Daylight pulls back a minute or two each evening and the jacket comes out of the closet before October is over. The season announces itself in every direction — shorter afternoons, cooler mornings, the particular grey of a sky that will not clear until March. Inside your bloodstream, something else is moving at its own pace, making no announcement at all.
You have searched for how fast vitamin D levels drop in winter and found the same answer from every direction: they drop. Lower in November than in July. Lower in January than in November. A direction without a speed — an answer that tells you which way the car is going but not how many miles per hour.
Clinical trials did draw blood at set intervals from people who took no supplement, the placebo groups. Their numbers do not give one speed.
How Fast Vitamin D Levels Drop in Winter
In a 2024 meta-analysis of ten vitamin D trials in athletes, nine trials ran in winter. In their placebo groups, the average level fell in three, by 1.6 to 4.5 ng/mL over 8 to 12 weeks, ended higher in five, and was unchanged in one, a trial that lasted eight days. The paper reports no rate of decline.
3 of 9
winter placebo groups whose average vitamin D level fell
One trial ran in autumn, not winter: professional rugby players in New Zealand. Their placebo group started at 38.0 ng/mL, read 34.0 after six weeks and 32.0 after twelve, on average about half a nanogram per milliliter a week. The group given vitamin D began at nearly the same point, 37.2, and ended at 45.6. Same trial. Same months. Opposite slope.
In vitamin D trials with athletes, levels without a supplement did not drop at one steady rate in winter: of nine winter placebo groups, the average fell in three, ended higher in five and held in one. In one autumn trial in New Zealand, a placebo group fell from 38.0 to 32.0 ng/mL over twelve weeks, an average of about 0.5 ng/mL a week. With a supplement, levels rose nearly 15 ng/mL more than with placebo.
— Han et al. 2024 · Frontiers in Nutrition · meta-analysis · n=354
A second meta-analysis, which includes that trial, split its trials by where the athletes started: placebo levels fell significantly in the three trials whose athletes started at or above 75 nmol/L (30 ng/mL), that trial among them. Its authors name the season, larger body or fat mass, low dietary intake, poor sun exposure, darker skin and indoor activity as possible reasons (Sist et al. 2023).
Same starting point · Opposite slope
One trial · 12 autumn weeks · New Zealand, 45–46.5°S
Han et al. 2024 · Frontiers in Nutrition · meta‑analysis · n = 354
What a supplement does is clearer: across the ten trials, levels rose nearly 15 ng/mL more than with placebo, and how quickly that happens has its own answer.
Every trial in this dataset tested athletes — not sedentary adults, not elderly populations — so what happens to you will depend on your starting level, your latitude, and the form of vitamin D you take. What these trials settle is narrower than a rate: without a supplement, some groups slipped and some did not; with one, levels rose.
What they cannot settle is the question that forms the moment a winter blood test comes back where it should be: if the level looks right, why doesn’t anything feel different?