Vitamin D is one of those nutrients I talk about all the time, and for good reason. It supports bone health, immune function, hormone balance, mood, and even plays a role in long-term disease prevention.
But here’s what most people don’t realize: There are actually two forms, vitamin D2 and vitamin D3. And the form you take matters more than you might think.
One is much more effective at raising and maintaining vitamin D levels in the body, while the other may be less helpful, and in some cases, can even interfere with your active D3 levels.
If you’re taking vitamin D (or thinking about it), this is an important distinction to understand.
In this article, I’ll cover:
- What the research discovered about vitamin D2
- The difference between vitamin D2 and D3
- The benefits of optimizing your vitamin D blood levels from a functional medicine perspective
- Why choosing vitamin D3 is essential for your future health
Vitamin D2 and D3: Not the Same
Vitamin D comes in two main supplemental forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol).
Vitamin D2 is derived from plants and fungi, such as yeast or UV-exposed mushrooms. It’s also been used for decades in food fortification.
If you ever receive a prescription for vitamin D deficiency, it’s likely ergocalciferol (D2).
The prescribed name is Drisdol®, typically in 50,000 IU capsules that patients take once a week for a limited time to correct the deficiency.
Vitamin D3, on the other hand, is the form your body naturally produces when your skin is exposed to sunlight. It’s the only form that I recommend to my patients (as a supplement).
Vitamin D3 Cholecalciferol (D3) can be prescribed in some forms, but it’s less common.
For many years, D3 was only available from animal-derived sources such as lanolin (from sheep’s wool) or fish oil, which meant vegetarians and vegans would turn to D2 as their only option.
That is no longer the case. Today, vitamin D3 is also made from lichen or algae as a vegan option.
What the Research Shows
A recent study published in Nutrition Reviews delivered an important warning about vitamin D2. The study found that supplementing with vitamin D2 (whether in supplement or prescribed form) actually reduced levels of vitamin D3 in the blood.
This means that when people took vitamin D2, their circulating concentration of 25-hydroxyvitamin D3 [25(OH)D3] dropped, in some cases even below the levels of those who didn’t supplement at all.
This demonstrates that vitamin D2 not only fails to provide the same benefits as vitamin D3, but it also may actively interfere with your body’s natural vitamin D levels.
This is the big difference between vitamin D2 and D3 – they act very differently in the body.
Vitamin D3 is the form that activates a key part of your immune system called type I interferon signaling. This is the pathway that is like a built-in alarm system for your body, helping you fight off viruses and bacteria before they take hold.
D3 also connects directly with your cells. Every single cell in your body has receptors for vitamin D, and when D3 binds to those receptors, it switches on genes that lower inflammation and strengthen immune defenses.
Vitamin D2 does not have these same effects.
Even more compelling, a large meta-analysis found that only vitamin D3 supplementation, not D2, was associated with reduced cancer mortality and lower all-cause mortality.
Could You Be Consuming Vitamin D2 From Fortified Foods?
Vitamin D3 is naturally found in only a small number of foods, which makes deficiency quite common, especially for those who avoid animal products or have limited sun exposure.
The richest natural sources include:
- Fatty fish such as salmon, sardines, mackerel, and tuna
- Cod liver oil (one of the most concentrated sources)
- Egg yolks
- Liver and other organ meats
- Dairy products -milk, butter, and cheese from grass-fed animals contain only small amounts.
Even with dietary sources, it’s difficult to reach adequate vitamin D levels through food alone.
Sunlight exposure is the most natural way to help the body synthesize vitamin D, though many factors like sunscreen use, time of year, latitude, and skin tone affect how efficiently this happens.
For most people, the best approach is a combination of responsible sun exposure, a nutrient-rich diet, and supplementation when needed, guided by lab testing to ensure that your levels are in an optimal range.
Because natural food sources are limited, the food industry fortifies processed foods with vitamin D2 (not D3).
Common examples of foods that are fortified with D2 include:
- Fortified plant-based milks (almond, oat, soy, coconut)
- Fortified cow’s milk and yogurts
- Fortified orange juice
- Fortified cereals and oatmeal
- Some margarines and plant-based spreads
This was largely a manufacturer’s decision of convenience. D2 is cheaper to produce, more stable during food processing, and for many years, it was the only option for vegetarian or vegan diets.
Knowing what we know today, fortifying with D2 does not raise vitamin D levels in the blood and does not provide the same health benefits as fortifying with D3, and in some cases, it could even be counterproductive.
Functional Medicine and Vitamin D Levels
You may go to your annual doctor visit, and you’re told that your vitamin D levels are within range.
“In range” means sufficient, which means that your D levels could be as low as 30 ng/mL.
In Functional Medicine, we look at what is optimal for health, and recommend maintaining blood serum 25-hydroxyvitamin D [25(OH)D] levels between 50 and 80 ng/mL to support overall wellness and health. Functional medicine considers these numbers optimal, while 30 ng/mL is sub-optimal.
