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Calcium, Vitamin D, and Sunlight

By Whitney Marema, RN


Calcium is fundamental to bone formation and maintenance, and one of the most important minerals in our body. The body is able to absorb calcium with the help of Vitamin D. The fat-soluble Vitamin D has long been known for its involvement in mineral metabolism and bone growth. In the absence of vitamin D, dietary calcium is not well absorbed.

A relatively small but vital amount of calcium is found in the blood and soft tissue where it plays multiple roles. Our body needs calcium to repair bones, help nerves function, make muscles contract, clot blood, and allow proper function of the heart. Calcium also plays a role in mediating secretion of insulin and hormones. When calcium intake is low or inefficiently absorbed, it is withdrawn from our bones and teeth in order to maintain normal biological functions. Weight gain can occur when calcium levels are not maintained due to the effect dietary calcium has in stimulating fat breakdown and preventing fat accumulation.

Several studies have found that many morbidly obese individuals have deficiencies in vitamin D before they have surgery.  Deficits in vitamin D in association with obesity are believed to be due to an accumulation of vitamin D in fat tissue, causing decreased circulating levels of the vitamin and reducing its ability to enhance calcium absorption.  Vitamin D deficiencies may also result from not spending enough time in the sunshine.  Exposure of the skin to the UV rays of the sun is important for the synthesis of vitamin D, and vitamin D levels are generally much higher in the summer than in the winter months. 

To determine the influence of environment on vitamin D levels in bariatric patients, a study compared the blood (serum) vitamin D levels of a group of morbidly obese pre-operative patients living in Michigan with those of a group of patients from South Florida.  The blood samples were drawn in the months of October through February.  The results did find, as have other studies, that vitamin D levels are inversely correlated to body size (body mass index), meaning that the more obese the individual, the lower their levels of vitamin D.  However, something else of significance was discovered from the study findings.  The group of patients from the northern U.S. had significantly lower vitamin D levels than did the South Floridians, regardless of body size. The data showed that 96% of the morbidly obese patients from Michigan had vitamin D deficiencies, whereas, only 39% of the patients from South Florida had low levels (vitamin D levels less than 20 mg/ml). 

Exposure to sunlight plays an important role in maintaining normal Vitamin D levels.  Since vitamin D is essential for proper calcium absorption, it is recommended that outdoor activity with exposure to sunlight be encouraged and a part of bariatric surgery patients’ routine.  Extra vitamin D supplementation (and, perhaps even artificial UV light exposure) during winter months for bariatric patients living in northern states is advisable.