Study backs caffeine sports benefits, but with caveat

A study looking at the blood flow changes when caffeine is used in conjunction with exercise came to some confusing results. Subjects’ aerobic performance improved, but the blow flow in their muscles didn’t.

The new research was published in the Journal of the International Society of Sports Nutrition and was the work of researchers at a university in Wuhan, China.

Caffeine is one of the oldest focus and performance-enhancing ingredients on the market and has been the subject of much research. A search on the PubMed database using the terms “caffeine effects” returns more than 2,500 results. Using the search term “caffeine exercise” yields more than 700 results.

Summary of existing research

The International Society of Sports Nutrition released a position paper on the subject in 2021. According to that document, the ISSN experts concluded:

The Chinese researchers noted that even with this wealth of research, there are aspects of caffeine’s mode of action that are still imperfectly understood. One of those aspects relates to caffeine’s effect on blood vessels.

What’s happening in the smallest blood vessels?

It has been well demonstrated that caffeine causes larger blood vessels to dilate, but this effect is less well understood in smaller vessels within muscle tissue, the Chinese researchers noted.

“The somewhat contradictory results raise intriguing concerns about how coffee affects the microvasculature of muscles, especially during exercise when proper blood flow control is crucial,” the authors wrote. “Furthermore, little is known about how these microvascular reactions relate to overall exercise performance.”

To understand these effects, and how they might influence performance, the researchers recruited 20 young, healthy and physically active young men. They designed their study in three arms, using the same cadre of participants for all.

In one arm, the subjects ingested a placebo, while the other two arms used a low dose and high dose of caffeine (three grams per kilogram of body weight, and six grams, respectively). All of the caffeine and placebo dosages were delivered as cups of coffee (Nescafe decaffeinated instant coffee in the case of the placebo). One of the selection criteria was that the subjects were not regular coffee drinkers, so the slight taste difference of a decaf coffee, which an experienced coffee user might detect, was presumably not an issue.

In each of the three lab visits, the subjects rested, then ingested the respective product, and subsequently rested again. Muscle reactivity was measured at rest and after coffee ingestion using near-infrared spectroscopy (NIRS).

The subjects then participated in a standard cycling ergometer test, in which the resistance was increased at intervals while maintaining a steady RPM until voluntary exhaustion.

NIRS was used to probe the blood flow in muscle tissue during the cycling test, too. All the tissue measurements were made in the largest of the quadriceps muscles.

Results raise further question

The results showed that the high dose of caffeine increased blood flow within the tiny vessels in muscle tissue at rest. And it also increased the aerobic performance on the cycling ergometer test. However, neither caffeine dosage increased blood flow over placebo within those fine vessels during the exercise portions of the test. So exactly how is caffeine boosting the performance of those muscles?

“In conclusion, in this study we found that acute coffee intake with high caffeine level (6 mg/kg body weight) significantly enhanced skeletal muscle reactivity at rest,” the authors concluded. “Maximal aerobic power was also improved, indicating the ergogenic potential of coffee. However, the improvement of exercise performance with coffee intake is not accompanied by alterations in muscle microvascular oxygen extraction.”

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