The Cooper Institute

Founded in 1970 by the "Father of Aerobics"
Kenneth H. Cooper MD, MPH


Vitamin D Levels, Cardiorespiratory Fitness, and Adiposity: Are They Related?

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Live well

Thursday, Mar 03, 2016

Because vitamin D can be formed in the skin upon exposure to sunlight, it is known as ‘the sunshine vitamin.’ Fortified dairy products (especially milk and margarine) as well as fatty fish are significant dietary sources of vitamin D. You may have heard that vitamin D deficiency is a global epidemic, and that deficiencies are related to an increased risk for several chronic health conditions. These include some cancers, cardiovascular disease, hypertension, diabetes, and osteoporosis. More recently, it has been shown that vitamin D levels are also related to muscle strength and balance. Vitamin D status is measured with a simple blood analysis called the 25-hydroxyvitamin D test. A vitamin D deficiency is defined as a blood level less than 20 ng/mL, while vitamin D insufficiency is defined as a level between 20-29.9 ng/mL. While blood levels of 30 ng/mL or higher are considered normal, the optimal blood level of vitamin D has not yet been established. The current RDA for vitamin D is 600 I.U. for individuals between the ages of 1 and 70 years, while individuals greater than 70 years need 800 I.U. daily. To put that into perspective, an 8 ounce glass of milk contains about 100 I.U. of vitamin D.  

Because cardiorespiratory fitness (CRF), obesity, and vitamin D status are important markers of health, my colleague and I decided to examine these relationships in a cohort of 1320 female Cooper Clinic patients1. None of the women were taking a vitamin D supplement, and all underwent a comprehensive physical exam including a maximal treadmill exercise test and measures of body mass index (BMI), waist circumference, and blood vitamin D level. Based on their age group, women were assigned to one of five CRF categories (quintiles). We observed a strong relationship between CRF and blood vitamin D levels (Figure 1), with higher blood levels of vitamin D seen across increasing levels of CRF.

Next, we divided the women into categories based on their Body Mass Index (BMI) and waist circumference. We observed a strong relationship between BMI and vitamin D levels, with lower levels of vitamin D observed among individuals with higher levels of BMI.  Compared to women in the normal weight BMI category, overweight and obese women were 1.6 and 4.1 times more likely to have vitamin D deficiency, respectively (Figure 2). A similar trend was observed for waist circumference and vitamin D deficiency. Women who were obese based on waist circumference were significantly more likely to have vitamin D deficiency than women with a normal waist circumference (data not shown).


Finally, we put it all together. We categorized women in the bottom 20% as ‘unfit’ and the remaining 80% as ‘fit’. We then examined vitamin D levels within each category of waist circumference. Fit women had significantly higher blood levels of vitamin D than unfit women within each category of waist circumference (Figure 3).

It is important to note that this is a cross-sectional study. Such studies simply provide a snapshot at a specific point in time; they do not prove cause-and-effect. Therefore, we cannot say for certain whether or not having low blood vitamin D levels has an adverse impact on CRF, or whether having a high blood vitamin D level has a favorable effect on CRF. However, it is likely that being overweight or obese does have an adverse effect on vitamin D levels because D is a fat-soluble vitamin. These vitamins are stored in adipose tissue, so it is logical to assume that much of the vitamin D in overweight/obese individuals is trapped inside body fat stores instead of circulating freely in the bloodstream. Having large amounts of vitamin D stored in fat cells is not beneficial to health; we need vitamin D to be in the bloodstream so that it can reach all tissues within the body.   

An important limitation to our study is that we did not have adequate data on sun exposure, sunscreen use, and diet for the group. Higher levels of vitamin D in fit women may have been due to either an increased exposure to sunlight and/or a higher dietary intake of vitamin D when compared to unfit women. Nevertheless, this was the first study to show a significant relationship between CRF, adiposity, and blood vitamin D levels. Further studies are needed to see if these relationships are cause-and-effect or just coincidental.

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Farrell, S.W. and Willis, B.L. Cardiorespiratory fitness, adiposity, and serum 25-hydroxyvitamin D levels in women. (2011). Journal of Women’s Health. 21(1):80-86.