Vitamin D, the sunshine vitamin that never works alone
Welcome to Nutrition A to Z! This is a brand new series where we take one nutrient at a time and really get to know it. What it does, where to find it, and my favorite part, which nutrients it teams up with.
Because here is something I believe with my whole heart: nutrients are not solo artists. They work in teams. When you understand the team, everything starts to make more sense.
And I could not think of a better nutrient to kick things off than vitamin D. Fall arrived yesterday, the days here in Iowa are getting shorter, and this is exactly the time of year I start thinking about D.
The quick version
Your body makes vitamin D from sunshine, then turns it into something that acts like a hormone.
It is a key player for strong bones, a balanced immune system, and healthy muscles.
More than 9 out of 10 Americans do not get enough from food alone.
It works best with its partners, especially vitamin K2 and magnesium.
What it is
Vitamin D is a fat-soluble vitamin, which means it loves to travel with a little healthy fat and your body can store it for later.
There are two main forms. D2 comes from plants and mushrooms. D3 is the form your own skin makes when sunlight hits it, and it is also found in foods like fish and egg yolks. D3 is the form I look for.
Now here is what makes vitamin D so clever. Once your body makes it or takes it in, your liver and kidneys transform it into an active form that behaves like a hormone. It sends messages throughout your body, and cells in tissues from your bones to your immune system have receptors ready to listen. As a hormone health coach, I think that is pretty amazing.
Vitamin D is not just a vitamin. It is a messenger.
What it does
Builds strong bones. Vitamin D helps your body absorb calcium from your food. Without enough D, even a calcium-rich diet has a hard time getting that calcium where it needs to go. This matters so much for women in their forties and fifties, because as estrogen shifts, bones need all the support they can get.
Keeps your immune system balanced. Vitamin D helps your immune system respond the way it should. That balance is a big reason I talk about it so often with the women I work with who are focused on autoimmune wellness.
Supports your muscles. Strong muscles need vitamin D too, and when levels run low, muscles can feel weak and achy.
Shows up for your mood. Have you ever noticed your mood dipping right along with the daylight? You are not alone, and researchers have been interested in the connection between sunshine, vitamin D and how we feel for a long time.
Where you get it
Sunshine
The sun is our original source, and I will always encourage you to get outside. But here is the catch for those of us in the middle of the country. From about November through early March, the sun sits too low in the sky at our latitude for your skin to make much vitamin D at all. Add in sunscreen, days spent indoors, and the simple fact that our skin makes less as we get older, and you can see how easy it is to fall short.
Food
I love building meals around nutrient-rich foods, and it is always where I start. Here are some of the best food sources of vitamin D, based on numbers from the National Institutes of Health.
Rainbow trout: 645 IU per 3 ounces
Sockeye salmon: 570 IU per 3 ounces
Mushrooms exposed to UV light: 366 IU per 1/2 cup
Fortified milk: 120 IU per 1 cup
Sardines: 46 IU per 2 sardines
Egg: 44 IU per 1 large
But I also want to be real with you. Even a beautiful, whole-food diet has a hard time delivering enough vitamin D. National survey data shows about 94 percent of Americans fall short of the recommended amount from food alone. On top of that, research comparing crops over the last several decades has found declines in some of the nutrients in our fruits and vegetables. Our food is simply not what it used to be.
Supplements
This is why I believe so strongly in smart supplementation. Not as a replacement for good food, but as the support that fills the gaps our modern lives and modern food leave behind. When I look at a vitamin D supplement, I look for three things:
D3, the same form your skin makes.
Its partners included, especially vitamin K2, which you will read about in a second.
Quality you can trust, ideally from a company that uses whole-food ingredients and is open about where they come from.
And take it with a meal that has some healthy fat, like eggs, avocado or salmon. Remember, D loves to travel with fat.
Synergy partners: vitamin D's dream team
This is the section I am most excited about, and every post in this series will have one. If you only take one thing away from today, let it be this: vitamin D does its best work when its partners show up too.
