Many components of the immune system exhibit change after intense and prolonged exertion, reflecting the physiologic stress the body is experiencing. These changes include decreases in blood cells counts and function of natural killer (NK) and T cells, nasal and salivary IgA output, blood polymorphonuclear respiratory burst activity (PMN-RBA), lung and peritoneal macrophage function, and the skin delayed-type hypersensitivity response and an increase in blood cell counts of neutrophils and monocytes and in plasma levels of pro- and anti-inflammatory cytokines and chemokines (20-26,33).
The influence of nutritional supplements and drugs on the immune response to heavy exercise workloads is an active area of research by multiple investigators (19-21,24-26). Carbohydrate supplementation attenuates exercise-induced increases in blood neutrophil and monocyte cell counts, plasma cytokines, and stress hormones but is largely ineffective against changes in other immune components including NK and T cell function and salivary IgA output (19-21,23). Carbohydrate may influence some aspects of immunity through a blood glucose and hormonal interaction (20,21). Quercetin supplementation in one study had a strong effect in decreasing upper respiratory tract infection (URTI) incidence after a 3-d period of intensified exercise (24,25). Quercetin may operate as a direct antiviral molecule without countering exercise-induced immune changes (25). Other nutritional supplements and drugs have had inconsistent, ineffectual, or counterproductive influences on exercise-induced immune changes (14,19).
The β-glucans are a heterogeneous group of polysaccharides found in the bran of oat and barley cereal grains, the cell wall of baker's yeast, and many kinds of mushrooms (2,3,10,13). They vary in macromolecular structure, solubility, molecular mass (50 to 2300 kDa), and biologic activity depending on the source from which they are derived and how they are prepared.
The β-glucans from yeast and mushrooms consist of d-glucose linked in the β-(1→3) position with small numbers of β-(1→6) side-branch glucose linkages. Oat and barley β-glucans are unbranched, with β-(1→3) and β-(1→4) linkages (2,3,13). Humans lack small intestine enzymes to separate the glucose molecules of β-glucans, and they pass to the large intestine undigested. Oat and barley β-glucan is a fermentable, viscous fiber that decreases LDL-cholesterol by interfering with enterohepatic recirculation of cholesterol and bile acids (18,28). Consumption of oat and barley products with at least 3 g of β-glucan soluble fiber per day is recommended as part of a heart healthy diet and has been given GRAS status by the Food and Drug Administration (28).
Receptors for β-glucan include scavenger receptors, lactosylceramide, dectin-1, and complement receptor 3 and are among the pattern recognition receptors that can recognize non-self-structures (2). These receptors have been identified on a wide variety of cell types including macrophages, dendritic cells, NK cells, neutrophils, some types of T cells, epithelial cells, vascular endothelial cells, and fibroblasts (3,30). Thus, despite the lack of enzymatic breakdown in the small intestine, the widespread distribution of β-glucan receptors throughout the body suggests some degree or unique method of absorption. Evidence using rodent models indicates that the bioavailability of β-glucans is approximately 4-5% and that soluble glucans can translocate from the gastrointestinal tract to the systemic circulation (6,9,30,31). The exact pathway by which β-glucans interact with the immune system is still undetermined, and no conclusive data from human subjects have been published demonstrating that β-glucans can enter lymphoid tissues or the circulation from the small intestine (29).
Studies with rodents, fish, poultry, and swine indicate that oral β-glucan ingestion stimulates innate immune defenses and antitumor responses and increases resistance to a wide variety of infections (1,4,5,12,13,15-17,32,35,36). The β-glucans may activate macrophages and neutrophils directly, stimulating their phagocytic, cytotoxic, and antimicrobial activities through several types of cellular receptors including dectin-1, lactosylceramide, Toll-like receptors 2 and 6, scavenger receptors, and complement receptor 3 (2,3,13). However, not all reports indicate that oral β-glucan ingestion exerts immunomodulatory effects, and a variety of factors may play a role including the type and timing of β-glucan supplementation (8,27).
Rodent studies indicate that oat β-glucan supplements offset the increased risk of infection associated with exercise stress through augmentation of macrophage and neutrophil function (4,5,16,17). In one study using mice, ingestion of oat β-glucan for 10 d before intranasal inoculation of herpes simplex virus 1 (HSV-1) countered the increase in morbidity and mortality and the decrease in macrophage antiviral resistance after exhaustive 140-min exercise bouts for three consecutive days (4). In a follow-up study using the same research design, these investigators showed that depletion of lung macrophages using clodronate negated the beneficial effects of β-glucan, indicating that these immune cells are at least partially involved (16).
