Scandinavian researchers in the 1960s were the first to demonstrate that the ability to exercise at a high intensity was related to the pre-exercise level of muscle glycogen.1 Body glycogen stores play an important role in intense exercise (70-85% of peak aerobic power) that is either prolonged and continuous (e.g., running, swimming, and cycling), or of an extended intermittent, mixed anaerobic-aerobic nature (e.g., soccer, basketball, ice hockey, or repeated exercise intervals). Endurance athletes have been urged to ingest plant sources of carbohydrates to optimize muscle and liver glycogen stores.1,34,35 At the high intensities necessary for athletic training and competition, the metabolism of body carbohydrate stores provides the major fuel for muscle contraction, and, when these reach low levels, fatigue occurs.34,35 About 500-800 grams of carbohydrate per day (or 8-10 g/kg body weight or 60-70% of energy intake) have been recommended for athletes training intensively for more than 60-90
minutes per day.1,34-36
A near-vegetarian diet is often needed to take advantage of high carbohydrate plant foods such as cereals, pasta, grains, dried fruits, and legumes. In one study of 347 marathon runners, more than 75% reported higher intake of fruits, vegetables, and whole grains, and lower intake of red meat and eggs when compared with pre-running dietary habits.37 (See Figure 12.2.) Nonetheless, in most studies, intake of carbohydrate by endurance athletes falls below recommended levels, although there are some noteworthy exceptions (e.g., Tarahumara Indian ultramarathon runners, and triathletes).137-41 The Tarahumara Indians, a Ute-Aztecan tribe inhabiting the rugged Sierra Madre Occidental Mountains in the north-central state of Chihuahua, Mexico, are extraordinary endurance runners who consume a simple, near-vegetarian diet composed primarily of corn and beans (75-80% of total energy intake is carbohydrate).38
Fruits ^Vegetables HHWhole Grains EH Red Meats
Fruits ^Vegetables HHWhole Grains EH Red Meats
Definitely higher Somewhat higher Somewhat lower Definitely lower
Definitely higher Somewhat higher Somewhat lower Definitely lower
Figure 12.2 In this study of 347 marathon runners, more than 75% reported higher intake of fruits, vegetables, and whole grains, and lower intake of red meat and eggs when compared with pre-running dietary habits.37
A. Iron, Zinc, and Trace Minerals
Some athletes, especially elite male and female endurance athletes, test positive for mild iron deficiency.1 Using serum ferritin as a criterion (less than 12 ^g/l), between 10% and 80% of female athletes, depending on the study, have been described as having mild iron deficiency (in contrast to 5-11% of non-athletic females).1 Concerns have been raised that an emphasis on plant foods to enhance carbohydrate intake by athletes may increase polyphenol (in certain vegetables), phytate (in bran), dietary fiber, and tannin (tea) intake to levels that reduce the bioavailability of several nutrients, including zinc, iron, and some trace minerals.1,39-43 Furthermore, heme iron from meat is two to three times more absorbable than non-heme iron from plant-based foods and iron-fortified foods, increasing the risk of sports anemia in vegetarian athletes who are already at high risk
for iron deficiency due to exercise-induced iron losses (via increased hemolysis, gastrointestinal bleeding, and high sweat rates).1-40 These concerns may be especially apparent in certain subgroups such as female adolescent athletes.41
Most studies of long-term vegetarians (non-athletes) who have avoided dietary extremes indicate that, despite the apparent lower bioavailability of some minerals, the iron, zinc, and trace element status (as measured in the serum, hair, and urine) appears adequate.42,43 Dietary intake of iron is typically above recommended levels in vegetarians, but serum ferritin levels and other iron status indicators are often lower than in non-vegetarians, although anemia is rare.11,44-49 It appears that the gastrointestinal tracts of vegetarians can adapt by increasing the absorption of iron and trace elements, although concerns have been raised that adolescents on a vegetarian diet may have suboptimal zinc status because of their high zinc requirements for growth.42,48
Although inhibitors of dietary iron absorption are present in plant-based foods, plant foods also contain enhancers of dietary iron absorption, such as vitamin C and citric acid found in fruits and vegetables.14,43 Although considered controversial, there is some evidence that a reduction in body iron stores may be associated with a reduced risk for both coronary artery disease and cancer.50-52 Thus, the lower ferritin levels found in vegetarians may actually be advantageous, but further research is needed before this hypothesis can be accepted.