When your vitamin D levels are truly optimal, your immune system is better equipped to support your overall health, not just fend off illness.
Vitamin D plays a role in many key processes, including:
- Supporting a balanced immune response and helping calm excess inflammation
- Improving how immune cells communicate and function
- Strengthening the body’s natural antimicrobial defenses
- Helping prevent the immune system from becoming overactive, which is why adequate vitamin D status is associated with both resilience against infections (including respiratory illnesses) and support for autoimmune balance
Beyond immunity, vitamin D also supports:
- Bone density and skeletal health
- Cognitive and brain health
- Hormone balance
- Protection against chronic disease
One important point I often share with patients: being told your vitamin D is “normal” doesn’t necessarily mean it’s optimal. The goal is to aim for levels that truly support health, not just avoid deficiency. That means supplementing thoughtfully, monitoring blood levels, and reassessing to be sure we’re in the optimal range.
When Vitamin D Levels Get Too High
Although rare, it is possible to get too much vitamin D, especially if high-dose supplements are taken for long periods without monitoring.
Vitamin D is fat-soluble, meaning that when excess amounts are stored in the body, rather than flushed out. This can lead to a condition called vitamin D toxicity or hypervitaminosis D.
Also, when vitamin D levels get too high, calcium absorption can go into overdrive. This can cause hypercalcemia (too much calcium in the blood), which may lead to nausea, vomiting, constipation, kidney stones, kidney damage, or, in severe cases, calcium deposits in the heart and blood vessels.
The good news is that this is very uncommon and usually only happens with extreme, prolonged over-supplementation, not with healthy sun exposure, food sources, or reasonable supplement doses.
By checking blood levels regularly and adjusting doses as needed, we can safely keep vitamin D in the optimal range (50–80 ng/mL), where the benefits are strongest, and risks remain minimal.
This is why I recommend to test, not guess.
When Low Vitamin D Means Something More
Low vitamin D can be a signal of deeper imbalances in the body that need to be addressed.
Chronically low vitamin D levels have been linked to leaky gut, autoimmune conditions, mood changes, weakened immunity, and hormonal imbalance.
In functional medicine, we look for why vitamin D remains low (with supplementation), which is typically more complex than just intake.
Here are some common underlying factors that can keep vitamin D from working optimally:
- Magnesium Deficiency – Magnesium is required to convert vitamin D into its active form in the body. Without it, vitamin D remains biologically inactive, meaning even if your blood test looks “normal,” your body may not actually be using it effectively. Over 60% of adults are deficient in magnesium, making this one of the most common hidden causes of low vitamin D function.
- Boron Deficiency. Boron is another underappreciated player in vitamin D metabolism. It helps regulate vitamin D activity and retention by slowing down its breakdown. Boron also supports testosterone, bone density, and calcium balance. Without enough boron, vitamin D is less stable and less effective.
- Cholesterol Levels. Vitamin D is synthesized from cholesterol in the skin after UVB exposure. If cholesterol is too low, your body lacks the raw material needed to make vitamin D. Low cholesterol doesn’t just impair vitamin D status; it also impacts hormone and bile acid production, both of which are essential for health and nutrient absorption.
- Oxidative Stress and Inflammation. High levels of oxidative stress accelerate the breakdown (catabolism) of vitamin D. Sources of oxidative stress include toxins, poor sleep, infections, gut imbalances, and poor diet. Chronic stress also speeds vitamin D turnover, leaving less available for the body to use. In these cases, the problem isn’t a lack of intake, but faster depletion.
- Liver and Kidney Health. Vitamin D has to be activated before your body can use it.
Your liver turns it into a stored form (what shows up on blood tests), and your kidneys turn it into the active form your cells actually use. If either the liver or kidneys are under stress, this process can slow down. That’s why vitamin D can look “normal” on labs but still not work effectively in the body.
- Genetic Variations. Some people carry genetic differences in the vitamin D receptor (VDR gene), making it harder for their cells to use vitamin D even when blood levels look fine.
- Obesity or High Body Fat. Because vitamin D is fat-soluble, it can become “trapped” in fat tissue, leaving less available for circulation and cellular use.
- Thyroid Dysfunction. Low thyroid function slows metabolism and conversion pathways, reducing the body’s ability to activate vitamin D.
- Parathyroid Hormone (PTH) Imbalance. PTH works closely with vitamin D and calcium. When it’s out of balance, vitamin D metabolism and utilization are affected.
- Medications. Certain drugs increase vitamin D breakdown, including anticonvulsants, glucocorticoids (such as prednisone), HIV medications, and fat-blocking drugs like orlistat.
- Aging. As we age, the skin produces less vitamin D from sunlight, and the kidneys become less efficient at converting it into its active form.
