For a full discussion of the nature of vitamin D (it is not actually a vitamin but a powerful hormone), its many known and suspected effects, and the ideal levels of the hormone, the reader is referred to specialist sources on the subject. It is a booming area of research.
One thing to note is that there are several forms of vitamin D and related compounds, but it is now well agreed that vitamin D3 is the best form to take orally and that serum 25-hydroxyvitamin D is the best and standard blood component to measure.
For many years, minimum recommended serum levels were based purely upon the level required merely to avoid the disease of rickets in childhood. However, recognition that vitamin D has many other beneficial effects, and that levels which are sufficient to avoid rickets, but barely any higher, have other adverse health effects, has resulted in the minimum recommended levels being raised.
Nevertheless, there is a major body of research evidence suggesting that recommended levels should be raised further. Numerous experts in the field argue that minimum levels should be much higher, frequently recommending that the ideal level should be around that experienced by hunter-gatherers who lived outdoors all day, usually with little clothing, and also by full-time outdoor workers, surfers, and the like. Since the body has an automatic upper limit “thermostat” for vitamin D levels, serum vitamin D levels for all these categories of people plateau around 70 ng/mL (175 nmol/L). This is therefore the sort of blood level that the vast majority of humans throughout history enjoyed until the Industrial Revolution. In most industrialised countries, agricultural workers fell to become less than 50% of all workers only around the year 1900.
Interestingly, blood serum levels of all our primate relatives are also around the 70 ng/mL (175 nmol/L) mark, again suggesting that these levels, very high compared to those of most Developed Country inhabitants, have been with us as the normal state for a very long time indeed!
So there is very little likelihood of adverse effects from vitamin D itself at these levels, and every likelihood of there being (currently) hidden long-term benefits, which is a keen area of research in relation to many diseases.
An additional factor to consider is the major decline which occurs in the body's ability to manufacture vitamin D as age advances. Many seniors have very low levels as a result. This aspect is well covered in many medical sites and references, and may well have direct relevance to the age trends shown by atrial fibrillation.
It is also the case that no adverse effects have been shown with long-term serum levels of 100 ng/mL (250 nmol/L).
That “high” vitamin D is a good thing hardly seems in doubt. The real question is: how much calcium intake is then ideal, at this higher vitamin D level?