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J. Mark Davis

The mechanisms of central fatigue are largely unexplored, but the central fatigue hypothesis suggests that increased brain serotonin (5-HT) can cause a deterioration in sport and exercise performance. There is now convincing evidence that exercise-induced increases in the plasma free tryptophan (f-TRP)/branched-chain amino acids (BCCA) ratio are associated with increased brain 5-HT and the onset of fatigue during prolonged exercise. Furthermore, when drugs are administered to alter brain 5-HT, they have the predicted effects on exercise performance. The influence of nutritional manipulations of f-TRP/BCCA on performance is less well established. The effects of BCCA supplementation on exercise performance are mixed, and the published studies often suffer from methodological flaws. Alternatively, dramatic reductions in f-TRP/BCCA and enhanced performance accompany carbohydrate feedings during prolonged exercise. However, it is difficult to distinguish between the effects of carbohydrate feedings on mechanisms that reside in the brain versus the muscles themselves.

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J. Mark Davis, Ralph S. Welsh, and Nathan A. Alderson


This study was designed to test the hypothesis that addition of chromium (Cr) to a carbohydrate-electrolyte drink would enhance the reported benefits of carbohydrate on exercise capacity during intermittent high-intensity shuttle running.


Eight physically active men performed 3 exercise trials while ingesting 6% carbohydrate-electrolyte (CHO), CHO plus chromium picolinate (400 μg) (CHO + Cr+3). or placebo (P) using a double-blind, counterbalanced design. Each trial consisted of 5 × 15 min bouts of shuttle running (walk, sprint, and run at 95 and 55% of estimated V̇O2max, separated by 3-min rest). This was followed by a fatigue test (running alternating 20-m lengths at 55 and 95% of estimated V̇O2, until fatigue).


During the standardized shuttle running, blood glucose was higher with both CHO and CHO + Cr+3 than P. Plasma free fatty acid was higher in P than both CHO and CHO + Cr+3 at 75 min of exercise and at fatigue. In the fatigue test, subjects ran longer with both CHO and CHO + Cr+3 than P.


The data confirm an ergogenic benefit of ingesting CHO during exercise designed to imitate sports like basketball, soccer, and hockey, but do not support the hypothesis that the addition of Cr would enhance this effect.

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Russell R. Pate, Bonnie J. Miller, J. Mark Davis, Chris A. Slentz, and Lisa A. Klingshirn

This study was designed to observe iron status and prevalence of iron deficient conditions in adult female habitual runners (n=111) and inactive females of comparable age (n=65). The runners were significantly lower (p<.05) than the reference group in mean serum ferritin (SF), total iron binding capacity, and red blood cell count, but significantly higher (p<.05) in mean corpuscular hemoglobin. The groups did not differ significantly in hemoglobin concentration, hematocrit, serum iron, percent saturation of transfenin, or red cell protoporphyrin. Chi square analysis indicated that iron depletion (SF <20 ng·ml-1) was significantly more prevalent (p<.005) in the runners than in the controls. Anemia was extremely rare in both groups. A multiple regression analysis revealed significant negative associations between serum ferritin and coffeeltea intake (p<.001) and running activity (p<.05). These results indicate that habitual runners, as compared with inactive women, are at increased risk for iron deficient states but that full-blown anemia is a rare consequence of this deficient iron status.

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J. Mark Davis, Catherine J. Carlstedt, Stephen Chen, Martin D. Carmichael, and E. Angela Murphy

Quercetin, a natural polyphenolic flavonoid substance present in a variety of food plants, has been shown in vitro and in animal studies to have widespread health and performance benefits resulting from a combination of biological properties, including antioxidant and anti-inflammatory activity, as well as the ability to increase mitochondrial biogenesis. Little is known about these effects in humans, however, especially with respect to exercise performance. The authors determined whether quercetin ingestion would enhance maximal aerobic capacity and delay fatigue during prolonged exercise in healthy but untrained participants. Twelve volunteers were randomly assigned to 1 of 2 treatments: (a) 500 mg of quercetin twice daily dissolved in vitamin-enriched Tang or (b) a nondistinguishable placebo (Tang). Baseline VO2max and bike-ride times to fatigue were established. Treatments were administered for a period of 7 days using a randomized, double-blind, placebo-controlled, crossover study design. After treatment both VO2max and ride time to fatigue were determined. Seven days of quercetin feedings were associated with a modest increase in VO2max (3.9% vs. placebo; p < .05) along with a substantial (13.2%) increase in ride time to fatigue (p < .05). These data suggest that as little as 7 days of quercetin supplementation can increase endurance without exercise training in untrained participants. These benefits of quercetin may have important implications for enhancement of athletic and military performance. This apparent increase in fitness without exercise training may have implications beyond that of performance enhancement to health promotion and disease prevention.