Although some athletes have been reported to be at risk for iron deficiency, iron deficient anemia is rare (about 2-5%, the same as measured in the general population). 1,53,54 There is a growing consensus, however, that mild iron deficiency has little or no meaningful impact on the health or performance capabilities of athletes.1,53,54 Low intake of dietary iron, increased hemolysis, decreased iron absorption, and increased iron loss in sweat, feces, and urine have all been implicated as factors that may reduce body iron stores in some athletes, especially females.1,53 In one large study of 1743 eastern Finnish men, the duration and frequency of physical activity were associated inversely with serum ferritin.51 (Figure 12.3). The authors speculated that a reduction in stored iron levels could be one mechanism through which exercise training decreases the risk of coronary artery disease.50,51
There has been some concern that vegetarian female endurance athletes may be at special risk for iron deficiency.39,40 A significant proportion (30-50%) of endurance athletes, especially women, have been reported to be semi-vegetarians, with a low intake of meat products.26,39 Snyder et al.26 compared iron status in nine non-vegetarian and nine semi-vegetarian female runners, and reported lower serum ferritin and higher total iron binding capacity in the semi-vegetarians. Serum iron, percent transferrin
Duration of Leisure Time Physical Activity (hours/wk)
Duration of Leisure Time Physical Activity (hours/wk)
Figure 12.3 In this study of 1743 eastern Finnish men, the duration and frequency of physical activity were associated inversely with serum ferritin. There is some evidence that reduced body iron stores are inversely related to coronary heart disease risk.51
saturation, and hemoglobin levels, however, were not different between the groups.
Seiler et al.28 has also reported lower ferritin levels in vegetarian vs. non-vegetarian runners (both male and female). However, no impairment in ability to compete in a 20-day, 1000-km race was measured in the vegetarian runners. During the 1000-km race, dietary iron intake was higher in the vegetarian group, but was of the non-heme variety from legumes, dried fruits, nuts, vegetables, and grain products. The high intake of vitamin C, together with the avoidance of tea, appeared to enhance iron absorption in that blood indicators of iron status were similar between the vegetarian and non-vegetarian runners.29 The Tarahumara Indians have been found to have normal blood hemoglobin levels, despite diets that are 90% corn and pinto beans.38,55 In the study of eight athletes who consumed a vegetarian or non-vegetarian diet for 6 weeks (cross-over design), despite an extremely high fiber intake while on the vegetarian diet (98 vs. 47 grams/day), blood hemoglobin, serum iron, and serum transferrin concentrations did not differ significantly between dietary reg-
Several studies have shown that acute exercise alters blood levels of trace elements, suggesting that exercise leads to a redistribution between body tissues.56,57 There is some evidence that an acute bout of exercise increases urinary excretion of zinc.56 However, most studies have failed to find that indicators of trace element status are different between athletes and non-athletes.57 Concerns have been raised that athletes, especially adolescents, who avoid meat may have difficulty in maintaining appropriate levels of zinc in their bodies.40 Insufficient data exist to determine whether these concerns are warranted. Beef, pork and poultry are major sources of zinc in the United States.58 Milk products, cereal products, legumes and nuts are also good sources of zinc, but zinc from these sources is less bioavailable than from meat.40,56,58
Vegetarian athletes are urged to increase good sources of iron and zinc in their diets (e.g., fortified breakfast cereals, legumes, nuts and seeds), include vitamin C sources with each meal and avoid heavy tea intake.40 Iron, zinc and trace element supplementation may be necessary for some vegetarian athletes with poor diets, but the supplements should be at a level no higher than 100% of the Recommended Dietary Allowance to avoid negative interactions with the absorption or function of other nutrients.56,59
While there have been some concerns about the trace mineral status of vegetarian athletes, their dietary practices may be of benefit in another area of current interest in sports nutrition. Research has been directed toward the interaction of exercise, generation of reactive oxygen species or free radicals, and antioxidant nutrients (primarily vitamins E, C, and A, and the mineral selenium).60-63 During exercise, oxygen consumption can increase 10- to 20-fold over rest to meet energy demands. Due to various means that are still being researched (e.g., increases in catecholamines, lactic acid, hyperthermia, and transient hypoxia), the rise in oxygen consumption results in an "oxidative stress" that leads to the generation of reactive oxygen species, such as the superoxide radical, hydrogen peroxide, and the hydroxyl radical.60-63 These reactive oxygen species are defined as molecules or ions containing an unpaired electron that cause cell and tissue injury. Reactive oxygen species have been implicated in certain diseases and the aging process.
