Individual differences in cardio-metabolic response to caffeine may not predict its benefits on endurance performance

Competitive athletes frequently explore the use of performance enhancing (i.e. ergogenic) agents to give them that extra edge. According to the National Institutes of Health^[1], international surveys revealed that 66% of nearly 4000 surveyed adult and adolescent elite track and field athletes reported using one or more ergogenic supplements. Even amateur individuals in the general population^[2] who are simply looking to maximize their workout time will often consider using products like “preworkout” supplements, sports and energy drinks, protein and amino acid supplements, and other agents.

Many of these products contain caffeine^[3], a central nervous system stimulant that can be found in varying amounts in a number of natural plant sources. Almost 90% of the general population consumes caffeine on a regular basis, with the most common sources^[4] being coffee, soft drinks, and tea. Improvements in mental alertness, concentration, fatigue, and athletic performance are well-established benefits^[5] of caffeine consumption up to 400 milligrams per day^[6].

Caffeine ingestion has been touted to boost energy and minimize fatigue during athletic activity, and a number of studies have indeed observed increased aerobic^[7], anaerobic^[8], and neuromuscular performance^[9] after consumption. While it is widely accepted that caffeine blocks adenosine receptors^[10] in the brain, thereby producing a more awake and alert state, other mechanisms of its activity are less understood. Some studies have shown that caffeine exerts endurance-promoting metabolic changes^[11] by inducing higher adrenaline output, leading to more fatty acids being burned for fuel while sparing glycogen stores. Caffeine may also have local effects on the muscular system by mobilizing calcium^[12] and increasing muscle contractile force^[13]. Furthermore, caffeine may improve endurance by boosting nitric oxide levels and enhancing blood vessel dilation^[14], which both permit increased oxygen- and nutrient-rich blood flow into muscles.

However, not all studies have shown these types of effects on athletic performance and not all studies have agreed on the precise mechanism by which caffeine may produce these changes in performance. Researchers have suggested that interindividual differences in biological response to caffeine may influence its effects on athletic performance. In an effort to remove this barrier, this study crafted a research design in which participants were grouped as either “high” or “low” caffeine responders using multiple measures.

Caffeine is touted as a performance-enhancing agent, but some people physiologically respond differently to it, which may explain its ergogenic variability. This study explores this hypothesis by splitting athletes into two groups, low responders and high responders, and putting them through a rigorous endurance challenge.