Human skin when exposed to summer sunshine creates 10,000~20,000iu Cholecalciferol.
Paler skinned humans survived better away from the equator because they make more vitamin d3 and store it for use in winter.
Hunter-gatherers maintain 25(OH)D at 50ng/ml 125nmol/l.
1000iu/daily Vitamin D3 for each 25lbs you weigh is a good starting level.
After 3~6 months get a postal 25(OH)D test from CityAssays (Birmingham UK NHS path lab) £25 (less if you bulk buy)
Then adjust intake less/more by 1000iu for each 10ng/ml 25nmol/l above/below 125nmol/l ~ 50ng/ml.
The trial mentioned above uses too little vitamin D3 AND for too short a time. The aim should be to attain and maintain a 25(OH)D level at/above 125nmol/l 50ng/ml throughout the trial period with 25(OH)D levels 3 monthly throughout that period.
We know magnesium improves production of Calcitriol as does the omega 3 DHA. So if we want to see Vitamin D working efficiently as humans evolved then correcting magnesium deficiency and ensuring a natural omega 3 <> omega 6 ratio is required alongside vitamin D supplementation. Modern foods / lifestyles promote inflammation so we need more anti-inflammatory resources to resolve inflammation.
A vitamin d trial that doesn't ensure Vitamin D co-factors are present is a waste of time and money.