Unexpected Downfall in Level of Vitamin D: Is Something Apprehensive

In recent years, I've been struck by the prevalence of posters urging regular vitamin D check-ups. Surprisingly, I've noticed that very few people actually maintain optimal vitamin D levels. This raises two possibilities: either there has been a sudden decline in vitamin D levels for which no satisfactory explanation has been provided, or this situation is part of a larger narrative.

Vitamin D has long been debated due to its dual classification as both a vitamin and a hormone. Unlike traditional vitamins, which must be obtained from external sources, vitamin D is synthesized by the body. This unique characteristic complicates its classification and raises questions about our understanding of its role in health.

A comprehensive article delves into the biochemical and metabolic aspects of vitamin D, shedding light on its importance. The purpose of this text is to present these facts clearly and analytically, empowering individuals to make informed decisions about their vitamin D health.

Basic Biology of Vitamin D

Facts to Examine

Optimal Level of Vitamin D

The requirement for vitamin D varies primarily with age, as well as during pregnancy and lactation. According to international standards, individuals aged 1 to 70 years need 15 micrograms of vitamin D daily, while those over 70 require 20 micrograms. 

Age-wise Requirement

Most people worldwide meet at least some of their vitamin D needs through sunlight exposure. [1] Type B UV (UVB) radiation, with a wavelength of approximately 290–320 nanometers, penetrates uncovered skin and converts 7-dehydrocholesterol to pre-vitamin D3, which then becomes vitamin D3. Several factors influence UV radiation exposure and vitamin D synthesis, including season, time of day, length of day, cloud cover, smog, skin melanin content, and sunscreen use. Notably, older adults and individuals with darker skin are less efficient at producing vitamin D from sunlight.[1] Additionally, UVB radiation does not penetrate glass, meaning that sunlight exposure indoors through a window does not contribute to vitamin D production. [2]

Research by Glerup et al. (2001) indicates that even with adequate dietary vitamin D intake, insufficient sunlight exposure will prevent individuals from maintaining the necessary levels of vitamin D3, the pro-metabolic form utilized by the body. Their findings suggest that daily oral intake of vitamin D for sunlight-deprived individuals should exceed 15 mcg, with a recommended intake of around 25 mcg/day to achieve normal levels of 25-hydroxyvitamin D.[3]

During initial sunlight exposure, provitamin D3 is efficiently converted to previtamin D3. However, prolonged exposure leads to the isomerization of previtamin D3 into lumisterol, which is less beneficial.[4] Therefore, extended sunbathing is not as effective as intermittent exposure. To maintain healthy blood levels, aim for 10–30 minutes of midday sunlight several times a week. Individuals with darker skin may require slightly longer exposure, depending on their skin's sensitivity to sunlight. Always avoid sunburn.[5]

External Sources and Their Amount

Natural dietary sources of vitamin D are limited in the typical Indian diet. For vegetarians, consuming at least 2 American cups (250 ml) of milk and one cup of curd, along with cereals, can help meet the 15 microgram requirement for those aged 1 to 70 years. For non-vegetarians, cod liver oil and fish, particularly from the Salmonidae family, are excellent sources of vitamin D. Incorporating fish into your diet once a week can alleviate concerns about vitamin D intake. However, it is important to note that no supplement can fully replace the benefits of sunlight exposure. 

Considering the data presented in the graph, it is evident that consuming 2 cups of milk and one serving of cereals alone cannot fulfill the vitamin D requirement unless fish or mushrooms are included in the diet. However, the absorption of vitamin D from these sources presents its own challenges, particularly regarding bioavailability. Therefore, the most effective option remains sun exposure, as artificial sources may not provide the same benefits.

Absorption of Vitamin D in the Gut

The absorption of vitamin D in the intestines is not a passive process; it involves specific cholesterol transporters.[6] Research indicates that vitamin D is absorbed primarily in the small intestine. Significant competitive interactions for uptake have been identified among fat-soluble vitamins (FSVs) such as vitamins D, E, and K, suggesting they share common absorption pathways. Notably, vitamin A can significantly reduce the uptake of other FSVs, while its absorption is not hindered by vitamins D and K and may even be enhanced by vitamin E. These findings are crucial for supplement formulation, as they highlight the need to optimize the absorption of fat-soluble vitamins.[7]

Given the changes in lifestyle and the constraints of official dress codes—particularly in the Indian context—it's essential to prioritize sun exposure whenever possible. While we may not be able to change these dress codes immediately, we can allocate 2-3 hours on weekends for intermittent sunbathing. This dedicated time can provide a sufficient amount of vitamin D for the week, especially considering that the half-life of vitamin D is approximately 15 days.

Even if you take vitamin D in pill form, it may not be effectively absorbed, and even if it is absorbed, it might not be converted into the metabolically active form that the body can utilize. Instead, the body tends to store it in its pre-vitamin form, converting it as needed. This is supported by research from Glerup et al. (2001), which demonstrated that even after administering sufficient doses of vitamin D, blood levels did not reach optimal levels.