Speaketh Holick (the godfather of vitamin D research on who and how much)

“For my patients who wish to take a vitamin D supplements, I recommend that they take a national brand of vitamin D3. However if they are vegans I will recommend they take vitamin D2.”[It probably doesn't matter much if you take D2 or vegan D3.]

“There continues to be contentious debate about what blood level of 25(OH)D is considered to be deficient and sufficient.”

“The Endocrine Society, National and International Osteoporosis Foundations and the American Geriatric Society chose to define vitamin D sufficiency as the blood level of 25(OH)D of at least 30 ng/mL [75 nmol/L]. They also considered a blood level up to 100 ng/mL [250 nmol/L] as perfectly safe. The Endocrine Society recommend a preferred range of 40–60 ng/mL [100–150 nmol/L]. This is the range that likely our hunter gatherer forefathers achieved while being exposed to sunlight on a daily basis. The body has a huge capacity to produce vitamin D. Exposure of half an adult body to about 50% of the amount of sunlight that would cause a mild sunburn 24 h later is equivalent to ingesting approximately 5000 IUs [125 µg] of vitamin D daily. This is consistent with the observation made in Maasai herders who maintained blood levels of 25(OH)D of 40–50 ng/mL [100–125 nmol/L]. To achieve and maintain this level would require an adult to ingest 4000–5000 IUs daily of vitamin D.”

Recommended vitamin D intakes in the absence of endogenous synthesis through sun exposure (according to Holick 2017)
babies (0 – 1 year old)
400–1000 IU (10–25 µg) per day
children (1 – 10 years old)
600–1000 IU (15–25 µg) per day
teenagers and adults
1500–2000 IU (38–50 µg) per day

[Note that if you and/or your children haver darker skin tones you should probably go for the upper margin of these recommendations (see Öhlund et al. 2017).]

“The IOM recommended the upper limit for most children and adults be at 4000 IUs daily. The Endocrine Society agreed that 4000 IUs daily is reasonable as the upper limit for children but for adults 10,000 IUs daily is more reasonable especially since obese people require 2–3 times more vitamin D to treat and prevent recurrent vitamin D deficiency.”

“There are a multitude of studies relating the health benefits of vitamin D and sun exposure for reducing risk for many chronic illnesses including deadly cancers, autoimmune diseases including multiple sclerosis, rheumatoid arthritis, Crohn’s disease in type 1 diabetes, cardiovascular disease, neurocognitive dysfunction, type 2 diabetes and infectious diseases.”

“The abstinence message of avoiding all direct sun exposure without sun protection by many dermatology societies has not resulted in a significant decline in the incidence of the most deadly form of skin cancer, melanoma. This is expected since most melanomas occur on the least sun exposed areas and occupational sun exposure decrease decreases the risk for melanoma. Furthermore obesity has been linked to an increased risk for both melanoma and nonmelanoma skin cancer. Excessive sun burning increases risk not only for the deadly melanoma but also for non-melanoma skin cancer.”

“The goal should be to have a blood level of 25(OH)D of at least 30 ng/mL; the preferred range being 40–60 ng/mL. This can be achieved by increasing everyone’s vitamin D supplementation to the levels recommended by the Endocrine Society as well as obtaining sensible sun exposure.”

“There is no need to be screening every one for the vitamin D status. It is much more cost effective to increase food fortification with vitamin D and encourage vitamin D supplementation and sensible sun exposure. However those individuals with fat malabsorption syndromes, those who had had gastric bypass surgery or have other risk factors or inborn or acquired disorders in vitamin D metabolism do require screening with followup measurements of 25(OH)D.”

Holick MF: The vitamin D deficiency pandemic: approaches for diagnosis, treatment and prevention. Rev Endocr Metab Disord. DOI 10.1007/s11154-017-9424-1 (2017)