1,000–2,000 IU/day of vitamin D3 (more if deficient or overweight), with a meal containing fat. Ideally adjust according to lab tests (target ~30–50 ng/mL).
3 trials · 175 participants: moderate improvement in fibromyalgia symptoms (especially with baseline deficiency).
See the study (Grade A) →Together with calcium it slightly reduces fractures, especially in older people or those with deficiency; on its own, barely.
See the study (Cochrane) →Reduces the risk of respiratory infections, more so if deficient.
View the study (meta-analysis) →Low vitamin D is associated with higher mortality; correcting the deficiency is the priority.
View the study (meta-analysis) →Inconsistent evidence; useful mainly if deficient.
View the study (review) →5 trials · 310 participants: supplementation raises plasma vitamin D levels but does NOT improve Hashimoto's signs or TSH. Correcting deficiencies remains reasonable for other reasons.
See the study (Grade D) →⏱ 3 min read
Vitamin D is, technically speaking, a steroid hormone and not a vitamin in the conventional sense of the term: it is produced in the body when the skin is exposed to ultraviolet B radiation from the sun, and its active form regulates the expression of more than 900 genes, approximately 5 percent of the protein-coding human genome, which makes it one of the broadest genetic regulators that exist and explains why its deficiency has been associated with such a heterogeneous and seemingly unconnected spectrum of diseases.
Approximately 70 percent of the United States population has vitamin D insufficiency or deficiency according to Endocrine Society criteria, and the problem extends similarly to much of the Western world; the highest-risk groups are people with dark skin, older adults, people with obesity, those living at high latitudes, those who work indoors, exclusively breastfed infants, and those taking certain common medications such as some antihypertensives, antibiotics, and antituberculosis drugs.
Vitamin D synthesis is a multistep process that requires not only sun exposure but also heat, and continues in the liver and kidneys through a series of hydroxylase enzymes that depend on magnesium for their activity; this means that magnesium deficiency can directly impair vitamin D metabolism even when sun exposure is adequate, establishing an interdependence between these two nutrients that has important practical implications for anyone who wants to optimize their vitamin D levels.
Vitamin D3 is significantly more effective than vitamin D2 at raising serum concentrations of 25-hydroxyvitamin D, the main circulating marker: a crossover trial in healthy young people found that D2 was about half as effective as D3 at maintaining serum levels, and another study found that D2 supplementation was associated with a decrease in 25(OH)D3 levels, suggesting that the two forms are not interchangeable and that D3 should be the default choice in any supplementation protocol.
People with higher brain concentrations of vitamin D were 25 to 33 percent less likely to have dementia or mild cognitive impairment at their last evaluation before death, according to a study that measured vitamin D directly in four brain regions of 290 participants who had donated their brains for research; those with higher brain vitamin D concentrations also showed better working memory, episodic memory, word recall, and perceptual processing speed.
In a study of more than 12,000 adults free of dementia at baseline, those who took vitamin D supplements over a ten-year follow-up period were 40 percent less likely to develop Alzheimer's disease, with a 33 percent reduction even among carriers of the APOE4 gene, which is the most important genetic risk factor for this disease; most interesting was that vitamin D appeared to counteract the excess Alzheimer's risk usually seen in women, suggesting sex-specific neuroprotective effects that deserve further research.
A finding that connects two of the most powerful tools in preventive medicine: regular physical exercise not only mobilizes vitamin D stored in adipose tissue but actively optimizes its metabolism throughout the body in ways that oral supplementation does not replicate; in a ten-week trial conducted during the winter, participants who exercised four sessions per week fully maintained their levels of active vitamin D, whereas sedentary participants experienced a 15 percent drop in that same form, with the risk of reaching deficiency by late winter.
There is an important biological interdependence between vitamin D and vitamin K that most supplementation protocols ignore: vitamin D promotes the production of several proteins that require carboxylation, a vitamin K-dependent process, in order to function properly; among these proteins, osteocalcin stands out, which promotes calcium accumulation in bones and teeth, along with matrix Gla protein, which inhibits vascular calcification, the process associated with the development of cardiovascular disease and kidney dysfunction, which makes vitamin K a logical complement to any high-dose vitamin D supplementation protocol.
Information, not medical advice. Scientific evidence reflects the quality of the studies.