Human adult physiology requires 3000 to 5000 international units of vitamin D3 per day to maintain baseline metabolic and steady-state dynamics (Heaney et al in 2003).
Vitamin D is more hormone-like in its action and not a true vitamin. It remains stable with heat and oxidation. It is not required in the diet if there is sufficient sunlight to allow the production from provitamin D molecules in the skin. The liver then converts it into the active vitamin D. Boron seems to be important for this process. Also parathyroid hormone will stimulate the kidneys to produce active Vit D3 in kidneys when blood calcium levels are low.
The primary role of Vitamin D is regulation of calcium and phosphorus absorption in the gut, regulation of calcium balance and stimulation of bone cell mineralisation.
Vitamin D seems to be also extremely important in immune function - research is ongoing.
How much Vitamin D do we need?
It is essential to test your Vitamin D levels even if you take a supplement or spend time in the sun.
Especially in view of the Covid-19 pandemic, you need to keep your Vitamin D levels in the optimal physiological range!
Source: Vasquez A. Vitamin D3 Dosing and Dogma: Mechanistic Explanation for the Relative Failure of the Bolus Depot. Int J Hum Nutr Funct Med 2020;8:5
Vitamin D from animal foods occur in liver, eggs, fatty fish, butter, and fortified foods. Vegetables are low in vitamin D. Ten minutes of summer sun exposure to the face and hands results in the production of about 400 IU Vit D.
Vitamin D deficiency leads to rickets in children and osteomalacia in adults. Today both are rare, but most notable in the elderly and in people who don't spend time in sunlight.
People with poor fat absorption may become vitamin D deficient, (e.g. gluten-sensitive enteropathy), as the vitamin is fat soluble.
Studies throughout the year 2020 showed that vitamin D could effectively treat clinical coronavirus infections – but not when delivered in bolus/depot doses.
Safety and toxicity
Vitamin D production through sun exposure does not produce toxicity, as it is closely regulated.
2,000-5,000 IU Vitamin D3 - or more depending upon genetic issues (e.g. Vitamin D receptor SNPs that result in poorly formed cell receptors for vitamin D - see KASHI Bone profile) and other factors like the latitude where you reside (the further away from the equator, the fewer months sun exposure), digestive function is considered to be safe. Vitamin D3 is the preferred form.
Functional medicine considerations
Patients who are seldom exposed to the sun should be evaluated for vitamin D deficiency. Recently research as expanded the important functional interactions of vitamin D tin the prevention of cancers, type 1 diabetes, heart disease, osteoporosis and persistent, nonspecific musculoskeletal pain.