Vitamin D in winter — why 75 µg is becoming the standard

From late October until roughly the end of March, the sun in the Netherlands sits too low in the sky for your skin to produce enough vitamin D. For five months, what you put in your glass, on your plate or in your supplement is all that is left. Seen in that light, the old guideline of 10 µg a day — which still appears on many multivitamins — has by now become a floor rather than a target.

In orthomolecular and functional-medicine practice we have watched the advice shift for years. This year the mainstream health sources appear to be moving along too: 50 to 75 µg a day is named more and more often as a realistic winter dose for adults who do not work outdoors and do not take a sunny holiday. Time to set that out properly.

What does vitamin D do again

Vitamin D is not a classic vitamin but functionally a hormone. It is made in the skin under the influence of UVB light and converted into its active form in the liver and the kidneys. It binds to receptors in virtually every cell type we know of.

Three EFSA-authorised health claims for vitamin D that we may mention here:

• Vitamin D contributes to the normal function of the immune system
• Vitamin D contributes to the maintenance of normal bones and muscles
• Vitamin D has a role in the process of cell division

It sounds mild. In practice it means that a chronically low status almost always shows up somewhere — usually in subtler symptoms than an outright deficiency diagnosis.

Why the old 10 µg guideline no longer works

The Health Council recommendation of 10 µg a day (400 IU) is based on preventing rickets and clear bone problems. That is a very different aim from an optimal serum 25(OH)D level for immune and muscle function. According to most international references, optimal levels lie between 75 and 125 nmol/L. To get there in a Dutch winter, with no outdoor work and no sunny holiday, you structurally need more than 10 µg.

Two research findings that underpin this shift:

1. Endocrine Society guideline 2024 names 50–100 µg a day as realistic maintenance for adults under 70 without sun exposure.
2. Cochrane review (2023) on vitamin D and respiratory tract infections: the effect was stronger with daily doses between 25 and 100 µg than with single high "bolus" doses.

Why 75 µg is often the right middle ground

For many adults with no known medical history, 75 µg a day in winter is a reasonable starting point:

• Comfortably within the EFSA upper limit (Upper Limit) of 100 µg a day for adults
• Enough to correct a moderately low level within 2–3 months without over-supplementing
• Small enough not to require routine blood testing for anyone without risk factors

For anyone who wants to sit above 75 µg a day — or to run a loading phase — consulting a doctor or an orthomolecular practitioner (plus a 25(OH)D measurement) is usually the right route.

And K2 alongside it?

Vitamin D increases calcium absorption from the gut. Vitamin K2 helps to get that calcium to the right place — into bone, and not into the arterial wall. For anyone using a dose above 50 µg vitamin D over a long period, a K2-MK7 addition of 75–180 µg a day is a common combination in protocols. Not obligatory; but logical.

When to be more careful

Vitamin D is fat-soluble; it accumulates. Hold back, unless you have consulted your doctor, if:

• You have sarcoidosis, hyperparathyroidism or another calcium metabolism disorder
• Your kidney function is less than optimal
• You already take a multivitamin containing vitamin D (add the doses together)
• You are pregnant or want to become pregnant — other guidelines apply then

What to do with this in practice

Three scenarios:

1. No symptoms, no risk factors, simply maintenance: 50–75 µg a day, October to April. Combining it with a source of fat (at a meal) increases absorption.
2. Suspected low status, or symptoms: have your 25(OH)D measured through your GP or an orthomolecular practitioner. Dose on the basis of the value.
3. Multivitamin user: check the label for how much vitamin D your multi already holds. Many multivitamins now land at 25–50 µg; there is no need to automatically stack another 75 µg on top of that.

Vitarium therapeutic editorial team

Which vitamin D formulas we take into the range, we choose on three criteria: a realistic dose for the Dutch winter, a transparent label declaration (vitamin D2 or D3, with or without K2), and a manufacturing partner with a track record. No mega-dose stunts, no hidden fillers.

In our protocol overview for practitioners and their clients, vitamin D often stands as a basic building block, alongside magnesium and a quality multivitamin. For anyone who wants all three in one concentrated formula: take a look at our Immuvir Lysine complex (vitamin C + zinc immune stack) and the separate vitamin D3+K2 in the range.

This article is informative and is not medical advice. If you have symptoms, use medication, are pregnant or have specific conditions: consult a doctor or an orthomolecular practitioner. Vitarium sells food supplements — not medicines.

Sources: Endocrine Society Clinical Practice Guideline on Vitamin D 2024 · Health Council of the Netherlands dietary reference values for vitamin D 2012 · EFSA Tolerable Upper Intake Levels (vitamin D) 2018 · Cochrane Review "Vitamin D for the prevention of acute respiratory tract infections" 2023.