Yes, running a marathon can make you temporarily shorter. In fact, running for just 30 minutes at moderate intensity can decrease your height. Why? Blame the discs in your spine. There are 23 of them, each separating vertebrae and simultaneously allowing movement, absorbing shock, and holding the vertebrae together. Among other things, the discs are made up of water, and when the discs compress beyond a certain pressure, they will leak water through the disc wall. The result is a loss in disc height and volume, and therefore a slight change in overall height, as intervertebral discs make up one-third of the height of your spinal column.
Although everyone’s vertebral column height decreases throughout the day, running speeds up the rate of shrinkage because the discs compress significantly more under the force of your stride. One recent study published in the journal Medicine and Science in Sports and Exercise found that running for 30 minutes at moderate intensity decreases disc height by about 6.3 percent. Each disc in the lower spine is about seven to 10 millimeters thick, so if we assumed each disc was that thick (a generous assumption, as normal discs get as small as three millimeters higher up) that’s a maximum about 10 to 15 millimeters off of your height after a half-hour jog on the treadmill.
The good news: Your intervertebral discs will take in fluid to regain their original size when the compression stops. And in 2010, researchers found that running may play a role in halting disc degeneration by aiding in the formation of new disc cells (caveat: that study was done in rats, and the optimal amount of running to promote disc health has yet to be determined).