What You Need to Know About How Your Body Really Uses Vitamin D
Vitamin D is one of the most talked-about nutrients today, and for good reason! It plays a critical role in immunity, hormone balance, bone health, mood, and even metabolic function. But one common question we hear is:
Can’t I just take a supplement instead of getting sunlight?
The truth is: vitamin D from a capsule isn’t quite the same as the vitamin D your body makes from the sun. Both can support healthy levels, but they’re processed differently by the body, and not everyone absorbs or uses vitamin D the same way.
If you’ve been supplementing and still feel off… or you’re unsure whether you even need a supplement, this article will walk you through what matters, what to watch out for, and when to seek expert support.
How the Body Actually Makes Vitamin D
Vitamin D isn’t just a vitamin, it acts more like a hormone. And it doesn’t come directly from food or supplements alone.
When sunlight, specifically UVB rays, hits your skin, it starts a chemical reaction that turns a compound called 7-dehydrocholesterol into cholecalciferol (vitamin D₃). This is the same form often found in supplements.
From there, the body still needs to activate it in two steps:
- In the liver, cholecalciferol is converted to 25-hydroxyvitamin D, the form most commonly measured in blood tests.
- Then in the kidneys (and other tissues), it’s converted again to 1,25-dihydroxyvitamin D, the biologically active form that binds to cells and supports essential functions.
This conversion process relies on healthy liver and kidney function, as well as other nutrient co-factors.
What About Sunscreen?
Sunscreen is designed to block UVB rays and those are the very rays your skin needs to make vitamin D. For example, an SPF 30 sunscreen can reduce your body’s ability to produce vitamin D by up to 95–98%.
That doesn’t mean you should avoid sunscreen altogether. But if you use it regularly, wear long sleeves, or spend most of your time indoors, it’s worth considering how much vitamin D you’re actually making from the sun and whether your current levels reflect your needs.
This is one of the first things we assess when determining whether supplementation or testing is appropriate.
Vitamin D Supplements: What Form Matters
There are two main forms of vitamin D available in supplements:
- Vitamin D₂ (ergocalciferol): Plant-based and less effective at raising blood levels. It’s sometimes used in prescriptions but not ideal for long-term use.
- Vitamin D₃ (cholecalciferol): The same form your body makes from sunlight. It’s more stable, better absorbed, and more effective at maintaining healthy levels.
Most high-quality over-the-counter supplements contain D₃. However, even if you’re taking D₃, it doesn’t guarantee your body is using it properly, especially if there are underlying issues with absorption, co-nutrient status, or genetics.
Why You Might Be Taking Vitamin D But Still Feel Off
We may see clients who’ve been taking a vitamin D supplement, yet their blood levels remain low, or they still experience symptoms like fatigue, mood changes, or low immunity.
Here’s why that happens:
- You may not be absorbing it well – Vitamin D is fat-soluble, so gut health and fat digestion are key.
- You may not be converting it well – The conversion process in the liver and kidneys can be affected by nutrient deficiencies, inflammation, or certain medications.
- You may have genetic variations – Your body’s ability to use vitamin D at the cellular level may be impacted by your DNA.
This is where personalization matters most.
If you’ve been supplementing without results, we can help you uncover the root of the issue.
Genetics and Vitamin D
Small changes in your DNA, called single nucleotide polymorphisms (SNPs), can significantly affect how your body absorbs, converts, and uses vitamin D.
Here are some key ones we look for:
- VDR (Vitamin D Receptor): These SNPs affect how well your cells can respond to activated vitamin D. Even with normal blood levels, someone with a VDR variation might not benefit fully from it.
- CYP2R1: This gene is involved in the conversion of vitamin D to its circulating form in the liver. Variants in CYP2R1 may reduce how efficiently your body converts vitamin D from sunlight or supplements.
These SNPs don’t mean something is “wrong”, but they help us understand why your levels might not reflect your actual vitamin D status, and what kind of support your body may need.
Our practice uses targeted genetic testing and interpretation to personalize your approach to vitamin D, ensuring that we address the full picture, not just the numbers.
Co-Nutrients That Make a Difference
Vitamin D doesn’t work alone. It relies on several key nutrients to be absorbed, activated, and used efficiently in the body:
- Magnesium – Essential for converting vitamin D into its active forms. Without enough magnesium, vitamin D supplementation may not be effective. There are many different forms of magnesium, like magnesium glycinate, citrate, malate, and threonate, and the right form depends on your unique needs. Some support sleep and relaxation, while others may support energy or digestion. Most people are deficient in magnesium due to soil depletion, stress, certain medications, and diets low in whole, unprocessed foods.
- Vitamin K2 – Directs calcium into the bones and teeth instead of soft tissues like arteries. This is crucial when supplementing with higher doses of vitamin D.
- Omega-3 fatty acids – Support cell membrane function and may enhance the activity of vitamin D receptors.
We often see better results when these nutrients are optimized alongside vitamin D, not in isolation.
How Much Vitamin D Do You Need?
There’s no universal answer. What’s right for one person may not be enough, or may be too much for someone else.
Some of the key factors that influence your needs include:
- Your current vitamin D status
- Skin tone and geographic location – Darker skin requires more sun exposure to produce the same amount of vitamin D as lighter skin.
- Time spent outdoors and clothing/sunscreen habits
- Gut health and fat absorption – Since vitamin D is fat-soluble, any issues with digestion or absorption can impact levels.
- Current health status – People with immune system dysfunction, autoimmune conditions, or chronic inflammation may require higher levels to support immune resilience.
- Age – As we age, our skin becomes less efficient at synthesizing vitamin D, and kidney function (important for activation) may decline.
- Nutrient intake – Especially magnesium and vitamin K2, which support vitamin D metabolism.
- Genetic variations (SNPs) – Certain variants can affect how vitamin D is transported, activated, or used at the cellular level.
When to Get Help
If you’ve been taking vitamin D and still feel off… or if you’re unsure whether you even need a supplement… it’s time to take a more strategic approach.
In our functional nutrition practice, we guide clients through:
- Comprehensive testing (including nutrient status and genetics)
- Understanding why current levels may not reflect true needs
- Reviewing supplement forms and dosages
- Identifying hidden factors that interfere with absorption or conversion
- Building a sustainable plan that works long-term
We’d love to help you move beyond guesswork. Reach out to schedule a session and get the clarity you deserve!
