What Is Vitamin D3? Benefits, Sources & How It Works

Vitamin D3 — also known as cholecalciferol — is a fat-soluble nutrient your body produces when UVB sunlight hits your skin, converting a compound called 7-dehydrocholesterol into previtamin D3. It belongs to a family of seco-steroids and serves as a prohormone, meaning it must be activated through two enzymatic steps before it can do its work in the body.

So what is Vitamin D3, exactly, in a practical sense? It's the form of vitamin D that research suggests is the most bioavailable. Studies indicate that Vitamin D3 could be more than three times as effective as vitamin D2 (the plant-derived form) at raising and maintaining serum 25(OH)D concentrations — the marker healthcare practitioners measure to assess your vitamin D status.

Few foods naturally contain meaningful amounts of vitamin D. Fatty fish like salmon, trout, and mackerel are among the best dietary sources, while beef liver, egg yolks, and cheese provide smaller amounts. Because dietary sources alone are typically insufficient to maintain optimal levels, and because factors like season, cloud cover, skin pigmentation, sunscreen use, and age all reduce cutaneous synthesis, supplementation may be an important consideration for many people.

Vitamin D3 as consumed in food or produced in the skin is biologically inert. It requires two hydroxylation steps to become active. First, the liver converts cholecalciferol into 25-hydroxyvitamin D — also called calcifediol — which is the major circulating form and the one measured in blood tests. Second, the kidneys convert calcifediol into calcitriol (1,25-dihydroxyvitamin D), the physiologically active hormone.

Calcitriol exerts its effects by binding to the vitamin D receptor (VDR), which is found in cells throughout the body — in bone, the immune system, muscle tissue, the cardiovascular system, and more. Through VDR-mediated genomic and non-genomic pathways, calcitriol influences gene expression related to Calcium absorption, bone remodeling, immune regulation, and cellular differentiation.

Because Vitamin D3 is fat-soluble, absorption in the small intestine is enhanced when taken with dietary fat, though some absorption occurs even without it. Notably, neither aging nor obesity appears to alter gut absorption of vitamin D, although both can affect overall vitamin D status through other mechanisms.

Vitamin D3 plays a well-established role in structural health. It promotes intestinal absorption of Calcium and phosphorus — minerals essential for bone mineralization — and helps regulate their blood levels. Historically, vitamin D deficiency was recognized as the cause of rickets, a childhood bone disease first described in the 1600s. The use of cod liver oil to address rickets dates back to the late 18th century, and by the 1930s the fortification of foods with vitamin D had largely eliminated rickets in developed countries.

Research also supports Vitamin D3's role in immune function. Upon conversion to calcitriol, it induces antimicrobial peptides like cathelicidin and β-defensins, which have broad-spectrum activity against bacteria, viruses, and fungi. It modulates macrophage and monocyte activity and helps adjust T and B cell function, favoring the development of regulatory T cells that support immune tolerance. A 2017 meta-analysis of 25 randomized controlled trials found that vitamin D supplementation had a protective effect against acute respiratory infections.

Observational studies have consistently linked low vitamin D levels to increased cardiovascular risk, though large-scale randomized controlled trials — including the landmark VITAL trial — have not confirmed that supplementation reduces major cardiovascular events in generally healthy populations. The mechanistic interest remains, as Vitamin D3 may modulate the renin-angiotensin system, endothelial function, and inflammation.

A Rich Scientific History

The story of Vitamin D3 spans centuries. In 1919, German researcher Kurt Huldschinsky demonstrated that ultraviolet light could cure rickets. By 1922, Elmer McCollum isolated the nutrient from cod liver oil and named it vitamin D. The biologically active form found in skin — D3 — was characterized in 1936 and confirmed as a secosteroid resulting from UVB irradiation of 7-dehydrocholesterol.

Many people in the United States consume less than the recommended amounts of vitamin D, according to data from the National Health and Nutrition Examination Survey. Several groups may be particularly likely to benefit from supplementation:

• People with limited sun exposure — those who live in northern latitudes, work indoors, or wear protective clothing
• Older adults — aging skin produces less Vitamin D3, and the kidneys become less efficient at the conversion to calcitriol
• Individuals with darker skin — higher melanin content reduces cutaneous vitamin D synthesis
• People with malabsorption disorders — conditions affecting fat absorption can impair vitamin D uptake
• Those in cloudy or winter climates — season, cloud cover, and shorter days limit UVB exposure

If you're unsure whether Vitamin D3 supplementation is right for you, our naturopath is available for a free consultation to help you think through your individual needs.

Take 2 tablets daily with a meal. Each serving of 2 tablets provides 100 mcg of Vitamin D3 (cholecalciferol). Taking your tablets alongside a meal that contains some dietary fat may support absorption, since vitamin D is fat-soluble.

This product comes in a bottle of 180 tablets and carries a non-GMO designation. As with any supplement, it's wise to consult your healthcare provider before beginning a new regimen — especially if you take medications that may interact with vitamin D, such as corticosteroids, certain seizure medications, orlistat, cholestyramine, or cardiac glycosides.