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William A. Burgess, J. Mark Davis, William P. Bartoli, and Jeffrey A. Woods

The effects of ingesting a low dose of CHO on plasma glucose, glucoregulatory hormone responses, and performance during prolonged cycling were investigated. Nine male subjects cycled for 165 min at ≈67% peak VO2 followed by a two-stage performance ride to exhaustion on two occasions in the laboratory. Every 20 min during exercise, subjects consumed either a flavored water placebo (P) or a dilute carbohydrate beverage (C). Blood samples were collected immediately before, every 20 min throughout, and immediately after exercise. Plasma was analyzed for glucose, lactate, free fatty acids (FFA), and various glucoregulatory hormones. VO2, RER, heart rate, perceived exertion, and exercise performance were also measured. Lactate, FFA, epinephrine, norepinephrine, ACTH, cortisol, and glucagon increased with exercise whereas glucose and insulin decreased (p≤05). Except for a small difference in glucose at 158 min of exercise and at exhaustion, no significant differences were found between drinks for any of the variabfes studied (p ≥ 05). Ingestion of 13 g carbohydrate per hour is not sufficient to maintain plasma glucose, attenuate the glucoregulatory hormone response, and improve performance during prolonged moderate intensity cycling.

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Edward E. Pistilli, David C. Nieman, Dru A. Henson, David E. Kaminsky, Alan C. Utter, Debra M. Vinci, J. Mark Davis, Omar R. Fagoaga, and Sandra L. Nehlsen-Cannarella

Immune changes in 75 younger (age 37.4 ± 0.9 years) and 23 older (57.0 ± 1.4 years) runners were compared after a competitive marathon, with blood samples collected pre- and immediately and 1.5 hr postrace. Race times were slower for the older group (4.7 ± 0.2 vs. 4.3 ± 0.1 hr, p = .015), but both groups performed at similar intensity (83.4 ± 0.9 vs. 82.9 ± 0.5% HRmax). The pattern of change in plasma cortisol, epinephrine, growth hormone, and blood leukocyte subsets did not differ significantly between the groups postrace. Blood lymphocyte counts were 20–24% lower in the older runners at each time point because of reduced T-cell counts. Postrace, plasma levels of IL-1ra, -10, -6, and -8 rose strongly in all runners, and salivary IgA secretion rate decreased, but no group differences in the pattern of change were noted. In conclusion, younger and older runners experienced similar hormonal and immune changes after running a marathon.

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Dru A. Henson, David C. Nieman, Andy D. Blodgett, Diane E. Butterworth, Alan Utter, J. Mark Davis, Gerald Sonnenfeld, Darla S. Morton, Omar R. Fagoaga, and Sandra L. Nehlsen-Cannarella

The influence of exercise mode and 6% carbohydrate (C) versus placebo (P) beverage ingestion on lymphocyte proliferation, natural killer cell cytotoxicily (NKCA), Interleukin (IL)-1ß production, and hormonal responses to 2.5 hr of intense running and cycling (~75% V˙O2max) was measured in 10 triathletes serving as their own controls. The C versus P condition (but not exercise mode) resulted in higher plasma glucose concentrations, lower plasma cortisol concentrations, reduced poslexercise lymphocytosis and NKCA, and a lessened T-cell reduction during recovery. No condition or mode effects were observed for concanavalin A and phytohemagglutinin-induced lymphocyte proliferation. Significant mode (but not condition) effects were observed for lipopolysaccharide-induced IL-1ß production over time. However, when expressed per monocyte, the mode effect was abolished and a sustained suppression in IL-1 ß/monocyte was observed in all sessions throughout recovery. These data indicate that carbohydrate ingestion significantly affects plasma glucose and cortisol concentrations, blood lymphocyte counts, and NKCA, whereas exercise mode has no effect on these parameters.