In the following paragraphs we will briefly discuss the individual vitamins and reported effects of exercise.
VITAMIN B1 (THIAMIN)
Vitamin Bi plays an important role in the oxidative conversion of pyruvate to acetyl CoA, an essential step in the energy production process from CHO.
For this reason the recommended requirements for vitamin Bi have been related to total energy expenditure and to CHO intake. The RDA is set at 0.5 mg/1000 kcal energy intake (131). It is accepted now that the vitamin B1 requirement of athletes may be slightly higher due to increased energy and CHO metabolism. Impairment of the maximal oxygen uptake resulting in increased CHO metabolism and lactate production has been shown in humans receiving a vitamin B1 deficient diet (10). Low intakes of this vitamin as well as biochemical deficiencies have been reported in sedentary subjects and also in athletes, especially in cyclists who consume large amounts of CHO solutions with refined CHO sources (13, 28, 58, 84, 165), although an almost linear relationship with energy intake has been seen (58). There are no controlled studies available on the effect of vitamin B1 supplementation on performance.
VITAMIN B2 (RIBOFLAVIN)
Vitamin B2 is involved in mitochondrial energy metabolism. The National Research Council relates B2 intake to energy intake. The recommended daily intake is 0.6 mg/1000 kcal, although it is stated that there is no evidence that the requirements increase with increased energy metabolism (131). Few studies have shown that vitamin B2 requirements for people involved in physical exercise may be increased (28). However, there are no studies that indicate low intakes of this vitamin in athletic populations. Studies in which vitamin B2 was supplemented in elite swimmers did not show any effects on performance (13, 58).
VITAMIN B6 (PYRIDOXINE)
Vitamin B6 plays an important role in protein synthesis. For this reason this vitamin is often assumed to be of crucial importance for strength athletes and bodybuilders. However, there are no data available to support an increased requirement for athletes. Accordingly, studies in which B6 was supplemented did not improve performance. Some studies indicated performance improvements after supplementation with combined preparations, also including substances that play a role in the citric acid cycle (Krebs cycle; see page 186). However, it is likely that any effect observed was not due to vitamin B6 but was caused by accompanying substances (13). A dietary B6 ratio of 0.016 mg/g protein intake appears to ensure acceptable values for B6 status in adults of both sexes. The RDA is set at 2.0 mg/day for males and 1.6 mg/day for females (131). Recent data are available, indicating insufficient intakes of B6 in different athletic populations (58).
VITAMIN B12 (CYANOCOBALAMIN)
Vitamin B12 functions as a coenzyme in nucleic acid metabolism and influence protein synthesis. Endurance cyclists and strength athletes surprisingly often use vitamin B12 because it is believed that this compound can have an analgesic effect on muscle soreness when used in mega-doses. Williams (199) and van der Beek (11) reviewed the literature up to 1985 and concluded that there is no evidence for any benefit of supplementation, a conclusion shared by others in more recent reviews. Both oral and parenteral supplementation did not influence any performance related parameters (86, 131). The RDA is 2.0 ^g/day (131). Deficits of this vitamin may occur in cases of impaired absorption due to lack of gastric factor (a factor required to make vitamin B12 bioavailable), or in subjects who do not consume any meat (only source for B12), as is the case in vegetarians. However, no data are available on vitamin B12 intake or on the existence of deficits in athletic populations.
Niacin functions as a coenzyme in NAD (nicotine adenine dinucleotide), which plays a role in glycolysis and is needed for tissue respiration and fat synthesis. The amino acid tryptophan can be converted to niacin: 60 mg of tryptophan has the same response as 1 mg of niacin and is therefore declared as 1 NE (niacin equivalent). Several authors have hypothesized that this vitamin could influence aerobic power, which is an important factor for endurance performance in athletes (199). However, it has been reported that mega-dose intake can also have adverse effects on performance. This may be induced by the inhibiting effect of nicotinic acid on the mobilization of free fatty acid (FFA) from stored triglycerides. During exercise a reduced FFA availability will enhance CHO utilization, which in turn will lead to a higher rate of glycogen depletion. This has been shown to enhance subjective fatigue and to impair performance (13,85). The RDA has been set at 6.6 NEs per 1000 kcal or at least 13 NEs at caloric intakes of <2000 kcal (131). No data are available on niacin intake or on deficiencies in athletic populations, or on effects of niacin supplementation on performance.
PANTOTHENIC ACID (PA)
Pantothenic acid is a component of acetyl CoA, the intermediate citric acid cycle metabolite of CHO and fat metabolism. Williams stated in 1985 that some reports suggested a beneficial effect of PA supplementation but that conclusive data were not available (199). This has not changed until now. No data are available on PA intake or on deficiencies in athletes.
Supplementation with pharmacological doses as high as 1 g/day did not result in any performance improvement (13). The National Research Council concludes that there is insufficient evidence to set a RDA for pantothenic acid. The safe daily intake level is assumed to be 4-7 mg (131).
Folate functions as a coenzyme in amino acid metabolism and nucleic acid synthesis. The RDA for folate amounts to approximately 3 yg/kg body weight, resulting in a daily RDA of 200 yg for males and 180 yg for females (131). There are no controlled studies available on the effect of folate supplementation on physical performance, or on folate intake in athletes (13). Plasma folate levels, which may reflect folate intakes, were observed to increase in Tour de France participants, who ingested substantial amounts of vitamin preparations (164). Williams (202) cited recent research that folate supplementation would restore normal folate status to runners who were folate deficient, but did not improve performance capacity.
