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And yet, a large percentage of people is low on vitamin D:

https://www.sciencedaily.com/releases/2012/01/120110102058.htm

Vitamin D deficiency is a real problem in Europe as levels in the blood are low in 50% to 70% of the population. Pérez-López points out that "healthcare professionals should be aware that this is a common problem which affects a large part of the population in Europe, even those who live in sunny places."

I would rather recommend to get out in the sun a Vitamin D is generally synthesized in the body from certain variant of cholesterol and taking Vitamin D supplements can create adverse effects IMHO ( https://web.stanford.edu/group/hopes/cgi-bin/hopes_test/vitamin-d3/ )

I agree with spending more time in the sun. Unfortunately, for many people that seems to be not sufficient.

Citing https://caloriebee.com/vitamins-supplements/Vitamin-D-Deficiency-Why-You-Need-to-Act :

Do you …

  • Use a high SPF sun barrier cream
  • Work long hours in an office environment
  • Cover up your arms, legs, face for religious or cultural reasons
  • Stay out of direct sunlight because of sensitive skin
  • Have a restricted diet that excludes fish, red meat or eggs
  • Stay indoors all the time through frailty or ill health (e.g. live in a care home or other institution)

If any of these apply to you, you may need to take Vitamin D tablets. You are not getting enough exposure to natural sunlight.

agree ...

Do you realize that you'd practically have to live in the sun to get your Vitamin D levels up to what some people might need? A more realistic/practical approach for this is Vitamin D supplements.

I am not sure of the time required to generate enough Vitamin D. Do you have references for the same ?