Early humans would have never faced vitamin D deficiency as they spent most of their time in the sunlight which is the best source of vitamin D. But today we hardly ever venture outside without fully covering our body and some body parts that are uncovered are also under the protection of sunscreens.

There can be other reasons behind the deficiency of vitamin D too. Vegetarians tend to lose out on vitamin D as most common sources are fish and fish oil, beef liver, egg yolk besides cheese. Persons with dark skin also soak less vitamin D due to pigmentation. Some disease also result in less absorption of vitamin D. Other reasons include obesity and problem with digestive tract.