Why Does Caffeine Affect Heart Rate?

Caffeine is found in coffee, soda, chocolate and energy drinks. Caffeine occurs naturally in more than 60 different plants and man-made caffeine is sometimes added to food, drinks and medicines. Approximately 90 percent of the people of the world use caffeine in one form or another and, in the United States, 80 percent of adults use caffeine every day, according to the U.S. Food and Drug Administration.

Central Nervous System

The autonomic nervous system regulates organ systems that are not under voluntary control, including the heart and heart rate. It controls the force of contraction, constriction and relaxation of the heart muscle. The autonomic nervous system is separated into the parasympathetic and sympathetic systems. It is the parasympathetic nervous system that causes changes in heart rate and blood pressure. Caffeine is a central nervous system stimulant and also affects the kidneys and releases free fatty acids from fat tissue.

Effect on Epinephrine

At the cellular level, caffeine will block the action of phosphodiesterase, the chemical which normally breaks down cyclic adenosine monophosphate. When caffeine stops the breakdown of cyclic adenosine monophosphate, it prolongs the effect and the response in the body. This response will prompt the release of norepinephrine and epinephrine, which increase the rate and the force of the heart contractions. This raises the heart rate and blood pressure and increases the delivery of oxygen to the brain and other tissues. It produces the "fight or flight" response in the body.

Caffeine Mimics Adenosine

Caffeine also blocks adenosine receptors. Caffeine has a similar molecular structure to adenosine and will fit the cellular receptor as well as adenosine does. This inhibits the release of neurotransmitters and works in concert with the release of norepinephrine, resulting in an increased heart rate.

Inconsistent Effects on Heart Rate

The U.S. Food and Drug Administration classifies caffeine as a drug and food additive that should not be used with individuals who have heart problems or people with anxiety because it can increase your heart rate and symptoms. However, Dr. A Suleman and Dr. N. Siddiqui published a review of the research literature in Priory.com, an online peer-reviewed medical journal, and found that another common response to the ingestion of caffeine is a slight decrease in heart rate in laboratory settings. This reduction in heart rate can occur 75 minutes after the caffeine has been ingested, even with individuals who have borderline hypertension. Reduction in heart rate was dose-related, meaning that administration of 250 to 350 mg of caffeine would produce a smaller decrease in heart rate than a lower dose of caffeine. According to the Food and Drug Administration, 5 oz. of coffee can contain up to 150 mg of caffeine and 12 oz. of Jolt Cola can contain 100 mg of caffeine.