Biotin is an essential part of enzymes that transport carboxyl units and fix carbon dioxide in tissues. The conversion of biotin to active coenzyme depends on the availability of magnesium and ATP. Biotin plays an essential role in CHO, fat, propionate and branched chain amino acid metabolism. Biotin is produced in the lower intestine by microorganisms and fungi. No data are available on the quantitative absorption and there are insufficient data to establish a RDA for biotin. A range of 30-100 yg/day is provisionally recommended as a safe daily intake for adults (131). There are no studies available on supplementation effects, or on biotin intake or on intestinal synthesis in athletes (13).
VITAMIN C (ASCORBIC ACID)
Vitamin C is probably the most studied vitamin. Vitamin C is a water soluble antioxidant. It scavenges free radicals that cause cell damage and protects vitamin E, another antioxidant, from destruction. It participates in many enzymatic reactions by acting as an electron transmitter, and is involved in the synthesis of collagen and carnitine (the latter is needed for the transport of long chain fatty acids across the mitochondrial membrane prior to oxidation). Vitamin C enhances iron absorption in the gut. It is also needed for the biosynthesis of some hormones (14, 68, 131). Early studies performed during the Second World War showed that insufficient vitamin C lowered physical performance capacity in soldiers and increased the sensation of exhaustion and muscle pains during and after hard physical work. However, many of the studies performed at that time have now been criticized for their poor methodology, control and statistical design. More recently well controlled double blind studies have shown that a state of moderate vitamin C deficiency does not reduce physical performance in single intensive bouts of exercise. There are some indications that vitamin C may enhance the rate of heat acclimation (13). This may be of benefit to athletes involved in endurance competitions in the heat around different parts of the world. However, vitamin C supplementation did not improve performance in controlled studies. A study in long distance runners has shown that the supplementation of vitamin C prior to the run results in a decreased occurrence of respiratory infections (236). In general, vitamin C intake in athletes is sufficient, with the exception of individuals consuming a low caloric diet (12, 13, 28, 58, 68). Vitamin C is also a powerful antioxidant. This aspect is dealt with in Chapter 9.
VITAMIN E (ALPHA-TOCOPHEROL)
Vitamin E is an antioxidant and scavenges free radicals to protect cell membranes from lipid peroxidation. It functions in concert with vitamin C, beta-carotene and selenium, and also protects red blood cells from haemolysis (14, 131, 171). In the period 1970-1980, special attention was given to this vitamin after reported beneficial effects of its supplementation on oxygen consumption and physical performance. As is the case with vitamin C, many of these studies were also not well controlled or suffered from poor statistical design. Critical analysis of the literature and more recent results from well designed double blind studies did not bring any solid evidence for performance improvement (13, 14, 84, 171, 206). It has been observed that endurance athletes in general have low vitamin E serum levels. This may be an indication of either marginal vitamin E supply with food or increased usage in antioxidant defence mechanisms.
There are indications that vitamin E supplementation elevates the testosterone/cortisol ratio suggesting that vitamin E has a stress reducing effect on the body. It is able to reduce lipid peroxidation in both animals and humans as measured by an enhanced appearance of penthane in exhaled air. Studies at high altitude indicate that vitamin E can influence metabolic performance parameters and reduce penthane (an indirect marker of free radical induced cell damage) exhalation, suggesting that vitamin E may have a protective effect. However, it is not known which tissues undergo lipid peroxidation most during exercise. It may be that the most important site is tissue that is prone to some ischaemia during exercise, such as the gastrointestinal system, but not muscle.
Since it became possible to measure the effects on free radical pathology, attention has been given to the antioxidant properties of vitamin E. This aspect is dealt with in Chapter 9.
VITAMINS A, D AND K
Although the importance of the fat soluble vitamins A, D and K for health is beyond doubt (131), there are no studies available which indicate any significant effect of these vitamins on biochemical or physiological parameters concerned with physical performance capacity. Since these vitamins are potentially toxic, when taken in high doses for a prolonged period of time (with the exception of vitamin K), and daily intake in Western civilized countries is generally sufficient, there is no need for supplementation (13, 83, 84, 199).
Vitamin K serves a function in bone mineralization. This was found after the observation that the intake of anticoagulant drugs (vitamin K antagonists) influences bone formation processes. Accordingly, the role of vitamin K on bone formation and the prevention of osteoporosis is currently under study (237). Recently the effect of vitamin K supplementation, 10 mg/day, has been studied in eight female endurance athletes, four of whom had been amenorrhoeic for more than one year, while the remaining four had been using oral contraceptives. Such female endurance athletes have depressed oestrogen levels and may develop mineral loss from bone to an extent comparable to postmenopausal women. It was observed that in all subjects increased vitamin K intake was associated with a 15-20% increase in markers of bone formation and a parallel decrease of 20-25% in markers of bone resumption, suggesting an improved balance between bone formation and loss (238). Further research seems to be justified to determine whether long-term vitamin K supplementation is of benefit to bone health for the female athletic population.
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