Aerobic exercise provides many health benefits, including a reduced risk of cardiovascular, metabolic, and other chronic diseases. Some of these benefits are due to adaptations in skeletal muscle that arise from the repair of cellular damage produced by the demand of exercise. One team of researchers aimed to determine the effects of antihistamines on muscle adaptation following acute and chronic high intensity interval training, often referred to as HIIT.

Histamine is an organic molecule that acts as a neurotransmitter in the brain and spinal cord, while also regulating gut and immune function. Much of the research surrounding histamine is in relation to its effects in allergies and inflammation; however, recent research has demonstrated the role of histamine as a mediator of exercise-induced physiological responses. Histamine release is stimulated by exercise and promotes increased blood circulation, which may be important for facilitating muscle adaptation.

The investigators conducted two phases of research including a randomized cross-over study of antihistamines and their effect in acute exercise and a double-blind, placebo-controlled, randomized study of chronic exercise training and antihistamine use. Participants in the acute exercise trial completed two exercise sessions on separate days. On one day, they took an antihistamine one hour prior to aerobic exercise and on the other day they took a placebo treatment. The researchers measured heart rate during exercise and blood pressure, heart rate, and arterial blood flow in the thigh before and after exercise.

In the placebo group, blood flow in the thigh was increased by approximately threefold 15 minutes following exercise and remained 50 percent higher two hours following exercise. However, post-exercise blood flow was significantly reduced in participants who took an antihistamine (about 35 percent). Histamine treatment did not alter blood pressure or heart rate before or after exercise compared to the placebo group.

For the study on chronic exercise, participants completed six weeks of high intensity interval training, which consisted of three weekly sessions of cycling. Half of the participants took an antihistamine one hour prior to exercise and the other half took a placebo. The researchers measured maximum exercise capacity by having participants exercise to exhaustion and measuring their oxygen output. They measured metabolic health by having participants complete an oral glucose tolerance test in which they consumed 75 grams of glucose (the amount of sugar in about two cans of non-diet soda) and had their blood glucose measured over two hours. They assessed vascular function by using ultrasound to measure the rate of blood flow in the thigh. Finally, the researchers took muscle biopsies from the thigh of participants in order to measure muscle adaptation.

Over the six weeks of training, resting heart rate tended to decrease in the placebo group but not in the antihistamine group, indicating a blunting of exercise-induced adaptation. Peak power output during exercise also increased in both groups; however, this increase was significantly greater in the placebo group (12 percent increase) compared to the antihistamine group (7 percent increase). The time to exhaustion also improved significantly with training in the placebo group (81 percent increase) compared to the antihistamine group (31 percent increase). Training improved glucose tolerance significantly in the placebo group (26 percent increase), but this effect was blocked in the antihistamine group (1 percent increase). Finally, participants in the placebo group experienced a 37 percent improvement in vascular function while those in the antihistamine group experienced a 14 percent decrease in vascular function. Maximal blood flow during exercise also increased due to training in the placebo group only.

In conclusion, antihistamine use blunted the functional adaptations in exercise capacity, whole-body glycemic control, and vascular function produced by exercise. Histamine activity is necessary for the integrative exercise training response in humans, potentially related to increases in blood flow.

• Link to full report.