There are conflicting views regarding vitamin D levels. I read up on vitamin D in the 1990s, so I am a bit out of touch now.
The (now defunct?) ‘Vitamin D Council’ recommend unusually high levels. I have ignored their publications because I dislike people and organisations that use terms like ‘council’ or ‘academy’ and give a false impression that they are an official body. I suspect they are well meaning learned people who want to campaign for a better knowledge of vitamin D. As a lobby group they may give a less balanced view.
On the other side the NHS recommends we take 10mg (400 IU) of vitamin D daily during the winter months. This is based on recommendations from the ‘Society for Endocrinology’ which is the UK club for endocrinologists. They define ‘deficient’ as <25nmol/l and ‘insufficient’ as <50nmol/l endocrinology.org/endocrino... .
The ‘Endocrine Society’ which is the USA club for endocrinologists define deficient as < 20 ng/ml (50 nmol/l) and insufficient as 21–29 ng/ml (52.5–72.5 nmol/l) academic.oup.com/jcem/artic... . (This paper is not easy to read and they leave out the decimal place sometimes! They also use four circles to denote the strength of evidence, the more plusses in the circles the stronger the evidence. This is explained halfway thought the paper!). The Endocrine Society recommend 600 – 800 IU daily as a minimum for white people who are not elderly, pregnant, or obese and are healthy. They recommend much higher doses for people who are deficient, e.g. 1,500 – 2,000 IU/d and up to 4,000 IU/d.
The USA recommendations take a balanced view and are supported with evidence. It’s possible higher vitamin D levels provide other benefits but at least some studies have found no benefit. e.g. for preventing cancer.
There have been several studies showing associations between SARS-CoV-19 infection and vitamin D status. It could be that people more susceptible to COVID-19 have illnesses that lower vitamin D levels or lifestyles that give reduced exposure to sunlight. My favourite study journals.plos.org/plosone/a... seems to show a clear link between vitamin D and SARS-CoV-19 infection with a vitamin D level around 50 ng/ml (125 nmol/l) being ‘optimal’.
I’ve no idea of my vitamin D level, in the past I’ve supplemented during the winter, but my main strategy has been to try and keep an all year round suntan which is not possible with travel restrictions. Up until now I have tended to follow the NHS guidance of 400 IU/d during winter with a bit extra for luck. I now think this is inadequate and the USA guidance is better and for as long as COVID-19 is around we should aim for a level approaching 125 nmol/l. I am now taking one or two 5,000 IU tablets per week as it is cheaper and easier to take a larger dose less often.