As the days get shorter and we head into fall and winter, vitamin D becomes an especially important vitamin to talk about.

We often think of vitamin D as the “bone health vitamin,” but vitamin D is involved in much more than that. It plays important roles in calcium absorption, muscle function, immune function, and the health of multiple tissues throughout the body.

So today we’re going to talk about why vitamin D matters, where you can get it, how much you actually need, who should consider testing, and whether supplementation makes sense.

Why Does Vitamin D Matter?

Let’s start with the biggest reason we care about vitamin D: your bones.

Bone Health

Vitamin D helps your intestines absorb calcium and phosphorus, two minerals that are absolutely essential for maintaining strong bones.

If vitamin D is severely deficient, your body has a harder time maintaining normal calcium balance. In adults, significant deficiency can contribute to osteomalacia, which is softening of the bones and can cause bone pain and muscle weakness.

And as we get older, maintaining bone health becomes even more important because bone loss accelerates, particularly around menopause when estrogen levels decline.

But, vitamin D doesn’t work alone here. Bone health is really a team effort involving adequate calcium, vitamin D, protein, resistance exercise, and hormones such as estrogen.

Vitamin D and Muscle Health

Vitamin D is also involved in normal muscle function.

Low vitamin D levels have been associated with muscle weakness and other musculoskeletal problems. This becomes especially important as we age because maintaining muscle isn’t just about looking strong, it’s about maintaining balance, mobility, independence, and reducing the consequences of falls.

But there’s an important distinction here, having adequate vitamin D is important; but taking huge amounts of vitamin D doesn’t necessarily make your muscles stronger. Clinical trials haven’t consistently shown that high-dose supplementation improves muscle strength in people who aren’t deficient.

Vitamin D and Your Immune System

Now, vitamin D isn’t just important for your bones. Many cells of the immune system have vitamin D receptors, which means vitamin D can directly influence how those cells function. It plays a role in supporting our innate immune defenses and helping regulate the immune response.

And this is where we have to separate biology from marketing.

Low vitamin D levels have been associated with a higher risk of respiratory infections, and some studies have found that supplementation may modestly reduce respiratory infection risk, particularly in people with very low vitamin D levels.

But the research isn’t completely consistent. Vitamin D is not an immunity booster that guarantees you won’t get sick, and clinical trials haven’t consistently shown that supplementation prevents respiratory infections in everyone.

So I think of it this way: you want your vitamin D levels to be adequate for normal immune function — but more isn’t necessarily better.

*What About Mood?

This is a really interesting one.

There are vitamin D receptors in the brain, and observational studies have found an association between low vitamin D levels and depression.

Now, I want to be clear here. This doesn’t necessarily mean that low vitamin D causes depression.

And when researchers have actually given people vitamin D supplements in clinical trials, the results haven’t shown that vitamin D reliably prevents or treats depression.

So if you’re feeling down, tired, or experiencing changes in mood, I wouldn’t say, “You just need vitamin D.”

There are many potential contributors to mood like sleep, hormone changes, stress, thyroid function, nutrition, medications. But maintaining adequate vitamin D is still part of overall nutritional health.

*What About Heart Disease?

This is another area where vitamin D has been surrounded by a lot of hype.

Low vitamin D levels have been associated with cardiovascular disease in observational research. This is because vitamin D is involved in biological processes related to blood vessels, inflammation, and blood pressure.

But here’s the important part, taking vitamin D supplements has not been shown in clinical trials to prevent heart disease or reduce the risk of dying from heart disease. 

So again, we’re talking about an important nutrient that the body needs, not a cardiovascular medication.

Where Do We Get Vitamin D?

So where do we get Vitamin D? There are three main ways we get vitamin D. Food, sunlight, and supplements.

And ideally, we should be getting enough through our food and sunlight exposure. 

Food Sources

There aren’t actually that many foods that naturally contain substantial amounts of vitamin D.

Some of the best natural sources include:

• Fish liver oils
• Fatty fish (e.g., trout, salmon, tuna, sardines, and anchovies)
• Beef liver
• Egg yolks
• Cheese, in smaller amounts
• Mushrooms, particularly mushrooms exposed to UV light

And then there are fortified foods, which are actually a major source of vitamin D in the American diet.

Milk is commonly fortified with vitamin D, and many plant-based milks are fortified as well. You can also find vitamin D added to certain breakfast cereals, yogurt, and orange juice.

So when you’re shopping, look at that Nutrition Facts label and specifically look for vitamin D.

What About Sunlight?

Your body has an amazing ability to actually make vitamin D when your skin is exposed to UVB radiation (a type of ultraviolet light from the sun). When UVB rays reach your skin, they trigger a chemical reaction that converts a form of cholesterol in the skin into vitamin D3. Your liver and kidneys then convert it into the active form your body uses. 

How much vitamin D your skin produces depends on factors like skin tone, location, season, time of day, age, cloud cover, clothing, and sunscreen use. There’s no one-size-fits-all amount of sun exposure.

*Does Everyone Need a Vitamin D Blood Test?

The test we’re talking about is 25-hydroxyvitamin D.

For generally healthy adults, routine vitamin D screening isn’t recommended. But there are people who are more likely to have low vitamin D or problems absorbing or utilizing it.

