I’ve gotten a lot of requests for a post on vitamin D, probably because there’s a lot of confusing stuff on it out there. 

Everyone has important questions, like:

What are the benefits of vitamin D?

Do we all need to take it?

How much should we take? Is the official recommendation too low?

Is it possible to take too much?

How about vitamin D and Covid? 

Here’s everything you need to know about vitamin D:

What is vitamin D?

Vitamin D is a fat-soluble vitamin that’s also a hormone. 

The body also gets vitamin D from the sun, which know sounds weird, but here’s how it happens:

The sun shines onto your exposed skin. 

Ultraviolet rays (UVB) from the sun cause 7-dehydrocholesterol, a type of cholesterol in our skin, to be converted into cholecalciferol, which is otherwise known as vitamin D3.

But that’s not it! The vitamin D3 goes to the liver, where it’s then converted to 25-hydroxyvitamin D3, or calcidiol. This form of vitamin D is still not functional in the body. 

The calcidiol then goes to the kidneys, where it’s converted into 1,25 dihydroxyvitamin D3, or calcitriol. This is the active form of vitamin D. Phew! What a ride.

This type of vitamin D synthesis is called endogenous production, because it happens within our bodies.

We can also get vitamin D exogenously, from food and supplements. Good sources of vitamin D are fatty fish, egg yolks, and fortified milk and plant-based beverages. 

Normal blood levels of vitamin D are 20-50 nanograms/milliliter, and this range is somewhat controversial. Many people believe that the lower end of the range should be higher. The issue is that we don’t actually know what the optimal level for blood vitamin D levels is. It likely differs between individuals. 

Nevertheless, levels less than 12 nanograms/milliliter indicate a vitamin D deficiency.

Severe vitamin D deficiency affects 5.9% of the US population, and 7.4% of Canadians. Severe vitamin D deficiency can cause bones to become brittle and porous. It can also cause muscle cramps and fatigue. It may also cause depressed mood. But you don’t have to have a severe deficiency of vitamin D to have low vitamin D levels in your body. 

Low vitamin D status is reported in 24% of people in the US, and 37% of people in Canada. 

You might not know if you have low vitamin D status, since there are no obvious symptoms. If you’re at risk for vitamin D deficiency, it’s good to have your levels tested occasionally. The test for vitamin D levels is called the 25-hydroxy vitamin D test.

Risk factors for vitamin D deficiency are:

Dark skin

Living in a colder climate

Being homebound

Being elderly

Having bariatric surgery

Not eating vitamin D-containing foods, such as fatty fish and fortified dairy

It’s also important to mention that magnesium, a mineral that many of us don’t get enough of, is essential for vitamin D metabolism. Everything is interconnected. 

What does vitamin D do?

Vitamin D is involved in a lot of processes, but its primary role is to increase intestinal absorption of calcium. If we don’t have enough vitamin D, this can impair calcium absorption, leading to weak bones. In fact, without enough vitamin D, calcium absorption from food is only around 10-15%. With adequate vitamin D stores, it’s 30-40%.

It also means that when our bodies need calcium for other processes, such as maintaining heart rhythm, muscle contraction, it will be taken from our bones. When calcium is leached from the bones, and isn’t replaced, the bones get weak. 

I’m sure you’ve heard people saying that we should be taking vitamin D to ward off Covid or to help minimize symptoms. There’s a good reason for this: It modulates both the innate and acquired immune responses.

We know that vitamin D regulates antimicrobial proteins and barrier functions to keep pathogens out of the body. It also helps reduce the proliferation cells that promote inappropriate inflammation, and helps to build cells that fight pathogens. 

How about vitamin D and Covid?

We know that many people who have severe Covid symptoms, are also low in vitamin D. We see this in other diseases and conditions, such as cardiovascular disease, diabetes, HIV, cancer, and autoimmune disease.

But does the condition cause low vitamin D? Or, does low vitamin D have a role in causing the disease? 

