Half of 793 office workers were low in vitamin D and four in ten women low in iron. Most had no idea, and the supplements were mostly going to the wrong people

Roughly half of adults in wealthy countries take at least one supplement, and vitamin D is near the top of the list. Most start on their own, to feel healthier, not because a test told them to. A cross-sectional study published in Nutrients on 30 October asked whether that habit actually reaches the people who are short of something.

The data came from a preventive-health programme a large, mostly office-based employer in Poland offered to 1,500 staff. 793 took part, three quarters of them women, with an average age of 39. Between late April and mid June 2026 they gave a blood sample and answered a structured medical interview that included an open list of every medicine and supplement they took. The researchers counted vitamin D as low below 30 ng/mL (with 20 ng/mL reported alongside, since the higher cut-off is debated), iron stores as low when ferritin was under 30 µg/L, and vitamin B12 as low under 200 pg/mL.

Shortfalls were common. 53% had vitamin D below 30 ng/mL and 24% below 20, in men and women alike. 38% of the women had low ferritin, against 2.5% of the men, yet only about one in eight of those with low iron stores was anaemic, so a standard blood count would have missed most of them. Low B12 was rare at 3%. Supplements were much rarer than shortfalls: 21% listed a vitamin D product, under 3% an iron product and about 2% a B12 product.

Vitamin D takers did look better. 77% of them were at 30 ng/mL or above, against 39% of non-takers, and their levels were about 13.5 ng/mL higher after allowing for age, sex, BMI, smoking and month of testing. But look at it from the other side. Only 9% of the people low in vitamin D had listed a vitamin D supplement, 2% of those low in iron listed iron, and none of the 24 people low in B12 listed B12. Among those who answered the question, 70% of the vitamin D group, 69% of the iron group and 86% of the B12 group reported neither a past diagnosis nor a supplement. Meanwhile a quarter of vitamin D takers were at 50 ng/mL or more, and both readings above 100 ng/mL belonged to supplement users.

Could anyone be picked out without a test? Not really. Age, sex, BMI and smoking predicted low vitamin D barely better than a coin toss (a C-statistic of 0.58, where 0.5 is chance). For iron, the model looked better until you notice it mostly said 'be a woman'; among women alone it fell to 0.57. The authors conclude that getting the right dose to the people who need it in practice requires measurement, while noting that their study did not test whether workplace screening is worth its cost, and that current Endocrine Society guidance advises against routine vitamin D testing in healthy adults under 75.

There are real limits. It is a single snapshot, so it cannot show that supplements caused the better vitamin D levels; takers smoked less and exercised more, and some may have started after an earlier low result. The open supplement list missed people: when people below target were asked directly, about a third of the vitamin D group said they took it, and the share with neither diagnosis nor supplement fell to 56%. Ferritin was read without an inflammation marker for nearly everyone, which can hide low iron, and B12 rested on one blood value. Those who signed up were volunteers from one company. Two of the three authors work for the firms that run the programme and its interview software, which the paper declares.