Like we just mentioned, if you live somewhere with relatively little sunlight, spend most of your time indoors, or routinely cover most of your skin, you may produce less vitamin D from sunlight.

If you have darker skin, higher amounts of melanin reduce the skin’s ability to produce vitamin D from UVB exposure, so you may produce less vitamin D from sunlight

Also, as we age, our skin becomes less efficient at making vitamin D from sunlight, and older adults may also spend less time outdoors.

And, people with overweight or obesity can have lower circulating vitamin D levels, partly because vitamin D is distributed into a larger amount of body tissue.

Some medications can also interfere with vitamin D metabolism. One example is phenytoin, an anti-seizure medication that can increase the breakdown of vitamin D.

And conditions such as Crohn’s disease, ulcerative colitis, and celiac disease can interfere with absorption of nutrients, including vitamin D, particularly when fat absorption is impaired.

Lastly, those who have had gastric bypass surgery, may absorb less vitamin D. This is because the surgery changes the anatomy of the digestive tract and can reduce absorption of vitamin D and other nutrients, which is why these patients often require nutritional monitoring and individualized supplementation.

In all of these types of situations, talking with your healthcare provider about whether testing makes sense is much more appropriate than simply taking vitamin D supplements on your own.

*So How Much Vitamin D Do You Actually Need?

For most adults, the recommended daily allowance is actually pretty modest.

For adults 19 to 70 years old (including those who are pregnant or breastfeeding) 600 IU per day is sufficient.  And for adults 71 and older, 800 IU per day.

And yes, women and men have the same recommended daily allowance. 

There is some research suggesting sex differences in vitamin D status and its relationship with musculoskeletal health, and women can have particular concerns around bone health, especially during and after menopause.

But I don’t want to overstate the evidence and tell women they need more vitamin D simply because they’re women. Reviews of sex differences have found conflicting results, and the current RDA is the same for adult men and women. 

For women, the bigger conversation is really about bone health across the lifespan, particularly as estrogen declines around menopause.

What About Vitamin D3 Supplements? 

Okay, so before we finish out this podcast, I’d like to briefly talk about vitamin D supplements and what to look for when choosing a product. 

When you look at vitamin D supplements, you’ll typically see two forms: vitamin D2 and vitamin D3.

Both can raise your vitamin D levels, but D3, or cholecalciferol, generally raises blood levels more effectively and keeps them elevated longer than D2, or ergocalciferol.

D3 is also the form your skin naturally produces when exposed to sunlight, and it’s found naturally in foods like fatty fish and egg yolks. D2, on the other hand, comes primarily from plants and fungi.

Fortified foods can contain either D2 or D3, depending on the product.

When I’m choosing a vitamin D supplement, I personally prefer vitamin D3 (Momentous brand specifically). 

Now, I know that’s more than the 600 IU recommended daily amount, but it’s still well below the established adult upper limit of 4,000 IU per day. As a vegetarian, I also find it difficult to get enough vitamin D from my diet. And during certain times of the year, when there’s less sun exposure, I’ve had low vitamin D levels in the past.

And I want to make an important distinction here. The RDA is not the same thing as the upper limit. The RDA is the amount established to meet the needs of most healthy people.

The upper limit is the highest daily intake that’s generally considered unlikely to cause harm for most adults.

For adults, that upper limit is 4,000 IU per day from all sources, food, beverages, and supplements. 

Also, if someone is truly vitamin D deficient, a healthcare professional may temporarily prescribe a dose above 4,000 IU to correct the deficiency. That’s a treatment situation, and dosing should be individualized and monitored rather than self-prescribed.

Other things you may see when you’re choosing a vitamin D supplement is that they may contain vitamin K2, and this is where things get a little more complicated.

Vitamin K is involved in proteins that help regulate where calcium is used in the body, including proteins involved in bone mineralization. Because vitamin D increases calcium absorption from the intestine, you may hear the combination described as helping “put calcium into your bones”.

And that’s a little too simplistic.

There is some interesting research on vitamin D and K together, particularly regarding bone health, but we don’t currently have strong evidence that every healthy person taking vitamin D needs to add vitamin K2 or that taking the combination prevents cardiovascular disease.

And vitamin K isn’t something I would casually add to everyone’s supplement routine because it can interact significantly with warfarin and other vitamin-K–sensitive anticoagulant therapy.

So if you see a D3 + K2 product, don’t assume that the K2 makes it automatically better. The important thing is making sure you’re getting an appropriate amount of vitamin D3. Whether you need additional vitamin K depends on your overall diet, medications, and individual health situation.

And lastly, it’s important to remember that vitamin D is fat-soluble. So taking a D3 supplement with a meal that contains some fat can improve absorption. You don’t need a huge amount of fat, just taking it with a normal meal rather than on an empty stomach is a simple approach.

And because vitamin D is fat-soluble, it means your body can store it. So unlike some water-soluble vitamins where excess intake is more readily excreted, taking very high amounts of vitamin D over time can cause toxicity and cause symptoms such as nausea, vomiting, weakness, dehydration, excessive thirst and urination, and kidney stones. 

Thanks again for listening to The Peptide Podcast. 

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Until next time, be well, and have a happy, healthy week.