We don’t really know. And because the studies done on people with these conditions and vitamin D levels are mostly observational, all we can do is draw a link between the two. We can’t really establish causation. For example, the elderly are frequently affected by covid and by low vitamin D levels. Which came first, the chicken, or the egg?

Vitamin D is also a negative acute-phase reactant, meaning that vitamin D levels fall during infection. If we don’t have pre-infection vitamin D levels for someone, it’s tough to determine whether they had low D status before getting ill, or if their low D is a result of the illness. 

The evidence around taking vitamin D to prevent or lower symptom severity in Covid-19 is underwhelming and/or inconclusive. (and here) (and here) 

A lot of evidence around vitamin D’s link with Covid seems to be based on correcting a vitamin D deficiency, rather than bolstering already-normal blood levels of D with more and more of it. 

A study published in the Lancet in January 2021 showed that people who had adequate blood levels of vitamin D didn’t get any significant benefit from taking vitamin D to prevent respiratory infections. It did find that vitamin D supplementation appeared to decrease duration of symptoms, but these findings weren’t clinically significant. 

Other, more questionable benefits of vitamin D include:

May lower risk of falls in the elderly.

May lower risk of cancer.

Studies are inconclusive. 

May help lower risk for cardiovascular disease.

Again, we can’t establish causation here.

Which vitamin D supplements should we be taking?

D3, which is widely available everywhere supplements are sold, is the best choice. D2 supplements are also available, but the body seems to use D2 less efficiently than it does D3. 

D3 is the type of vitamin D that our body synthesizes from sunlight. D2 is found in plants, and isn’t made by the human body.  

Because vitamin D is fat-soluble, you should take it with a source of fat – ideally, around mealtime. 

How much do we need?

There are people out there who are taking up to 20,000 IU of vitamin D a DAY! Is this smart? Will mega dosing vitamin D really help us more than taking the standard dose?

The RDA for vitamin D for adults is 600-800 IU/day. But as I mentioned above, some people are going far and beyond that amount. And honestly, the RDA is surrounded by a lot of controversy. There are many, many doctors and other professionals who believe that 600IU is too low to correct and maintain adequate blood levels of vitamin D. 

But what if you take too much?

Because it’s a fat soluble vitamin that’s stored in our fat cells (and not peed out like a water soluble vitamin, such as vitamin C), it’s rare – but possible – to overdose on D. 

Vitamin D toxicity can result from super-high doses (more than 10,000 – 40,000 IU a day, but sometimes less), taken for long periods. It’s also important to note that vitamin D toxicity manifests itself in an overload of calcium, which causes weakness, arrhythmias, and kidney damage, among other things. 

The tolerable upper limit for vitamin D is listed at 4000IU/day and really, there isn’t any research that suggests that more than that is effective for anything. In fact, aiming for vitamin D sufficiency (ie: if you’re deficient, correcting that, and maintaining adequate, normal blood levels of vitamin D) by taking 1000IU-2000IU a day has been suggested. 

This is the thing:

I feel like there’s a lot we don’t know about vitamin D and disease prevention. But what we do know is that people who get sick with a variety of illnesses, seem to be low in vitamin D. Whether vitamin D is the cause or just an effect of that, we don’t know, since it’s tough to pinpoint cause and effect and each nutrient. And often, we don’t have pre-illness D levels to compare to. 

But vitamin D is required for health, and many of us don’t get enough. But studies point to the benefits of correcting low levels of vitamin D, versus supplementing already-adequate levels with high doses of D.

Maintaining your intake of vitamin D to keep your levels normal, is a good idea – so I do recommend that you take vitamin D, especially in the winter. And especially if you’re in a high-risk group. Don’t mega dose. 

But don’t expect vitamin D to be a replacement for good health hygiene, vaccines, or masks. Vitamin D, or a combination of vitamin D and ANYTHING ELSE, is not effective against illness, without these other precautions.


Here’s a recipe with a good source of D: my Asian-Style Salmon Poke Bowls.