Introduction
Vitamin D is often called the “sunshine vitamin,” but what if I told you that's only half the story? Unlike other vitamins, Vitamin D functions more like a hormone, influencing everything from bone health to immune function, mood, and metabolism.
Despite its importance, there’s a lot of confusion surrounding Vitamin D and you may be wondering:
Do you actually need to test it?
Can you get enough from food?”
How does it interact with other essential nutrients like magnesium and Vitamin K2?
Is supplementation necessary? If so, how much?
What are reputable supplements on the market?
Inside this blog article we’ll explore the multifaceted nature of Vitamin D, debunking common misconceptions, explaining its function as a hormone, revealing its relationship with other essential minerals, and guiding you towards optimal supplementation. With so much conflicting information available on the net it’s easy to misunderstand how to utilize Vitamin D to your benefit. Let’s clear things up!
“We live in Northeast Ohio, everyone’s Vitamin D is low!”
While my team and I coach clients from all over the globe, I personally live outside of Cleveland, Ohio where I get a fair amount of referrals from local physicians. Recently a new client came in who was referred for weight loss. Her main complaint? She couldn’t pick her head up off the pillow to get moving most mornings - she was experiencing severe fatigue!
As she struggled to get through the calls, I asked her when the last time her Vitamin D levels were checked. She explained that Vitamin D was not included in her annual labs. She was told not to test it and just assume it was low since the sun rarely shines in Cleveland in the winter.
Since I rarely provide guidance on supplementation without testing, especially Vitamin D, I asked if she’d be willing to get her levels tested if I sent her an order. She agreed. We tested them and her levels were low.
At a level of 18 (below 20 is considered deficient per conventional and functional standards), she needed pretty aggressive supplementation to help bring her levels up. Once she began to replenish her Vitamin D levels, she had more energy, started sleeping better, thinking more clearly and her mood improved. The best thing? This gave her motivation needed to improve her eating habits - a shift she would not have been able to make without this discovery!
This brings us to one of the biggest misconceptions about Vitamin D: that you don’t have to test it, but rather should just assume it’s low. And worse yet - should just blindly supplement.
Nearly 42% of adults and 70% of children between six and eleven years old in the US have low Vitamin D stores (1). In spite of these rates, many don’t know that their Vitamin D needs a boost. They may feel tired or fatigued, just like my client did, and attribute it to something else, such as feeling tired due to being overweight. This makes testing - NOT GUESSING pretty important!
Some other common myths include:
“You can’t get too much.”
Vitamin D is fat-soluble, meaning it’s stored in your body. Although it's rare, it's possible to get too much Vitamin D, which can cause calcium buildup in soft tissues (hypercalcemia), leading to kidney stones or cardiovascular issues, or a slew of other problems.
“You can get enough from the sun.”
I always found it to be so ironic that my days living in Arizona, one of the sunniest states in the US, produced the lowest Vitamin D levels that I ever had. I was outside almost daily. Yet, when I tested my levels, they were shockingly low. How could that be?
Sun exposure isn’t enough if you can’t absorb it properly. Factors like gut health, liver function, and magnesium levels affect how well you process Vitamin D.
Sunscreen blocks Vitamin D synthesis. Wearing SPF 30 reduces Vitamin D production by 95-98% (don’t tell my dermatologist)!
Skin pigmentation matters. People with darker skin require 3-5 times more sun exposure to produce the same amount of Vitamin D as someone with lighter skin.
Latitude makes a difference. If you live above 37°N latitude (e.g., New York, Chicago), your body can’t make enough Vitamin D from the sun during winter months.
Even those in sunny states like Florida, Texas, and Arizona can struggle with low levels due to lifestyle habits—staying indoors, working office jobs, or slathering on sunscreen year-round.
If sunshine was all it took, Vitamin D deficiency wouldn’t be a concern in warm climates—but it is.
“You can get enough from food alone.”
Some foods contain Vitamin D, such as fatty fish, egg yolks, and fortified dairy, but they provide only small amounts. For example, you’d need to eat about 10 servings of salmon per week to meet optimal Vitamin D levels! Not only is consuming this much salmon in a week’s time unattainable, it’s not safe. Salmon is not recommended more than twice a week due to levels of mercury it contains, a particularly important factor for pregnant women who are only recommended to consume 8-12 ounces of high mercury fish in a week’s time (2).
