Studies that supposedly indicate higher doses of Vit. D are not effective for preventing cancer, heart disease, etc are bogus, as are any recommendations based on them.
Recommended dosage is 4000 IU and as high as 10,000 IU daily is considered a safe dose. This should be adjusted based on the results of blood tests. Vit. D serum levels should be at least 40ng/ml; max about 60ng/ml. Sun exposure is considered to be the best source, vitamin D-3 the next best. Calcium daily intake also needs to be around 1500mg per day (probably from all sources).
The most important study found that 1100 IU per day Vit D with 1500mg Calcium (at 3:30-3:40 in video) reduces cancer rates by 80% for most common types of cancer: breast, lung, colon, etc.
Source is Vitamin D research at UC San Diego (and probably elsewhere).
See this Dr Mercola article and also: https://www.grassrootshealth.net/webinars/
It's a sad situation when new findings that can prevent so much human suffering and death are not in the interest of the corporate cancer industry. (What other explanation could there be for deliberately writing and disseminating "studies" that exclude the most important recent findings?!)