The information below has been provided by my friend Dr David Anderson, former professor of endocrinology at the University of Manchester and professor of medicine at the Chinese University of Hong Kong.
David has now formed the Murine Institute of Clinical Excellence.
One international unit of vitamin D is the amount required each day for an immature mouse (weight 10 grams) to avoid rickets.
It has been demonstrated that an adult mouse of weight 30g requires 3 units of vitamin D each day.
This achieves a blood level of 30 ng/ml (75 mol/L).
It has been demonstrated several times that this is the ideal minimum blood level for adult humans to avoid critical or fatal Covid-19.
Let us size up from a mouse to a human.
A 30g mouse to a 60kg human (or 60kg mouse!)
60kg = 60,000g.
60,000 ÷ 30 = 2,000.
An adult human weighing 60kg is 2,000 times the weight of an adult mouse.
3 units x 2,000 = 6,000 units.
An adult human being weighing 60kg requires vitamin D 6,000 units daily.
A child aged 10 weighing 30kg requires 3,000 units each day.
An obese adult weighting 100kg requires 10,000 units each day.
Units or micrograms?
Keep to units to avoid confusion.
Official semi-scientific organisations tend to use mass units, and so they would recommend for example 100 micrograms of vitamin D.
This is a very small weight as 1 microgram = 1 millionth of a gram.
100mcg of vitamin D is 4,000 units.
The immature mouse requires I unit per day, which is 100mcg divided by 4000, which is 1 millionth of a gram divided by 40, which is just 25 billionths of a gram.
This is a difficult number handle, and 1 unit is easier.
100 micrograms is abbreviated to 100mcg or 100µg.
Many people do not know about micrograms as such units do not appear in normal human life. There might thus be confusion with milligrams, one milligram (1mg) being one thousandth of a gram.
So, with vitamin D, 100mcg = 4,000 units,
but 100mg = 4,000,000 units.
Confusion between mcg / µg and mg can lead to a thousand times the correct dose of vitamin D being taken. This is how vitamin D excess can occur. It only occurs because of a major dosing error, and confusion of weight,
Vitamin D excess certainly does not result from 4,000 units per day.
There are many manufacturers and many different capsule / tablet strengths. Just work out which one suits you best in terms of understanding how much vitamin D you will be taking.
Remember, the dose is not critical. If you have 5,000 unit capsules rather than 4,000 the difference is not important.
Also, a 4.000 unit capsule contains only 100 millionths of a gram of vitamin D. Most of the oil in the capsule will be olive oil. So no matter how much vitamin D is in a capsule, all the capsules tend to be the same size.
Keep to units.