Vitamin K2, the traffic director
Here is the easiest way I know to explain it. Vitamin D opens the door and brings calcium into your body. Vitamin K2 is the traffic director that tells that calcium where to go.
K2 switches on special proteins that help move calcium into your bones and teeth, and help keep it out of places it does not belong, like your blood vessels and soft tissues. Vitamin D even helps your body make more of those proteins. They really are a team.
Research on the pair is exciting, too. A review of studies found that taking vitamin K together with vitamin D3 increased bone mineral density. That is exactly why so many quality supplements now pair D3 with K2.
Where to find K2: natto (fermented soybeans) is the superstar. You will also find it in aged cheeses like Gouda, egg yolks, butter, and liver. K2 is a little different from K1, the kind in leafy greens, so it is worth knowing both.
One important note: if you take a blood thinner like warfarin (Coumadin), talk with your doctor before adding vitamin K in any form, because changes in vitamin K can affect how that medication works.
Magnesium, the activator
Your body needs magnesium to turn vitamin D into its active form. Without enough magnesium, vitamin D cannot fully do its job, and national survey data shows about half of Americans do not get enough magnesium from food. Magnesium is our very next letter in this series, so stay tuned, because it deserves its own spotlight.
Calcium and healthy fats, the supporting cast
Vitamin D and K2 are all about getting calcium where it belongs, so calcium-rich foods like leafy greens, sardines, and yogurt round out the team. And healthy fats help your body absorb D and K2 in the first place.
Who tends to run low
Most of us, especially this time of year. But you are more likely to need extra support if you:
Spend most of your day indoors
Live in the northern half of the country (hello, Midwest)
Have darker skin, which needs more sun to make the same amount
Are in your forties, fifties or beyond
Carry extra weight, since vitamin D gets tucked away in body fat
Have a condition that affects how you absorb fat, like Crohn's or celiac disease
Partner with your provider
I am a big believer in teamwork, and that includes you and your healthcare provider. Finding the right amount of vitamin D for your body is personal, and more is not always better. Here are some great questions to bring to your next appointment:
Based on where I live and my lifestyle, what do you think about vitamin D for me?
Should I be taking vitamin K2 along with my vitamin D?
Do any of my medications affect vitamin D, vitamin K or magnesium?
I am in perimenopause or menopause. How can I best support my bones right now?
Is the supplement I am already taking the right amount for me?
My take
I love vitamin D. I love that it gets us outside, and I love that it is such a beautiful example of how our bodies work in harmony.
But if there is one thing I hope you remember, it is this: balance matters. Vitamin D on its own is only part of the story. Pair it with K2, make sure magnesium is on board, eat real food with healthy fats, and get outside whenever the sun cooperates. That is the holistic approach I believe in, and it is exactly how I love helping women take care of themselves.
Next up in Nutrition A to Z: magnesium, the mineral that helps put vitamin D to work.
Want to go deeper on how nutrients team up? My free Common Supplement Combinations guide is waiting for you in my free resources library, along with my Bioavailability Guide. Join the list and they are all yours.
This post is for education only and is not medical advice. I do not diagnose or treat any medical condition. Please talk with your healthcare provider before starting any supplement, especially if you are pregnant, nursing, taking medication, or managing a health condition.
Where this comes from
National Institutes of Health, Office of Dietary Supplements: Vitamin D and Vitamin K fact sheets
Linus Pauling Institute, Oregon State University: Vitamin D and skin health; Micronutrient Inadequacies in the US Population (NHANES 2007 to 2010)
Aaseth and colleagues, Nutrients, 2024: "The Importance of Vitamin K and the Combination of Vitamins K and D for Calcium Metabolism and Bone Health"
Davis and colleagues, Journal of the American College of Nutrition, 2004: changes in USDA nutrient data for 43 garden crops, 1950 to 1999