Vitamin D, also known as the "sunshine vitamin", is essential for good health. However, around 42% of the US population is deficient (even higher in certain groups!). (1)
Your skin produces Vitamin D in response to sun exposure, so why are so many people deficient? Keep reading to find out why, and how much you should take!
What is Vitamin D?
Vitamin D is actually a hormone! It's produced in the skin from exposure to the sun's UV rays (very little comes from diet, with the exception of sources like cod liver).
The liver and kidneys activate it, meaning that they turn it into a form your body can use.
It controls blood calcium, the immune system, and so much more.
Vitamin D is a fat-soluble vitamin, so your body can store it and use it over time.
There are two forms of D found in supplements: vitamin D2 (ergocalciferol) and D3 (cholecalciferol). D3 has been found to be 87% more potent at raising blood D concentration compared with D2, and leads to 2-3 times greater storage. (2)
Getting enough vitamin D is associated with numerous benefits you can read about here.
Why Are Vitamin D Deficiencies So Common?
If you're reading this, you may be low in Vitamin D and wondering why - especially if you spend plenty of time outside!
Some reasons are obvious: we spend more time indoors, wear sunscreen, live in cities, far from the equator, and the air is polluted.
But, since vitamin D is a hormone, there are other complexities.
As we age, our kidneys sometimes can't convert vitamin D to its active form.
Digestive problems like Crohn's and even celiac disorder prevent our digestive tract from absorbing it from diet.
Being overweight is also linked to low absorption.
Plus, you need enough vitamin K, magnesium, and zinc in your diet (or supplements) for proper absorption!
How Much Vitamin D Should I Take?
The answer to this depends on a lot of factors, such as age, where you live (latitude, but also whether you live in a city), sun exposure, and how often you use sunscreen.
In general, the NIH recommends 400 IU/10 mcg per day for babies, and 600 IU/15 mcg per day for those older than 1 (until you hit 70, in which case it's 800 IU or 20 mcg). (3)
However, studies have shown that higher intakes are sometimes necessary to raise blood vitamin D to a healthy level.
One randomized controlled trail found the optimal dose to be 3800-5000 IU per day to maintain a healthy vitamin D blood status. (4)
Another study gave hospital-bound patients long-term doses of 5,000-50,000 IU per day with no adverse effects and improvements to the patients' conditions. (5)
You should consult with your doctor before supplementing with a higher dose than the recommended values.
How do I get enough vitamin D?
Spending adequate time outside at peak sun (the middle of the day) is a great way to get enough vitamin D, but for the reasons mentioned previously, it's not an option for everyone (and may not be adequate, if you live in a city or far from the equator).
Consuming cod liver oil is a classic remedy for supplementing vitamin D.
How often should I take vitamin D?
Vitamin D is fat-soluble, so your body can store it for months and use it over time.
Conventional wisdom is that you can take it once a week or even once a month.
Technically that's true, but studies show it's not the most effective.
In randomized clinical trials, patients were given either doses of 600 IU/day, 4200 IU/week, or 18,000 IU/month. Note that this is the same dose per day but at different intervals.
We would expect to see the same vitamin D status from all groups, but in reality the groups who took the vitamin D daily had the best results. (6)
So we'd recommend you take it as regularly as possible - ideally, a little each day.
Whether you get enough vitamin D from sunlight or supplement, this vitamin (more accurately called a hormone) is critical for many processes in the body.
(1) Prevalence and correlates of vitamin D deficiency in US adults