Physical Activity, Body Weight Status, and Serum Vitamin D Levels in Healthy Women

Many years ago, it was thought that the sole function of vitamin D (the sunshine vitamin) was to assist with calcium and phosphorus absorption from the gut into the bloodstream. Today, we know that the function of this fat-soluble vitamin extends far beyond that function. Unfortunately, vitamin D deficiency is extremely common in U.S. women. Also very common among women are sedentary lifestyle and obesity. Importantly, each of these three lifestyle factors is associated with an increased risk of a number of adverse health outcomes including cardiovascular disease, diabetes, and some cancers. However, a thorough examination of the relationship among vitamin D, physical activity, and body weight status is lacking.

Accordingly, Cooper Institute researchers recently reported on 7553 healthy women who received a comprehensive physical exam at the Cooper Clinic between 2006 and 2018. Blood vitamin D level, physical activity level, and body weight status were assessed at the time of the exam. Physical activity was assessed via a detailed questionnaire, while body weight status was assessed using four different methods including body mass index (BMI), waist circumference, waist to height ratio, and percent body fat. Participants were divided into 4 categories of physical activity based on current guidelines: <500 (not meeting guidelines), 500-1000 (meeting guidelines), 1001-2500 (>1-2.5 times guidelines), and >2500 (>2.5 times guidelines) MET-minutes/week. Body weight status was categorized using standard clinical cut points. Our findings were as follows:

• Among these women, the higher the level of physical activity, the higher the blood levels of vitamin D. More specifically, vitamin D levels were 31.3, 33.6, 34.3, and 36.6 ng/mL across the four categories of physical activity, respectively.
• Using BMI as the indicator of body weight status, when compared to normal weight women, overweight and obese women were 1.8 times and 3.1 times more likely to have vitamin D deficiency, respectively. Similar results were observed when using the other three measures of body weight status.
• Overall, the highest vitamin D levels were seen in normal weight women who were the most physically active, while the lowest vitamin D levels were seen in obese women who were the least physically active.

Those findings were expected. What made the current study novel is that we also examined vitamin D levels within all possible combinations of physical activity and body weight status. By doing so, we sought to better understand the impact of physical activity on blood vitamin D levels within different categories of body weight status. The Figure below shows our results based on BMI as the indicator of weight status.

The three BMI categories are shown on the horizontal axis of the Figure. Blood vitamin D levels are shown on the vertical axis. The shaded areas show blood vitamin D levels across the four physical activity categories within normal weight, overweight, and obese women.

Notice that within each of the BMI categories, vitamin D levels were higher across increasing categories of physical activity. We observed the same trends when examining waist circumference, waist to height ratio, and percent body fat. Therefore, regardless of the method used to assess body weight status, vitamin D levels were higher with increased levels of physical activity in normal weight, overweight, and obese women. These findings suggest that blood vitamin D levels are directly related to physical activity level, regardless of body weight status. Additionally, the risk of having low vitamin D levels is substantially reduced by being more physically active, even among overweight and obese women.  

At this point, you may be wondering why more physically active women had higher blood vitamin D levels than less active women, regardless of body weight status. There are three possibilities: 1) active women may have had greater sunlight exposure than inactive women. 2) active women may have had a higher dietary vitamin D intake than inactive women. 3) vitamin D is stored in fat cells, and fatty acids are released into the blood from fat cells during physical activity. It is possible that vitamin D is released by fat cells into the blood during physical activity as well. Since we were unable to measure sunlight exposure and only a small percentage of these women completed a dietary assessment, further studies are needed in this area.  

Based on our results, we have a few recommendations:

1. All women are strongly encouraged to meet the 2018 Physical Activity Guidelines for Americans. The Guidelines recommend accumulating a minimum of 150 minutes each week of moderate intensity aerobic activity (e.g. brisk walking). Additionally, a minimum of 2 days each week of strength training is recommended in order to obtain maximal health benefits.
2. All women should strive to achieve a reasonable body weight. For example, a normal BMI for adult women is between 18.5-24.9 kg/m². Use this link to calculate your BMI.  
3. All women should undergo testing of blood vitamin D levels and follow their primary health care provider recommendations with regard to vitamin D supplementation.  

Reference
Farrell, S., Meyer, K., Leonard, D., Barlow, C., Shuval, K., Pavlovic, A., DeFina, L., Haskell, W.  Physical Activity, Adiposity, and Serum Vitamin D Levels in Healthy Women: The Cooper Center Longitudinal Study. Journal of Women’s Health. Published online ahead of print, March 30, 2022.

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