 mg of niacin is
equivalent to 60 mg of dietary tryptophan). The
requirement for niacin also usually is linked to
energy intake which means that athletes who
have a large energy intake need a proportionally
higher niacin intake. The RDA for niacin in the
general population has been set at 6.6 mg ·
4.2 MJ–1 (1000 kcal–1) or about 14–19 mg · day–1 for
adults. The RDA for niacin for Chinese athletes
has been set at 10 times that for riboflavin: 25 mg ·
day–1. Since niacin is widely distributed in plant
and animal food sources, most athletes show no
evidence of niacin deficiency except those who
have a chronically reduced dietary intake for
weight control. Niacin deficiency symptoms may
easily be solved by a well-balanced diet without
recourse to specific supplementation. Good
sources of niacin include poultry, meats, grain
products, peanuts, yeast, fish, etc. (see Chapter
20).
Pyridoxine (vitamin B6)
Vitamin B6 exists in five forms: pyridoxine,
pyridoxal, pyridoxamine, pyridoxal phosphate
(coenzyme form), and pyridoxamine phosphate
(coenzyme form). The pyridines function in
protein and amino acid metabolism, gluconeogenesis, and in formation of haemoglobin, myoglobin, and cytochromes, and are also a
285
component of glycogen phosphorylase which
plays a key role in glycogenolysis. Because
exercise stresses metabolic pathways that use
vitamin B6, it has been suggested that the
requirement for this vitamin is increased in
athletes and active individuals (Manore 1994).
Vitamin B6 is essential in coenzymes related to
nitrogen metabolism, and the requirement for
vitamin B6 is closely related to dietary protein
intake. Currently the RDA for vitamin B6 is
2.0 mg · day–1 for men and 1.6 mg · day–1 for
women. If the dietary protein is more than 100 g ·
day–1, then the intake of vitamin B6 should be
more than 2 mg · day–1 (US National Research
Council 1989; Manore 1994). Although the
requirement for vitamin B6 appears to increase
when a high protein diet is consumed, vitamin B6
is found in meats and other animal foods, and
sufficient may be provided if these foods are the
major source of protein in the diet. The incidence
of acute toxicity of vitamin B6 is low. Intakes of
117 mg of vitamin B6 for more than 6 months can
result in neurological impairment (US National
Research Council 1989). Chronic ingestion of 2–
6 g pyridoxine · day–1 has been shown to result in
sensory neuropathy (Clarkson 1991). Some
studies reported that athletes have low or marginal dietary intakes of vitamin B6 (Hickson et al.
1987; Weight et al. 1988b; Guilland et al. 1989),
whereas others reported mean intakes at or
above the RDA (Faber et al. 1991; Singh et al.
1992a, 1992b, 1993). Young female athletes and
those participating in sport training emphasizing
low body weights need to be monitored regularly. Deficiencies should be corrected by a wellbalanced diet, and if necessary, small amount of
supplements within the RDA (Rokitzki et al.
1994a). There is no evidence suggesting that supplementation enhances athletic performance, so
the use of vitamin B6 as an ergogenic aid is contraindicated. Good food sources include meat,
poultry, fish, whole grain, peanuts, soybeans,
seeds, yeast and eggs (see Chapter 20).
Cobalamin (vitamin B12)
Vitamin B12 is involved in a variety of metabolic
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nutrition and exercise
processes. It is an essential component in the formation and function of red blood cells. Because
of this role, it is sometimes thought by athletes
and their coaches that vitamin B12 supplement
should enhance the oxygen-carrying capacity of
the blood and improve performance in those
events where oxidative metabolism is important.
In practice, however, vitamin B12 supplementation will only help in cases of pernicious anaemia
or macrocytic anaemia, and will not show benefits for the athlete with iron deficiency anaemia
or for athletes whose iron stores are replete. In
spite of this, vitamin B12 injection is a common
practice in sport, and it has been noted that some
athletes have been receiving 1000 mg about 1 h
before competition (Ryan 1977). The RDA for
vitamin B12 for the general adult population is
2 mg · day–1, and the average diet contains about
5–15 mg · day–1, so deficiency is rare. As for the
general population, deficiency in athletes is rare,
except for those who are complete vegetarians.
This group may be susceptible to vitamin B12
deficiency as the vitamin is found only in animal
protein including meat, poultry, fish, egg, milk
and milk products, and some fermented soybean
products (see Chapter 20). Most vegetarians,
however, are well aware of the need to ensure an
adequate intake.
