Remember Tiny Tim from A Christmas Carol? He might have been suffering from rickets, a bone disease caused by deficiency of Vitamin D. Charles Dickens often based his characters on real diseases that he saw around him, and rickets was very common among children in England at the time, because they simply weren’t getting enough Vitamin D. It wasn’t a dietary deficiency: people can make Vitamin D when their skin is exposed to sunlight, but in Britain at the time, air pollution was literally so bad that it prevented people from getting even the small amount of sun exposure needed to avoid rickets.
Rickets causes unusually soft and deformed bones, hence Tiny Tim’s need for crutches. Today, few babies actually get rickets (although it still does happen!) but a lot of people are still suffering from a lower-level Vitamin D deficiency that might contribute to bone problems later in life, poor mental health, immune deficiency or autoimmune disease, and trouble maintaining a healthy weight. Here’s a look at what you need to know about the importance of Vitamin D, how to get it, and when to take a supplement.
What Is Vitamin D?
“Vitamin” D is actually a hormone, and if you really want to impress your friends and appall your enemies, you can pull out the full name: 1,25 dihydroxyvitamin D [1,25(OH)2D], or calcitriol. But call it Vitamin D for now, because the interesting part is what it does.
Vitamin D is important for all kinds of processes.
- Healthy bones and teeth: Vitamin D helps your bones absorb calcium. Severe deficiency causes a disease called rickets, but even mild deficiency is a risk factor for osteoporosis and fractures later in life. For example, this study found that many women who came to a hospital with hip fractures had “occult” Vitamin D deficiency: it wasn’t bad enough for anyone to notice any obvious signs...until they broke a bone.
- Mental health and brain function:brain development in children, for mood and mental health throughout life, for forming memories and making decisions, and for maintaining brain function in old age. Low levels of Vitamin D are associated with diseases from depression to Alzheimer’s Disease.
- Immunity and Autoimmunity: This review of recent studies goes over the role of Vitamin D in the immune system autoimmunity: it’s crucial for healthy immune function, and it may play a role in some autoimmune diseases. For example, a lot of studies suggest that lower Vitamin D levels are associated with a higher risk or worse symptoms of Multiple Sclerosis, and that supplementing may slow the disease.
- Weight and appetite: here it’s a little murkier because it’s not clear whether Vitamin D is a benefit for weight and metabolic health, or whether it’s something else in the sunlight that delivers the Vitamin D. But there’s likely a connection; you can read more about it here.
Some research has also linked lower levels of Vitamin D to cancer and other diseases, but so far it’s not clear whether that’s causal or just an association (so people who have higher Vitamin D levels are just healthier, and healthier people tend to get less cancer, but it’s not the Vitamin D specifically).
Getting Vitamin D
So Vitamin D is important. Now the big question: how to get enough? There are actually two different forms of the hormone:
- D2: 25-hydroxyvitamin D
- D3: 1,25-dihydroxyvitamin D
D3 is the form that the human body needs. You can get it from food, or you can make it yourself from sunlight.
How Much Vitamin D do we Need?
This one is a bit of a wild ride through studies that all conflict each other. Some studies get really precise about blood levels of Vitamin D, and try to quantify exactly what level of Vitamin D in the blood is optimal. The Institute of Medicine recommends that almost all people will be vitamin D-sufficient at a blood level above 50 nmol/L (20 ng/mL), but no more than 125 nmol/L (50 ng/mL). The advice from there would be to just keep getting more Vitamin D until you hit the optimal blood levels.
That’s just great if you know how many nanograms of Vitamin D you have per milliliter of your blood, but it’s just not very helpful for most of us since most people don’t know their blood levels of Vitamin D and don’t want to go having blood drawn every few months to check.
In the real world, it makes more sense to look at what consumption of Vitamin D will get most people into a healthy range. In the USA, the RDA of Vitamin D is 600 IU for healthy adults, except for people over 70 where it goes up to 800. But quite a bit of evidence shows that more may be better, especially since a lot of people have trouble absorbing Vitamin D. And there’s also the problem of malabsorption. A lot of people may do better with more Vitamin D because they have trouble absorbing it.
