Subjects

Ten trained, male professional athletes (8 cyclists and 2 triathletes) from the same sports team (club) were recruited to participate in the study throughout winter season training in a training camp in the south of China following their training in the north of China. The biometrics of the training subjects are shown in Table 1. Their mean training period was 6.3 ± 1.6 years. They ranked in the top 20 percent of national competition records, and even were champions in Asian games. As professional athletes they trained for 5-6 days a week, and basically participated in national and Asian competitions such as Taiwan/Hong Kong/Hainan/Qinghai Lake bicycle races each year.

Table 1 Biometrics of the training subjects Full size table

The study was approved by the Ethical Board of National Institute of Sports Medicine (NISM) and was in compliance with the WMA Declaration of Helsinki. The study protocol was approved by the Review Board of NISM. All athletes signed the consent form before the study.

Study design, VO 2 max test and food consumption

A 10-week self-controlled, crossover design with two 4-week phases of consuming whole almonds and isocaloric cookies in a randomized feeding trial fashion and a 2-week washout period between two phases was conducted (Figure 1). Eight cyclists and two triathletes were randomly assigned to almond- (ALM) and cookies-consuming (COK) groups with equal athlete number after the baseline (BL) performance test.

Figure 1 Study design. Ten trained male athletes (8 cyclists and 2 triathletes) participated in a 10-week self-controlled, crossover trial during winter season training with training for 3-5 hours per day, 5-6 days a week (see the section of Exercise training regimen and Additional file 4). Dietary treatments consisted of two intervention phases of 75 g raw whole almonds or 90 g isocaloric cookies per day for four weeks each, and a 2-week washout period between two phases. VO 2 max test was undertaken one week prior to the baseline performance test. The time points for performance tests, blood collection and dietary record are indicated with black arrows. The red arrow shows the missed necessary performance test due to modification of athletes’ training plan. Full size image

The individual VO 2 max test was determined using an incremental workload (Watts) on a cycling ergometer (Lode Excalibur Sport, Groningen, Netherlands) 1 week prior to BL performance test. A 100-watt initial workload and 25-watt increment per minute were applied. Respiratory gas was measured using a Cortex MetaMax® 3B remote-controlled system (Cortex Biophysik, Germany), which provides a reliable gas exchange analysis based on the principle of breath-by-breath analysis [28, 29]. The system was calibrated with a calibration gas (5% CO 2 , 15% O 2 , BAL. N 2 ) (Air Liquide Healthcare America Corporation, Plumsteadville, PA, USA) prior to the VO 2 max test and each performance test.

After the BL performance test, subjects began to consume raw whole almonds (75 g/d as described by Xiao et al. [30]) as 1.8 times the FDA’s claim considered to meet the athletes’ need for intensive training or isocaloric starch-based commercial cookies (90 g/d), which was split equally into three portions fed before three main meals. We chose cookies as the placebo because they are carbohydrate-containing convenient snacks beneficial to exercise training and commonly used by the subjects. Additionally, 90 g of isocaloric cookies have a similar weight but a very different nutritional profile as 75 g of almonds (Additional file 1). In consideration of the unknown effective dosage of almonds for athletes we did not use a lower feeding of almonds. We recognized that the subjects were also aware of almonds as a kind of healthy food, while they seldom had it as snacks due to relative high cost.

Almonds were generously provided by the Almond Board of California. Nutrition information for 75 g almonds and 90 g cookies are presented in Additional file 1.

Exercise performance test

Subjects stopped their regular training one day before performance test. They reported to testing room either at 8:30 am or 2:30 pm, 1.5 h after standard breakfast or lunch. Each subject used the same indoor stationary bicycle trainer in all 3 performance tests and followed the same testing protocol with the same settings (Additional file 2, a representative video). The same trainer was also used for their routine training. The test consisted of 10 min of warm-up at 30% VO 2 max, 115 min of steady-state (SS) cycling at 50-60% VO 2 max, 20 min of time trial (TT) at all-out effort following a 10-min relaxation (for collection of urine). Expired gas composition and temperature, HR, ambient temperature and humidity during whole TT were monitored using Cortex MetaMax® 3B System and Polar 725 heart rate monitor. Carbohydrate (CHO) and fat utilization was calculated based on the equation built in the software by selecting an assumed 15% total energy expenditure derived from protein.

The rating of perceived exertion (RPE) using the 6-20 Borg scale was surveyed at 20-min intervals throughout the test. The pre- and post-testing body mass (BM) with removal of their racing suit was checked using an electronic BM scale. Urine sample was collected during 10-min relax time of the performance test for volume determination. To ensure subjects were enthusiastic about the test and performed at their highest level, they were informed at the beginning of the test that a prize would be awarded to the winner cycling the longest distance during TT.

Blood samples collection and biochemical measurements

Venous blood was collected from anticubital arm vein into vacutainer tubes before the performance tests. Heparin plasma and serum were obtained after centrifugation at 3000 × g for 10 min. Samples were stored at -80°C until analyses. Finger blood was obtained via puncture for glucose determination at 0, 60, 125 and 155 min during the test.

