Am I talking to a particularly obtuse LLM?

Roughly 20% of the UK population (1 in 5 people) is clinically deficient (<20 ng/mL). Similar in Ireland.

If we extrapolate slightly from 'clinically deficient' to 'subnormal' then 94 % of children, 94 % of teenagers and 90 % of adults on the island of Ireland have inadequate vitamin D intakes. This is because the Sunlight UVB radiation needed to make vitamin D is only available in Ireland during the months of April through October.

For us and the other latitudes which experience a ‘vitamin D winter’ that stretches from the end of October to late March, almost everybody in Ireland needs a supplement of vitamin D and 15 µg (600 IU) is recommended on a daily basis.