- Gut Microbiome Imbalance. Beyond leaky gut, imbalances in the gut microbiome can interfere with nutrient absorption, including vitamin D and its cofactors.
- Chronic Illness Load. Conditions like diabetes, autoimmune disease, and persistent infections increase the body’s demand for vitamin D, leading to faster turnover and depletion.
Low vitamin D is often a signal of deeper imbalances.
Before starting vitamin D3 supplementation, it’s important to consider: What else might be influencing your vitamin D status?
Addressing nutrient deficiencies, balancing the microbiome, healing leaky gut, supporting liver and kidney health, lowering inflammation, and balancing stress all improve how your body activates and uses vitamin D.
This is why I encourage testing, not just for vitamin D levels, but also looking at magnesium status, inflammation markers, and overall micronutrient status and balance.
Why Vitamin D3 Matters More During Cooler Months
Sunlight is one of the most natural and effective sources of vitamin D3. When your skin is exposed to UVB rays, it produces D3 on its own.
But during the fall and winter months, that process slows dramatically. Shorter days, less direct sunlight, colder weather that keeps us covered up, and factors like latitude, skin tone, and sunscreen use all limit how much vitamin D3 our bodies can make naturally.
That’s why supplementation becomes so important in the cooler months. Optimal vitamin D3 levels are protective against seasonal infections such as colds and flu. This is also the time of year when most of us spend more hours indoors, and holiday travel often means crowded environments and greater exposure to viruses. Keeping your vitamin D3 levels in the optimal range can help strengthen your defenses when you need them most.
If you’ve been prescribed vitamin D, check which type it is. As mentioned earlier, many prescriptions are written for ergocalciferol (vitamin D2), which research shows is less effective and may even lower D3 levels in the body.
To truly support your immune system, bones, heart, and brain, vitamin D3 is the preferred and most bioactive form of vitamin D.
When it comes to dosage, individual needs vary. For many adults, 5,000 IU daily can be effective and well tolerated, though some may require more or less depending on absorption, genetics, diet, and sun exposure.
The safest and most accurate approach is to measure your blood levels (25-hydroxyvitamin D) with your healthcare professional and adjust supplementation to maintain optimal levels.
Vitamin D3 is the only form I recommend to patients, either with or without vitamin K2.
When I recommend vitamin D3, I often pair it with vitamin K2 because of the way these two nutrients work together. Vitamin D3 helps your body absorb calcium, while K2 directs that calcium into the bones and teeth instead of letting it build up in blood vessels. This partnership supports both strong bones and cardiovascular health.
However, vitamin K2 is not appropriate for everyone. If you are taking blood-thinning medications, such as warfarin (Coumadin®) or similar drugs, vitamin K2 can interfere with how these medications work. In these cases, D3 without K2 is the safer option. This is why it’s so important to individualize supplement recommendations and work closely with your healthcare provider before starting or adjusting any regimen.
In addition to being immune-boosting and protective, optimal vitamin D levels help maintain strong bones and reduce the risk of osteoporosis and fractures as we age.
Research is also connecting vitamin D3 to cancer prevention and longevity.
Best Paleo Food Sources of Vitamin D3
While supplementation is often necessary to reach and maintain optimal vitamin D levels, it’s also worth noting that you can get vitamin D from food as well.
For those following a Paleo diet, nutrient-dense foods are a great way to boost vitamin D levels. Unlike many modern fortified products that use vitamin D2, whole-food sources provide natural vitamin D3.
Including these foods in your diet can help support your vitamin D status, especially when combined with safe sun exposure.
- Fatty fish (especially wild-caught): salmon, mackerel, sardines, anchovies, tuna (limit to once a week due to high mercury content)
- Cod liver oil (traditional, nutrient-dense source — also rich in vitamin A and omega-3s)
- Egg yolks (from pasture-raised or omega-3-enriched hens have the highest levels)
- Grass-fed liver and organ meats (contain smaller amounts, but still beneficial)
- Pasture-raised beef and pork fat (minimal, but naturally occurring D3)
- Shellfish like oysters and shrimp (modest amounts, plus added minerals like zinc)
If you need inspiration for healthy meals to cook at home, click here for my collection of nutritious paleo recipes (all are gluten and dairy-free).
Final Thoughts
Vitamin D is essential for health, but the form matters. Vitamin D2 is still widely prescribed and added to foods, yet research shows it may lower blood levels of the very nutrient we depend on for immune protection, bone strength, and disease prevention.
Vitamin D3, in contrast, is the body’s natural form and the one consistently shown to deliver real health benefits.
As always, work with your healthcare professional, check your levels regularly, and choose supplements that provide the form your body truly needs. By prioritizing Vitamin D3, you can better support your immunity, protect your bones, and safeguard your health well into the future.
References:
Vitamin D’s Effect on Immune Function
Role of Vitamin D Beyond the Skeletal Function: A Review of the Molecular and Clinical Studies
