Barring any mildew or mold growth on your neglected workout clothes, it’s likely the itch is related to an exercise-induced histamine response.
Histamine is a substance found in the body that’s most commonly associated with allergic reactions, itching, and inflammation. But researchers are just beginning to understand that histamine also plays a role in exercise.
Previously, a popular theory on exercise-induced itchiness had to do with blood vessel expansion, or vasodilation. The body expands blood vessels during exercise to help bring more oxygen and nutrients to working muscles, and to eliminate carbon dioxide and waste products. The idea is when blood vessels expand after a period of relative inactivity, the surrounding nerves, unused to the phenomenon, mistakenly register the expansion as itchiness.
The problem with that theory: “I’ve read no studies that support that,” says Stanford exercise physiologist Dr. Anne Friedlander.
However, new research suggests that vasodilation may, indeed, have something to do with the itch. Histamine is a known vasodilator; it increases blood flow to injured or infected tissues to help the immune system get at the problem area. And one Japanese study showed that histamine may be released during exercise to help protect the body against exercise-induced fatigue or exhaustion.
Unfortunately, histamine also sends itch signals to the brain, which means an increase histamine production in your body could induce general feelings of itchiness, which seems to be what you’re experiencing. (For more on how histamine induces itch, check out this study.)
One simple way to test this hypothesis: try taking an antihistamine before working out to see if the itch stops.
On a positive note, your own anecdotal evidence suggests the more regularly you exercise, the less sensitive you’ll be to the histamine response. So keep it up to keep the itch away.