The Effects of Vitamin D Deficiency on the Immune System
If you’re reading this right now feeling run down, inflamed, extra achy, extra tired, or you’re in the middle of a flare after the stress of the season… I see you!
And yes — I know the winter immune “go-to” is usually vitamin C and zinc. Those can absolutely be supportive. But there’s one nutrient that is consistently overlooked, especially in winter months, and it plays a much deeper role than most people realize: Vitamin D!
I know what you’re thinking. That’s just for bone health isn’t it? No my friend. It’s not just for bones. Not just for mood. It’s for immune function. For inflammation. For autoimmune support. For the body’s ability to respond appropriately to stress, viruses, and chronic triggers.
So let’s break this down in a way that feels grounded, hopeful, and actually useful — with enough science to build your confidence, but not so much that your eyes glaze over and you fall asleep on me. 🤣
Vitamin D Isn’t “Just a Vitamin” — It Acts Like a Hormone
Vitamin D is technically a fat-soluble vitamin, but functionally it behaves like a hormone in the body, which really is kind of wild to me!
Here’s why that matters to you:
When your body has enough vitamin D, it can convert it into an active hormone-like form that communicates with cells through something called the vitamin D receptor (VDR). And the wild part?
The VDR is found throughout your body — including on many immune cells. So when vitamin D is low, it doesn’t just impact one thing. It can ripple through multiple body systems.
A quick (but important) overview of vitamin D activation:
Most people think vitamin D is simple: go in the sun, take a supplement, boom — you’re good. But the truth is, vitamin D is one of those nutrients that requires your body to do a whole chain of steps correctly in order to actually use it. And if any step in that chain is compromised, you can be doing “all the right things” and still end up with low levels — or levels that look okay on paper but aren’t functioning well in the body.
Here’s the process in a way that actually makes sense:
Step 1️⃣: Your body has to GET vitamin D (sunlight, food, or supplements)
There are a few ways vitamin D enters your system:
- ⚫ Sun exposure (UVB rays): Your skin produces vitamin D when UVB hits it. This is why it’s called the “sunshine vitamin.”
- ⚫ Food sources: Fatty fish, egg yolks, cod liver oil, and fortified foods can contribute (but it’s hard to rely on food alone).
- ⚫ Vitamin D supplements: Usually vitamin D3 is the preferred form.
But here’s the catch: even if you get vitamin D in, your body still has to activate it.
Step 2️⃣: Your liver converts it into the “storage form”
Once vitamin D enters your body, it goes to the liver. Your liver converts it into a form called 25-hydroxyvitamin D, also written as 25(OH)D. This is the number we typically measure on a blood test when your doctor says, “Your vitamin D looks fine.”
So if you’ve ever had labs run and your vitamin D was low, this is the step where the issue shows up most clearly. But — and this is a big but — this is the storage form, not the active form. Think of it like having money in your bank account… but not being able to access it.
Step 3️⃣: Your kidneys convert it into the ACTIVE hormone form
After the liver step, vitamin D goes through another conversion — mostly in the kidneys — into its active form called 1,25-dihydroxyvitamin D, or calcitriol.
This is the form your body actually uses like a hormone. This is the form that can bind to vitamin D receptors (VDR) throughout the body — including on immune cells — to help regulate:
- ⚫ immune responses
- ⚫ inflammation and cytokine activity
- ⚫ antimicrobial defenses
- ⚫ calcium balance
- ⚫ gene expression
- ⚫ and more
So it’s not enough to “have vitamin D.” You need to be able to convert it and activate it, then your cells need to be able to respond to it.
Why Vitamin D Is Crucial for Immune Health (Not Just Winter Colds)
Your immune system isn’t supposed to be “on” all the time — and it’s not supposed to be asleep either. A healthy immune system is like a well-trained guard dog:
- ⚫ Alert when it needs to be
- ⚫ Calm when it doesn’t
- ⚫ Able to identify what’s dangerous
- ⚫ Able to stand down when the threat is gone
Vitamin D helps regulate that balance.
Vitamin D supports BOTH major branches of immunity:
- ⚫ Innate immunity = your body’s first line of defense
- ⚫ Adaptive immunity = your learned, targeted immune response
Let’s unpack both.
Vitamin D and Innate Immunity (Your Body’s First Line of Defense)
Innate immunity is your immune system’s “front door security.” It includes:
- ⚫ epithelial cells (barrier cells in the respiratory tract, gut lining, skin)
- ⚫ macrophages
- ⚫ natural killer cells
- ⚫ certain antimicrobial compounds that help fight invaders quickly
Vitamin D helps strengthen this front line in a few key ways:
1) Supports barrier protection in the respiratory tract and gut
Your immune system can’t do its job if your barriers are weak. The respiratory tract and gut are both “high traffic” areas — and vitamin D helps support the integrity of those barrier cells so pathogens have a harder time getting through.
