Vitamin D2 (ergocalciferol) is produced in fungi by the conversion of ergosterol under UV irradiation.

Vitamin D3 (cholecalciferol) is produced in animal bodies (including humans) through the metabolism of 7-dehydrocholesterol under UV irradiation.

Both Vitamin D2 (ergocalciferol) and Vitamin D3 (cholecalciferol) can convert to calcifediol (25(OH)D3), but it is vitamin D3 that is much more effective at this.

Cholecalciferol induces faster calcifediol production and is sustained for longer and at higher concentrations than ergocalciferol. This is why cholecalciferol is used in dietary supplements.

References:

Tripkovic L, Lambert H, Hart K, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012;95(6):1357-1364. doi:10.3945/ajcn.111.031070

Any dysfunction in liver function will significantly impair the body’s ability to absorb and assimilate and properly activate Vitamin D.

In people with encephalopathy or severe gut problems, it can take up a lot of time to raise Vitamin D levels.

The two-step activation of Vitamin D requires the presence of magnesium in the body. The mineral must be available in adequate amounts, and supplementation is not always necessary. Yet, in pregnant women, people who exert their bodies while performing their work or as a hobby, those with high emotional stress, those who struggle to cope with stress, and people with neurological problems, magnesium levels may need to be supplemented regularly.

Cholecalciferol (Vitamin D3) is used in dietary supplements. This is the same compound that the human body synthesises on its own when exposed to sunlight.

Cholecalciferol is further converted by the body successively into calcifediol and then calcitriol.

The most common test performed to measure Vitamin D levels is the measurement of calcifediol, also called 25(OH)D3.

Calcifediol is a metabolite, i.e. a product of the metabolic conversion of cholecalciferol (Vitamin D3), which takes place in the liver under the hydroxylation process. If, therefore, despite supplementation with an adequate dose of Vitamin D3, 25(OH)D3 levels do not rise, this may indicate liver-related problems and the fact that the liver is unable to carry out the hydroxylation on its own despite the available supply of cholecalciferol. If this happens, seek medical advice.