Using a research design similar to these rodent studies, we investigated the effects of oat β-glucan supplementation on chronic resting immunity, exercise-induced changes in immune function, and URTI incidence in human endurance athletes. Trained cyclists were chosen as subjects to ensure that the exercise duration and frequency used in the rodent studies could be attained (4,5). Our review of the published literature led us to hypothesize that 5.6 g·d−1 of oat β-glucan compared to placebo during an 18-d period would augment immune function during normal training, decrease URTI incidence rates, and counter immune changes after a period of intensified exercise. The 5.6-g·d−1 dose of oat β-glucan was based on an extrapolation of the dose used in the rodent studies (4,5,16,17).
Forty trained male cyclists were recruited as experimental subjects through local and collegiate cycling clubs. Written informed consent was obtained from each subject, and the experimental procedures were approved by the institutional review board of Appalachian State University.
At 2 to 3 wk before the first test session, subjects reported to the university Human Performance Lab for orientation and measurement of cardiorespiratory fitness. V˙O2max was determined using a graded maximal protocol (25-W increase every 2 min starting at 150 W) with the subjects using their own bicycles on CompuTrainer™ Pro Model 8001 trainers (RacerMate, Seattle, WA). Oxygen uptake and ventilation were measured using the MedGraphics CPX metabolic system (MedGraphics Corporation, St. Paul, MN). HR was measured using a chest HR monitor (Polar Electro, Inc, Woodbury, NY). Basic demographic and training data were obtained using a questionnaire.
Subjects agreed to avoid the use of large-dose vitamin/mineral supplements (>100% of recommended dietary allowances), herbs, and medications known to affect immune function during the entire 31-d study. During orientation, a dietitian instructed the subjects to follow a diet moderate in carbohydrate (using a food list) during the weekend before and the 3-d intensified exercise period. Subjects recorded food intake in a 3-d food record before the first exercise test session. The food records were analyzed using a computerized dietary assessment program (Food Processor; ESHA Research, Salem, OR).
The cyclists were randomized to β-glucan or placebo groups. Nineteen of 20 subjects randomized to the β-glucan group and 17 of 20 subjects randomized to the placebo group completed all phases of the study. Under double-blind procedures, subjects received β-glucan (5.6 g·d−1) or placebo 600-mL beverage supplements for 14 d before, during a 3-d period of intensified exercise, and 1 d after the training period (thus, 18 d in total). The 600-mL β-glucan beverage supplement was prepared by the Gatorade Sports Science Institute (Barrington, IL) using Gatorade and 10.37 g of Oatvantage, a 54% oat β-glucan concentrate (GTC Nutrition, Golden, CO). The Gatorade and Oatvantage were combined using heat and high-shear mixing techniques. The placebo beverage was prepared in the exact same manner except that cornstarch was substituted for Oatvantage. Subjects ingested the 600-mL supplement beverages in two 300-mL doses each day before their first and last meals on an empty stomach.
Before and after the first 14 d of supplementation, subjects provided blood samples at 8:00 a.m. after an overnight fast and having avoided exercise training for at least 12 h. Subjects then came to the laboratory for three consecutive days and cycled for 3 h from 3:00 to 6:00 p.m. at approximately 57% maximal watts (Wmax). This workload was chosen in accordance with previous research in our laboratory using a similar design (25). Subjects reported to the laboratory not having ingested energy in any form after 12:30 p.m. During the 3-h exercise period, subjects consumed water at a rate of 0.5-1.0 L·h−1 with no other beverages or food allowed. Blood samples were immediately obtained after completing the third exercise bout (6:00 p.m.) and then again 14 h later (8:00 a.m., overnight-fasted). Subjects continued ingestion of the supplements for one additional day and trained normally throughout a 2-wk period while being monitored for incidence and severity of URTI.
During the test sessions, experimental subjects cycled using their own bicycles on CompuTrainer™ Pro Model 8001 trainers with the exercise load set at approximately 57% Wmax. Metabolic measurements were made every 30 min of cycling using the MedGraphics CPX metabolic system to verify workload.
