Reason of deficiency of vit d3?

Most likely you: Are not getting enough sunlight exposure. And the dermatologists may disagree, but the use of sunscreens has possibly made this problem more common-although our society's indoor-oriented sedentary lifestyle is a primary factor. Diet is minor contribution. However sunnlight exposure needs to be in moderation! 15-30 min daily not during higher uv periods. Less if very light skinned. No sunburn!

It's the sources.: The major sources of vitamin D3 are dairy and sun exposure. Now, we generally tell people to avoid the sun because of its effects on the skin and development of cancer. And not everyone tolerates dairy so well, or even likes dairy. This is why vitamin D deficiency is so rampant. So the supplements of D3 have become useful sources.