2) Supports antimicrobial defense
Vitamin D helps stimulate compounds like antimicrobial peptides (think: built-in microbial defense) that may help your immune system respond to viruses and bacterial infection more effectively. This is one reason low vitamin D levels are repeatedly associated with increased risk of respiratory infections and respiratory tract infections.
Vitamin D and Adaptive Immunity (Where Autoimmunity Comes In)
Adaptive immunity is more complex. It involves:
- ⚫ T cells
- ⚫ B cells
- ⚫ dendritic cells (these are antigen-presenting cells)
- ⚫ immune “memory” and targeted response
This is the branch that learns and adapts — and it’s also the branch that can go off track in autoimmune disease.
Vitamin D influences how the adaptive immune response behaves, including how aggressively it responds and whether it stays regulated. When vitamin D is low, the immune system may be more likely to lean inflammatory, reactive, and dysregulated — which can matter a lot if you’re dealing with chronic illness or autoimmune tendencies.
Vitamin D Deficiency and Autoimmune Diseases
This is one of the biggest reasons I care so much about vitamin D in my work. In functional practice, I regularly see low vitamin D levels in people navigating conditions like:
- ⚫ rheumatoid arthritis
- ⚫ systemic lupus erythematosus
- ⚫ inflammatory bowel disease (including Crohn’s disease and ulcerative colitis)
- ⚫ autoimmune thyroid patterns
- ⚫ and many other autoimmune disorders
Is vitamin D the only factor? No. But is it often a key missing piece that makes symptoms worse and immune regulation harder? Absolutely.
Low vitamin D status has been associated in numerous studies with:
- ⚫ higher autoimmune risk
- ⚫ increased disease activity
- ⚫ increased inflammation
- ⚫ worsened quality of life
And if you’ve been in chronic survival mode for years — inflamed, exhausted, reactive to food, sensitive to stress, and constantly “on edge” — vitamin D is one of those foundational supports that can help the body shift toward more regulation.
Why Vitamin D Deficiency Is So Common (Especially in Winter)
Even if you eat well or take supplements sometimes, vitamin D deficiency is incredibly common. Some major risk factors include:
- ⚫ living in a low-sun region (hello winter)
- ⚫ being indoors most of the day
- ⚫ heavy sunscreen use
- ⚫ dark skin (higher melanin reduces vitamin D synthesis)
- ⚫ higher body mass index (vitamin D can become “sequestered” in fat tissue)
- ⚫ gut issues (malabsorption)
- ⚫ inflammatory bowel disease
- ⚫ celiac disease
- ⚫ liver disease
- ⚫ older adults (reduced synthesis and activation)
And here’s the kicker: you can be deficient and not even know it!
Symptoms of Low Vitamin D Levels (What It Can Feel Like)
Vitamin D deficiency doesn’t always come with a flashing warning sign. But here are common symptoms I see in real life:
- ⚫ fatigue (including chronic fatigue patterns)
- ⚫ frequent colds or infections
- ⚫ low mood / seasonal depression
- ⚫ muscle weakness
- ⚫ increased body aches
- ⚫ poor recovery
- ⚫ autoimmune flares
- ⚫ brain fog
- ⚫ sleep disturbances
And for many people with chronic illness, it becomes this vicious cycle: low vitamin D → more immune stress → more inflammation → worse gut health → worse absorption → even lower vitamin D
That’s why testing matters!
How to Test Vitamin D Status (What to Ask Your Doctor For)
If there’s one thing I want you to take away from this blog, it’s this:
Don’t guess. Test.
Ask for a 25-hydroxy vitamin D blood test (this is the standard marker for vitamin D status).
Typical ranges:
- ⚫ <20 ng/mL: deficiency (often severe symptoms)
- ⚫ 20–30 ng/mL: insufficiency
- ⚫ 30–50 ng/mL: “normal” in conventional medicine (but not always optimal)
- ⚫ 50–80 ng/mL: often considered a more functional “sweet spot” for immune health (varies by person)
And yes — you can have “normal” labs in a conventional sense and still be functionally struggling.
How to Get Enough Vitamin D (Sunlight, Food, and Supplements)
Let’s talk options.
1) Sun Exposure (Vitamin D Synthesis)
Sun exposure is the most natural source. But it’s also the hardest in winter — and not reliable if:
- ⚫ you live north of a certain latitude
- ⚫ it’s cloudy, icy, or you’re indoors
- ⚫ you’re wearing long sleeves constantly
- ⚫ you use sunscreen (which is important for skin cancer prevention, but blocks vitamin D production)
Also: there’s a real balance here. We want smart sun exposure without burning — and for people with skin cancer risk, this should be guided carefully.