Blood samples were drawn from an antecubital vein with subjects in the supine position. Routine complete blood counts were performed by our clinical hematology laboratory using a Coulter STKS instrument (Coulter Electronics, Hialeah, FL) and provided hemoglobin and hematocrit for the determination of plasma volume change using the method of Dill and Costill (7). Other blood samples were centrifuged in sodium heparin or EDTA tubes, and plasma was aliquoted and then stored at −80°C before plasma cytokine analysis.
Plasma cytokine measurements.
Total plasma concentrations of interleukin (IL) 1 receptor antagonist (IL-1ra), IL-6, IL-8, and IL-10 were collected in EDTA and determined using quantitative sandwich ELISA kits provided by R&D Systems, Inc (Minneapolis, MN). All samples and provided standards were analyzed in duplicate. High-sensitivity kits were used to analyze preexercise samples of IL-6 and IL-10. The minimum detectable concentration of IL-1ra was <22 pg·mL−1, IL-6 <0.70 pg·mL−1, IL-6 (high sensitivity) <0.039 pg·mL−1, IL-8 <3.5 pg·mL−1, IL-10 <3.9 pg·mL−1, and IL-10 (high sensitivity) <0.5 pg·mL−1. To improve sensitivity in the detection of IL-8, we used SOFTmax™ analysis software (Molecular Devices, Sunnyvale, CA). When applicable, pre- and postexercise samples were analyzed on the same assay plate to decrease interkit assay variability.
Leukocyte mRNA extraction and cDNA synthesis.
The QIAampRNA Blood Mini Kit Protocol (Cat. no. 52304; Qiagen, Valencia, CA) was used to extract mRNA. Four-milliliter aliquots of whole blood collected in EDTA were purified for RNA from each subject. Briefly, erythrocytes were selectively lysed, and leukocytes were recovered by centrifugation. Samples were briefly centrifuged through a QIAshredder spin column, ethanol was added to adjust binding conditions, and the sample was applied to a QIAamp spin column. RNA was bound to the silica gel membrane during a brief centrifugation step. Contaminants were washed away, and total RNA was eluted in 30 μL of RNase-free water.
The extracted RNA (7.5 μL of sample) was dissolved in diethylpyrocarbonate-treated water and was quantified spectrophotometrically at a 260-nm wavelength. RNA was reverse-transcribed into cDNA in a 50-μL reaction volume containing 19.25 μL of RNA in RNase-free water, 5 μL of 10× RT buffer, 11 μL of 25 mmol·L−1 MgCl2, 10 μL of deoxy-NTP mixture, 2.5 μL of random hexamers, 1 μL of RNase inhibitor, and 1.25 μL of multiscribe reverse transcriptase (50 U·μL−1). Reverse transcription was performed at 25°C for 10 min, 37°C for 60 min, and 95°C for 5 min, followed by quick chilling on ice, and storage at −20°C until subsequent amplification.
Quantitative real-time polymerase chain reaction analysis.
Quantitative real-time polymerase chain reaction (RT-PCR) analysis was done as per manufacturer's instructions (Applied Biosystems, Foster City, CA) using TaqMan® Gene Expression Assays. DNA amplification was carried out in 12.5 μL TaqMan Universal PCR Master Mix (AmpliTaq Gold DNA Polymerase, Passive Reference 1, Buffer, dNTPs, AmpErase UNG), 1 μL of cDNA, 9 μL of RNase-free water, and 1.25 μL of 18S primer (VIC) and 1.25 μL of a primer (FAM; for endogenous reference and target cytokine) in a final volume of 25 μL per well. Human control RNA (calibrator RNA) was also used and served as a calibrator for each plate. Samples were loaded in a MicroAmp 96-well reaction plate. Plates were run using ABI Sequence Detection System (Applied Biosystems). After 2 min at 50°C and 10 min at 95°C, plates were coamplified by 50 repeated cycles, of which one cycle consisted of a 15-s denaturing step at 95°C and a 1-min annealing/extending step at 60°C. Data were analyzed by ABI software using the cycle threshold, CT, which is the value calculated and is based on the time (measured by PCR cycle number) at which the reporter fluorescent emission increases beyond a threshold level (based on the background fluorescence of the system), and it reflects the cycle number at which the cDNA amplification is first detected. We have previously reported the detailed methodology concerning the dual amplification technique (21,23). Samples were run in duplicate, and the intra- and interassay coefficients of variation were determined to be 1.69% and 1.65% for the ΔCT, respectively.
