The recognition of vitamin D has long been linked to preventing rickets, and in recent years, its role in assisting calcium absorption has also gained widespread acknowledgment. However, the functions of vitamin D extend far beyond these aspects. In this post, we will delve into the fundamentals of vitamin D, covering the cutoff points for deficiency/insufficiency, its physiological functions, its connection to various diseases, and ways to replenish vitamin D in our daily lives. The topic of supplementation will be addressed in our next post.
Vitamin D Deficiency is Widespread
Vitamin D deficiency affects about 40% of the US population (<20ng/ml) and around 75% of people globally have insufficient vitamin D levels (<30ng/ml) [1].
Among the elderly, the deficiency rate is above 50%, while prescription vitamin D-containing supplement use is low at 4.4% [2].
Pregnant women are at high risk, with 76% being deficient at childbirth despite the fact that they took a prenatal vitamin and drank two glasses of milk with 600 IU of vitamin D a day. [3].
People with darker skin tones, including Hispanics and blacks, are also more susceptible to deficiency due to hindered vitamin D synthesis even with sun exposure [4].
Physiology of Vitamin D
Vitamin D, a fat-soluble steroid, binds to cell receptors to exert its effects. Its biologically significant forms are D3 and D2. Vitamin D needs conversion to active metabolites to function. After liver metabolism to 25-(OH)D, it becomes active vitamin D (1,25-(OH)2D) in the kidneys. The active form, calcitriol, stimulates the small intestine to increase calcium absorption. Insufficient vitamin D results in 10%-15% absorption, while sufficient levels lead to 30%-40% absorption [5].
Active vitamin D also suppresses the level of parathyroid hormone (PTH). Vitamin D can enter parathyroid cells and bind to the vitamin D receptor (VDR), reducing the secretion of PTH. Since PTH promotes the transformation of osteoblasts, increasing bone resorption, it leads to an increase in blood calcium while also causing bone calcium loss. Regulating PTH levels helps maintain bone health.
Vitamin D’s impact on bone health is well-known, but its profound connection to chronic diseases, autoimmune disorders, and mental health is equally significant. Immune cells in the body have vitamin D receptors, and both influence each other. Inflammation-related diseases deplete vitamin D, while deficiency affects the immune response and leads to other symptoms. Though not the main protagonist, vitamin D plays a crucial supporting role in overall health.
Vitamin D levels are measured by 25-hydroxyvitamin D in the blood, but “cut-off” values vary widely among labs. Relying solely on lab-provided “cut-offs” is insufficient. Established standards consider <20ng/ml as deficient and <30ng/ml as insufficient, primarily focusing on calcium absorption and bone health. Recent research, supported by numerous scholars, suggests that levels around 40-60 ng/ml may be necessary for effective relief in many chronic diseases.
Ways to Increase Vitamin D Levels
Apart from supplementation, there are mainly two ways to boost our body’s vitamin D levels:
1 Consume foods rich in vitamin D.
2 Get more sunlight exposure to stimulate the body’s natural production of vitamin D.
The units of vitamin D in supplements are International Units (IU), with 1000 IU equal to 25 mcg.
Regarding blood tests for vitamin D levels, the internationally recognized deficiency level is:
20ng/ml = 50nmol/L
While vitamin D can be obtained from food sources, there aren’t many natural foods rich in vitamin D. Examples include oily fish, liver, egg yolks, milk, cheese, and mushrooms.
Vitamin D is not strictly a vitamin since our bodies can synthesize it. 7-dehydrocholesterol, a type of steroid found in the sebaceous glands of our skin, converts into vitamin D3 under sunlight or UV exposure. Outdoor exposure to sunlight is a practical, “safe,” and free method to obtain vitamin D. Sun exposure of 5 to 30 minutes per session, resulting in the minimum erythemal dose, may produce enough vitamin D to meet the body’s needs.
Healthy adults, during sunny summer days from 10 am to 3 pm, exposing their hands and legs to the sun twice a week, can synthesize approximately 10,000 to 20,000 IU of vitamin D, which is then safely stored in the body’s fat tissue without causing toxicity.
Individuals with darker skin may require 5 to 10 times more sun exposure to achieve the same effect. Cloudy weather, shade from trees, or staying indoors can weaken the intensity of UV rays, so outdoor activities may need to be extended by two times.
A study conducted in China shows infants with 2 hours of daily outdoor activity increased serum vitamin D levels from 29.7 ng/mL to 38.1 ng/mL in 2 months, while the control group showed no change.
