Do Antihistamines Affect Calories & Metabolism?

What are Histamines?

Allergens in the environment, such as pollen and mold, cause allergic reactions that start with the production of histamines. Histamines are neurotransmitters that cause your body to begin an inflammatory response to an allergen. This response involves blood-vessel growth, swelling and triggering various receptors to eliminate the irritant. The expansion of blood vessels and triggering of receptors cause skin redness, itchiness and headaches as the body works to fight off the allergen.

Histamine Production and Exercise

When you exercise, mast cells -- containing large amounts of histamines -- release histamine granules, causing an allergic reaction. Taking medication or eating food to which you are allergic before exercising may trigger histamine production, according to a 2008 study published in the "Journal of Investigational Allergology and Clinical Immunology." Exercise-induced histamine production can result in urticaria, angioedema or anaphylaxis, which involve varying degrees of red and itchy hives, swelling and airway constriction.

What are Antihistamines?

Antihistamines come in the form of tablets, liquids, eye drops and nasal sprays. They block certain histamine receptors, working to reduce and even eliminate the reactions generally caused by the associated allergen. Rantidine, also called Zantac, reduces stomach acid buildup by blocking receptor H2. Fexofenadine, also called Allegra, can be used to block receptor H1, reducing sneezing, itchy eyes and runny nose.

Calories and Metabolism

Although certain drugs have been linked to weight gain, no studies have turned up results linking antihistamines to calorie intake or metabolism. Some antihistamines make you drowsy, thus temporarily slowing your metabolism, but the effects are not lasting. Studies have shown that other medications that block histamine receptors, such as the antipsychotics olanzapine and clozapine, have a significant effect on weight gain. However, because antihistamine medication is taken less frequently and in a much smaller dosage, the effect on weight gain is negligible, as demonstrated in a 2006 "Proceedings of the National Academy of Sciences of the United States of America" article.

Warnings

Some antihistamines can make you drowsy and should not be used when exercising, especially if your workout routine involves lifting or using machines. Stick to non-drowsy antihistamines before your workout and save the drowsy ones for bedtime. If you prefer to take drowsy antihistamines, read the bottle and talk to your physician to determine how long you should wait after taking the antihistamine before it's safe to exercise.