Calculations for relative quantification.
Quantification of mRNA expression for leukocyte IL-8, IL-10, and IL-1ra was calculated using the ΔΔCT method as described by Livak and Schmittgen (11). This method uses a single sample, the calibrator sample, for comparison of every unknown sample's gene expression. This method of analysis and quantification has been shown to give similar results as the standard curve method (34). Briefly, ΔCT (CT(FAM) − CT(VIC)) was calculated for each sample and calibrator. ΔΔCT (ΔCT(calibrator) − ΔCT(sample)) was then calculated for each sample and relative quantification was calculated as 2ΔΔCT. Initial exclusion criteria consisted of FAM CT ≥ 40 and VIC CT ≥ 23.
Blood cell counts and hormones.
Blood samples were drawn from an antecubital vein with subjects in the supine position. Routine complete blood counts were performed in our clinical hematology laboratory, and these provided leukocyte subset counts, hemoglobin, and hematocrit. Other blood samples were centrifuged in sodium heparin or EDTA tubes, with plasma aliquoted and then stored at −80°C. Blood samples assayed for epinephrine were drawn into chilled tubes containing EGTA and glutathione and were centrifuged; the plasma is then stored at −80°C until analysis. Plasma concentrations of epinephrine were determined by competitive enzyme immunoassay (Labor Diagnostika Nord, Nordhorn, Germany) using the microtiter plate format. The intra- and interassay coefficients of variation for epinephrine were 11% and 14%. Sensitivity of the epinephrine assay for the plasma samples is 11 pg·mL−1.
Natural killer cell activity.
Natural killer cell activity (NKCA) was assessed using a modification of a flow cytometry assay as we have described previously (25). Peripheral blood mononuclear cells (effector cells = 3.75 × 106 cells·mL−1) were isolated from heparinized blood by density gradient centrifugation with Ficoll-Hypaque. K562 target cells (1 × 106 cells·mL−1) were labeled for 20 min with 3,3-dioctadecyloxacarbocyanine perchlorate (DiO) reagent [0.01 mL of 3 mmol·L−1 DIOC18(3) (Sigma Chemical Company, St. Louis, MO) in DMSO per milliliter of cell suspension], washed, and resuspended to a concentration of 5 × 105 cells·mL−1. Effector and target cells were added to individual tubes (final volume = 0.9 mL) to yield E/T ratios of 60:1, 30:1, 15:1, 7.5:1, 3.8:1, and 1.9:1. All tubes received 0.1 mL of a 500-μg·mL−1 solution of propidium iodide (PI; Sigma Chemical Company). The tubes were vortexed, and the cells were pelleted by centrifuging for 15 s and were then incubated for 2 h at 37°C in a 5% CO2 incubator. At the end of the incubation, the tubes were vortexed and placed on ice, and the cells were analyzed on a flow cytometer for 30 min. DiO-labeled target cell membranes emit a green fluorescence (FL1) and PI-labeled compromised cells emit a red fluorescence (FL3). The percentage of target (green) cells that were also compromised (red) was determined. Control tubes received target cells and PI alone and effector cells and PI alone. The results were acceptable if the percentage of compromised cells in these controls was <5%. Plots of the percentage of compromised target cells at the various E/T ratios were constructed, and linear regression analysis was performed. The results are expressed as percent lysis of DiO-stained K562 cells at a 20:1 E/T ratio.
Phytohemagglutinin-stimulated lymphocyte proliferation.
The mitogenic response of lymphocytes was determined in whole blood culture using phytohemagglutinin (PHA) at optimal and suboptimal doses previously determined by titration experiments. Heparinized venous blood was diluted 1:10 with complete media consisting of RPMI-1640 supplemented with 5% heat-inactivated fetal bovine serum, penicillin, streptomycin, sodium pyruvate, l-glutamine, β2-mercaptoethanol, and Mito+™ Serum Extender (Cat. no. 355006; Becton Dickinson Immunocytometry Systems, San Jose, CA). PHA was prepared in RPMI-1640 media at a concentration of 1 mg·mL−1 and was then further diluted with complete media to the optimal and suboptimal working concentrations (6.25 and 3.13 μg·mL−1, respectively). A 100-μL aliquot of the diluted blood was dispensed into each of triplicate wells of a 96-well flat-bottom microtiter plate. To each well, 100 μL of the appropriate mitogen dose was added. Control wells received complete media instead of mitogen. After a 72-h incubation at 37°C and 5% CO2, the cells were pulsed with 1 μCi of thymidine (methyl)-3H (New England Nuclear, Boston, MA) prepared with RPMI-1640. After pulsing, cells were incubated for an additional 4 h before harvesting. The radionucleotide incorporation was assessed using a Wallac 1209 RackBeta liquid scintillation counter (LKB Wallac, Inc., Gaithersburg, MD) with the results expressed as experimental minus control counts per minute (cpm).