During summer, vitamin D is abundant, but it may become insufficient in winter. In a study of 68 children and adolescents aged 7 to 18, 89.7% experienced declining vitamin D levels during winter, with 14.7% reaching deficiency (<20 ng/mL) and 36.8% reaching insufficiency (<30 ng/mL) [6].
However, not all professionals agree that vitamin D should be supplemented through sun exposure. Dr. Leffell from Yale School of Medicine is concerned that attempting to obtain enough vitamin D through sunlight exposure may increase the risk of skin cancer [7].
Many ladies wouldn’t risk skin damage for vitamin D. Sunscreen and glass block UV rays, reducing vitamin D production. Unless you work outdoors without sunscreen, relying on sunlight alone may not suffice.
Is vitamin D overdose toxic?
Excessive vitamin D intake can lead to toxicity, mainly causing hypercalcemia and arterial calcification, increasing cardiovascular disease risk. However, achieving vitamin D toxicity is challenging. Most cases involved toxic doses of 1.2 to 2.4 million IU in children, equivalent to 10,000 to 20,000 IU daily. Some reports suggest toxicity may occur with daily intake of 5,000 IU for over 6 months.
Sun exposure doesn’t lead to excessive vitamin D levels. According to Dr Holick, who served as the chair for the Endocrine Society’s Practice Guidelines on Vitamin D, ultraviolet radiation regulates both vitamin D and its precursors, degrading excess vitamin D [8].
Summary
Vitamin D Deficiency is Widespread: Around 40% of the US population and 75% globally have insufficient vitamin D levels (<30ng/ml). The elderly, pregnant women, and people with darker skin tones are particularly at risk.
Physiology of Vitamin D: Vitamin D is a fat-soluble steroid, converted into an active form (calcitriol) that aids calcium absorption and bone health. It also plays a significant role in chronic diseases and immune function.
Ways to Increase Vitamin D Levels: Apart from supplementation, consume foods rich in vitamin D or get sunlight exposure. Sun exposure varies based on skin tone, time, and weather conditions.
Research on Vitamin D: Infants with outdoor activity showed increased vitamin D levels. Vitamin D may become insufficient in winter, affecting many individuals.
Vitamin D Overdose: Excessive intake may lead to toxicity, but achieving it is difficult. Sun exposure does not cause excessive vitamin D levels. Dr. Holick’s research supports this claim.
References:
[1] Reddy, P., & Edwards, L. R. (2019). Magnesium Supplementation in Vitamin D Deficiency. American journal of therapeutics, 26(1), e124–e132. https://doi.org/10.1097/MJT.000
[2] Moyersoen, I. et al. (2019). A Novel Approach to Optimize Vitamin D Intake in Belgium through Fortification Based on Representative Food Consumption Data. The Journal of nutrition, 149(10), 1852–1862. https://doi.org/10.1093/jn/nxz119
[3] Aspell, N. et al. (2019). The Prevalence and Determinants of Vitamin D Status in Community-Dwelling Older Adults: Results from the English Longitudinal Study of Ageing (ELSA). Nutrients, 11(6), 1253. https://doi.org/10.3390/nu11061253
[4] Holick M. F. (2011). Vitamin D: a d-lightful solution for health. Journal of investigative medicine : the official publication of the American Federation for Clinical Research, 59(6), 872–880. https://doi.org/10.2310/JIM.0b013e318214ea2d
[5] Alshahrani, Fahad & Aljohani, Naji. (2013). Vitamin D: Deficiency, Sufficiency and Toxicity. Nutrients. 5. 3605-16. 10.3390/nu5093605.
[6] Shakeri, H. et al. (2017). Do sufficient vitamin D levels at the end of summer in children and adolescents provide an assurance of vitamin D sufficiency at the end of winter? A cohort study. Journal of pediatric endocrinology & metabolism : JPEM, 30(10), 1041–1046. https://doi.org/10.1515/jpem-20
[7] Moriarty,C. (2018), Vitamin D Myths ‘D’-bunked, https://www.yalemedicine.org/news/vitamin-d-myths-debunked
[8] Holick, Michael et al. (2007). Vitamin D and Skin Physiology: A D-Lightful Story. Journal of bone and mineral research : the official journal of the American Society for Bone and Mineral Research. 22 Suppl 2. V28-33. 10.1359/jbmr.07s211
