Vitamin D and Its Role in Health and Nutrition

What is Vitamin D?

Vitamin D is a fat-soluble vitamin that is necessary for bone health and overall immunity. It plays an important role in calcium absorption and as well as regulating both the innate and adaptive immune responses. Recent research suggests that it may also play a role in protecting against some chronic diseases.

As a fat-soluble vitamin, fat micelles help facilitate the passive diffusion into the small intestine as well as allow for storage of Vitamin D in the liver and fat cells. Vitamin D exists in two primary forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). The most bio-available form is vitamin D2, which is obtained from plant and fungi sources that have been exposed to UV light. Alternatively, vitamin D3  is synthesized in the skin of humans when exposed to sunlight. It is the only vitamin that can be absorbed through external source.⁵

Both forms of Vitamin D are biologically inactivated when ingested or absorbed. When ingesting Vitamin D2, it is absorbed in the small intestine. In this form, the body must activate it in the liver and kidneys through the process of hydroxylation (see Figure 1 below). In contrast to ingesting, vitamin D3 is absorbed into the blood stream after it is synthesized by sunlight, converting 7-dehydrocholesterol in human skin which is hydroxylated in the liver and kidneys.⁵

Figure 1. The Metabolism of Vitamin D

Vitamin D exists in two primary forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). These forms differ slightly in their chemical structure and origin. Vitamin D2 is produced by plants and fungi when exposed to ultraviolet (UV) light, while vitamin D3 is synthesized in the skin of humans and animals upon exposure to sunlight. Both forms are biologically inactive and require two hydroxylations in the body for activation. The first occurs in the liver, converting vitamin D to 25-hydroxyvitamin D [25(OH)D], also known as calcidiol or calcifediol. The second occurs primarily in the kidney, forming the physiologically active 1,25-dihydroxyvitamin D and 24,25(OH)2D, also known as calcitriol. Image Source: Vitamin D Sources, Metabolism, and Deficiency: Available Compounds and Guidelines for Its Treatment

Vitamin D’s Role in Human Nutrition

Research shows that vitamin D plays a significant role in bone health, mental health , and the proper function of both the immune system and muscle function:

1. 1. Bone health: Transcellular transport of calcium in the bone and kidney is mediated by vitamin D. It acts as a precursor for calcium (crucial for strong bones) absorption in the gut. Additionally, it acts as a stabilizer to maintain serum calcium and phosphate concentrations to enable bone mineralization and regeneration of both osteoclasts and osteoblasts for the regeneration of bone tissue. It also mediates bone mineralization by saturating the plasma with calcium and phosphate through its binding proteins. Bone mineralization and preventing unwanted involuntary contracting of muscles are the key functions of vitamin D.^4,6,7
   2. Immune health: It plays a crucial role in regulating both the innate and adaptive immune responses. It modulates the activity of immune cells, such as B cells, T cells, and antigen-presenting cells, and promotes a balanced immune response. Vitamin D deficiency is associated with an increased susceptibility to infections and autoimmune diseases.⁸
   3. Muscle function: Vitamin D supports muscle function, ensuring proper muscle movement and strength.⁷
   4. Mental health: Emerging evidence suggests a link between vitamin D and mental health. Studies have shown an association between low vitamin D levels and an increased risk of depression, anxiety, and cognitive decline. Vitamin D’s neuroprotective properties may contribute to its role in mental well-being. It plays a prominent role as a neuroprotective agent, reducing neuroinflammation, supporting serotonin synthesis, and improving brain plasticity.^9,10
   5. Other Potential Roles: Vitamin D has other roles in the body, including modulation of cell growth, neuromuscular function, and glucose metabolism.¹¹ Ongoing research is also exploring the potential benefits on other health conditions including heart disease, diabetes, and musculoskeletal diseases like multiple sclerosis.¹²

Recommended Daily Intake & Food Sources of Vitamin D

According to FDA, the Recommended Dietary Intake (RDA) is 400IU-600IU for males and females in the age range of 1-50 years of age. If the Daily Value (DV) of a food product has 20% of DV of the nutrient, it considered a rich source. Serum 25(OH) D is the optimized marker of vitamin D.