Whole blood PMN oxidative burst activity assay.
One hundred microliters of whole blood collected in heparin was dispensed into two 12 × 75-mm tubes labeled 1 and 2. Twenty-five microliters of working dihydrorhodamine 123 (DHF-123) solution (Sigma Chemical Company) was added to each of tubes 1 (endogenous respiratory burst activity) and 2 (stimulated respiratory burst activity). The DHF-123 was prepared from a stock solution of 5 mg·mL−1 as follows: 10 μL of the stock solution was added to 990 μL of a 1:1 ratio of phosphate-buffered saline and glucose. The final concentration of DHF-123 was 1 μg·mL−1. The tubes were incubated at 37°C for 15 min. After the incubation period, 10 μL of the working phorbol 12-myristate 13-acetate (PMA) solution (Sigma Chemical Company) was added to tube 2. The PMA was prepared from a stock solution of 1 mg·mL−1, 10 μL of the stock solution was added to 990 μL of a 1:1 ratio of phosphate-buffered saline and glucose. The final concentration of PMA was 100 ng·mL−1. Tubes were incubated again for 15 min at 37°C. After the incubation period, the tubes were placed on ice. Samples were processed on Q-Prep Instrument (Beckman Coulter, Fullerton, CA) using ImmunoPrep Reagent (Beckman Coulter). This allowed for erythrocyte lysis, as well as stabilizing and fixing of the blood cells in one step without the trauma of mixing, vortexing, and centrifuging of samples. Samples were analyzed on a flow cytometer (Epics XL-MCL; Beckman Coulter) using forward scatter and side scatter and were gated on granulocytes. A total of 5000 cells were counted in each reaction tube. The mean channel number for the fluorescence peaks was determined, and the PMN oxidative burst activity was calculated by subtracting the mean channel number of the endogenous respiratory burst activity from the mean channel number of the stimulated respiratory burst activity.
Subjects filled in a health log during the 18-d supplementation period and the 2-wk period after intensified exercise, listing codes for symptoms of URTI. The following health codes were recorded daily in accordance with previous investigations by our research team (25): 1) No health problems today; 2) Sick with cold symptoms (runny, stuffy nose, sore throat, coughing, sneezing, and colored discharge); 3) Sick with flu symptoms (fever, headache, general aches and pains, fatigue and weakness, chest discomfort, and cough); 4) Sick, with nausea, vomiting, and/or diarrhea; 5) Muscle, joint, or bone problems/injury; 6) Allergy symptoms; and 7) Other health problems (describe). A URTI episode was recorded if cold or flu symptoms persisted for 2 d or longer.
Data are expressed as mean ± SE. Data in Table 1 were compared between groups using Student's t-tests. Data in Tables 2 and 3, and Figures 1-4 were analyzed using a 2 (groups) × 4 (time points) repeated-measures ANOVA. When Box's M suggested that the assumptions necessary for the univariate approach were not tenable, the multivariate approach to repeated-measures ANOVA was used (Pillais trace). When interaction effects were significant (P ≤ 0.05), pre- to postexercise changes were calculated and compared between oat β-glucan and placebo groups using Student's t-tests, with significance set at P ≤ 0.05. The total number of days with URTI symptoms was compared between groups using Student's t-tests. The incidence of URTI during the 2-wk period after intensified exercise was compared between groups using chi-square analysis. The effect size for URTI during the 2-wk follow-up period was calculated using Cohen's d or the difference between the two means of the groups divided by the pooled SD for those means.
