Prolonged running sustains the activity of the body's own cannabinoid substances. A study in BMC Medicine showed that anandamide levels rise throughout a marathon and remain elevated 45 minutes after the finish. Elevated levels were also noted after ultramarathons of up to 230 kilometers.
The "runner's high" is a state that some people experience during or after endurance exercise: euphoria, reduced anxiety and pain, calmness. It was previously explained by endorphins, but there is no convincing evidence that these large molecules cross the blood-brain barrier - the protective filter between the blood and the brain - in noticeable quantities.
Endocannabinoids are small fat-soluble molecules that are produced as needed and cross this barrier. They act on the same receptors as THC, the main psychoactive substance in cannabis. Anandamide and 2-arachidonoylglycerol (2-AG) are the best studied.
Earlier animal experiments and clinical work pointed to the role of this system. The new study, led by Michael Siebers, a psychiatrist at the University Hospital in Essen, was conducted in real-world conditions. In the first study, 19 experienced runners ran a marathon over three laps around Lake Baldeney in Essen. A month or two later they walked the same route. Blood was taken at the start, at the 14 and 28 kilometer marks, immediately after the finish and after rest; participants rated euphoria, anxiety and pain.
In the second study, 36 athletes took part in the TorTour de Ruhr ultramarathon over 100, 160 and 230 kilometers. Their blood was taken and their mood assessed before and after the race.
During the marathon, anandamide levels rose gradually and at the midpoint were higher than during walking, and 45 minutes after the finish they remained elevated. 2-AG levels at the start of the race were lower than during walking, but by the end they had risen and remained high during recovery. In the ultramarathoners, both substances after the finish exceeded their baseline values at all distances. After the marathon, anandamide levels were almost twice as high as after the ultramarathons - possibly because of the greater intensity of the shorter race.
Mood also changed: during the marathon, euphoria was higher than during walking, but at the 28 kilometer mark the difference disappeared; anxiety decreased toward the end, and pain increased after 28 kilometers. In the ultramarathoners, anxiety fell after the finish, pain rose, and euphoria did not change significantly.
The study did not test whether the runners whose anandamide rose more had more vivid euphoria. Daniel McCartney of the University of Sydney, who was not involved in the work, noted that the data are consistent with the idea of anandamide's contribution but do not establish a causal link. Among the limitations are comparison with walking rather than rest; self-selected pace; small samples; the absence of blood tests during the ultramarathons; brief mood scales; and measurement of substances in the blood rather than in the brain.
Siebers emphasizes that the body can produce its own cannabinoid substances through physical activity, but the "runner's high" is not the result of a single molecule but a complex interaction of signaling systems, and not everyone experiences it.