For Paleo specifically, a particular problem is gut health. Vitamin D is a fat-soluble vitamin, so to absorb it you have to have a gut that can absorb dietary fat. Many people who try Paleo have gut problems that prevent them from absorbing dietary fat properly. Those problems include…
- Celiac Disease
- Crohn’s Disease
- Some forms of ulcerative colitis
- Some kinds of liver disease
- Cystic fibrosis
People with obesity may also need more Vitamin D, because their body fat stores suck up a lot of Vitamin D and hide it away in the fat. This study suggests that people with obesity need about 40% more Vitamin D - if the RDA is enough for people at a medically normal weight, then the numbers for people with obesity would be 840 IU for people under 70 and 1120 for people over 70.
If you’re in one of these groups, it’s especially important to get enough Vitamin D for your actual needs.
Getting Vitamin D from Food
Some foods contain Vitamin D, but unless you eat a lot of them all the time, they probably won’t provide enough to really live on. The most Vitamin D-rich foods are fatty fish that live in cold waters, like salmon and sardines.
- Wild-caught salmon: 1694 IU per 6-oz serving
- Canned mackerel: 497 IU per 6-oz serving
- Farm-raised salmon: 411 IU per 6-oz serving
- Herring: 364 IU per 6-oz serving
(Here’s more about the difference between wild-caught and farm-raised salmon). Other fish contain very small amounts of Vitamin D, but nothing really noticeable. So if you ate 6 ounces of wild-caught salmon every other day, you’d be able to get enough Vitamin D from food, but for most people that’s just not going to happen. Realistically speaking, most of us will have to rely at least partly on the other two sources: sunlight and supplements.
Getting Vitamin D from Sunlight
Sunlight is the way that humans were designed to get most of our Vitamin D. When sunlight hits your skin, you can synthesize it yourself from cholesterol. Studies have shown that a healthy adult can get most of her Vitamin D from 5-30 minutes of direct sunlight while the sun is high in the sky - it doesn’t take a huge amount.
Sunscreen prevents this from happening, so sun exposure with sunscreen doesn’t count for Vitamin D. You can read more about sunbathing and get a closer look at the studies behind all those claims here.
Getting Vitamin D from Supplements
Food doesn’t provide enough Vitamin D for most adults, and it’s just not realistic for all of us to get enough sun exposure, especially in the winter when a lot of people spend all the best sunlight hours indoors. That leaves supplements.
Vitamin D supplements are one of the most common Paleo recommendations precisely because Vitamin D is so important and so hard to get. It’s reasonable that even a person eating a great diet might need one - here’s a guide to choosing high-quality supplements so you can make sure you’re actually getting what you pay for.
How Much is Too Much?
Overdosing is really only an issue with supplements. Even if you stay out in the sun for a long time, your body has its own way of shutting down Vitamin D production; you’ll just stop making it when you don’t need it. And unless you’re eating multiple pounds of salmon every day, getting toxic amounts of Vitamin D from food is extremely unlikely.
WIth supplements, the USDA set the upper intake limit at 4,000 IU per day for all healthy people over 9, including pregnant women. Sunshine can provide as much as 10,000 IU per day, and some researchers do argue that 10,000 IU should be the upper intake limit, but the thing about sunshine is that it has an automatic off switch if you’re getting too much for your particular body. Supplements don’t come with that kind of safety, so they call for a little more caution.
Summing it Up
It’s very clear that Vitamin D is important, but it’s not totally clear yet how much is the optimal amount to get from food + sunshine + supplements. It’s possible that some of the associations between Vitamin D intake and health could be caused by confounding factors (like other things in sunshine). But overall, the Paleo recommendation is…
- Eat plenty of fatty, cold-water fish like salmon and sardines.
- Get out in the sun when possible (remember that wearing sunscreen prevents the absorption of Vitamin D so time spent wearing sunscreen doesn’t count!)
- Take a Vitamin D supplement if it’s right for you.