“If I take a multivitamin, I’m covered.”
Most multivitamins contain only 400-800 IU, far below what many people need. Plus, Vitamin D absorption depends on fat intake, which brings me to my next point…
“The fat-free craze didn’t impact Vitamin D levels.” (Spoiler: It did.)
In the 1990s, the fat-free diet craze took over, convincing millions of people that all fat was bad. But Vitamin D is fat-soluble, meaning it needs dietary fat to be properly absorbed.
By stripping fat from foods (and replacing it with sugar, processed salt and additives), we unintentionally made it harder for our bodies to absorb Vitamin D. Could the aftermath of a fat-free nation be one of the real reasons why we see widespread Vitamin D deficiency today?
Regardless, here’s what you need to know about Vitamin D, why it’s essential, and how to optimize your levels.
Vitamin D - as a Vitamin
Vitamin D has a lot of jobs, but one of its main roles is to help absorb and maintain calcium and phosphorus. For example, with Vitamin D’s help, calcium is absorbed in the small intestine. Without it - the body only absorbs about 10-15% of dietary calcium, whereas with sufficient Vitamin D, absorption can increase to 30-40% (3).
While calcium gets the most attention, Vitamin D also plays a key role in absorbing phosphorus, a mineral essential for bone structure and energy production.
And because of these jobs, Vitamin D is essential in preventing bone diseases in adults and children, as well as osteoporosis.
Vitamin D - as Hormone—Here’s Why
While the name “vitamin” is literally in the name “Vitamin D,” it’s so much more than that and here’s why. Most vitamins act as cofactors in enzymatic reactions—meaning they help the body perform chemical processes but don’t send direct signals to cells. In contrast, hormones are signaling molecules that regulate biological functions by binding to receptors and triggering responses.
Vitamin D acts as a hormone because:
✔ It is produced by the body rather than solely obtained from food.
✔ It binds to receptors in nearly every cell, influencing gene expression.
✔ It regulates essential functions like calcium balance, immune response, and cell growth.
How Vitamin D Becomes a Hormone
Unlike traditional vitamins that the body cannot synthesize, Vitamin D is made in the skin when exposed to sunlight. From there, it undergoes multiple transformations to become a fully active hormone that regulates calcium, immunity, and much more.
Step 1: The Body Produces Vitamin D from Sunlight
When UVB rays hit the skin, they convert a cholesterol precursor (7-dehydrocholesterol) into Vitamin D3 (cholecalciferol).
This process makes Vitamin D unique among “vitamins” because the body creates it just like it creates hormones.
Step 2: The Liver Converts Vitamin D Into a Storage Form
Once in the bloodstream, the liver converts Vitamin D3 into 25-hydroxyvitamin D (25(OH)D)—the main circulating form of Vitamin D.
This is the form that doctors measure in blood tests to assess your Vitamin D levels.
Step 3: The Kidneys Activate Vitamin D Into a Hormone
The kidneys convert 25(OH)D into 1,25-dihydroxyvitamin D (1,25(OH)₂D), also called calcitriol. Calcitriol is the active, hormonal form of Vitamin D.
How Does Vitamin D Act Like a Hormone?
Once Vitamin D is converted into calcitriol, it functions just like other hormones by binding to Vitamin D receptors cells. When calcitriol binds to these receptors, it turns genes on or off, influencing over 200 different genes that control bone health, immune function, and even mood regulation.
This is why Vitamin D is so much more than just a vitamin—it’s a powerful hormone that influences nearly every system in the body!
Because of its widespread influence, Vitamin D deficiencies are linked to immune dysfunction, depression, muscle weakness, and chronic diseases like diabetes and heart disease.
How to Optimize Your Vitamin D Levels
Get Tested
Before supplementing, it’s best to check your 25(OH)D levels through a blood test. Consult your primary care practitioner and ask for a lab order to get levels tested if you have not had them checked in 1 year. Or - contact our office to be seen and we can test it for you!
