Often dubbed the “sunshine vitamin,” vitamin D plays a key role in bone health and in maintaining a healthy immune system. It also helps keep calcium in balance by increasing its absorption and reducing its loss through the urine. In fact, it behaves more like a hormone than it does a vitamin.
The best source of vitamin D is exposure to sunlight. When sunlight hits your skin, vitamin D precursors on the skin’s surface are converted to vitamin D precursors. This “pre-vitamin D” then travels to the liver and kidneys where it’s activated and turned into active vitamin D your body can use.
New research shows that your vitamin D level may have potential health benefits that extend beyond calcium balance and immune health. For example, a study published in the European Journal of Preventive Cardiology found that vitamin D plays a role in cardiovascular fitness. In other words, vitamin D may impact how the heart responds to cardiovascular exercise. Can vitamin D even enhance exercise performance?
Aerobic Capacity and Cardiovascular Fitness
The best marker of cardiovascular fitness is a measurement called V02 max. V02 max is a measure of how much oxygen an individual can utilize during exercise. During a workout, your lungs and cardiovascular system must continuously deliver oxygen to working muscles. As exercise intensity increases the demand for oxygen rises. But, there are limitations to this oxygen delivery. As the intensity increases, oxygen delivery and utilization will peak at some point and start to plateau. This point at which oxygen delivery plateaus is V02 max, a measure of cardiovascular or aerobic capacity of fitness.
Some people have a higher V02 max than others and can sustain sub-maximal exercise longer as a result. A person’s V02 max is impacted by how efficiently the heart and lungs take in oxygen and pump it to the working muscles. Strong lungs and a healthy heart equate to a higher V02 max and better aerobic fitness.
Another variable that impacts V02 max is how efficiently muscle cells can extract oxygen from the bloodstream and convert it to ATP, the energy currency of all cells. Organelles inside the muscle cells called mitochondria make the ATP that fuels exercise. As such, the total number of mitochondria inside muscle cells impacts V02 max. More mitochondria and higher levels of enzymes that make ATP enhance aerobic capacity (V02 max). One of the positive adaptations to endurance training is an increase in the total number of mitochondria and mitochondrial enzymes inside muscle cells. So, you can improve your V02 max with training.
What Role Does Vitamin D Play?
According to researchers at Virginia Commonwealth University, higher levels of vitamin D may positively impact aerobic capacity. Researchers looked at 1,995 participants as part of the National Health and Nutrition Survey (NHANES) trial that ran from 2001-2004. Some of the male and female participants were healthy while others had hypertension or diabetes.
After comparing the cardiovascular fitness (as measured by V02 max) with the vitamin D levels of the study participants, the researchers made some interesting observations. Those with the highest blood levels of vitamin D had 4.3-fold higher VO2 max measurements. Although this is correlational and doesn’t show that the greater vitamin D level caused the higher measurements of V02 max, it’s an interesting observation.
You might wonder whether some other factor could explain these findings. It’s possible, but even after the researchers controlled for a number of factors that impact aerobic capacity, the benefit was still there, although not as strong. The difference in aerobic capacity between the highest and lowest vitamin D levels was around 2.9-fold. That’s substantial!
Cardiovascular Fitness Matters!
Having a higher V02 max is important for reasons other than being able to sustain aerobic exercise longer. Studies link a higher V02 max with decreased mortality. In fact, a 2009 study published in the Journal of the American Medical Association correlated higher V02 max with a reduction in all-cause mortality. Another study that looked at 800 middle-aged men and followed them for 45 years found that low aerobic fitness was second only to smoking as a predictor of death. Poor physical fitness was associated more strongly with mortality than health conditions like hypertension and elevated cholesterol.
How’s Your Vitamin D Level?
Are you curious about your own vitamin D level? The best way to find out is to check a 1,25 hydroxy-vitamin D level, a simple blood test your physician can perform. Depending on where you live, how much time you spend outside, your age, skin color, and your body weight, your vitamin D level may be suboptimal.
How high should it be? According to the Institute of Medicine, 20 ng/ml or above is adequate for bone health, but some experts argue that a level of 30 ng/ml or above is a healthier goal. The problem is we don’t yet know what the ideal vitamin D level is, only what is too low. Some studies show that vitamin D levels correspond to a U-shaped curve. Too low is unhealthy but so is too high. For example, Danish studies found that a vitamin D level above 56 ng/ml was associated with a 40% higher risk of mortality relative to a level of 30 ng/ml. However, people with low levels were at the highest risk of dying. Their risk was 200% higher.
Experts question whether these results pertain to the population as a whole, as many people in Danish countries use cod liver oil as a source of vitamin D. Cod liver oil is high in vitamin A, and vitamin A can be toxic at higher levels. The higher mortality could have been due to the excess vitamin A the people in Danish countries get rather than vitamin D. What is clear is that low levels of vitamin D are linked with a greater risk of death. You don’t want to be below 20 ng/ml and 30 to 40 ng/ml may better, but the optimal level has yet to be established.
The Bottom Line
There’s still much to learn about what the optimal vitamin D level is but it is important to avoid a deficiency state. Being deficient could impact your exercise performance as well. So, know your level and make sure you’re getting enough!
Science Daily. “Vitamin D levels in the blood linked to cardiorespiratory fitness”
European Journal of Preventive Cardiology, 2018; 204748731880727 DOI: 10.1177/2047487318807279.
JAMA. 2009 May 20;301(19):2024-35. doi: 10.1001/jama.2009.681.
Health Day. “Lack of Fitness Second Only to Smoking as Predictor of Early Death: Study”
Harvard Health Publishing. “Vitamin D: What’s the Right Level? “
Vitamin D Council. “High and low vitamin D levels cause mortality?”