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Did you know DNA affects your fight-or-flight response?

How your fight-or-flight hormones affect endurance ability

It’s the final Olympic races of the returning 100m champ Usain Bolt and the 5000m winner Mo Farrah. As we watch each of them at the start lines, it’s easy enough to imagine their mental state, excitement laced with a large dose of nerves. In response to these emotions, their adrenal glands (two small organs located just above their kidneys) are working overtime, flooding their bodies with hormones especially adrenaline, making them ready for “fight-or-flight”.

Fight-or-flight is the body’s natural response to stress. It starts with the release of hormones like adrenaline, which elevates heart rate, blood pressure and sugar levels, giving us the courage to stay put and fight or the energy to run away. The question is, how does a sprinter like Bolt whose race lasts a mere 9 seconds, and an endurance runner like Farrah who runs for well over 9 minutes, benefit from the exact same response? The key to this question is genetic variation in a specific gene called ADRB2, which relays the signal of adrenaline and affects athletic performance and how our bodies respond to exercise.

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The ADRB2 gene makes the beta-2 adrenergic receptor, which is found on the surface of cells. When released into the bloodstream adrenaline binds to this receptor and starts off a sequence of events, including elevating our heart rate and stroke volume (amount of blood pumped out of the heart per beat), bronchodilation (expanding the air passages in the lung), as well as increased fat breakdown to power our bodies. Exercise itself induces the release of adrenaline, and ADBR2 is central to exercise-induced changes in our bodies.

However, prolonged activation of the signal by adrenaline will increase both heart rate and blood pressure for a long period of time, and can cause damage to our bodies. So, turning off this signal through a process called ‘receptor desensitization’ is just as important as turning it on. A desensitized beta-2 adrenergic receptor can no longer bind adrenaline, essentially shutting off the effects of the hormone.

As you might imagine, runners have an expertly trained ‘flight’ response. However, there’s a clear difference between a sprinter and an endurance runner. These differences include the types of muscle they use for running, as well as the source of energy they burn when they run. DNA changes in the ADRB2 gene can account for at least some of this variation. Studies show that endurance athletes are more likely to have a version of ADRB2 known as Arg16 (rs1042713 A), linking it to endurance performance. One study involving Olympus marathon runners found that athletes with the Arg16 polymorphism had faster marathon completion times compared to their counterparts with the Gly16 allele.

Furthermore, patients with heart failure responded better to exercise (higher increases in oxygen carrying capacity) when they had the Arg16 version of ADRB2. In contrast, the Gly16 version of ADBR2 may benefit power/strength athletes, highlighted by the over-representation of this version among sprint athletes. The effect of ADBR2 on athletic performance is clear, and popular theories suggest that Arg16 may promote endurance by influencing the number of receptors on the surface of cells, or by modifying receptor desensitization. However, despite these insights, precisely how ADBR2 variation is relevant to athletic performance is still largely a mystery.

ADBR2 is one of the many genes that have been linked to athletic performance. What sets it apart from other genes is the fact that it influences an ancient physiological response that helps us to fight or run away from danger. Regardless of how they influence performance, as the list of ‘fitness genes’ continue to grow, it’s becoming increasingly apparent that genetics has a large say in whether or not we excel in sports, right down to whether one becomes a sprinter or a marathon runner.

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