Pantothenic acid
Pantothenic acid is a part of CoA, thus making it important in metabolism involving the
Krebs cycle. The RDA for pantothenic acid is 4–
7 mg · day–1 (Williams 1985). Inadequate intakes
of pantothenic acid are rare for the individual
who has a normal diet, because it is widely distributed in foods including animal and plant
foods such as eggs, yeast, whole grains, etc. It is
not known if exercise increases the requirement
for pantothenic acid. Results of studies on the
effect of pantothenic acid supplementation on
performance are equivocal (Nice et al. 1984; Litoff
et al. 1985; Clarkson 1991). Nice et al. reported
that supplementation with 1 g pantothenic acid ·
day–1 (10 000% RDA) for 2 weeks had no effect on
a treadmill run to exhaustion, pulse rate, blood
glucose levels, or several other blood measures
in highly trained distance runners; it was concluded that pantothenic acid in pharmacological
dosages has no significant effect on human exercise capacity (Nice et al. 1984).
Folic acid (folate)
Folic acid acts as an coenzyme functioning in
DNA synthesis for red blood cell formation, and
is also important for nucleotide and amino acid
metabolism. A deficiency state may cause
anaemia, and at least in theory, a deficiency may
affect aerobic endurance performance. The RDA
has been set at 400 mg · day–1 for non-trained adult
males (US National Research Council 1989). The
RDA set by the FAO/WHO is 200 mg · day–1. No
study is known to have been performed on the
effects of folic acid supplements on physical performance. Since folate is present in large
amounts in vegetables, fruits, and animal foods,
a balanced diet would appear to provide adequate amounts of this vitamin (see Chapter 20).
Vitamin B complex
Because of the close association of the vitamins in
the B complex, the effects of deprivation of or
supplementation with various combinations of
the B vitamins have been studied. Results of
some of the early studies showed that a deficiency of the B complex vitamins over a period of
time, a few weeks at the most, could create a definite decrease in endurance capacity (Berryman
et al. 1947). It is extremely unlikely that athletes
on a well-balanced diet will encounter this level
of deficiency. However, the effects of vitamin B
complex supplements remain contradictory and
further study is needed to determine the usefulness of vitamin B complex supplemention for
athletes (Read & McGuffin 1983; Clarkson 1991).
Ascorbic acid (vitamin C)
Vitamin C functions in the biosynthesis of collagen, catecholamines, serotonin and carnitine. It is
also a powerful antioxidant which may aid intra-
vitamins: effects of exercise on requirements
cellular oxidation-reduction reactions. Vitamin C
also helps non-haem iron absorption, transport
and storage. The deemed benefits of the effects of
vitamin C supplements include stimulation of
immune function and resistance to infection
(Chen 1988) and a reduction in fatigue and
muscle soreness, enhancing performance capacity and protecting cells from free radical damage
(Kanter 1994), and thus it is perhaps the most
widely used and studied of the vitamins.
The US RDA for vitamin C is 60 mg · day–1 (US
National Research Council 1989), but recommended intakes vary widely between countries.
In some countries, specific recommendations
have been made for athletes, and the RDA for
vitamin C has been set at 140 mg · day–1 during
training and 200 mg · day–1 during competition
periods for Chinese athletes on the basis of maintaining vitamin C in a saturation status as shown
by urinary output (Chen et al. 1962, 1963, 1992).
Most athlete groups studied have been reported
to exceed the RDA for vitamin C, but a small percentage of athletes, particularly young gymnasts,
have been found to have a less than adequate
intake of vitamin C (Loosli et al. 1986; Chen et al.
1989). Megadoses of vitamin C can cause iron
loading, may affect the availability of vitamin B12
from food, and may also promote the formation
of urinary stones, yet high intakes of vitamin C
are relatively harmless (Clarkson 1991). A single
bout of exercise may increase blood levels of
ascorbic acid but decrease the ascorbic acid
content of other tissues (Chen et al. 1965; Gleeson
et al. 1987). Increases in plasma ascorbic acid
levels correlate significantly with the increase in
plasma cortisol, suggesting that exercise may
cause ascorbic acid to be released from the
adrenal gland or other organs into the circulation
along with the release of cortisol. The effect of
vitamin C supplementation on physical performance has been investigated intermittently over
the past 50 years, but the results of these studies
have been contradictory. The possible benefits of
vitamin C supplementation on exercise-induced
muscle damage remain doubtful and need
further study.
Vitamin C is present in fresh fruits and vegeta-
287
bles, primarily the citrus fruits such as oranges,
grapefruit, lemons and limes. Other good
sources are broccoli, green peppers and greens.
