Vitamin D in Winter: Who Needs It?

Your inner sunshine, all year round.

What if you were told that vitamin D deficiency is common in a country where the sun shines most days of the year?

It sounds contradictory, but a lifestyle spent between home, office and car means many of us don't get enough. And as winter days grow shorter, the same question comes back.

• What is vitamin D, and what does it do in the body?
• How much do you need each day?
• Who is most at risk of deficiency?
• Does everyone need a supplement?
• And what's the risk of taking too much?

Let's explore the science behind vitamin D.

1. What is vitamin D?

Vitamin D is a fat-soluble vitamin. The skin makes it when exposed to the sun's ultraviolet B (UVB) rays, and it's also found in some foods.

One of its key roles is helping the body absorb calcium, which is why it's linked to bone health. It also plays a part in how muscles, nerves and the immune system work.

The takeaway: Vitamin D is essential for bones and involved in many other body functions.

2. Where do we get it?

Vitamin D has three sources:

• Sunlight: the biggest natural source, but affected by time of day, clothing, skin tone and sunscreen.
• Food: such as fatty fish (salmon, tuna, sardines), egg yolks, fortified milk and breakfast cereals, and some mushrooms.
• Supplements: usually as vitamin D3 (cholecalciferol) or D2 (ergocalciferol).

The takeaway: Food alone rarely covers your needs, and the sun doesn't reach everyone equally.

3. How much do you need each day?

According to the NIH Office of Dietary Supplements (NIH ODS):

Age group	Recommended daily amount
Infants up to 12 months	10 mcg (400 IU)
1 to 70 years	15 mcg (600 IU)
Over 70 years	20 mcg (800 IU)

The safe upper limit for adults is 100 mcg (4,000 IU) a day, unless a doctor advises otherwise.

The takeaway: Most adults need 600–800 IU a day, and more isn't necessarily better.

4. Why does vitamin D come up more in winter?

In winter the days are shorter, the sun sits lower, we wear more clothing and spend more time indoors.

All of this may reduce how much vitamin D the skin makes, especially in countries further from the equator.

The takeaway: Winter doesn't cause deficiency on its own, but it cuts the natural chances of getting enough.

5. Who is most at risk of deficiency?

The NIH lists several groups:

• Breastfed infants, because breast milk is low in vitamin D.
• Older adults, because the skin makes less with age.
• People with little sun exposure, such as those who work indoors or cover most of their skin.
• People with darker skin, because melanin reduces how much the skin makes from sunlight.
• People with obesity, or who have had weight-loss surgery.
• People with conditions that affect fat absorption, such as Crohn's disease or celiac disease.

The takeaway: Vitamin D needs differ from person to person; there's no single answer for everyone.

6. How do you know if you're low?

Mild deficiency often has no clear symptoms. The reliable way to know is a blood test that measures 25-hydroxyvitamin D, or 25(OH)D.

Severe, long-term deficiency can lead to softening of the bones in adults (osteomalacia) and rickets in children.

The takeaway: Don't guess your level; a blood test is the most accurate way to know.

7. What does the research say?

Claim	What the evidence supports
Bone health	An essential role alongside calcium; severe deficiency weakens bones
Preventing respiratory infections	May slightly reduce risk, mainly in people who are deficient; results are inconsistent
Treating an infection once it starts	Doesn't appear to treat existing infections
Energy and mood	Evidence is limited and inconclusive

• Deficiency is harmful ≠ high doses are helpful
• Correcting a deficiency helps ≠ everyone needs the same supplement dose

The takeaway: The clearest benefit comes from correcting a deficiency, not from raising a level that's already normal.

8. Can you take too much?

Yes. Because it's fat-soluble, vitamin D can build up in the body.

According to the NIH, excess intake can cause nausea, vomiting, weakness, confusion, dehydration and kidney stones. Very high doses can harm the kidneys and heart.

Sunlight doesn't cause this build-up; high-dose supplements taken without supervision do.

The takeaway: High doses need medical supervision and blood tests before and after.

9. Medication interactions to watch

The NIH notes that vitamin D may interact with some medicines, including:

• Orlistat, used for weight loss.
• Statins, used to lower cholesterol.
• Thiazide diuretics.
• Steroids such as prednisone.

The takeaway: If you take regular medication, check with your pharmacist before adding a supplement.

10. Vitamin D in Saudi Arabia: why does this matter?

Many studies suggest vitamin D deficiency is common in the Kingdom, despite plentiful sunshine.

Possible reasons include:

• Avoiding the sun during the long hot months.
• Spending most of the day in enclosed, air-conditioned spaces.
• Clothing that covers most of the body.
• Few vitamin D-rich foods in the everyday diet.

The irony is that the Kingdom's mild winter may be the best time of year for outdoor activity.

If you choose a supplement, make sure it's registered with the Saudi Food and Drug Authority (SFDA).

The takeaway: The sun is available; our lifestyle decides how much of it we actually get.

11. Five evidence-based habits

1. Use the mild winter weather for walks or outdoor exercise, while still protecting your skin from the sun.
2. Add fatty fish, eggs and fortified foods to your diet.
3. Ask for a vitamin D test if you're in a higher-risk group.
4. If you need a supplement, stick to the dose your doctor or the label sets.
5. Take it with a meal that contains some fat, since it's a fat-soluble vitamin.

The takeaway: Sun, food and testing first; a supplement when it's needed.

12. How to choose a vitamin D supplement

Six questions before you buy:

1. Do I actually need it, or are sun and food enough?
2. Is the dose clearly listed in both mcg and IU?
3. Is the daily dose within the safe limit, or taken under medical supervision?
4. Is it vitamin D3 or D2?
5. Does it interact with any medication I take?
6. Is the product authentic and registered with the SFDA?

The takeaway: The right supplement matches your real needs, not the highest dose on the shelf.

FAQs

Is sitting in the sun behind a window enough?

No. Glass blocks most of the UVB rays the skin needs to make vitamin D.

Does sunscreen cause vitamin D deficiency?

It may reduce how much the skin makes, but sun protection still matters, and vitamin D can be made up through food or supplements when needed.

Do I need a test before taking a supplement?

It's recommended, especially if you're considering doses above the recommended amount.

What's the difference between D3 and D2?

Both raise blood levels, and some studies suggest D3 may keep them up for longer.

When should I take the supplement?

With a meal that contains some fat, at whatever time suits you and is easy to remember.

Does my baby need a supplement?

Doctors often recommend one for breastfed infants, and the pediatrician sets the dose.

Does vitamin D boost immunity?

It may slightly lower the risk of respiratory infections in people who are deficient, but it doesn't appear to treat an infection once it starts.

Can I take a high weekly dose instead of a daily one?

Some doctors prescribe this for deficiency, but it should only be taken with a prescription and medical follow-up.

The Avnzor takeaway

Vitamin D matters for bones and other body functions, and how much you need varies from person to person. Sun and food first, then testing, then a supplement at the right dose when needed.

At Avnzor, we believe the right supplement starts with knowing what you actually need, not with big promises.

Your inner sunshine, all year round.

Discover vitamin supplements from Avnzor, with free delivery and no minimum order.

Disclaimer: This article is for general education and is not a substitute for advice from a doctor or pharmacist. Consult one before taking any supplement if you have kidney or heart disease or high calcium levels, take regular medication, or are pregnant or breastfeeding. Doses for children and infants are set by a doctor, and adult supplements are not intended for anyone under 18.