Shorter days, less sun: the vitamin D question in winter
Nutriens editors
By November, many of us leave for work in the dark and come home in the dark. Most of the light in between reaches us through a window. Somewhere in that season, a question tends to come up: should I be thinking about vitamin D?
It’s a fair question, and it has a plain answer that doesn’t need any hype. Here is what the National Institutes of Health says about vitamin D and the sun, what food can and can’t do, and why the most useful first step is a conversation with your doctor about testing.
Where vitamin D comes from
According to the NIH Office of Dietary Supplements (ODS) Vitamin D fact sheet, your body gets it in three ways. Your skin makes it when ultraviolet B (UVB) rays from the sun reach it. A small number of foods contain it naturally. And some foods and supplements have it added.
For many people, the sun has done a lot of the work without them noticing: a walk at lunch, a weekend outside, a summer in short sleeves. That’s the part that changes when the days get shorter.
Why winter changes the picture
The ODS fact sheet lists the things that affect how much UVB reaches your skin and how much vitamin D you make. In winter, several of them shift at once:
- Season and length of day. Shorter days mean fewer hours of strong sun.
- Time of day. If you’re indoors from morning to evening, you miss the hours when the sun is highest.
- Cloud cover and smog. Both cut the UVB that gets through.
- Sunscreen and clothing. Sunscreen still matters, and it limits UVB. In winter, coats and scarves cover most of your skin anyway.
- Skin tone. Skin with more melanin makes less vitamin D from the same sun.
- Age. Skin makes vitamin D less efficiently as we get older.
- Windows. UVB doesn’t pass through glass, so a sunny desk by the window doesn’t make vitamin D.
None of this means something is wrong with you. It means the easy source of the summer months gets smaller for part of the year, and that’s worth knowing.
What food can do
Food helps, but few foods are rich in vitamin D. The ODS fact sheet names the best natural sources as fatty fish, such as trout, salmon, tuna and mackerel, and fish liver oils. Beef liver, egg yolks and cheese have small amounts. Some mushrooms contain vitamin D too, especially those exposed to UV light.
In the United States, fortified foods provide most of the vitamin D in people’s diets, according to ODS: most milk, many breakfast cereals and plant milks, and some brands of orange juice and yogurt.
If you eat fatty fish often and drink fortified milk, food covers part of it. If you don’t, it covers less. That’s a reason to look at your own plate, not a reason to worry.
How much people need
The ODS fact sheet sets the Recommended Dietary Allowance at 15 mcg (600 IU) a day for adults up to age 70, and 20 mcg (800 IU) a day after 70. Those are amounts for healthy people, from all sources together.
Your own needs can differ. That’s why the number on a fact sheet is a starting point for a conversation, not a personal plan.
The first step: ask your doctor about testing
You can’t tell your vitamin D level by how you feel or by how grey the sky is. The ODS fact sheet explains that a blood test for 25-hydroxyvitamin D, written 25(OH)D, is the main way to check vitamin D status.
So if winter has you wondering, ask your doctor whether a test makes sense for you. Bring a list of everything you already take, including any multivitamin, because vitamin D shows up in more than one bottle. Your doctor can tell you what your number means and whether you need anything beyond what you do now.
You’ll see vitamin D sold as the answer to many things. We stick to what our label says and what the NIH fact sheet supports, nothing more.
Where a supplement fits
If you and your doctor decide a supplement makes sense, Sunlight Foundation is vitamin D3, 50 mcg (2,000 IU) as cholecalciferol, in one softgel a day. D3 is the form your skin makes from sunlight. A bottle of 100 softgels lasts 100 days, which covers most of a winter.
The label says: "Supports bone health, maintains immune function & increases vitamin D.*" That’s the whole claim.
Two things to know before you buy. The softgel is gelatin, so it isn’t vegetarian. And the vitamin D is dissolved in soybean and corn oil, because it’s fat-soluble. The label lists no allergens.
If you already take Daily Foundation, it has 10 mcg of vitamin D, so the two together give you 60 mcg a day.
For what each number on the label means, read 2,000 IU, 50 mcg, 250% DV: reading a vitamin D3 label. For gelatin, the oils and who should check first, see the Sunlight Foundation FAQ.
A short winter checklist
- Notice how much daylight you actually get on a workday, and how much of it comes through glass.
- Look at your plate: fatty fish, eggs and fortified foods.
- List every supplement you take and add up the vitamin D on each label, in micrograms.
- Ask your doctor about testing before you start or change anything.
Your next step
If your doctor says a daily vitamin D3 fits, it’s one softgel a day. See Sunlight Foundation.
References
- National Institutes of Health, Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals.
- National Institutes of Health, Office of Dietary Supplements. Vitamin D: Fact Sheet for Consumers.
- Sunlight Foundation Supplement Facts label.