There is little doubt that a severe deficiency of
vitamin C would have an adverse effect on work
performance: the feelings of weakness and lassitude and the possibility of iron deficiency
anaemia would certainly not be beneficial
(Hodges 1980). Exercise may increase moderately the body’s need for vitamin C, but to what
extent exercise training will change an athlete’s
requirement for vitamin C is still not entirely
clear. However, the inclusion of additional fruits
and vegetables in the athletes’ diet is advised.
Vitamin A (retinol)
Vitamin A designates several compounds including retinol, retinaldehyde and retinoic acid.
Vitamin A plays a major role in maintenance of
proper vision and epithelial tissues, and is also
involved in the development of bones and teeth
as well as playing an important function in the
body’s immune response. b-carotene, the major
carotenoid precursor of vitamin A, plays a role as
an antioxidant. The need for vitamin A can be
met by intake of carotenoid precursors commonly found in plants. The RDA for vitamin A is
expressed in retinol equivalents (RE); one
RE equals 1 mg retinol or 6 mg b-carotene. The
RDA for vitamin A is 1000 RE (1000 mg retinol or
6000 mg b-carotene) for adult males and 800 RE
(800 mg retinol or 4800 mg b-carotene) for adult
females (US National Research Council 1989).
Russian research suggested that extra vitamin A
is needed in athletes requiring good visual acuity
and alertness and during periods of stress
(Williams 1985). The RDA for vitamin A for
Chinese athletes was set at 1500 RE · day–1 (Chen
et al. 1992).
The vitamin A intake of elite athletes has generally been found to be adequate, although it has
been reported that 10–25% of the athletes investigated were ingesting less vitamin A than the
RDA (Clarkson 1991). Studies that have assessed
the vitamin A, C and E status of athletes have
found that most had adequate blood levels of
288
nutrition and exercise
these vitamins (Weight et al. 1988b; Guilland et al.
1989; Fogelholm et al. 1992). Serum vitamin A
levels of 5% of 182 athletes investigated had a
value of less than 30 mg · dl–1 (Chen et al. 1992).
There has been no evidence of serious biochemical deficiencies of vitamin A existing in athletes.
It is unlikely that vitamin A supplementation will
enhance performance. Vitamin A supplementation is not necessary for athletes on an adequate
diet (Williams 1985; Clarkson 1991). Whether the
antioxidant role of b-carotene can reduce exercise
damage due to free radical activity remains to be
studied.
Vitamin A is one of the fat-soluble vitamins
and hence may be stored in the body for considerable periods of time, unlike water-soluble vitamins. Overdosage over a period of time may
cause a condition known as hypervitaminosis,
characterized by anorexia, hair loss, hypercalcaemia, and kidney and liver damage (Aronson
1986). Sustained daily intakes exceeding 15 000
mg · day–1 of retinol can produce signs of toxicity
(US National Research Council 1989). However,
high doses of b-carotene are not generally considered to be toxic (US National Research
Council 1989; Clarkson 1995). Bodily stores are
available for short-term deficiency periods, and
thus, no significant decrements would be
revealed during short periods of reduced dietary
intake of vitamin A. Good sources of vitamin A in
the diet include liver, fish liver oils, butter, whole
milk, cheese and egg yolk. Rich sources of the
carotenoid are dark-green leafy vegetables, the
yellow or orange fruits and vegetables (see
Chapter 20).
Vitamin D
Vitamin D represents any one of several sterol
compounds in the body; vitamin D2 (ergocalciferol) is the result of the irradiation of ergosterol.
D3 (cholecalciferol) is the naturally occurring
compound in the skin, formed by exposure to the
sunlight. The major function of vitamin D is its
hormone-like action in the process of mineralization of bones and teeth and the regulation of
calcium metabolism. It promotes absorption of
calcium from the intestine and helps to prevent
calcium deficiency. The RDA for vitamin D is
5 mg · day–1 for adults (US National Research
Council 1989), and no separate recommendations appear to have been made for athletes.
Overdoses of vitamin D are potentially toxic and
result in hypercalcaemia and hypercalciuria (US
National Research Council 1989). Levels of five
times the RDA are considered dangerous; intakes
of 50 mg · day–1 (2000 IU · day–1) for a prolonged
time may pose considerable risk. Hypervitaminosis D leads to loss of weight, vomiting,
nausea, lethargy and loss of muscle tone; calcium
released from the bones may deposit in the soft
tissues, in the walls of the blood vessels and in
the kidneys.
