The first of this two-part series, published in the June 2012 issue of Food Technology, reviewed the general principles of nutrition and athletic performance. The broad nutrient and food groups, general categories of caloric requirements, hydration, and electrolyte needs were examined.
A spectrum of studies and position statements among credible sports nutrition organizations underscore the importance of proper nutrition and adequate hydration as fundamental principles for optimal athletic performance. This column examines the evidence associated with specific foods and dietary components, and how their intake may affect performance relative to muscle power, cellular energetics, and immune function in well-trained competitors.
Nitrate supplementation is a popular practice. Common dietary nitrate sources from fruit and vegetables include beetroot juice (~143 mg/100 g), celery (>250 mg/100 g), spinach (23.9–387.2 mg/100 g), carrot (92–195 mg/100 g), cabbage (25.9–125 mg/100 g), and endive (100–250 mg/100 g) (Santamaria, 2006). The typical nitrate consumption from fruits and vegetables in the United States is about 73–157 mg/day, which is likely to increase following the promotion of these foods for improved health.
Some recent studies suggest the consumption of nitrate, particularly from beetroot juice, may increase athletic performance among well-trained cyclists and runners (Lansley et al., 2011; Vanhatalo et al., 2010; Larson et al., 2007). Improved performance among elite athletes was assessed by a) power output and b) reduced oxygen cost, following short periods of consumption up to about 700 mg nitrate, primarily from beetroot juice or supplement.
Relative to these intake levels, JECFA European Commission’s Scientific Committee on Food set an ADI (acceptable daily intake) for nitrate at 0–3.7 mg/kg body weight or about 260 mg/day for a typical 70.6 kg person. Similarly, the U.S. EPA established a Reference Dose nearly twice this level. A recent assessment of acute and chronic intake of nitrates from foods, especially fruit and vegetables, suggests an ADI for nitrate of 642 mg NO3/day based on NHANES 2007–2008 data. However, despite these conservative recommendations, some investigators suggest, in the absence of documented benefits of about 500 mg nitrate/day, that caution should be directed to athletes and those presenting cardiovascular disease and hypertension (Allen, 2011; Jones, 2011).
What food scientists and physiologists understand about beetroot juice and other nitrate-containing foods is that any benefits in athletic performance are likely mediated through the metabolic conversion of dietary nitrate (NO3) to biologically active nitrite (NO2) and then to nitric oxide (NO). Nitric oxide functions to regulate local blood flow and capillary gating and to participate in muscle contractility, glucose and calcium balance, and mitochondrial respiration (Vanhatalo et al., 2010). Beetroot juice also contains polyphenols such as resveratrol and quercetin, which may enhance aerobic capacity at the mitochondrial level (Lansley et al., 2010). Dietary nitrates via sources of fruit and vegetables, seem to lower the oxygen cost of exercise by reducing the total ATP demand of muscle force production—less ATP yields the same amount of work (watts). The rate of degradation of phosphocreatine reserves may decline, possibly due to increased efficiency of calcium transport by the sarcoplasmic reticulum Ca-ATPase, thereby increasing tolerance to high-intensity exercise (Bailey et al., 2010).
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While these kinds of results among well-trained athletes are interesting, the impact of nitrate-rich foods on the performance of the weekend physical activity enthusiast is uncertain. It may be that these individuals would do well simply to achieve modest muscle tone and mass along with some minimum cardiovascular integrity by avoiding the extreme or high-risk levels of exertion, even though dietary nitrates may result in transient augmentation of performance.
Beta-glucan and Immunonutrition
It has long been recognized that elite training or regular/sustained intensive exercise is associated with lower threshold or increased risk for upper respiratory tract infections (URTI) (Carpenter et al., 2012). These athletes typically present readily measurable reductions in neutrophils, NK cells, T-cells, and B-cells. Concomitant reductions in the cytokine response have also been noted in association with a spectrum of exercise and lifestyle stressors as well as dietary interventions (König et al., 2000; Mackinnon, 1997).
Beta-glucans are chains of polysaccharides of D-glucose monomers linked by β-glycosidic bonds. They occur in diverse forms, are almost ubiquitous, and are derived from cellulose in plant sources, the bran of oats (linear forms), and branched forms found in the cell wall of Saccharomyces cerevisiae, in certain mushrooms, and even in bacteria. These forms maintain structural integrity through the post-consumption digestion process, yet may be to some extent metabolized by select microbial flora in the digestive tract. What is clinically remarkable about β-glucans, specifically the β 1,3/1,6 gluco-polysaccharide used in the most recent studies/trials, lies in their capacity to activate neutrophils (Rop et al., 2009).
The most recent work with β-glucan interventions demonstrated a) statistically significant higher concentrations of monocytes, neutrophils, NK cell activity, polymorphonuclear respiratory burst activity, and lymphocyte proliferation and b) significantly higher levels of cytokines (IL-2, IL-4, IL-5, and IFN-gamma) following challenges simulating bacterial stimulation (Carpenter et al., 2012). Placebo-controlled studies indicate higher levels of IL-4 to IL-6 and IL-7 to IL-10 following exercise in subjects pre-treated with β-glucan (Chen & Seviour, 2007). More importantly, a well-designed study that followed marathon athletes through a meaningful pre-event treatment period and a controlled appropriate post-race observational phase found a significantly decreased URTI incidence compared with placebo (Talbott & Talbott, 2009).
