Endurance athletes may benefit from supplements such as carbohydrates, post-workout protein, electrolytes/fluid, caffeine, and nitrates.
Carbohydrate intake is important for endurance athletes before, during, and after exercise to fuel working muscles, offset fatigue and promote recovery.
Protein intake is critical for building muscle and accelerating recovery during the post-workout period 
Fluid and electrolyte replenishment is important to replace sweat losses during endurance exercise and prevent performance decrements associated with dehydration. In addition, new research on fluid intake shows that drinking electrolyte-containing water helps prevent muscle cramps during exercise [ 2
Caffeine has been shown to increase endurance, spare glycogen, promote fat oxidation, reduce fatigue, and decrease perceived effort in doses ranging from 3-9 mg/kg body weight [ 3
Nitrates (beetroot juice) have been shown to enhance endurance performance by reducing oxygen cost, increasing time to exhaustion, and improving VO2max [ 3 3
3]. The recommended dosage for nitrates is 300-600 mg 90 minutes before exercise [
Thomas, D. T., Erdman, K. A., & Burke, L. M. (2016). Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Journal of the Academy of Nutrition and Dietetics, 116(3), 501–528. https://doi.org/10.1016/j.jand.2015.12.006
ScienceDaily. (2021, March 18). Muscle cramp? Drink electrolytes, not water, study shows. ScienceDaily. https://www.sciencedaily.com/releases/2021/03/210318101536.htm.
Vitale, K., & Getzin, A. (2019). Nutrition and Supplement Update for the Endurance Athlete: Review and Recommendations. Nutrients, 11(6), 1289. https://doi.org/10.3390/nu11061289