The body is equipped with a sophisticated defense system to scavenge oxygen reactive species.1,60-63 Antioxidant enzymes (e.g., glutathione per-oxidase, superoxide dismutase, catalase) provide the first line of defense, with antioxidant nutrients providing a second line. Because strenuous and prolonged exercise promotes reactive oxygen species production, considerable concerns have been raised among experts regarding the ability of the body to cope with the increased oxidative stress.
Most studies have shown that chronic physical training augments the physiological antioxidant defenses in several tissues of the body.60-63 The activity of the various antioxidant enzymes is enhanced by physical training, helping to counter the exercise-induced increase in reactive oxygen species. In general, antioxidant supplementation does not appear necessary, and has not been consistently shown to improve performance, minimize exercise-induced muscle cell damage, or maximize recovery. However, until more is known, people who exercise regularly and intensely are urged to ingest foods rich in antioxidants (fruits, vegetables, nuts, seeds, and whole grains) to augment the body's defense system against reactive oxygen species.
All essential and nonessential amino acids can be supplied by plant food sources alone, as long as a variety of foods is consumed and the energy intake is adequate to meet needs.14,64 The American Dietetic Association has advised that conscious combining of various plant foods within a given meal is unnecessary.14
Interest in the influence of dietary protein intake on athletic performance has been evident since the days of the ancient Greeks and Romans.1 Athletes consumed meat-rich diets in the belief that they would achieve the strength of the consumed animal. In 1842, the great German chemist and physiologist, Justus von Liebig, reported that the primary fuel for muscular contraction was derived from muscle protein, and he suggested that large quantities of meat be eaten to replenish the supply. However, a number of studies during the late 1800s, which measured urinary urea excretion, failed to confirm his results and the concept became established that changes in protein metabolism during exercise are nonexistent or minimal at best.1-4 Studies since 1970 using modern technology and improved techniques, however, have concluded that protein is a much more important fuel source during exercise than previously thought.65 Research based on nitrogen balance and protein kinetic methodology have clearly shown that athletes benefit from diets containing more protein than the current RDA of 0.8 grams per kilogram of body weight per day. Strength athletes probably need about 1.6-1.7 g/kg, and endurance athletes about 1.2-1.4 g/kg.65
Although most vegetarian diets meet or exceed dietary recommendations for protein, they often provide less protein than non-vegetarian diets12,26,44,49 and concerns have been raised that vegetarian athletes may have intakes that fall below the added demands created by heavy exertion.39-65 Most athletes are able to meet these extra demands without protein supplementation by keeping dietary protein intake near 15% of total energy intake.65 The vegan athlete can achieve optimal protein intake by careful planning, with an emphasis on protein-rich plant foods such as legumes, nuts and seeds, and whole grain products.
