What amount would you say is the max?
32 ng/ml is the ideal level. In experiments, increasing it further doesn’t benefit.
Once the blood level reaches 32 ng/ml, the body starts deactivating vitamin d.
That’s a good question, but it depends on the person. People who have been found to have extremely low levels of D have to take massive doses to get to the normal range. Dark skinned people absorb less D from the sun than others, ditto for obese people. We don’t get as much in the winter, especially in the northern regions. Dermatologists have been nagging us for years to stay out of the sun, and that matters.
This article is helpful:
https://www.healthline.com/nutrition/how-much-vitamin-d-to-take#How-much-vitamin-D-should-you-take?
“The National Institutes of Health (NIH) recommends an average daily intake of 400–800 IU, or 10–20 micrograms (22Trusted Source, 23Trusted Source).
However, some studies find that the daily intake needs to be higher if you aren’t being exposed to the sun or have darker skin tones.
Depending on who you ask, blood levels above 20 ng/ml or 30 ng/ml are considered as “sufficient.”
One study involving healthy adults showed that a daily intake of 1,120–1,680 IU was needed to maintain sufficient blood levels (23Trusted Source, 24Trusted Source).
In the same study, individuals who were vitamin D deficient needed 5,000 IU to reach blood levels above 30 ng/ml.
Studies in postmenopausal women with vitamin D levels below 20 ng/ml found that ingesting 800–2,000 IU raised blood levels above 20 ng/ml. However, higher doses were needed to reach 30 ng/ml (25Trusted Source, 26Trusted Source).
People who are overweight or have obesity may also need higher amounts of vitamin D (27Trusted Source, 28Trusted Source).
All things considered, a daily vitamin D intake of 1,000–4,000 IU, or 25–100 micrograms, should be enough to ensure optimal blood levels in most people.”
As an aside, my dad was a veterinarian and would sometimes see puppies who suffered the effects of lack of vitamin D because the owners mistakenly thought they should be kept inside. Puppies need sunlight, too.