https://www.sciencealert.com/can-vitamin-d-slow-aging-a-new-study-says-yes-but-theres-a-catch
"There's also no agreement on the right dose. The Augusta researchers used 2,000 IU daily – much higher than the current recommended intake of 600 IU for under-70s and 800 IU for older adults. Yet other research suggests just 400 IU might help prevent colds...''