There are a few studies that find no significant differences between β-glucan and placebo groups across immunologic markers or their clinical correlates (i.e., URTI incidence) (Walsh et al., 2011; Nieman et al., 2008). Threats to validity are myriad in all these investigations including source of the β-glucan, timing, duration and character of administration, and of pre-treatment phase, medical history, and other characteristics of the athlete-subjects including possible interactions between β-glucan and other bioactive substances or pharmacologic agents not logged, variations in the intensity and character of the competitive event, duration of post-event observation, and the nature of the assays and flow-cytometric and instruments employed at each stage.
Branched-chain Amino Acid Supplementation
Branched-chain amino acids (BCAA) are essential amino acids (leucine, isoleucine, valine) characterized by nonlinear aliphatic side chains. During the past two decades, athletes have increasingly and consciously consumed BCAAs as a training table constituent. The dominant theme of research and clinical experience with the BCAA supplementation demonstrates that mechanisms are divergent and results inconclusive.
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One of the claims that drives continued use of BCAAs involves a possible decrease in “central fatigue,” a central nervous system phenomenon. The hypothesis for central fatigue, which was published in the late 1980s and early 1990s has to do with enhanced carrier (albumin) mediated carriage and entry into the central nervous system (CNS). A higher plasma level of BCAAs results in decreased binding of tryptophan (TRP), altered BCAA/TRP ratio, transport competition, and possibly decreased CNS serotonin (Meeusen & Watson, 2007). Because serotonin plays a significant role in sleep onset and mood, mitigation of fatigue, anergy, and depression might be possible (Blomstrand et al., 1988) and might lead to increase in exercise capacity and mental performance (Blomstrand, 2006). Several randomized, controlled trials were inconclusive, with most failing to demonstrate any performance benefit to the user of BCAA supplements despite increases in plasma BCAAs administered before, during, and after exercise (Sharp & Pearson, 2010; Negro et al., 2008; Watson et al., 2004; Blomstrand, 2001; Madsen et al., 1996; van Hall et al., 1995).
Another provocative line of investigation reported reduction of lactate dehydrogenase (LDH) in conjunction with an increase in testosterone producing an anabolic profile and attenuating training or competition-induced muscle damage (Sharp & Pearson, 2010). A recent Italian study with very small numbers of human subjects suggests that BCAA supplementation enhances recovery of mononuclear cell proliferation and alters cytokine synthesis such that the lymphocyte response to prolonged and intense exercise leads to a more adaptive Th1 immune regulation (Negro et al., 2008).
Several double-blinded studies examined the effects of BCAA supplementation during periods of resistance training and running (Sharp & Pearson, 2010; Greer et al., 2007; Kersick et al., 2006). These studies indicated subjective decreases in muscle soreness and fatigue, statistically significant increases in speed over marathon distances, and significant increases in bench and leg press performance. Other studies reported similar increases in performance with placebo and with whey and casein protein (Kersick et al., 2006). Most acid whey protein powder supplements contain 200–250 mg BCAA per g protein (USDA, 2012).
Probiotics are live microorganisms, which when consumed in adequate amounts, confer a health effect on the host (WHO, 2001). Probiotics should meet the World Health Organization (WHO) guidelines, which include knowledge of dosage for health benefits, mode of action, genetic stability, antimicrobial resistance, and data on the molecular level of interaction between probiotic, host flora, and host mucosal cells (WHO, 2002).
A systematic review of the literature failed to identify any investigation that directly looked at athletic performance and probiotics (Nichols, 2007). Only two articles speculated about probiotics augmenting T-lymphocyte responses of fatigued athletes. In a study not included in the review, following a 4-week course of L. acidophilus, athletes with impaired immune performance demonstrated an increased capacity to produce salivary interferon-gamma (Clancy et al., 2006). Another relevant study demonstrated L. casei may improve post-exercise immune function (Pajol et al., 2000). More recent investigations among active and competitive athletes suggest daily intake for 11–16 weeks of some probiotic strains may reduce the frequency of URTI, and may be more beneficial among males (Gleeson et al., 2011; West et al., 2011).
In an as yet unpublished report (Shing et al., 2011) based upon a randomized, double-blind, crossover, placebo-controlled study in Australia, it was found that heat stress linked with intestinal permeability or “leakiness,” as measured by serum lipopolysaccharide (LPS) was significantly increased when 12 runners trained >35°C and humidity >40%. When compared with placebo, a 4-week administration of high-dose, nine-strain probiotic cocktail was strongly associated with a significant decrease in the level of serum LPS. Previous investigations indicate intestinal permeability and gut inflammatory sequelae may be reduced through modulation of gut microflora (Frazier et al., 2011).
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Exploring the literature on specific nutritional components and athletic performance raises important issues. Most investigators and athletes would agree that nutrition does make a difference in performance. What seems unresolved are questions about the clinical significance of specific nutrients and substances that mediate everything from mitochondrial metabolism to immunogenetics.
Future research must address the following issues: efficacy, long-term safety, the regulatory environment, and the ethical context on a case-by-case basis as well as in the gestalt. More applied research is encouraged in efforts to gain insights into athletic performance and unintended acute and chronic consequences within the personal and medical context.
There is already a growing and interesting clinical and sports nutrition literature focusing on nitrates and cardiovascular disease, on β-glucans and specific oncologic and inflammatory processes, on branched-chain amino acids and neurological pathologies, and probiotics and inflammatory diseases including obesity. The entire discussion of nutrition and athletic performance gives us an optimistic window on some of the promises and challenges of nutrition and food science in the new millennium.
References cited in this column are available from the authors.
Peter Pressman, M.D., Contributing Editor
CDR, Medical Corps, U.S. Navy, Director Expeditionary Medicine, Task Force for Business & Stability Operations
Roger Clemens, Dr.P.H., Contributing Editor
Chief Scientific Officer, Horn Company, La Mirada, Calif.