Allergies are an unfortunate fact of everyday life for a lot of us. Recent surveys suggest that over 100 million Americans have allergies, and that number continues to rise as allergies grow in prevalence. Consequently, millions rely on antihistamines to manage their symptoms, and many take them chronically.
That’s why I was a bit alarmed by a recently published study suggesting that taking antihistamines could interfere with adaptations to exercise - meaning the physiological processes through which your body gets fitter and stronger. So...should those of us who battle allergies be concerned?
I'll get into that, but first let's look at why antihistamines might affect exercise adaptations in the first place.
Histamine: A Multi-tasking Compound
Most of the time, when we talk about histamine, it is in the context of allergy, and with good reason. A lot of histamine is carried in immune cells known as mast cells. These mast cells are concentrated in areas where the body interfaces with the outside world, like the nose, throat, and skin.
When mast cells are exposed to an antigen, like dust or pollen, they release histamine, which kicks off a bunch of local immune responses to help make the antigen go away. Most of the time, this manifests, for us, as inflammation, itching, runny nose, etc.
Antihistamines work by blocking histamine receptors, which prevents histamine from reaching its molecular targets and producing those annoying symptoms. Since most antigens are actually harmless, blocking the action of histamine isn't a big deal in this situation.
But here is where things get complicated.
You see, histamine is a primitive molecule. In fact, histamine and its receptors arose before the immune system as we know it, and even prior to the evolution of multicellular organisms. That should be a big clue that they are pretty darn important, and have roles outside of the allergic response.
And indeed, histamine is a participant in a wide array of inflammatory processes - including those associated with exercise.
A Biochemical Dilemma
Going back to the 1950s, researchers observed that histamine dilated blood vessels. Which isn’t surprising. In fact, you could argue that the fundamental job of histamine is to push blood flow to specific parts of the body, like in response to an invading allergen.
Scientists also noticed that levels of histamine rose after aerobic exercise. This makes sense too. After you work out, blood vessels that feed the previously active muscles get wider, resulting in greater blood flow and lower blood pressure. This is a super important adaptation for exercise recovery, because blood carries nutrients and oxygen to your muscles, helping rebuild them and make them stronger. And it looks like histamine may be driving the bus in this process.
So, you can probably see where I’m going with this. If we block the action of histamine in order to stop allergy symptoms, are we also inadvertently suppressing critical molecular pathways needed to reap the benefits of physical activity?
Putting It to the Test
To examine the short- and long-term effects of antihistamines on exercise, researchers affiliated with Ghent University and the University of Copenhagen performed two experiments.
In the first study, they had eight healthy participants ride an exercise bike for 40 minutes on two different occasions. On one of these lab visits, they rode the bike without taking antihistamines. So, this would be the control condition.
On another occasion, they had them do the exact same exercise for the same duration after taking two classes of over-the-counter antihistamines.
When they took the antihistamines, the natural boost in blood flow that normally follows exercise was significantly dampened, compared to placebo. In fact, the total post-exercise rise in blood flow was cut by 35%.
So, how much does this reduction matter? And what does it mean for your training over time if you take these medications chronically?
Impact of Long-term Antihistamine Use on Fitness Gains
The researchers then recruited 18 participants and assigned them to a 6-week training program on the bikes, during which they exercised three times per week. These subjects were split into two groups, and took either placebo or the aforementioned antihistamines for the duration of the program.
At the end of the program, the researchers put all of the participants through a cycling test where they had them ride the bike to the point of exhaustion to see how much fitter they had gotten through the training program.
Now, everyone got better, but the antihistamine showed markedly less improvement. For instance, they didn’t experience nearly as great of an increase in peak power output compared to the control group (+7% vs +12%) at the end of the training regimen. Basically, they were doing the exact same exercise program, but not getting nearly the same results for their efforts.
When the researchers took a look at muscle biopsies, they found the source of the problem. When you exercise regularly, your body adapts by building more mitochondria. More mitochondria result in greater energy production, and better resistance to fatigue, enhancing performance over time.
Typically researchers can get a good idea of the amount of mitochondria in muscle by measuring the activity of the enzyme citrate synthase. When these scientists did that, they found that the control group showed more than twice the growth in CS activity that the antihistamine group had (+33% vs +14%).
Impact of Long-term Antihistamine Use on Glycemic Control
The researchers were also interested in how alterations in blood flow due to antihistamines might affect one of the most important health benefits of regular exercise, and that is blood sugar control.
Aerobic exercise elicits a rise in insulin sensitivity, which leads to lower blood sugar, since insulin is helping shuttle blood sugar out of the blood and into your hungry muscles. This also leads to lower levels of insulin, since your body is more sensitive to the hormone and therefore your pancreas doesn’t need to churn out as much.
Sure enough, the placebo group showed a substantial increase in insulin sensitivity after the 6-week exercise program (+26%). And bear in mind that this was in a group of healthy people whose measures of insulin sensitivity were already well within the normal range. When they were given an oral glucose tolerance test, their total blood sugar and insulin levels were significantly lower than before they had trained (−11 and −30% respectively), reflecting their bodies' improved metabolic function.
In contrast, the antihistamine group showed no improvements in these glycemic parameters at all. In fact, they got slightly worse!
As the researchers summarize, "We found that the chronic [histamine] blockade severely impaired multiple clinically relevant adaptations to exercise training, i.e., exercise capacity, glycemic control, and vascular function."
Practical Takeaway
Before you throw out your Zyrtec, one major caveat is in order here. The antihistamine dosing in this study was pretty high. Actually, very high.
The subjects for both experiments were given 540 mg of Allegra, which is more than three times the usual dose for the 24-hour version of Allegra. That's a whole lot of antihistamine.
Would the researchers have found the same impairment if they had used more common dosages of these medications? Or if they had only used drugs to block either H1 or H2 receptors, rather than both? Hard to say for sure.
In the meantime, everybody's risk/benefit assessment is going to be a little different.
Allergies suck. And the symptoms of allergies can interfere with exercise, albeit indirectly. For one thing, it's obviously pretty tough to engage in hard exercise if you are having trouble breathing! Symptoms of allergies can also make it difficult to get restorative sleep, which is absolutely crucial for proper exercise recovery.
So I would say that the tradeoffs of the medications depend on how much you are affected by allergies and how frequently. One thing you can try to do is identify what exactly you are allergic to, and only take antihistamines when you know that you’re going to be exposed. This might mean you only use the drugs seasonally, or even just during certain days of the week (like if you go outside more during the weekend, for instance).
Finally, don’t forget that physical activity itself has been shown to improve allergic symptoms - so keep that in mind and harness the power of positive thinking and no sneezing next time you’re seeking gains!
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