Clinical Assistant Professor, Department of Family Medicine, University of Wisconsin; Clinical Assistant Professor of Medicine, Arizona Center for Integrative Medicine, University of Arizona, Tucson

Dr. Kiefer reports no financial relationships relevant to this field of study.

SYNOPSIS: Ten men ran 1,500 meters six seconds faster and were slightly less fatigued after ingesting 12 mg of capsaicin than after ingesting a placebo capsule.

SOURCE: de Freitas MC, Cholewa JM, Gobbo LA, et al. Acute capsaicin supplementation improves 1,500-m running time-trial performance and rate of perceived exertion in physically active adults. J Strength Cond Res 2018;32:572-577.

Well into the summer exercise and event season, patients may ask their healthcare providers what would provide that extra “boost,” that nudge toward greater endurance or strength, and better times running, cycling, or swimming. Assuming optimal nutrition, sleep, and aerobic training, active adults may consider any number of supplements for sports or exercise enhancement.¹ One familiar example is creatine,² but there are many others.¹

To expand on the clinical evidence for the use of supplements to affect exercise physiology, de Freitas et al chose a specific “middle distance” activity, the 1,500-meter run. Rather than a sprint or marathon, the 1,500-meter run is most limited by muscle fatigue as mediated by several physiological parameters that affect muscle force. (See Table 1.)

Into this milieu, the authors of a series of animal and human clinical trials found benefits for the use of capsaicin. Capsaicin is a compound found in species of cayenne pepper (Capsicum spp., Family Solanaceae) or chili pepper.³ There are some data for its analgesic use topically,⁴ but it also may shift muscle physiology when used orally. The authors cited mixed results from clinical trials that included humans doing exercise. Their own work found benefits with 12 mg of isolated capsaicin, whereas other groups using cayenne pepper powder (3 g, providing 28 g of capsaicin) were not able to show improvements in athletes’ performance. Clearly, more data were needed.

de Freitas et al conducted this randomized, double-blind study in 10 men (average age 23.5 years) in Brazil who were not regular runners in the previous six months, were not taking any performance-enhancing substance, and did not have any heart or musculoskeletal contraindication to participation. They were instructed to not change their diet or add supplements to their daily regimen.

There were three visits. During visit 1, researchers measured height, weight, and bioimpedance. One week later, during visit 2, participants completed a 24-hour dietary recall and were randomized to either a placebo capsule (50 g of starch) or capsaicin (12 mg) 45 minutes before the 1,500-meter run, which was done on an outdoor 400-meter track as fast as the participants could. After the run, each participant completed a survey instrument called the Rate of Perceived Exertion (a scale from 6-20) and had blood analyzed for lactate. Seven days later, for visit 3, each participant repeated the dietary recall and the 1,500-meter run, ingesting the alternate intervention (placebo or capsaicin), to complete the crossover analysis.

With respect to results, there was no difference in nutrition as per visit 2 and visit 3; presumably, major shifts in the diet before the time trials could have affected the primary outcomes of this study. With respect to running times, the placebo group time was listed as 376.7 seconds, whereas the capsaicin group came in at 371.6 seconds (P = 0.009). Of note, the times in both arms were quite fast, between six- and seven-minute miles. In addition, the Rate of Perceived Exertion in the placebo group was 18.8 vs. 18.0 in the capsaicin group (P = 0.022). Furthermore, in each arm of the trial, there was a predictable rise in blood lactate after exercise, which decreased over time; there was no difference in this change between the groups. There was no comment on side effects or other adverse events.

COMMENTARY

Exercise physiology, as with all other aspects of our physiology, is complex. Many variables are involved in promoting the optimal function of muscles in coordination with the cardiovascular and pulmonary systems. These researchers used a small trial to show small benefits on running time and perceived exertion for a 1,500-meter run with capsaicin supplementation. The statistical significance may override clinical significance in this case; these runners were fast and they gained six seconds over the course of about a mile. We may be hard pressed to find applicability for this intervention to a standard clinical practice. Furthermore, we are missing commentary of tolerability and safety of the intervention. The authors commented on gastrointestinal limitations to capsaicin doses greater than 30 mg, and there are certainly some people who have an increase in gastroesophageal reflux disease with spicy foods, including cayenne pepper. We should keep in mind that this intervention may not work for everyone.

It is possible that oral capsaicin does nudge the physiology in significant ways. The authors cited prior research that capsaicin is an agonist of the TRPV1 receptor; this receptor may activate the sympathetic nervous system and foster improved energy utilization that could enhance exercise performance, as well as lead to analgesia, another benefit when facing the rigors of physical activity. The lack of change in blood lactate between capsaicin and placebo arms negates some potential mechanisms of action, such as muscle glycogen preservation during activity.

This is a specialized use of a plant extract, one unlikely to be replicated through culinary means. Not only is it unreasonable to think about eating a spicy, chili pepper-laden meal right before a mile run, but the content of capsaicin in chili peppers is variable.⁵ There probably is no substitute for healthy nutrition, regular training, and adequate rest to promote exercise performance, although the results of this small clinical trial point to perhaps interesting future research for substances that may have physiological effects.

REFERENCES

1. Jenkinson DM, Harbert AJ. Supplements and sports. Am Fam Physician 2008;78:1039-1046.
2. Butts J, Jacobs B, Silvis M. Creatine use in sports. Sports Health 2018;10:31-34.
3. Schulz V, Hansel R, Blumenthal M, Tyler VE. Rational Phytotherapy: A Reference Guide for Physicians and Pharmacists. Berlin: Springer-Verlag; 2004.
4. Frias B, Merighi A. Capsaicin, nociception and pain. Molecules 2016;21:E797.
5. Barbero GF, Liazid A, Azaroual L, et al. Capsaicinoid contents in peppers and pepper-related spicy foods. Int J Food Prop 2015;19:485-493.