Sports Medicine: Just the Facts

CHAPTER 14 • NUTRITION 87

and sex. Athletes who perspire heavily or engage in

physical activity in hot conditions may be prone to

increased losses of calcium in sweat. If an individual

consumes calcium supplements, no more than 500 mg

should be consumed at any one time to enhance

absorption (Bergeron et al, 1998) (see Table 14-3).

• During and immediately following exercise, there is a
  transient shift in potassium from the intracellular to
  the extracellular fluid space, which returns to normal
  approximately 1 h after exercise. Transient shifts in
  potassium may indicate that athletes need more potas-
  sium in their diets than what is recommended
  (Millard-Stafford et al, 1995).


• Increased intake of sodium is recommended, espe-
  cially for individuals exercising in hot, humid envi-
  ronments. Adequate sodium intakes are necessary to
  maintain fluid balance and prevent muscle cramping;
  however, sodium needs can typically be met by
  adding salt while eating or eating foods that are
  known to be high in sodium. Chloride needs of ath-
  letes may also be increased compared to sedentary
  individuals. Foods containing sodium often also con-
  tain chloride (Convertino et al, 1996).


• Iron deficiency is a common nutrient deficiency, and
  30–50% of athletes, especially female athletes, may be
  at risk of poor iron status. Females are at increased risk
  of iron depletion and even iron deficiency anemia
  because of menstruation, sweat losses, low consumption
  of iron-containing foods, and myoglobinuria from
  muscle stress during exercise. Iron deficiency, as a result
  of decreased iron stores, negatively impacts exercise
  performance as a result of decreased maximal oxygen
  consumption. Adequate intake of iron daily will help to
  ensure optimal performance (Schena, 1995).
  
  
  • Zinc intake is less than optimal for approximately 25%
    of females in the United States (CSFII, 1994–1996)
    (Ma and Betts, 2000), and it has been estimated that
    about 50% of female distance runners also have less
    than optimal intakes (Deuster et al, 1989); however,
    few studies have been conducted that assess long-term
    changes in zinc status as a result of exercise training.
  
  
  
  

CARBOHYDRATE LOADING,

GLYCOGEN RESYNTHESIS, MUSCLE

MAINTENANCE—CHO/PRO RATIO

• Individuals training for any sport must replace carbo-
  hydrate on a regular basis.
  
  
  1. The modified carbohydrate loading regimen still
     used today involves consumption of a diet initially
     consisting of 60% carbohydrate. The athlete also
     manipulates the amount of exercise they perform
     on a daily basis in a downward fashion (from 90
     min down to 20 min) until the day before the event.
     The day before the event, the individual rests and
     consumes a diet containing 70% carbohydrate.
     This method is typically advocated for individuals
     participating in events lasting longer than approxi-
     mately 90 min (Sherman et al, 1981).
  
  
  2. Recent studies have observed improved perform-
     ance when carbohydrate has been ingested before
     high intensity and intermittent exercise lasting less
     than 60 min (Below et al, 1995; Davis et al, 1997;
     Jeukendrup et al, 1997).
     3.Following exercise, carbohydrate should be
     ingested immediately to ensure rapid muscle
     glycogen resynthesis. Athletes should consume
  
  
  
  

TABLE 14-3 Dietary Reference Intakes for Selected Minerals

NUTRIENT LIFE STAGE GROUP RDA†/AI‡ UL§ SELECTED FOOD SOURCES

Calcium Males Milk, cheese, yogurt, calcium-fortified foods
19–50 y 1000 mg/d 2500 mg/d
Females
19–50 y 1000 mg/d 2500 mg/d

Iron Males Meat and poultry (heme iron); fruits, vegetables,
19–50 y 8 mg/d 45 mg/d fortified grain products (nonheme iron)
Females
19–50 y 18 mg/d 45 mg/d

Zinc Males Red meats, fortified cereals
19–50 y 11 mg/d 40 mg/d
Females
19–50 y 8 mg/d 40 mg/d

SOURCE: National Academy of Sciences (1997; 2001).
†Recommended dietary allowances are set to meet the needs of most (97%) individuals in an age and gender group.
‡Adequate intakes are believed to meet the needs of all individuals in a life stage group, but lack of data prevent being able to specify an
RDA—indicated with asterisk (*).
§Tolerable upper intake level is the maximum level of daily nutrient intake that is likely to pose no risk of adverse effects.