While the majority of the our body’s vitamin D is synthesized through sun exposure, there are a variety of plant and animal sources that contain vitamin D. However, these food-based sources are not fully sufficient, thus sunlight exposure, supplementation, and eating fortified foods can become a necessity in order to achieve the RDA of this vitamin.

Animal based-products like fish, especially, trout, salmon and tuna are some of the best sources of vitamin D. Beef liver, egg yolk and cheese and milk have vitamin D2 as well. In terms of plant based of sources, mushrooms, soy, nuts, almonds, can be reliable sources.

Supplementation of Vitamin D Beyond Food

In many parts of the world, individuals may not be able to get sufficient amounts of vitamin D2 and D3. With the Sun being the primary source of vitamin D3, regions that experience long periods with limited sunlight (those in higher latitudes with longer winters) create conditions necessary for supplementation of vitamin D3, especially in the winters. These vitamin D2 and D3 supplements are in the form of capsules as well as in dissolvable powders for inclusion in beverages.

In higher latitude regions, UVB ray lamps are also available to mimic sunlight. Additionally, many food providers practice fortification of milk and fruit juices creating a reliable source on a regular basis. In the U.S., milk is sometimes fortified with vitamin D, mostly in the form of D3. This can allow for up to 120 IU of vitamin D3 in one cup (240ml) of fortified milk.

Risk of Vitamin D Deficiency

The Institute of Medicine (IOM) considers a serum 25(OH)D level below 20 ng/mL to be a Vitamin D deficiency. With sedentary lifestyles, it has become increasingly important to screen vitamin D levels. Immunity and depression are directly linked to levels of vitamin D.^1,2

Vitamin D deficiency in toddlers and young adults can cause rickets and slow brain development. Deficiency in adults causes osteomalicia (brittle bones) making them more susceptible to fractures. According to the NIH, 2.3 million people suffered osteoporotic fractures in 2015, which could be linked to vitamin D deficiency. Women undergoing menopause are also more likely to undergo osteoporosis and bone degradation making vitamin D deficiency an added concern.^1,3

Vitamin D and Menopause

During menopause, estrogen decreases leading to a decrease in calcium absorption in the small intestine. This leads to weakening of bones and making them susceptible to fractures and eventually osteoporosis. This is directly related to decrease in bone mineralization. Vitamin D facilitates reabsorption of calcium and phosphorus, which help with calcification, bone mineralization, and regulation of the parathyroid hormone to maintain blood calcium and phosphorous.

Vitamin D is an essential nutrient that plays a crucial role in maintaining overall health. It is vital for calcium absorption, bone health, immune function, and mental well-being. Vitamin D deficiency is a common problem that can lead to various health issues. While sunlight is the best source, it is important to balance sun exposure to reduce risk of overexposure to UV rays. Adequate intake of vitamin D can be achieved through a combination of diet, sunlight exposure, and supplementation. It is important to note that older adults may have trouble absorbing vitamin D, and toxicity can occur in some individuals even with lower doses of supplementation.¹² Ongoing research continues to explore the potential benefits further refining our understanding of its role in human health. While vitamin D is essential, further research is needed to fully understand its role in various health conditions and to establish clear guidelines for supplementation.

• References
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  3. Mei, Z., Hu, H., Zou, Y., & Li, D. (2023). The role of vitamin D in menopausal women’s health. Frontiers in physiology, 14, 1211896. https://doi.org/10.3389/fphys.2023.1211896
  4. Fleet J. C. (2022). Vitamin D-mediated regulation of intestinal calcium absorption. Nutrients 14 (16), 3351. 10.3390/nu14163351
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