D is for dark – all about vitamin D

Photo of mushrooms with text overlay that reads: vitamin D what you need to know


D is for Dark

I live way the heck up north, so by this time of year the sun is already setting by around 3pm, and soon enough it’ll be 2pm! Even though we’ve lived here for nearly eight years, I never quite get used to the darkness.

What does darkness have to do with nutrition? Vitamin D of course!

Vitamin D, it turns out, isn’t a vitamin at all – it was misclassified when it was first discovered in the 1920’s, and the name stuck. What it is, in fact, is a pro-hormone. Pro-hormones are intra-glandular precursors of hormones which, though not actually hormones themselves, amplify the effects of existing hormones.

Almost all cells and tissues in the body are able to convert vitamin D into its active hormone form – there are 2800 known binding sites on our DNA!

Vitamin D helps to form our bones and teeth, helps maintain bone health, helps regulate blood calcium levels, and contributes to our metabolism, nervous system, and immune system function.

D2 vs D3

In nutrition terms, vitamin D is used to refer to either D2 (ergocalciferol) or D3 (cholecaliferol). D3 is what’s produced by our skin upon exposure to sunlight, where vitamin D is synthesized from cholesterol when sun exposure is adequate (with enough UVB). D3 is also found in some foods, such as fatty fish and egg yolks, in fortified foods such as milk and orange juice, and in supplements. D3 supplements are manufactured through the UV irradiation of a substance derived from sheep’s wool.

D2 is the vegan version, which is made mainly from mushrooms or yeast irradiated under UV light and is used to fortify non-dairy beverages such as soy and almond milk, and is also found in some supplements. That D2 is less potent than D3 is widely acknowledged.

Is it sunny enough?

Anything that blocks UVB rays will also block vitamin D production by our skin. This includes sunscreen, clothing, complexion, age, windows, etc.

When I was doing my undergrad degree in nutrition I attended a lecture on vitamin D and pregnancy and learned that where we lived (Vancouver at that time) you could stand outside buck-naked all winter long and never produce enough vitamin D. True story. If you live above 42° North latitude (think Portland, Oregon or just north of Rome, Italy) your skin can’t make any from about October – April due to lack of UVB. And really, most of us aren’t outside in shorts and a t-shirt until around June. That’s a good chunk of the year we’re not producing any vitamin D.

So why is vitamin D it so dang important?

We know the most about vitamin D as it relates to bone health, as it helps the body to absorb calcium. A vitamin D deficiency leads to nutritional rickets, a condition that causes deformed and weak bones in infants and children. In adults not getting enough Vitamin D can increase the risk of developing osteoporosis.

Aside from bone health, vitamin D has been linked with diabetes, cardiovascular disease, depression, breast, colon, and prostate cancer, autoimmune diseases, influenza, and the common cold.

But what about the naysayers?

Despite the all of these claims regarding the benefits of vitamin D there is still a lot of disagreement amongst the scientific community regarding it’s efficacy as a supplement. The only proven benefit is in helping calcium to build strong bones, but we do know that vitamin D helps regulate the immune system and neuromuscular system. Exactly in what way is a matter of debate.

Vitamin D and vegetarians

Some studies have found vegans to have lower vitamin D intakes than lacto-ovo vegetarians or omnivores, yet any person who does not include good sources in their diet or take supplements can be at risk for not getting enough, especially in northern climates where sunlight exposure is limited.

If you’re found to be deficient in vitamin D and choose to take the vegan supplement D2, you may be advised to take a higher dose than if you were to take D3, which is not vegan but is generally believed to be more potent.

How much do I need?

For adults the recommended daily intake of Vitamin D from all sources including sunlight, fortified, foods, and supplementation is 600 IU / 15 mcg. More recent research suggests that higher intakes may be beneficial, and some experts are recommending 800 IU or more per day for the average adult, with higher intakes recommended to treat deficiency. For children and infants the recommendation is 400 IU / 10 mcg.

If you’re concerned about vitamin D deficiency, speak to your doctor about getting tested.

Can I take too much?

Yes, but it’s pretty unlikely that you will. Vitamin D is one of the fat soluble vitamins (along with vitamins A, E, and K) which means that we store rather than excrete the excess.

Excess vitamin D can cause the body to absorb too much calcium, which in turn can lead to kidney damage. Exactly how much is too much is a matter of debate. Some researchers report using up to 10,000 IU per day without observing problems, however the tolerable upper limit has been set at 4000 IU per day for those over 9 years of age. A conservative recommendation is to stay below 2000 IU per day.

Should I supplement?

Truthfully, I don’t know. I never took supplements when I lived in Vancouver, but now that I live in Stockholm, which is 59° North (even more north than Juneau, Alaska!) I’m serious about it. I take a lighter supplement in the lighter months (10mcg / 400 IU) and a stronger supplement in the deep dark winter months (30mcg / 1200 IU). My little guy also gets D drops year round.

I do believe I’ve noticed an improvement in my winter immunity since I’ve been supplementing, but that is merely an anecdotal observation.

The vast majority of research continues to show that vitamin D does reduce the risk of many chronic diseases. More research is needed, but in my opinion future research will shed light on the exactly how and why vitamin D is beneficial.









  1. Poorna says

    Hey Katie,
    Great post as always ! Your posts are one among the few things that brightens up the dark days for me 🙂
    I have been speculating starting supplements. But now it leaves no choice. Two hours of sunlight so far in Nov, that’s just terrible :(.
    I ordered an energy lamp as well, one of those clinically tested ones. Let’s see if it will help at all ! But I am definitely starting supplements.

  2. Kiersten says

    I just got some bloodwork results today and everything is fine – except my Vitamin D levels are super low. So this is very timely for me. I love your nutrition posts!

    • Katie Trant says

      Thanks Kiersten! Vitamin D is especially important in your situation, and Chicago is right on that 42° line, so a winter supplement would be a good idea for you regardless. Talk to your doctor about D2 vs D3, as there is a difference in potency.

  3. Linda @ Veganosity says

    I got depressed just thinking about having only two hours of sunshine in an entire month! No more complaining about the sun setting at 4:30 here in Chicago. I have been taking Vitamin D for years, but I never knew it was a hormone, or that there was a vegan version. Thanks for the information!

    • Katie Trant says

      Yeah, it’s been a dreary month. Hopefully we’ll get a good dumping of snow soon and things will brighten up! Yep, if you’re strictly vegan then D2 is what you’re after. I take D3 since it’s more potent, and I know some vegans who still choose to take D3, especially during pregnancy. The choice is up to you, I’d just do a little background reading on the difference.

  4. Debbi Basini-Herman says

    Thank you for your article on Vitamin D. I would highly recommend anyone going to their doctor for routine blood work, request their Vitamin D levels be checked. There is a range. From the information received from the doctor, easy to figure out what amount should be taken each day. I also would suggest a “good” vitamin. It is worth it to pay more for vitamins after researching and finding the brand you like. Thank you….Debbi Breast Cancer Survivor of 9 years and I take a D3 every day.

    • Katie Trant says

      Absolutely it’s worth the money for a reliable and reputable supplement! And yes, anyone concerned that they might be deficient should see a doctor for blood work. In northern climates it’s widely acknowledged that most people will be deficient, so taking a supplement within the RDA range is advised for the general population.


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