Subject characteristics and performance data for the 36 cyclists randomized to β-glucan and placebo groups and completing all phases of the study are summarized in Table 1. No significant differences were found between groups for age, body composition, or maximal performance measures. Subjects in the β-glucan and placebo groups came into the study averaging 242 ± 27 and 270 ± 29 km·wk−1 of cycling and 1.8 ± 0.2 and 1.7 ± 0.3 h per training bout, respectively. Thus, the 3-d intensified exercise period (9 h of exercise) represented a 70% increase in duration per training bout, and a substantial increase overall in exercise workload and intensity. Subjects in β-glucan and placebo groups were able to maintain a mean power output of approximately 56-57% Wmax at an oxygen consumption of approximately 68% V˙O2max and a cadence slightly greater than 85 rpm during the 9 h of exercise (Table 1). Humidity averaged 28.7 ± 0.4% and temperature averaged 22.3 ± 0.1°C during the three exercise bouts. Plasma volume change did not differ between groups after exercise on the third day and averaged less than 2% due to ingestion of 0.5-1.0 L of water per hour of exercise (data not shown).
Three-day food records before the 3-d exercise period revealed no significant group differences in energy or macronutrient intake (data not shown). Energy intake was 12.0 ± 0.5 MJ·d−1 (2867 ± 116 kcal·d−1) with carbohydrate representing 56.4 ± 1.4%, protein 16.6 ± 0.5%, and fat 26.4± 1.1%, for all subjects combined. Total dietary fiber intake was 28.8 ± 1.8 g·d−1, with soluble fiber intake averaging 3.3 ± 0.4 g·d−1. Thus, subjects in the β-glucan consumed an additional 5.6 g·d−1 of soluble fiber for a mean total of 8.9 g·d−1.
The β-glucan and placebo groups did not differ in responses to a symptom questionnaire (administered on the last day of supplementation) evaluating digestive health (constipation, heartburn, bloating, diarrhea, and nausea), feelings of hunger, energy levels, ability to focus and concentrate, and overall well-being (data not shown).
Significant main time effects were measured for each of the four plasma cytokines in Table 2 due to increases immediately after exercise on the third day, but the pattern of change over the supplementation and exercise periods did not differ significantly between β-glucan and placebo groups.
Significant time effects were measured for total blood leukocytes and plasma epinephrine, but groups did not differ in the pattern of change (Table 3). The β-glucan and placebo groups did not differ over time for PMN-RBA, NKCA, and PHA-stimulated lymphocyte proliferation (PHA-LP; concentration = 3.13 μg·mL−1; Table 3 and Fig. 1). PMN-RBA decreased 35% (P = 0.011), NKCA decreased 14% (P = 0.075), and PHA-LP decreased 21% (P = 0.001) 14 h after exercise compared to baseline, for all subjects combined.
The pattern of change over time did not differ significantly between groups for leukocyte IL-8, IL-10, and IL-1ra mRNA expression (P = 0.823, P = 0.563, and P = 0.238, respectively; Figs. 2-4). Expression of mRNA for IL-8, IL-10, and IL-1ra increased significantly above baseline immediately after exercise (all P < 0.01) and that for IL-10 and IL-1ra 14 h after exercise (P < 0.001 and P = 0.029, respectively), for all subjects combined.
Figure 5 compares the number of sick days accumulating over time as reported by an individual in each group. The average number of sick days per subject did not differ during the 5-wk study for the β-glucan and placebo groups (3.9 ± 0.7 and 3.7 ± 0.6, respectively, P = 0.786; effect size: d = 0.10; Fig. 5). URTI incidence rates during the 2-wk period after intensified exercise did not differ significantly between groups (9/19 or 47% for β-glucan and 6/17 or 35% for placebo, χ = 0.16, P = 0.693).
Oat β-glucan (5.6 g·d−1) compared to placebo ingestion by endurance athletes during an 18-d period did not alter self-reported URTI incidence or immune function before and after a 3-d period of intensified exercise. The oat β-glucan beverage supplement increased soluble dietary fiber 1.7-fold compared to placebo and was well tolerated with no group differences in self-reported gastrointestinal symptoms. The research design of this study allowed us to investigate whether oat β-glucan would alter chronic and/or acute exercise-induced changes for a wide variety of immune parameters including NK and T cell function, PMN-RBA, plasma cytokine levels, and leukocyte cytokine gene expression. However, in contrast to what has been previously reported in rodents, oat β-glucan had no effect on any of these outcome measures including illness rates (4,16).
