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Micronutrients play an important role in energy
production, hemoglobin
synthesis, maintenance of bone health, adequate immune function, and
the
protection of body tissues from oxidative damage. They are also
required to help
build and repair muscle tissue following exercise. Theoretically,
exercise may
increase or alter the need for vitamins and minerals in a number of
ways.
Exercise stresses many of the metabolic pathways in which these
micronutrients
are required, thus exercise training may result in muscle biochemical
adaptations that increase micronutrient needs. Exercise may also
increase the
turnover of these micronutrients, thus increasing loss of
micronutrients from
the body. Finally, higher intakes of micronutrients may be required to
cover
increased needs for the repair and maintenance of the lean tissue mass
in
athletes. It is assumed that the current RDAs and Dietary Reference
Intakes (DRIs)
are appropriate for athletes unless otherwise stated (2,68,69).
Athletes at the
greatest risk of poor micronutrient status are those who restrict
energy intake
or use severe weight-loss practices, eliminate one or more of the food
groups
from their diet, or consume high-carbohydrate,
low-micronutrientdense diets.
Athletes participating in these types of behaviors may need to use a
multivitamin and mineral supplement to improve overall micronutrient
status.
Supplementation with single micronutrients is discouraged unless clear
medical,
nutritional, or public health reasons are present, such as the
supplementation
of iron to treat iron deficiency anemia or folic acid to prevent birth
defects.
The B-complex vitamins have 2 major functions directly
related to exercise.
Thiamin, riboflavin, vitamin B-6, niacin, pantothenic acid, and biotin
are
involved in energy production during exercise (4,70-74), whereas folate
and
vitamin B-12 are required for the production of red cells, protein
synthesis,
and in tissue repair and maintenance (75). Limited research has
examined whether
exercise increases the need for some of the B-complex vitamins,
especially
vitamin B-6, riboflavin, and thiamin (70,71,73,75,76). Available data
were not
sufficiently precise to set separate recommendations for athletes or to
quantitatively link recommendations to energy expenditure (69).
Nevertheless,
the data available suggest that exercise may slightly increase the need
for
these vitamins perhaps up to twice the current recommended amount (72).
These
increased needs can generally be met by the higher energy intakes
required of
athletes to maintain body weight.
The antioxidant nutrients‹such as vitamins A, E, and C,
beta carotene, and
selenium‹play an important role in protecting the cell membranes from
oxidative damage. Because exercise can increase oxygen consumption by
10- to
15-fold, it has been hypothesized that chronic exercise produces a
constant
"oxidative stress" on the muscles and other cells (77,78). In
addition, muscle-tissue damage caused by intense exercise can lead to
lipid
peroxidation of membranes. Although there is some evidence that acute
exercise
may increase levels of lipid peroxide by-products (79), habitual
exercise has
been shown to result in an augmented antioxidant system and a reduction
of lipid
peroxidation (77). Thus, a well-trained athlete may have a more
developed
endogenous antioxidant system than a sedentary person (80). Research
examining
whether exercise increases the need for the antioxidant nutrients is
equivocal
and controversial; thus, there is no clear consensus on whether
supplementation
of antioxidant nutrients is necessary (77,79,80). The lack of consensus
is
especially true for the athlete with adequate or above-adequate blood
levels of
the antioxidant vitamins (77). Those athletes at greatest risk for poor
antioxidant intakes are athletes following a low-fat diet, those who
restrict
energy intakes, or those with limited dietary intakes of fruits and
vegetables.
The primary minerals low in the diets of
athletes‹especially female
athletes‹are calcium, iron, and zinc. (11,81). Low intakes of these
minerals
can usually be attributed to energy restriction or avoidance of animal
products
such as meat, fish, poultry, and dairy products. Calcium is especially
important
for the building and repair of bone tissue and the maintenance of blood
calcium
levels. Inadequate dietary calcium increases the risk of low bone
mineral
density and stress fractures. Female athletes are at greatest risk for
low bone
mineral density if energy intakes are low, dairy products are
eliminated from
the diet, and menstrual dysfunction is present (8,22). Vitamin D is
also
required for adequate calcium absorption, regulation of serum calcium
levels,
and promotion of bone health. The 2 primary sources of vitamin D are
fortified
foods, such as milk, and the production of vitamin D by ultraviolet
conversion
in the skin. Athletes who live at northern latitudes or who train
primarily
indoors throughout the year‹such as gymnasts and figure skaters‹may be
at
risk for poor vitamin D status, especially if foods fortified with
vitamin D are
not consumed (82). These athletes would benefit from vitamin D
supplementation
at the level of the DRI (5 mg/day or 200 international units [IU]
vitamin D)
(68).
Iron plays an important role in exercise as it is
required for the formation
of hemoglobin and myoglobin, which bind oxygen in the body, and for
enzymes
involved in energy production. Iron depletion (low iron stores) is one
of the
most prevalent nutrient deficiencies observed in athletes, especially
female
athletes. The impact of iron depletion on exercise performance is
limited, but
if this condition progresses to iron deficiency anemia (low hemoglobin
levels),
exercise performance can be negatively affected (4,81).
The high incidence of iron depletion in athletes is
usually attributed to
poor energy intakes; avoidance of meat, fish, and poultry that contain
iron in
the readily available heme form; vegetarian diets that have poor iron
bioavailability; or increased iron losses in sweat, feces, urine, or
menstrual
blood. Athletes‹especially females, long-distance runners, and
vegetarians‹should
be screened periodically to assess iron status. Changes in iron storage
(low-serum ferritin concentrations) will occur first, followed by
low-iron
transport (low-serum iron concentrations), and eventually iron
deficiency anemia
(low hemoglobin and hematocrit concentrations). Because reversal of
iron
deficiency anemia can require 3 to 6 months, it is advantageous to
begin
nutrition interventions before iron deficiency anemia can develop.
Although
depleted iron stores are more prevalent in female athletes, the
incidence of
iron deficiency anemia in female athletes is similar to the 9% to 11%
found in
the general female population (81,83).
A transient decrease in ferritin and hemoglobin may be
experienced by some
athletes at the initiation of training. These decreases are the result
of an
increase in plasma volume, which causes hemodilution and appears to
have no
negative effect on performance (81). If an athlete appears to have iron
deficiency anemia but does not respond to nutrition intervention, then
low
hemoglobin values may be the result of changes in plasma volume, and
not poor
nutritional status (4). Chronic iron deficiency anemia resulting from
poor iron
intake can seriously affect health and exercise performance and needs
medical
and nutrition intervention.
In the United States, it is estimated that the zinc
content of the food
supply is approximately 12.3 mg of zinc per person, with 70% of the
zinc coming
from animal products (84). Based on survey data, approximately 90% of
men and
81% of women have zinc intakes that are below the 1989 RDAs (15 mg and
12 mg,
respectively) (85). This nutritional shortfall is also seen in
athletes,
particularly females (11). The impact of these low zinc intakes on zinc
status
is difficult to measure, because clear assessment criteria have not
been
established and plasma zinc concentrations may not reflect changes in
whole-body
zinc status (86). Because of the role zinc plays in growth, building
and repair
of muscle tissue, and energy production, it is prudent to assess the
diets of
active females for adequate zinc intake.
Nutrition and Performance:PART 1;
Nutrition For Active AdultsPART 2, Energy Needs: PART 3, Body
Composition: PART 4,
Carbs, Fat,
Protein , REFERENCES
CARBOHYDRATES: How Much?;
Nutrition and Performance:PART
1;
Nutrition For Active Adults PART
2, Energy Needs: PART
3, Body Composition: PART
4, Carbs, Fat, Protein, REFERENCES
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