Vitamin D and the Immune System

Over the past few years, significant emphasis has been placed on the role of vitamin D, which reflects the growing need to ensure that the public is taking adequate levels of this essential nutrient since adverse health conditions results from suboptimal levels of this vitamin worldwide⁵. Vitamin D is an essential steroid hormone that the body needs to function effectively. And, as a result, the amount of vitamin D in ones system has important implications for young children and adults alike. This vitamin is required for bone development and growth in children and for maintenance of bone in adults¹². In addition, vitamin D is also used to prevent osteoporosis and fractures in theelderly¹². Both young and old require vitamin D to maintain calcium absorption and skeletalintegrity¹². Vitamin D can be obtained from exposure to the sunlight or by ingesting foods rich in vitamin D. For example, sun exposure (ultraviolet light) is absorbed through the skin which stimulates the conversion of 7-dehydrocholesterol to previtamin D, which is spontaneouslyconverted to vitamin D^4,12. However, although sun exposure alone may provide adequate vitamin D intake in some countries, this is not so in the northern United States where ultraviolet light in the winter provides insufficient quantities of vitamin D intake and as a result will not be able to protect against osteoporosis and fractures¹⁴. Consequently, vitamin supplements and products fortified with vitamins have been used to ensure that adequate levels can be consumed when ultraviolet light is not sufficient. While certain foods such as dairy products, eggs, and fish contain a small amount of vitamin D, fortified foods such as milk, infant formula, and cereals are among the sources which have the most vitamin D in the United States¹².

Chronic vitamin D deficiency has serious adverse consequences, which may include an increased risk of hypertension, multiple sclerosis (MS), cancers of the colon, prostate, breast, and ovary, and type 1 diabetes^2,4,9,10. Although, severe vitamin D deficiency, which also causes rickets and osteomalacia, have for the most part been eliminated as one of the common scourges affecting the population, it is evident that more needs to be done to control the problem among the sick and elderly¹². For example, the findings from a study conducted by Thomas et al., shows just how prevalent vitamin D deficiency can be. In that study, researchers evaluated hypovitaminosis D in medical inpatients. Researchers assessed vitamin D intake, ultraviolet light exposure, and risk factors for hypovitaminosis D and measured serum 25-hydroxyvitamin D, parathyroid hormone, and ionized calcium in 290 consecutive patients on a general medical ward¹¹. Results showed low serum 25-hydroxyvitamin D concentrationsin 57 percent of 290 patients hospitalized on a medical service¹¹. In particular, a total of 164 patients (57 percent) were considered vitamin D–deficient (serum concentration of 25-hydroxyvitamin D,_15 ng per milliliter), of whom 65 (22 percent) were considered severely vitamin D-deficient (serum concentration of 25 hydroxyvitamin D, _8 ng per milliliter)¹¹. In addition, researchers found that serum 25-hydroxyvitamin D concentrations varied inversely to parathyroid hormone concentrations¹¹. Moreover, results also showed that 67% of patients who consumed less than the recommended daily amount of vitamin D and 37% of the patients with intakes above the daily amount were found to be vitamin D deficient¹¹. Researchers concluded that the incidence of hypovitaminosis D is common in medical inpatients, and includes patients who exceed the daily recommended intake of vitamin D as well as for patients who have no apparent risk factors for vitamin D deficiency¹¹. The results of this study suggest two conclusions: First, it shows that many people do not ingest the daily recommended amount of vitamin D suggested by the Foodand Nutrition Board of the Institute of Medicine (i.e. 200 IU daily for people19 to 50 years old, 400 IU daily for those 51 to 70 years old, and 600 IU daily for those 71 years old or older)¹². Second, even the recommended “adequate-intake levels” may be too low¹².

While vitamin D is an important hormone used in the regulation of calcium intake, there exists mounting evidence which supports the link between vitamin D and its active role in the immune system. For example, epidemiological studies have shown that multiple sclerosis (MS) is more prevalent in temperate high latitudes when compared to those at the equatorial latitude^3,7. In another study conducted by Munger et al., researchers assessed the protective effect of vitamin D on MS⁶. Two large cohorts of women: the Nurses’Health Study (NHS; 92,253 women followed from 1980 to 2000)and Nurses’ Health Study II (NHS II; 95,310 women followedfrom 1991 to 2001) were used in this study⁶. Diet was assessed at baseline and updatedevery 4 years thereafter⁶. During the follow-up, 173 cases of MS with onset of symptoms after baseline were confirmed⁶. Results of this study show that vitamin D has a protective effect on MS⁶. In another study, which examined past sunlight exposure and its association with increased risk of MS, researchers found that a higher sun exposure in childhood and early adolescents reduces the risk of MS¹³. The study was based on a population-based case-control design in Tasmania at latitudes 41-3°S¹³. Participants included 136 cases with MS and 272 controls, which were randomly drawn from the community and matched based on gender and birth year¹³. Specific results showed that high sun exposure during the ages of 6-15 years (average 2-3 hours or more a day in summer during weekends and holidays) was correlated with a reduced risk of MS (adjusted odds ratio 0.31, 95% confidence interval 0.16 to 0.59)¹³. In addition, it seemed that higher exposure in winter was more important than higher exposure in summer¹³. Another study which examined vitamin D3 and its role in regulating gene expression patterns which would resolve inflammation was assessed⁹. A case-controlled study design was implemented in which experimental mice were injected with autoimmune encephalomyelitis (EAE), which induces acute inflammation (EAE is an induced disease and is a widely used model for MS)⁹. Mice were also injected with a placebo and compared to the experimental group. Results showed that the administering 1,25-(OH)2D3 (vitamin D₃) to mice with severe EAE caused a rapid reduction in the inflammation and cell burden in the central nervous system (CNS) and reversed the clinical disease signs in the experimental mice⁹. This study provides new insights into how vitamin D3 may regulate CNS inflammation⁹.

Autoimmune diseases have an unknown etiology¹. And as a result, there may be a host of factors which predisposes individuals to an increased risk of disease. For example, multiple interactions may take place between genes and the environment, which in turn may complicate how researchers are able to detect the exact mechanisms behind autoimmune diseases¹. However, studies which are genetically based may be able to indirectly associate a link between environmental risk factors such as low ultraviolet radiation (thus, low vitamin D intake) and increased risk of autoimmune disease by showing that individuals with an observed genetic vulnerability who have low levels of ultraviolet light exposure (low vitamin D) are more likely to develop disease⁸.

References