Allergy medication may be sabotaging your exercise gains

Most people hear “histamine” and think of allergy symptoms – stuffy noses, watery eyes, and sneezing fits. But histamine does a lot more than trigger hay fever. It turns out this tiny molecule may play a big role in how our bodies respond to exercise.

Researchers at the University of Oregon have demonstrated that blocking histamine during workouts can seriously blunt fitness gains.

In a recent study, the team found that people taking high doses of antihistamines before exercise improved only half as much as those who didn’t take allergy medication.

Histamine isn’t just about allergies

Histamine has been around for a long time – it’s found in everything from single-celled organisms to humans. In all these forms of life, it helps respond to stress.

“In exercise, it actually seems to be playing a very similar role of facilitating our adaptation to stress,” said study co-author John Halliwill, a professor of human physiology.

In humans, histamine is part of the immune system. When the body senses something harmful – like pollen – immune cells called mast cells release histamine. This sends a signal to create inflammation in the area, which can help fight off the threat.

Allergy medication prevents exercise gains

The same mechanism causes allergy symptoms, and that’s why drugs like Claritin or Zyrtec, which block histamine, are used to treat them.

But inflammation isn’t always bad. In fact, it’s part of how muscles grow stronger after exercise. When you work out, you cause tiny amounts of damage to your muscles. The body repairs that damage, and in doing so, it builds new muscle tissue.

In this new study, researchers showed that when histamine was blocked during exercise, this repair-and-growth process didn’t work nearly as well.

Training gains cut in half

The study tracked 16 men and women over six weeks. Everyone followed the same cycling routine: three to four sessions per week on stationary bikes. But there was one key difference.

Half of the participants were given high doses of antihistamines before each workout. The other half received a placebo.

By the end of the study, those in the placebo group had nearly twice the workout gains compared to those taking allergy medication. Their blood flow also improved more. That’s a sign that their bodies were adapting better to the workouts.

What didn’t change much between the groups was VO2 max – the maximum amount of oxygen a person can use during exercise. That’s often considered a gold standard for measuring aerobic fitness.

However, the researchers noted that this finding may not be reliable because the study group was small or because six weeks wasn’t long enough to see those kinds of changes.

Link between histamine and exercise

The connection between histamine and exercise isn’t brand new. Scientists first started suspecting a link in the 1970s. But the idea was largely ignored until recently.

Now, researchers are starting to understand the details. It seems to begin with mast cells – the same ones involved in allergy responses – which are spread throughout skeletal muscles.

When muscles are working hard, those cells release histamine. It’s still unclear what exactly sets them off.

What histamine does in the body

Histamine helps dilate blood vessels, increasing blood flow to the muscle. And even after the workout ends, histamine keeps working.

It triggers an immune response that brings inflammation to the muscle – the kind that helps it recover and grow stronger.

“We’ve got a whole village of cell types that are turning on programs to remodel and restructure and improve the function of the skeletal muscle-organ system,” Halliwill said. “Mast cells and the histamine that they release are a major coordinator of all those cell types.”

Histamine also affects genes. During exercise, it amplifies the expression of thousands of genes that help muscles rebuild and adapt.

When histamine is blocked, that gene response is muted. Fewer new proteins get made. That means less recovery, less muscle adaptation, and ultimately, smaller gains in fitness.

Allergy medication and exercise impact

This isn’t the first study to raise questions about allergy medication and exercise. Another research team, working separately, found a similar result when they looked at high-intensity interval training.

Professor Halliwill said both sets of findings point in the same direction: blocking histamine seems to reduce how much your body benefits from aerobic exercise – whether that’s biking, running, or swimming.

But that doesn’t mean you need to stop taking allergy medication. Halliwill made it clear that the doses used in this study were far higher than what you’d get from an over-the-counter drug like Claritin.

More research is needed to know whether low daily doses of allergy medication have any real effect on fitness. For now, the safest move is to stay informed and wait for more evidence.

If you’re training for an event or chasing personal bests, it might be worth talking with a doctor or trainer about how your meds could factor in. But there’s no need to panic over your allergy pills – at least not yet.

The full study was published in the journal Journal of Applied Physiology.

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