Although vitamin D’s claim to fame is linked to bone health, a growing body of evidence suggests that vitamin D deficiency is also associated with well-known heart disease risk factors, such as high blood pressure, obesity, high serum triglyceride levels, and diabetes. But what about heart rate?
A Quick Primer on Heart Rate
Before you can understand how vitamin D influences heart rate, it’s essential to grasp how the heartbeat works. The heart has billions of muscle cells (myocytes) that contract during each heartbeat. To make sure that each cell contracts in concert with one another at the right moment, the heart uses an electrical signal that moves from cell to cell, similar to the wave in a sports stadium.
Calcium plays a critical role in the heartbeat. In fact, calcium particles have an electrical charge and rush into the myocyte for each contraction contributing to the electrical activity. Once inside the myocyte, calcium binds to the muscle-moving machinery, creating mechanical muscle contraction. When calcium has done its job, it leaves the heart cell, allowing muscle relaxation. This repeatedly occurs, with each beat, from birth, until death. Calcium is what makes the heart tick. When the movement of calcium malfunctions, causing too much or too little calcium to enter the cell, this can lead to abnormal electrical signals and possible heart rhythm disorders.
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In addition to calcium influx, the autonomic nervous system (ANS) is fundamental in controlling the heartbeat. The ANS is a part of the nervous system that controls internal body processes, such as heart rate and vasodilation “automatically.” This way, we don’t have to consciously contract our heart muscle or widen or narrow our blood vessels. Our body does this all on its own.
After the ANS receives information about the body and environment, it can respond by stimulating the heart rate (sympathetic activation) or slowing the heart rate (parasympathetic activation). Overall, the sympathetic and parasympathetic divisions of the ANS work together to ensure that your heart rate is controlled appropriately.
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I know what you’re thinking. That’s a lot of information about heart rate. But what does vitamin D have to do with it?
How Does Vitamin D Affect the Heart?
Vitamin D has been found to directly affect many different cells throughout the body through the action on vitamin D receptors (VDR). Although the heart has not historically been considered one of those tissues, scientific evidence suggests otherwise.
Early studies have shown that the VDR gene is expressed in the heart. Furthermore, human studies have investigated the impact of vitamin D status on cardiovascular outcomes and found severe vitamin D deficiency (serum vitamin D levels below 25 nmol/L) to be associated with a three-fold higher risk of sudden cardiac death and increased cardiovascular events compared to those with sufficient vitamin D levels.
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So although associations don’t always mean causation, there is good reason to believe vitamin D plays a role in heart function. But how?
How Vitamin D Affects Myocytes
Vitamin D levels have been correlated to muscle cell contractility and muscle strength throughout the body. Why would the heart muscle be any different?
As previously mentioned, studies have shown that the VDR gene is expressed in the heart. Moreover, cardiac ventricular tissue also expresses the 1alpha-hydroxylase enzyme, implying that it can produce 1,25-dihydroxyvitamin D3 (the active form of vitamin D in the body) from 25-hydroxyvitamin D (the primary circulating form of vitamin D in the body). These points suggest that vitamin D plays a direct and essential role on the heart’s cells.
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Animal models provide more insight into vitamin D’s role in cardiac muscle function. Studies have shown that vitamin D acts directly on atrial and ventricular myocytes. Animal models have demonstrated that cardiac myocytes isolated from VDR knockout mice show accelerated contraction rates, and that vitamin D directly affects contractility in the wild type mice but not in VDR knockout cardiac myocytes. This evidence suggests that vitamin D and VDRs play a role in heart rate and contractility.
How Vitamin D Affects Calcium
Vitamin D and calcium go hand in hand. Like Bonnie and Clyde, they usually work together to get the job done. One of vitamin D’s most infamous roles is to increase calcium absorption from the intestine.
More specifically, vitamin D has been reported to regulate every step of the intestinal transcellular calcium transport process. It induces the expression of membrane calcium channels (to allow calcium from the gut into the cell), calcium-binding protein (to protect calcium as it moves throughout the cell), and calcium extrusion (removal of calcium once it’s no longer needed).
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Once calcium is in the bloodstream, it assists in nerve transmission, muscle contraction, blood vessel contraction, and dilation. Low calcium levels, or hypocalcemia (which can be secondary to low vitamin D levels), can cause irregular heartbeat. Severe hypocalcemia can even lead to life-threatening cardiac events. Therefore, vitamin D can affect the heartbeat indirectly by its influence on calcium. Vitamin D affects the amount of calcium your body absorbs, while calcium helps generate electronic impulses and muscle contractions that regulate heartbeat.
Vitamin D Affects Autonomic Activity
As discussed earlier, the heart rate is controlled by the two branches of the ANS. One study found that low serum levels of vitamin D were associated with a decline in cardioprotective vagal tone.
Vagal tone is a measure of parasympathetic nervous system control over heart rate. Therefore, decreased vagal tone is associated with less input from the parasympathetic nervous system and less control over heart rate.
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Some studies suggest that vitamin D may influence vagal output by diffusing across the blood-brain barrier and binding directly to nuclear vitamin D receptors in the adrenergic neurons located in the spinal cord and brain tissue. While this research is promising, further study is needed to better understand vitamin D’s effects on the ANS.
Excessive Vitamin D and Atrial Fibrillation
Taking too much vitamin D is not good either, as it can lead to hypercalcemia, or high calcium levels, which can also cause irregular heartbeat. Atrial fibrillation (A-fib), one of the most common forms of irregular heartbeat, is a condition in which the heart’s upper chambers quiver instead of pump rhythmically.
A-fib decreases the heart’s pumping ability and is linked to an increased risk of stroke, heart failure, and even death. Research has found that those with vitamin D levels in “excess,” defined as higher than 100ng/dl, had a significantly higher risk for developing atrial fibrillation compared to those whose levels were between 40ng/dl and 100ng/dl.
Moreover, excessive calcium can build up in the arteries (just like those fatty deposits called plaques) and increase the risk for coronary artery disease. However, long-term studies have concluded that hypercalcemia is unlikely to occur by eating a heart-healthy diet that includes calcium-rich foods.
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Risk of hypercalcemia is more common when taking calcium supplements. When it comes to vitamin D and calcium levels, too much or too little of either can lead to heart irregularities. Like Goldilocks in the story “Goldilocks And The Three Bears,” we want to get it just right.
Irregular Heartbeats
An irregular heart rhythm can mean your heart beats too fast (tachycardia) or too slow (bradycardia). It can be disorganized (atrial fibrillation or ventricular fibrillation), or even appear to skip a beat (premature contractions). Not every irregular heartbeat will look the same. Still, symptoms may include chest pain, shortness of breath, fluttering or pounding in your chest, lightheaded or dizziness, pale skin, or sweating.
A number of things may cause an abnormal heartbeat, including illness or injury that affects the heart muscle, some types of medications, too much or too little vitamin D and calcium, and health conditions associated with risk factors like high blood pressure, smoking, diabetes, chronic stress, being overweight, and excessive use of alcohol or drugs.
While irregular heartbeats are often harmless, sometimes they can be dangerous. If you feel you may have an irregular heartbeat, be sure to seek medical attention to learn more about your potential risks.
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