2) Food Sources (Helpful, but Usually Not Enough)
Vitamin D food sources include:
- ⚫ fatty fish (salmon, sardines, mackerel)
- ⚫ egg yolks
- ⚫ liver
- ⚫ some fortified foods
But most people don’t eat enough of these consistently to raise levels significantly — and if you have gut malabsorption or inflammatory bowel disease, food sources may not be enough.
3) Vitamin D Supplementation
This is where most people actually move the needle — especially in winter. But supplement quality and form matter.
Vitamin D Supplements: D2 vs D3
There are two common forms:
- ⚫ Vitamin D2 (ergocalciferol)
- ⚫ Vitamin D3 (cholecalciferol)
In most cases, D3 is preferred because it raises and maintains levels more effectively (and it’s the form your skin makes from sunshine). So if you’re supplementing, look for vitamin D3.
Why Vitamin D3 Should Be Paired with K2
This is a big one. Vitamin D increases calcium absorption. That’s great… as long as calcium is being directed to the right place. Vitamin K2 helps direct calcium into bones and teeth (where you want it), and away from soft tissue like arteries. This is why D3 + K2 is often recommended together, especially if you’re taking moderate to higher doses.
Think of it like this:
- ⚫ D3 opens the gate for calcium to enter the body
- ⚫ K2 acts like a traffic cop telling calcium where to go
This matters for:
- ⚫ bone health
- ⚫ cardiovascular support
- ⚫ long-term safety
How Much Vitamin D Do You Need? (International Units + Real Talk)
This depends on your:
- ⚫ blood test results
- ⚫ body size
- ⚫ absorption
- ⚫ sun exposure
- ⚫ health status
- ⚫ autoimmune burden
That said, many people in winter need more than they think. General common ranges used in practice:
- ⚫ maintenance: 1,000–2,000 IU daily for some
- ⚫ many adults: 2,000–5,000 IU daily
- ⚫ higher doses may be used short-term for deficiency under guidance
The most important thing: work with a healthcare provider, and re-test after 8–12 weeks. Because vitamin D is fat-soluble, it can accumulate. Too much vitamin D can cause issues, especially if someone is taking mega doses without testing!
Vitamin D and Gut Health (This Is Bigger Than Most People Realize)
If you’re one of my chronic illness clients, you already know the gut is a big deal. Vitamin D supports gut health in multiple ways:
- ⚫ supports the gut barrier (helps reduce permeability issues)
- ⚫ supports immune signaling in gut-associated lymph tissue
- ⚫ influences the gut microbiota
- ⚫ may impact inflammatory patterns in IBD
For individuals dealing with:
- ⚫ inflammatory bowel disease
- ⚫ Crohn’s
- ⚫ ulcerative colitis
- ⚫ food sensitivities
- ⚫ chronic inflammation
…vitamin D status is one of those foundational supports that can influence both symptoms and resilience.
Vitamin D and Respiratory Infections (Winter Reality)
In recent years, multiple human studies and clinical trials have explored vitamin D supplementation and respiratory infection risk.
A consistent trend: those with low vitamin D levels tend to have a higher risk of infections, and supplementation may offer protective effects, especially for people who start out deficient.
This doesn’t mean vitamin D is a magic shield. But it does mean if your levels are low, correcting that deficiency may help support your immune response more effectively.
Vitamin D and Respiratory Infections (Winter Reality)
If you’re reading this and thinking, “Okay… I need to support my immune system but I’m overwhelmed,” here’s a gentle way to start:
- 1) Order a vitamin D blood test (or request it from your doctor)
- 2) Aim for sunlight when possible (even 10 minutes matters)
- 3) Increase vitamin D-rich foods where you can
- 4) Start a quality D3 + K2 supplement if appropriate
- 5) Re-test to confirm you’re improving
- 6) Pair with basics: hydration, protein, sleep, and nervous system support
No pressure. Just progress.
The Vitamin D I Use and Recommend
If you want to take the guesswork out of it, this is the exact type of supplement I use and recommend to my 1:1 clients:
✅ Vitamin D3 + K2
✅ High quality
✅ Easy to take
✅ Great for winter + chronic illness support
Click below to order the vitamin D supplement I personally use and recommend!
Final Thoughts: This Isn’t About Perfection — It’s About Support
Especially if you’ve been sick for a long time, I want you to remember this:
Your body isn’t broken. It’s communicating! And sometimes the most powerful “next step” isn’t a complicated protocol — it’s correcting something foundational that your immune system depends on. Vitamin D is one of those foundations. And if you want deeper support — labs, root cause digging, and a plan that fits your unique body (not generic advice)?
With love and Balance,
Kristie 🖤