Vitamin D

Vitamin D is the supplement UK guideline bodies most consistently recommend, and the one consumer marketing most consistently overshoots. The SACN position for UK adults is 10 micrograms (400 IU) daily through autumn and winter to maintain 25-hydroxyvitamin D above 25 nmol/L. The NHS, NICE, and the 2024 Endocrine Society guideline diverge on whether routine screening is warranted, on optimal target levels, and on whether higher daily doses are justified outside specific clinical contexts. The library entries on this page report what each body says, what the underlying trials show, and where the evidence does and does not support the claims commonly made for vitamin D in cardiovascular, immune, and musculoskeletal contexts.

What this page covers

The vitamin D entries cover dosing for the general UK population, dosing for repletion when 25(OH)D is low, and the practical questions clinicians and informed patients actually face: D3 vs D2, whether vitamin K2 should be co-administered, what the upper limit is, how absorption changes with the meal it is taken with, and which populations have higher requirement (darker skin, housebound, veiled, elderly, pregnant and breastfeeding, and people on enzyme-inducing medications).

Where the guidance currently sits

The SACN 2016 report set the UK Reference Nutrient Intake at 10 micrograms (400 IU) daily for everyone over the age of one, framed as a maintenance dose to keep serum 25(OH)D above 25 nmol/L year-round. NHS guidance follows SACN. NICE largely defers to SACN for general population recommendations. The 2024 Endocrine Society guideline shifted away from routine screening of asymptomatic adults and away from empirical higher-dose supplementation in the absence of documented deficiency, which contrasts with widespread consumer messaging suggesting most adults need 2,000 to 5,000 IU daily.

The SACN tolerable upper limit for adults is 100 micrograms (4,000 IU) daily without clinical supervision. Doses above this are used in repletion protocols for documented deficiency under clinical oversight, but routine dosing at this level for asymptomatic adults is not what the guideline bodies recommend.

Where the evidence and the marketing disagree

Vitamin D supplementation reliably raises serum 25(OH)D in deficient individuals, and reliably reduces fracture risk in older adults at risk of falls when combined with calcium. Beyond that, the picture is more equivocal than supplement marketing suggests. Large randomised trials including VITAL have not shown the cardiovascular, cancer, or all-cause mortality reductions that observational studies hinted at. Vitamin K2 is frequently co-promoted with D3 on the basis that K2 directs calcium to bone rather than soft tissue, but the clinical trial evidence supporting this combination is limited and the populations studied are narrow. The library entries say so where they say so, with sources visible.