Free fatty acid (FFA), pyruvic acid (PA), and total antioxidant capacity (TAOC) in plasma were determined using commercial kits (Randox Laboratories Ltd, Crumlin, UK), and an auto-biochemical analyzer (Hitachi, Tokyo, Japan). Plasma VE, malondialdehyde (MDA) and arginine levels, xanthine oxidase (XOD) and glutathione peroxidase (GPx) and superoxide dismutase (SOD) and creatine kinase (CK) activities, and blood urea nitrogen (BUN) and nitric oxide (NO) were measured using spectrophotometric kits (Jiancheng Bioengineering Institute, Nanjing, China). Serum insulin (Ins) and cortisol (Cor) concentrations were measured using radioimmunoassay kit (Jiuding Diagnostic, Tianjin, China). Blood glucose (BG) was determined using handheld blood glucose analyzer (One Touch, LifeScan, Inc. Milpitas, CA).

Diet and dietary record

All subjects lived in a winter training camp and dined in the same canteen throughout the study, and were advised by a registered dietician to follow a diet with 60% total calories from CHO, 15% from protein, and 25% from fat for 2 days before each performance test. Generally subjects had a typical Chinese breakfast consisting of one chicken egg, two servings of steamed breads or noodles, deep-fried dough sticks, rice congee, bean milk, some meat, some vegetables and appetizers, and lunch and dinner consisting of meat, steamed rice, steamed breads, noodles, soup, milk, fruit and vegetables, etc.

To assess dietary intake throughout the study, a 2-day food record was conducted at week 2, 4, 8, and 10. Chinese food database (issued by Chinese Society of Nutrition) was used for nutritional analysis (Additional file 3). During the regular training, subjects were allowed to drink 6% CHO-electrolytes-vitamins (without VE) beverage (Competitor, Beijing, China) with an average amount of 1500 ml/d. Ten minutes prior to the performance test, subjects checked their BM after emptying bladder, and ingested 2.0% CHO-electrolytes-vitamins (without VE) beverage at 6 mL/kg BM for the pre-testing hydration, 2.5 mL/kg/15 min during SS. No beverage was provided during TT. Subjects did not take any other dietary supplements throughout the study.

Exercise training regimen

Basically, all subjects had their road cycling training together, whereas two triathletes had their run and swim training in the same training site throughout the study. Briefly, based on their training plan, subjects trained 5-6 days a week with incremental increase in training amount and intensity throughout the study. Detailed content of daily and weekly training was made by coaches on each weekend. The typical daily cycling training regimen consisted of 60-200 km (even 220-250 km) road endurance cycling, 2-3 km*N (N = 2-8) timing sprint cycling on the flat road and sloping fields. Exercise intensity was monitored by HR. Eight cyclists had a weekly road cycling distance of 2840 km and 3110 km during two phases, respectively (Additional file 4). Two triathletes had an average 380-km of road cycling weekly during two phases.

Limitation of the present study

The original study design included four performance tests performed by subjects before and after each intervention phase during the study. Regretfully, subjects did not undergo VO 2 max test prior to the 2nd intervention phase and the performance test at the beginning of week 7 due to a modified training arrangement. Thus, baseline values of the performance test at the start of the 2nd phase were not available. However, the following 4 points may be helpful to support that the drawback should not affect significance of study outcomes observed at the end of the intervention phases. First, we originally had a crossover design, that is to say, when ALM or COK was compared with BL, there were 5 subjects in each group at the first intervention phase. Second, we had blood biochemistry tests at the end of washout (the end of 6th week). With the exception of a higher FFA, biochemical outcomes after washout at 6th week (MDA 3.7 ± 0.4; XOD 12.5 ± 0.8; TAOC 15.5 ± 1.6; GPx 0.39 ± 0.02; SOD 55.8 ± 0.6; VE 25.2 ± 2.2; CK 237.3 ± 46.4; Cor 19.3 ± 0.8; Hb 143.6 ± 2.7; PA 0.49 ± 0.07; FFA 0.20 ± 0.02; arginine 0.076 ± 0.003; NO 96.7 ± 13.2; Ins 5.0 ± 0.9) were not statistically different from the BL values (see Table 2, their units are the same as shown in Table 2 presented, n = 10). Third, half-life of some nutrients or primarily functional components present in almonds supports that the carry-over effect of the first intervention should be minimal if there was any, e.g, the half-life of α-tocopherol, quercetin, diverse polyphenols and arginine is 57 h [31], 11-25 h [32, 33], 1-18 h [34] and 1.5-2.0 h [35], respectively. Finally, subjects all lived and trained in the same training camp throughout the study.

Table 2 Blood biochemistries pre-performance tests Full size table

Statistical analysis

According to the balanced crossover design we combined the data of the same treatment in two phases for statistical analysis.

All results are expressed as mean ± SE except when specified elsewhere. Two-way ANOVA was performed to analyze the differences among groups. Significance was analyzed using post hoc least significant difference (LSD) test. All statistical analyses were performed using SPSS 13.0 software. Differences were considered significant at P < 0.05.