Vitamin D in winter: why your blood level tells you more than the dose

Your vitamin D level follows the seasons. In a study of more than 27,000 pregnant women, deficiency was considerably more common in winter than in autumn, and how much vitamin D someone takes says little about their own blood level, because weight, skin type, genes and season shape it too. If you want to know where you stand, you measure the blood level.

Many people count International Units (IU) on the pack in winter. The research shows why the same amount can lead to very different levels in two people.

Key points at a glance

• In a study of 27,166 pregnant women in Shenzhen, about one third had vitamin D deficiency, and a further 42.4% were in the insufficient range.
• In the same study, the odds of deficiency in winter were about 3.7 times those in autumn (odds ratio 3.69).
• Besides season, body weight (BMI), skin type, hair colour, genes and vitamin D intake are linked to the blood level.
• A large systematic review for the Endocrine Society guideline (2024) found no studies that directly tested the benefit of vitamin D screening.
• One blood level before winter and a second one afterwards show whether your approach actually works for you.

Why is the vitamin D level lower in autumn and winter?

The vitamin D level in the blood varies with the season, and this pattern shows up in very different populations. What is measured is 25-hydroxyvitamin D (25(OH)D, or vitamin D 25-OH), the value that reflects vitamin D status over weeks to months.

A study of 27,166 healthy pregnant women in Shenzhen, published in the Journal of Maternal-Fetal & Neonatal Medicine (2022), found marked seasonality: deficiency was about 3.7 times as likely in winter as in autumn. The level was also linked to outdoor temperature. The warmer it was, the higher the women's vitamin D level.

The pattern is not a one-off. A cohort study of 4,368 pregnant women in southeast China in Clinical Nutrition (2021) found higher levels when blood was drawn in summer and autumn. Even among 4,509 children aged up to 8 years in 5 African countries (BMC Medicine, 2021), levels were lower in winter or during the long rains, and also in South Africa, the country at the highest latitude in the study.

Why does the dose not reveal your blood level?

The amount you take is only one of several factors that determine your vitamin D level. The same dose meets different bodies, skin types and genes.

A prospective cohort of 387 patients with cutaneous melanoma in Acta Dermato-Venereologica (2022) examined many possible influences. Significantly linked to vitamin D status were body mass index (BMI), the season of blood sampling, vitamin D supplementation and certain skin and hair colours. The African children's study adds a genetic layer: children with a particular variant of the vitamin D binding protein (Gc2) had lower levels on average.

A link with body weight appeared in several groups, but the direction depended on the population. In the Chinese pregnancy cohort, a higher pre-pregnancy BMI was even linked to higher levels. That is exactly why a rule of thumb helps little: your own blood level shows whether your approach gets you where you want to be.

All of these data come from observational studies. They show associations, not causes.

Does vitamin D help, and where does the evidence stop?

The benefit depends strongly on who receives vitamin D. A systematic review of 151 studies, prepared to support the Endocrine Society guideline and published in the Journal of Clinical Endocrinology and Metabolism (2024), mainly looked at giving vitamin D without prior measurement.

In healthy adults aged 19 to 74, it found no significant effect on the outcomes examined. In people older than 75, there was a very small reduction in mortality with high certainty of evidence. In adults with prediabetes, meaning mildly raised blood sugar, moderate-certainty evidence suggested slower progression to diabetes. According to the review, high single doses given at longer intervals may increase falls compared with lower daily dosing.

One limit belongs here in plain terms: the authors found no studies that directly tested the benefits and harms of screening with the blood level. The blood level therefore answers a practical question: where do you stand, and does your approach change the level the way you expect? The southeast China cohort shows that this is measurable. Those who took vitamin D more often had higher levels, and more than 3 intakes per week were linked to a clearly lower risk of deficiency.

Why does a trend show more than a single value?

A single vitamin D value is a snapshot of something that shifts with the season. A value from August says little about where you stand in February.

Think of the vitamin D level as a water level that rises and falls over the year. One measurement shows the level on one day. Two or more measurements show how far it drops for you in the dark months and whether what you do keeps it steady. Because status changes over weeks to months, an interval of about 90 days works well for a meaningful comparison.

It makes sense to look at the value alongside body fat percentage. BMI appears in the studies as an influence, but it cannot separate fat from muscle. Body fat percentage shows actual body composition and helps put changes over time into context.

When to see a doctor?

Talk to your doctor if you are unsure whether and how you should take vitamin D, or if you have already been taking large amounts for a long time. This applies especially during pregnancy, in older age or with known health conditions. A blood level is information to discuss, not a diagnosis. Whether to take vitamin D, in what form and how often is decided by your doctor based on your overall situation.

How Chirayou can help

CHIRAYOU is a doctor-led online program for preventive health. The at-home blood test measures vitamin D (25-OH) from a small capillary blood sample, with no clinic visit and no fasting. The same test covers other markers such as ApoB, fructosamine, hsCRP and vitamin B12. Doctors discuss the results with you.

In the 90-day program (Test, Plan, Act), further markers are added, such as body fat percentage via a smart scale. After 90 days you measure again and see in the trend whether your plan has moved your level. A measurement replaces guessing from the dose.

Your next step: start with the CHIRAYOU blood test before or at the start of winter and set your baseline.

The bottom line

Vitamin D follows the seasons, and how far your level drops in winter depends on more than the dose on the pack. A baseline before winter and a re-test after 90 days show you where you stand and whether your approach works.