Keep in mind that conventional ranges for Vitamin D are listed below, but functionally it is best to optimize levels between 50-70 ng/ml. Those with autoimmune conditions may benefit from levels closer to 80 ng/ml (3).
Optimal range: 40-60 ng/mL
Deficient: Below 30 ng/mL
Toxicity risk: Above 100 ng/mL
Get Vitamin D from the Sun (If Possible)
Best time for sun exposure: 10 a.m. – 3 p.m.
Expose skin without sunscreen for 10-30 minutes, depending on your skin type and location.
Get Vitamin D from your food
As mentioned earlier, optimizing Vitamin D levels through food alone can be challenging since very few foods naturally contain significant amounts. However, this doesn’t mean that you shouldn’t have Vitamin D foods on your radar, or that they won’t help you maintain adequate levels.
Natural sources of Vitamin D include: Fatty fish like salmon, mackerel, sardines and tuna. Aim for wild-caught as they tend to have higher levels than farm-raised. Cod liver oil, available in liquid or capsule form, eggs yolks and beef liver.
Fortified foods include: Dairy products like milk and yogurt. Some plant-based milks contain added Vitamin D such as soy, almond and oat. Breakfast cereals have been fortified with Vitamin D nearly since their inception, however, also can contain additional ingredients that are highly inflammatory like added sugars and preservatives. This makes leaning on them as a good source of Vitamin D less of a viable option.
Be diligent in how you take it!
For best absorption, pair Vitamin D with healthy fats (avocados, nuts, olive oil, ghee). Take in the morning as sunlight helps synthesize it. If you happen to miss the morning time, take it at the next possible time. Consistency is more important than perfection!
Considerations
The Relationship Between Vitamin D and Other Minerals
Vitamin D doesn’t work alone—it interacts with key minerals like calcium, magnesium, and Vitamin K2 to regulate important bodily functions.
Magnesium is essential for converting Vitamin D into its active form in the body.
Vitamin D increases Magnesium demand; as Vitamin D levels rise, more Magnesium gets used up, potentially depleting available stores.
Vitamin K2 ensures that calcium is directed to bones and not deposited in arteries.
Calcium absorption depends on Vitamin D, but too much Vitamin D without K2 can lead to arterial calcification.
This is why a balanced approach is necessary when supplementing and why supplementing blindly can be a dangerous game of Russian roulette!
My Favorite Vitamin D Supplements and Why
When it comes to Vitamin D supplementation in higher doses (5,000 IU’s and above) it’s always recommended to get your levels tested. If you need a higher dose, work with a practitioner like a Registered Dietitian Nutritionist (PS- that’s us) to determine the right amount and product for you.
That said - if you’re looking for a high quality low-moderate daily dose, you’ll find my top picks below! They provide safe and effective support. Plus, when you purchase through this link, you’ll receive our discount!
Combines D3 and K2 for better calcium regulation
Liquid form for better absorption
1,000 IU
Sourced from sustainably harvested fish oil
Easy-to-chew gummy, suitable for adults and children
Low sugar option
1,000 IU
Easy absorbable capsule
Slightly higher dosage at 2,000 IU’s
Doesn’t contain Vitamin K for those taking medications that contradict with it
Ensures proper Vitamin D activation
Great for muscle relaxation and bone health
High-quality, hypoallergenic and vegetarian formula
Conclusion
Vitamin D is so much more than just a vitamin—it’s a powerful hormone that influences immunity, bone health, metabolism, and even mental health. Advocate for your right to be tested and seek guidance in supplementation. Staying on top of Vitamin D is a critical part of your healthcare plan!
Do you take Vitamin D supplements? Have you noticed a difference in your energy, immunity, or overall health? Let me know in the comments!
Or - if you’re unsure where to start with Vitamin D and need more comprehensive support, we're here to help! We now accept insurance, making it easier for you to get the guidance you need. Apply to work with us here!
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Hi, I'm Amanda!
Functional weight loss coach, registered dietitian nutritionist and mom of one picky eater with multiple food allergies! I'm a big believer that if you can learn to fish -- you can eat for life! So this blog is dedicated to helping you learn to toss out the food myths that result in unnecessary food stress so that you can become a confident, healthy eater!
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