Vitamin D deficiencies are rare in athletes with
adequate intake of dairy products and exposure
to sunlight, but those who have inadequate milk
consumption and lack of sunshine may be at
some risk for inadequate vitamin D nutriture. No
known controlled research has been conducted
on the role of vitamin D in physical performance
(Van der Beek 1991). The role of vitamin D in providing calcium for newly forming muscle tissue
is not clear and needs further investigation
(Clarkson 1991). Vitamin D is present in fish liver
oil and milk fortified with vitamin D (see
Chapter 20).
Vitamin E
Vitamin E is a fat-soluble vitamin. Its activity is
derived from a number of tocopherols, the most
active one of which is a-tocopherol. Vitamin E
functions as an antioxidant of polyunsaturated
fatty acids in cellular and subcellular membranes, and thus it serves as a free radical scavenger to protect cell membranes from lipid
peroxidation. The RDA for vitamin E is 8–10 mg
of a-tocopherol · day–1. Vitamin E is relatively
non-toxic up to 800 mg · day–1 (US National
Research Council 1989). Dietary records showed
that between one third and one half of the athletes investigated consumed less than two thirds
of the RDA (Loosli et al. 1986; Guilland et al.
1989). Guilland reported that the mean vitamin E
vitamins: effects of exercise on requirements
intake of athletes was 77% of the RDA. However,
vitamin E deficiencies are rare in athletes with
a well-balanced diet. Although megadoses of
vitamin E are relatively harmless, some individuals experience gastric disturbances and weakness when taking supplements ranging from 200
to 1000 IU (Clarkson 1991). Acute exercise has
been shown to result in an increase of plasma
levels of tocopherol, and the author suggested
that tocopherol was mobilized from adipose
tissue into the blood to be distributed to exercising muscles; however, this study did not correct
for haemoconcentration and the small increase in
plasma tocopherol was back to baseline after
10 min rest (Pincemail et al. 1988). This response
to exercise has not been reported in another
study (Duthie et al. 1990). It is not clear if the disparate findings are due to different exercise
loads, different testing methods or to other
factors.
Many studies have reported a significant effect
of vitamin E supplementation on exercise performance, but the actual benefits are doubtful since
many of these experiments were not well controlled. Those studies that have been well controlled have generally shown that vitamin E
supplementation has no effect on performance
(Shephard et al. 1974; Watt et al. 1974; Lawrence
et al. 1975). On the contrary, supplements of
vitamin E showed a beneficial effect on
maximum oxygen uptake and a partially protective effect on cell membranes at high altitude; it
was reported that mountain climbers with a
vitamin E supplement working at an altitude of
5000 m exhaled lower levels of pentane, a marker
of lipid peroxidation, and exhibited a higher
anaerobic threshold than controls (SimonSchnass & Pabst 1990). Another study also
showed that the impairment of blood flow
parameters was attenuated by vitamin E
supplementation in mountaineers at altitude
(Simon-Schnass & Korniszewski 1990). The positive effect may be due to the antioxidant properties of vitamin E. At high altitudes, vitamin E
may counteract the effect of the increased lipid
peroxidation of red blood cell membranes
caused by the decreased availability of oxygen
289
(Williams 1989). Vitamin E may also play a role in
reducing muscle damage and oxidative stress, as
shown by a reduction in muscle-specific enzyme
levels in serum after strenuous exercise (Rokitzki
et al. 1994b). However, results are equivocal as to
whether muscle damage can be reduced by
vitamin E supplementation (see Chapter 20).
In short, there has been much debate on the
vitamin requirements of athletes, yet carefully
controlled studies are limited.
Conclusion
The following is a summary of the main
viewpoints.
1 Vitamin deficiencies may result in decreased
exercise performance, and it has been demonstrated that vitamin supplements improve performance in persons with pre-existing vitamin
deficiencies.
2 Vitamin supplements are generally unnecessary in athletes consuming well-balanced diets.
3 Athletes participating in strenuous training
may need monitoring of vitamin status even if
consuming the RDA levels of vitamins.
4 Vitamin supplements should be suggested for
athletes in special conditions including those
who are on a weight loss diet, or have eating disorders, or low energy intakes. Supplementation
is only warranted when there is reasonable
evidence to suggest that a deficiency may be
present.
5 Excessive vitamin intake, especially of the fatsoluble vitamins, can be accumulated to a level
that may be toxic. Prolonged excessive intake of
water-soluble vitamins also may be harmful and
cause nutritional imbalances. Attention to food
choices, rather than specific supplementation, is
the preferred option.
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