Edward Giovannucci, MD, Professor of Nutrition and Epidemiology at Harvard University explained in his 2006 article "Ask the Expert: Vitamin D and Chronic Disease Prevention," available on the Harvard School of Public Health website:

“Several randomized trials found that individuals who received 800 IU per day of vitamin D lowered their risk of osteoporosis; trials that provided only 400 IU per day did not show this benefit. It is reasonable to postulate that more than 800 IU per day would provide even more benefit, but this is not proven…

Milk alone is unlikely to be an adequate source of vitamin D. True, milk is fortified in the U.S. (it is not fortified in most countries). Each glass of fortified milk should contain about 100 IU of vitamin D (but on average, it may contain only 50 IU). So someone would have to drink at least 8 glasses of milk per day to get 800 IU of vitamin D. Moreover, most experts now conclude that 1,000 to 2,000 IU per day of vitamin D may be what we need for optimum health.”