Our research design was patterned after a animal model where mice ingested oat β-glucan for 10 d before three consecutive days of fatiguing exercise at 75-90% V˙O2max for 140-150 min (4,16,17). Oat β-glucan had strong effects in countering exercise-induced decrements in macrophage antiviral resistance and neutrophil function and in attenuating HSV-1 morbidity and mortality rates. In these studies, HSV-1 was inoculated intranasally at the end of the third day of exercise that coincided with the last day of oat β-glucan feeding. The HSV-1 dose yielded a 30-40% mortality rate among control mice during 21 d of follow-up. Our study was conducted during the winter of 2007, and self-reported URTI rates were 47% and 35% in the β-glucan and control groups, respectively, similar to the rates we reported in a prior study conducted at the same time of year (25).
Many factors may explain why results differ between our study and those of rodent-based investigations. Although our oat β-glucan supplementation regimen was patterned after the rodent studies, a higher dose may be needed in humans to see similar effects. The 600-mL·d−1 beverage delivery system for oat β-glucan used in this study approached the upper limit of what our subjects could tolerate, and a higher dose would require the use of supplement bars or something similar. Despite the presence of β-glucan receptors on NK cells, neutrophils, and some types of T cells, β-glucan may have its primary influence on macrophages, an immune cell whose function is difficult to measure in humans (4,5,16,17). The β-glucans may exert stronger effects in response to a direct and overwhelming viral challenge in contrast to low-dose environmental pathogens. Intranasal inoculation of HSV-1 is an established experimental model of respiratory infection in mice, but it is not a common respiratory virus in humans (4). Another study showed that oat β-glucan induced prolonged survival time and enhanced macrophage activity in mice injected with large doses of Staphylococcus aureus (36). Additional rodent studies are needed with rhinoviruses, coronaviruses, and influenza viruses to test the influence of oat β-glucan on these human-specific pathogens at less-than life-threatening doses.
Another issue is the lack of conclusive evidence that oat β-glucan is capable of influencing immunity through the human small intestine. Nearly all that is known regarding β-glucan bioavailability and interaction with intestinal macrophages comes from cell culture and rodent research models (9,30,31). These studies indicate that gastrointestinal mucosal dendritic cells may sample or interact with soluble β-glucans via projections across the epithelium and then migrate via afferent lymphatics to the mesenteric lymph nodes where immune modulation is initiated (30,31). Gastrointestinal macrophages may also engulf β-glucans, shuttle them to reticuloendothelial tissues and the bone marrow, and then degrade and secrete small β-glucan fragments that bind to receptors on bone marrow granulocytes (9). A subpopulation of intestinal epithelial cells and gut-associated lymphoid tissue cells seems capable of actively binding and internalizing β-glucans, which then leads to small but significant increases in blood β-glucan levels (30).
Human research is needed to determine whether oat β-glucan is capable of influencing systemic immunity through gastrointestinal pathways (6,29). One in vitro study of ileostomic content from six ileostomic patients consuming an oat β-glucan-enriched diet (5 g·d−1 for 2 d) showed, in contrast to a control diet, significantly increased IL-8 production and intercellular adhesion molecule 1 expression in small intestinal and colon cell lines after stimulation (29). However, as cautioned by the investigators, further evaluation is needed to determine whether these in vitro immune-stimulating effects on enterocytes contribute to enhanced in vivo host protection against invading pathogens in humans.
The exercise-induced immune changes seen in this study are comparable to those in previous studies including increases in blood total leukocytes, plasma cytokines, and blood leukocyte IL-8, IL-10, and IL-1ra mRNA expression (14,20-26,33). PMN-RBA, NKCA, and PHA-LP were decreased modestly 14 h after the third 3 h bout of exercise. Postexercise elevations in blood leukocyte IL-10 and IL-1ra mRNA, but not IL-8 mRNA, were still measurable 14 h after exercise, suggesting an extended anti-inflammatory effect.
In summary, the endurance athletes in this study ingested 5.6-g·d−1 β-glucan or placebo for 14 d while training normally, with no apparent augmentation of their resting immune function. Continued supplementation during a 3-d period of intensified exercise had no effects on how their immune systems responded to the physiologic stress. Self-reported illness rates were comparable between β-glucan and placebo groups during the entire study. Based on these data, the use of oat β-glucan in athletes for the purpose of influencing immunity or risk of illness is not warranted. These data contrast with findings from heavily exercised mice, and may be related to differences in the ability of oat β-glucan to interact with macrophages and other immune cells through the small intestine in humans.
Supported by a grant from the Gatorade Sports Science Institute. Results of the present study do not constitute endorsement by ACSM.
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