Researchers face challenges when conducting randomized clinical trials on vitamin D supplements due to the primary source of this vitamin being sunlight. The body synthesizes vitamin D when skin is exposed to UVB rays, although dietary sources—such as oily fish, egg yolks, mushrooms, and certain fortified products—contribute less significantly.
Vitamin D status is commonly assessed by measuring levels of 25-hydroxycholecalciferol (25(OH)D) in the blood, a metabolite produced by the liver. However, there is no consensus on what constitutes the “ideal” level of this vitamin. Even if a standard were established, determining the necessary intake of vitamin D through diet or sunlight exposure is complex. Individual responses to UV rays vary based on skin melanin content and other factors, complicating the assessment of additional requirements after dietary intake.
While there is more agreement on defining vitamin D deficiency—identified as blood levels below 30 nanomoles per liter—this does not clarify how to effectively address or prevent such deficiencies. Health authorities, such as the UK government, recommend that individuals take a daily vitamin D supplement of 10 micrograms during the autumn and winter months. However, this general advice does not account for personal variables such as age, existing blood levels, or skin pigmentation, leaving individuals to navigate their vitamin D needs with limited guidelines. Further research is necessary to fully understand the role of vitamin D in health and how best to achieve adequate levels.
Source: https://www.technologyreview.com/2025/11/21/1128206/vitamin-d-bodies-bone-health-immune/