Your Eye Color Has An Unexpected Effect On Your Heart

Our heart rate does not stay constant throughout the day. Much like the waves of the ocean, it rises and falls depending on whether we are at rest or expending energy, explains Johns Hopkins Medicine. A resting heart rate between 60 and 100 beats per minute is considered healthy (via Mayo Clinic). However, age, body size, one's emotional state, levels of physical activity, health conditions, and even outside temperature can all influence our heart rate. As it turns out, the color of our eyes may also have a unique effect on our heart rate.

The determination of our eye color can largely be traced to a particular area along chromosome 15, in which two specific genes are found that are responsible for melanin production (via MedlinePlus). It's melanin that gives our eyes their pigmentation. People who have more melanin in the iris (the ring surrounding the eye's pupil) will have darker eyes. Having lesser amounts of melanin in the iris means that a person will have lighter eyes. Research has shown that, in some specific instances, eye color may be linked with changes in heart rate.

In an early 1988 study published in the Journal of the Autonomic Nervous System, researchers injected atropine into the muscles of 20 healthy male adults. Eight of the participants had brown eyes, another eight had blue eyes, and four individuals had hazel eyes. Atropine is a drug used to treat symptoms of bradycardia, or a slowed heart rate of fewer than 50 beats per minute accompanied by unstable patient vital signs, according to 2023 updated research published in StatPearls.

Using two different devices to administer the drug, the researchers measured participant pupil dilation and heart rate within the first hour and a half of the intervention. The research findings showed that men with brown eyes experienced faster heart rate acceleration than those with hazel or blue eyes. The researchers theorized that melanin, such as that found within the iris, may modify the body's response to atropine. This was reinforced in 2022 research published in the Journal of Controlled Release, in which the researchers point out that certain drugs — such as atropine, timolol, and levofloxacin — bind to melanin, thereby heightening the drug's concentration and remaining in the body for longer periods of time. 

Researchers from another early 1978 study published in the British Journal of Opthamology looked at over 300 patients undergoing surgical treatment for squinting. The researchers noted that the body's oculocardiac reflex is common during such procedures, in which a patient's heart rate slows down as a result of pressure on the eyeball. The research findings showed that the reflex occurred more frequently in patients with brown eyes and hazel eyes undergoing anesthesia than in individuals with blue or grey eyes. In light of these results, the researchers emphasized the particular importance of monitoring the heart rate of patients with brown or hazel eyes undergoing eye surgeries.

Additionally, the study team found that abnormal heart rates (arrhythmias) were observed more frequently in brown-eyed and hazel-eyed patients in tandem with the body's oculocardiac reflex compared to patients with grey or blue eyes. While more research is needed in regards to the potential relationship between eye color and arrhythmias, the researchers stated that further study could help determine whether eye color may play a role in identifying high-risk heart patients in need of intensive coronary care.