Creatine supplementation has been urged as an ergogenic aid for athletes who engage in repeated bouts of short-term, high-intensity exer-cise.66,67 Creatine is found in large quantities in skeletal muscle and binds a significant amount of phosphate, providing an immediate source of energy in muscle cells (adenosine triphosphate or ATP). The intent of consuming supplemental creatine is to increase the skeletal muscle's creatine content, in the hope that some of the extra creatine binds phosphate, increasing muscle phosphocreatine content. During repeated bouts of high intensity exercise (for example, five 30-second bouts of sprinting or cycling exercise separated by 1-4 minutes of rest), the increased availability of phosphocreatine may improve resynthesis and degradation rates, leading to greater anaerobic ATP turnover and highpower exercise performance.66
The estimated daily requirement for creatine is about 2 grams. Non-vegetarians typically get about 1 gram of creatine a day from the various meats they ingest, and the body synthesizes another gram in the liver, kidney, and pancreas using the amino acids arginine and glycine as precursors. Vegetarians have a reduced body creatine pool, suggesting that lack of dietary creatine from avoidance of meat is not adequately compensated by an increase in endogenous creatine production.67
Various studies have shown that consuming about 20-25 grams of creatine per day for 5 to 6 days in a row significantly increases muscle creatine in most people, especially those with low levels to begin with, such as vegetarians.66-68 Four to five daily doses of 5 grams each are usually consumed by dissolving creatine in about 250 ml of a beverage throughout the day. Each 5-gram dose of creatine is the equivalent of 1.1 kg of fresh, uncooked steak. Creatine supplementation up to 8 weeks has not been associated with major health risks, but the safety of more prolonged creatine supplementation has not been established.66-71
Some, but not all studies have shown that supplemental creatine improves power performance during repeated bouts of short-term sprinting, cycling, or swimming.66-71 A recent randomized, double-blind study with vegetarian subjects failed to demonstrate any effect of creatine on power performance.69 Only about one-half of studies have shown a significant effect of creatine on power performance, and when a positive effect is reported, it is only 5-8% above placebo effects.1,67,71 Additional laboratory and field research is needed to help resolve the conflicting findings regarding the ergogenic efficacy of creatine supplementation.71 Creatine supplementation has no effect on aerobic exercise metabolism and performance.66,67 At this point, there is no justification for creatine supplementation by vegetarian athletes involved in power sports.
High fiber, low fat vegetarian diets have been associated with reduced blood estrogen levels and increased menstrual irregularity.72-75 Large volumes of exercise have also been related to menstrual irregularity.1,76-78 Approximately 5-20% of women who exercise regularly and vigorously, and up to 50-65% of competitive athletes may develop oligo-amenor-rhea.76,77 The causes are hotly debated, but may include the effect of exercise itself on the hypothalamic-pituitary-ovarian axis, low energy intake, and depleted fat stores in the female athlete.1,76-78 Amenorrheic athletes typically display reduced levels of estradiol and progesterone and have hormonal profiles more similar to those of postmenopausal women. The reduced levels of endogenous estrogen associated with athletic amen-orrhea may prevent the formation of adequate bone density.1,76,77 The syndrome of amenorrhea, disordered eating (and often excessive exercise), and osteoporosis is called the "female athlete triad."76
Although all physically active girls and women could be at risk for developing one or more components of the athlete triad, participation in the following sports is a major risk factor:1,76
Two reports (published as letters) suggested that a significant proportion of female athletes with amenorrhea were vegetarians.79,80 However, these were descriptive studies that were not able to determine whether the cause was the vegetarian diet, heavy exercise training, lower energy intake or other factors. There is increasing evidence that low energy intake, not diet quality, is a major cause of oligo-amenorrhea in female athletes, and that when brought into positive energy balance, hormonal profiles return to normal and menstruation resumes.81 There are many parallels between the amenorrhea induced by anorexia nervosa and by strenuous athletic training, with both causing an increased secretion of antireproductive hormones, which inhibit the normal pulsatile secretion pattern of gonadotropins. Hanne et al.24 have observed that when vegetarian female athletes are properly nourished, menstrual cycle function is normal, when compared with matched non-vegetarian controls.
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