In a House of Commons debate on 14 January, the backbench Conservative MP said it was 'no coincidence' that the UK has among the worst death rates from COVID-19 and high levels of vitamin D deficiency.
The MP is among around 200 doctors, scientists and public figures - including Labour MP Rupa Huq - who have signed a letter making the case for increased vitamin D intake to protect against COVID-19.
The letter, published late last year, reads: 'Research shows low vitamin D levels almost certainly promote COVID-19 infections, hospitalizations, and deaths. Given its safety, we call for immediate widespread increased vitamin D intakes.'
The government provides vitamin D supplementation to 2.7m people through autumn and winter. In advice backed by NICE, patients in at-risk groups are offered 10 microgram daily doses - equivalent to 400 international units (IU).
Mr Davis told parliament that this level of supplementation 'falls far short' of the amount required to have an impact on COVID-19. The letter signed co-signed by the Tory MP calls on governments and health systems to recommend to adults a daily vitamin D intake of 4000 IU.
However, a rapid review published last month by NICE said there was currently not enough evidence to support taking vitamin D to treat or prevent COVID-19.
Responding to a speech by Mr Davis in parliament, health minister Jo Churchill reiterated the NICE stance.
She told MPs that 'nothing should be taken off the table', but said that there remained 'significant gaps in the current evidence' and that 'studies to date haven’t reached the high level of data quality required to revise the guidance'.
Ms Churchill highlighted 70 trials currently underway in the UK and elsewhere that will 'answer key questions'.