The big picture: Vitamin D is a fat-soluble nutrient primarily synthesized through skin exposure to sunlight and found in certain foods. It is essential for calcium absorption and bone mineralization, making it critical for bone health. A deficiency in vitamin D can lead to bone density loss, muscle weakness, immune dysfunction, and chronic conditions like cardiovascular disease and diabetes.

Vitamin D3 vs Vitamin D2: Vitamin D has two main forms: D3 (cholecalciferol) and D2 (ergocalciferol). While both contribute to health, D3 is more effective in raising blood levels of the active form of vitamin D.

Vitamin D2 is a version of vitamin D that is largely synthesized and added to food products as a fortificant. It's not naturally abundant in most foods, though it can be derived from certain fungi and yeasts.
Vitamin D3 is the natural version of vitamin D that our bodies produce when our skin is exposed to sunlight, specifically UVB rays. Additionally, D3 is naturally found in some animal-based foods, e.g. fatty fish.

Vitamin D3 (25-OH) vs Vitamin D3 (1,25-Dihydroxy): Vitamin D3 can be further broken down into two forms which are used to assess health.

Vitamin D3 (25-OH): Known as 25-hydroxyvitamin D, this is the storage form of vitamin D created in the liver from dietary or sunlight-derived vitamin D. It serves as the primary indicator of vitamin D status in the body, as it reflects vitamin D reserves.
Vitamin D3 (1,25-Dihydroxy): Known as 25-hydroxyvitamin D, this is the storage form of vitamin D created in the liver from dietary or sunlight-derived vitamin D. It serves as the primary indicator of vitamin D status in the body, as it reflects vitamin D reserves.

Sunlight, Skin, and Vitamin D3 Production: Vitamin D3 synthesis in the skin, triggered by exposure to UVB rays, is a vital source for the body since dietary intake alone is often insufficient. Factors influencing this process include:

UVB radiation and skin: UVB rays, in the medium-wavelength “band B,” initiate the conversion of 7-dehydrocholesterol (7-DHC)—a cholesterol-derived compound stored in the skin—into pre-vitamin D3, which then becomes vitamin D3. However, UVB exposure varies significantly depending on latitude, altitude, cloud cover, and pollution levels, with UVB decreasing as you move further from the equator.

Skin pigmentation and barriers: Melanin in the skin, as well as clothing and sunscreen, reduce UVB penetration, limiting D3 production. This effect is more pronounced in individuals with darker skin tones, who may require up to six times more sun exposure than lighter-skinned individuals to produce the same amount of vitamin D.
Sun exposure and cancer risk: While UVB exposure is necessary for vitamin D synthesis, excessive exposure increases the risk of skin cancer. Therefore, short, frequent sun exposures (rather than prolonged periods) are recommended to optimize vitamin D production without excessive UV exposure. Extended sun exposure also quickly plateaus vitamin D synthesis, so moderate exposure is more efficient and safer.
Geographic and seasonal variation: From November to February, regions above 35°N latitude (such as much of North America and Europe) experience reduced UVB exposure, necessitating alternative vitamin D sources. In contrast, during March to October, sufficient UVB for vitamin D synthesis is available regardless of latitude.

Role in musculoskeletal health: Vitamin D is a well-established marker for musculoskeletal health due to its central role in calcium and phosphate metabolism.

Once it is produced in the skin or taken in through food or supplements, vitamin D undergoes conversion in the liver and then the kidneys to produce the active form called calcitriol, which acts as a hormone, regulating various processes including calcium and phosphate metabolism in the body.

This active form of vitamin D acts on the intestines and kidneys increasing the absorption of calcium and phosphate which can then be used to make bone. So to absorb calcium efficiently, an adequate amount of vitamin D must be present.
Deficiency of vitamin D leads to decreased bone density and increased fractures. It can also adversely affect muscle strength and lead to muscle weakness and pain (myopathy).

Beyond bone health: Vitamin D’s role extends far beyond musculoskeletal support due to its presence in almost every cell in the body.

Cardiovascular health: Studies have shown that vitamin D deficiency doubles the risk of heart attacks, increases the likelihood of stroke and coronary heart disease, and is associated with higher cardiovascular mortality. Vitamin D’s regulatory effects on blood pressure, cholesterol levels, and inflammation contribute to its cardiovascular benefits.
Metabolic health: Deficiency is linked to insulin resistance and pancreatic dysfunction, increasing the risk of type 2 diabetes. Observational studies have shown that low vitamin D levels raise the risk of type 2 diabetes by approximately 43%.
Immune function: Vitamin D has anti-inflammatory and antimicrobial effects, making it essential for immune resilience. Optimal vitamin D levels have been linked to a reduced risk and severity of COVID-19, influenza, and other infections.
Cognitive and mental health: Deficient vitamin D levels have been associated with faster cognitive decline, dementia, and Alzheimer’s disease. Studies suggest that sufficient vitamin D may help reduce depression symptoms and enhance cognitive performance.

Managing Deficiency: Given the challenges of achieving sufficient vitamin D from sunlight alone—especially in high-latitude locations, winter, or for those with high melanin levels—supplementation is often necessary. Vitamin D3 supplements, including calcifediol, can elevate vitamin D levels more rapidly and are often recommended for maintaining optimal status. Balanced UVB exposure, however, may provide additional health benefits beyond bone health, potentially reducing risks for certain cancers and autoimmune or mental health disorders.