Cardiorespiratory fitness is the ability of the circulatory and respiratory systems in the body to supply oxygen to the muscles during physical activity. Improved cardiorespiratory fitness enables the heart to better pump blood and increases the amount of oxygen that is distributed throughout the body. Good cardiorespiratory fitness reduces the risk of heart disease, diabetes, and stroke.
The main measurement of cardiorespiratory fitness is maximal oxygen consumption (VO2max). VO2max reflects how effective the oxygen transport system is able to move blood and how effective cells throughout the body are able to take in and use oxygen in energy production. It is usually tested during exercise to evaluate an individual’s overall physical fitness and endurance.
VO2max declines with age. This is mainly because as we age, we are less able to exercise and perform physical activities. This means that aging limits the body’s ability to deliver blood and use oxygen for energy, which increases the risk of cardiovascular disease.
Vitamin D intake is associated with cardiovascular health. Research shows that higher vitamin D levels may help reduce blood pressure, help prevent clogged arteries, and reduce inflammation in the cardiovascular system.
Vitamin D status, too, declines with age. This is due to the fact that aged skin is thinner and less able to produce vitamin D from the sun. Older people are also less likely to get outside and get sun exposure.
Because both measurements of cardiorespiratory fitness and vitamin D levels seem to decline as we age, researchers in the present study wondered, might vitamin D levels correlate with cardiorespiratory fitness?
To answer their question, the researchers recruited 67 healthy African-American and European-American women between the ages of 60 and 74 years old. The women were all defined as sedentary, meaning they did not exercise more than once per week.
The women underwent treadmill exercise to determine VO2max. Additionally, the researchers measured respiratory quotient during moderate exercise on three consecutive mornings. Respiratory quotient is a ratio used to determine which source of fuel is being used for energy (carbohydrate or fat). A lower respiratory quotient is an indicator that fat is being oxidized and used for energy. Finally, the researchers took fasting blood samples to determine serum vitamin D levels.
Here is what the researchers found:
- VO2max and vitamin D levels were both significantly lower among the African-American women compared to the European-American women.
- After adjusting for ethnicity, higher vitamin D levels were associated with greater VO2max only among the African-American women.
- A higher vitamin D level was non-significantly associated with a lower respiratory quotient among both ethnic groups, indicating that vitamin D may play some role in oxidizing fat for energy in the body.
The researchers concluded,
“A novel observation of this study was the association between serum 25(OH)D and VO2max in African Americans specifically, independent of body composition. Vitamin D status also appeared to influence substrate oxidation, such that serum 25(OH)D was inversely correlated with all measures of respiratory quotient during submaximal exercise.”
The observational design of the study means that we can’t say for sure if low vitamin D causes poorer cardiorespiratory fitness. Also, the researchers only looked at a small group of older adult women, meaning that the results may not apply to other populations.
This research raises question of yet another possible role for vitamin D in cardiovascular health. Future randomized controlled trials looking at the effects of vitamin D supplementation on cardiorespiratory fitness and fat oxidation are needed to elucidate this relationship.