INTRODUCTION

One of the important goals of athletes and researchers is to improve and develop the level of physical fitness. In this regard, the use of regular or unique training programs in each sport increases the physical performance and physical fitness of individuals. To maximize the effects of exercise training, sports nutrition is also considered. With the advancement of sports physiology, metabolism, and nutrition, it has been proven that diet and nutritional intake affect the performance of athletes. Today, the use of dietary and sports supplements to improve body composition and performance has become highly popular among people, especially athletes, while the benefits of dietary supplements, as people believe, have not been fully proven.

There is significant evidence supporting the role of nitrate-containing dietary supplements in increasing nitric oxide (NO)-induced vascular dilation through an oxygen-independent pathway that affects arterial hemodynamics. NO is a compound that causes vasodilation, improves calcium management, enhances athletic performance, and increases the speed of skeletal muscle contraction. Dietary nitrates (NO3-) act as a precursor to NO and are present in large amounts in some plants, such as beetroot and spinach.

The high presence of NO3-, iron, zinc, magnesium, plant steroids, and other vital compounds, such as vitamins B and C, in red spinach hypothesizes that spinach consumption improves cardiovascular activity, neurotransmission, and strength and delays fatigue. However, considering the limited and inconsistent studies on the observed effects of spinach on aerobic and anaerobic performance, as well as the scarcity of red spinach species in Iran, the present study decided to examine the effect of short-term consumption of the native green spinach species available and inexpensive on the aerobic and anaerobic performance of young athletes.

METHODS

The present applied study was conducted using a randomized design with a pretest-posttest and placebo group. The statistical population consisted of men and women aged 18 to 26 in Rasht, with a normal body mass index (between 18 and 24.9 kg/m2) and a history of regular exercise in the past 3 years. After announcing the call for proposals and specifying the criteria for entering the research, 20 people volunteered, and finally, 14 individuals (7 women and 7 men) completed the protocol based on the criteria for entering the research.

The subjects were asked to avoid strenuous activity and caffeinated foods 24 hours before the test. During the pre-test session, participants completed a 10-minute general warm-up followed by stretching. They then performed the 6-minute Astrand sub-test on the Monarch aerobic ergometer model LC4, took a 15-minute rest, and proceeded with the 30-second Wingate test on the Monarch foot ergometer model 894E. After another 15-minute rest, they conducted the 30-second Crank test on the Monarch hand ergometer model 891E, followed by another 15-minute rest. Finally, the Bruce maximum test was conducted on the Cosmos T150 treadmill, and the test data was recorded. The leaves of green spinach (Spinacia oleracea) from a specific harvest in the city of Rasht were purchased. After being wiped and washed, they were naturally dried in the sun and turned into a soft powder using a milling machine. The spinach supplement group in the trials was asked to add 50 g of spinach powder to their dishes daily for 14 days, along with their usual diet, such as soups, stews, salads, and yogurt. The placebo group (rice flour+green food powder) also took the placebo during the same period. The allocation of spinach or placebo supplements was conducted in a blind manner; therefore, participants were unaware of which group they belonged to. Forty-eight hours after the last supplement, the tests were repeated under the same conditions as the pre-test. During this time, variables related to aerobic power were measured in the sub-maximal test, while anaerobic power components and the fatigue index were recorded for both the upper and lower body during the maximum test. After confirming the normal distribution of data with the Shapiro-Wilk test, a combined variance analysis test and Bonferroni paired comparisons were used to analyze the results. All statistical analyses were carried out in SPSS software version 26, with a significance level of α=0.05.

RESULTS

Based on the results of calorie intake and the percentage of nutrient intake, no difference was found between the subjects. According to the findings, it was observed that aerobic power did not significantly change in the placebo group compared to the pre-test (P=0.385), while in the spinach group, it was about 16% higher (P=0.026). However, there was no significant difference between the two groups (P=0.703). Maximal oxygen consumption values, examined by the maximum test, showed that there was no significant change in the placebo group (P=0.747) and the spinach supplement group (P=0.240) compared to the initial values of each group, and no significant difference was observed between the groups (P=0.079).

The results of the present study demonstrated that the components of upper and lower body anaerobic performance, including peak power, average power, minimum power, maximum speed of racing, and maximum speed retention, did not significantly change in the groups studied, and there was also no difference between the two groups (P>0.05).

CONCLUSION

The results of the present study indicated that a daily intake of 50 mg of dried green spinach could not improve the aerobic and anaerobic performance of active individuals. Therefore, this amount of supplementation is not recommended for individuals who intend to improve sports performance, especially aerobic and anaerobic power. However, further research is needed for more accurate results.

Ethical Considerations

Compliance with ethical guidelines

This study was approved by the Ethics Committee in Research at Gilan University (IR.GUILAN.REC.
1401.026). The principles of the Declaration of Helsinki, including obtaining informed consent from the participants and the confidentiality of their information, have been observed.

Funding

There is no funding support.

Authors’ Contributions

The authors contributed equally to the conceptualization and writing of the article. All of the authors approved the content of the manuscript and agreed on all aspects of the work.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgments

We are grateful to all those who provided scientific consulting for this study.