In temperate regions, where the sun seldom makes an appearance during the winter months, to keep our bodies healthy, we need to compensate for the lack of sunlight.
In the UK, for example, it is recommended that everyone over the age of four consider taking a daily supplement of vitamin D during the autumn and winter months (October to March) to support immune function, bone health and overall well-being. But are we supplementing with the right vitamin D?
Taking vitamin D2 instead of D3 can actually lower your body’s levels of active vitamin D, leaving you more vulnerable to fatigue, poor immunity and calcium imbalance, Dr. Joseph Mercola warns.
Vitamin D3, the same form your body makes from sunlight, is far more effective at raising and maintaining healthy vitamin D levels than D2, especially in those who are deficient.
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How Taking the Wrong Vitamin D Actually Lowers Your Levels
Many people assume that all vitamin D supplements work the same way, but research shows that taking the wrong form actually makes your deficiency worse. Vitamin D is a fat-soluble hormone-like nutrient required for strong bones, immune balance, hormone regulation and cellular repair.
You make it naturally when your skin is exposed to sunlight, yet millions of people turn to supplements without realising that not all forms of vitamin D behave the same once they enter your bloodstream. Unlike D3, which your skin makes from sunlight, vitamin D2 comes mainly from fortified foods and mushrooms exposed to ultraviolet light.
Because it’s cheaper to produce, D2 is often used in fortified plant-based milks and multivitamins. But once absorbed, it appears to accelerate your body’s disposal of D3, disrupting the natural balance your cells depend on for proper function. Understanding why D2 suppresses D3 is key to knowing which supplement supports your health and which undermines it.
Vitamin D2 Depletes the Active Form Your Body Needs Most
A meta-analysis published in Nutrition Reviews examined 20 randomised controlled trials on vitamin D2 supplementation and its impact on blood levels of vitamin D3, the form your body actually uses.1 Out of these, 11 studies qualified for statistical analysis and included more than 650 participants. The goal was to find out if taking vitamin D2 (ergocalciferol) affects your body’s circulating levels of vitamin D3 (cholecalciferol).
• Study participants taking vitamin D2 experienced a sharp drop in their vitamin D3 levels. Across the combined data, those who took vitamin D2 had about 18 nanomoles per litre (nmol/l) lower concentrations of vitamin D3 than participants in control groups who took nothing. In other words, people trying to boost their vitamin D by taking D2 ended up lowering the active form that actually powers their immune, hormonal and metabolic systems.
• Healthy adults were most commonly studied, but the results were consistent across age and gender. Most trials included healthy men and women, and one involved both young and older males. Regardless of age or sex, the same trend appeared – vitamin D2 supplementation triggered a measurable decline in vitamin D3 status. Even when doses and study durations varied widely, from a few weeks to several months, the downward effect on vitamin D3 levels held steady.
• The drop wasn’t a random fluctuation – it reflected how the body regulates vitamin D metabolism. Researchers believe that vitamin D2 activates enzymes in the liver that accelerate the breakdown of vitamin D3.
Specifically, D2 appears to stimulate a protein responsible for dismantling both D2 and D3 into inactive fragments for excretion. When this enzyme becomes more active, your vitamin D3 doesn’t just fade – it’s chemically destroyed faster, leaving you with less of the form that supports bone, heart, and immune health.
• Vitamin D3 is superior in supporting immune and metabolic functions. Vitamin D3 stimulates genetic pathways that control inflammation, calcium absorption, and hormone regulation, while D2 lacks this same potency.
In earlier research cited by the authors, only D3 was shown to boost the activity of interferons – immune molecules that help your body recognise and fight infections.2 D2 not only failed to produce this benefit but also appeared to suppress the immune response in some cases.
• The findings expose why supplement quality matters more than quantity. Many people buy vitamin D2 because it’s cheaper, vegan-friendly and widely added to processed foods. Yet this review makes it clear that choosing D2 for cost or convenience could work against your health goals. If your goal is to restore energy, strengthen immunity and stabilise mood or hormones, D3 – not D2 – is the form that your body recognises and uses efficiently.
Vitamin D3 Proves Far More Effective Than D2 in Correcting Deficiency
Understanding why vitamin D3 works more efficiently helps you choose a supplement that aligns with your biology rather than fights against it, setting the stage for restoring healthy, lasting vitamin D levels. Vitamin D3 is the same form your body naturally makes when your skin meets sunlight. It’s also found in animal foods like fatty fish, egg yolks and grass-fed dairy.
Because it’s bioidentical to what your body produces, your liver converts it into its active form – 25(OH)D – more efficiently. D2, on the other hand, comes from UV-exposed yeast and mushrooms. Though technically a form of vitamin D, it’s metabolised differently, resulting in lower and less stable blood concentrations. In simple terms, your cells recognise D3 as “homegrown” and handle it far better than the plant-based version.
• A large clinical trial confirmed that vitamin D3 raises blood levels faster and more efficiently than D2. In a randomised controlled trial conducted in the Middle East, 250 adults with vitamin D deficiency or insufficiency were assigned to take either vitamin D2 or D3 once a week for 12 weeks.3 Each capsule contained 50,000 international units (“IU”) – a dosage often prescribed in cases of deficiency.
After just eight weeks, those taking vitamin D3 showed an average increase of 18.74 nanograms per millilitre (ng/ml), compared to only 5.88 ng/ml in the D2 group. By week 12, the gap had widened even more, with the D3 group averaging 20.76 ng/ml while the D2 group rose only to 7.93 ng/ml.
• Deficiency risk is especially high for certain groups. Worldwide, about 1 billion people are vitamin D deficient, with 50% having vitamin D insufficiency.4 However, in some groups, the prevalence is even higher. If you have darker skin, excess body fat or take medications that speed up vitamin D metabolism (like certain anti-convulsants or steroids), your vitamin D stores drop faster.
Those who wear protective clothing, spend little time outdoors or live in institutions are also vulnerable. Chronic gut conditions such as inflammatory bowel disease and celiac disease compound the problem by reducing nutrient absorption. For these people, choosing D3 is not a matter of preference – it’s a matter of effectiveness.
• For those unable to get consistent sun exposure, D3 supplementation fills the gap efficiently. If you live in a region with long winters or simply can’t be outdoors often, vitamin D3 supplementation is often necessary. Its chemical structure matches what your skin naturally produces, ensuring your body recognises and uses it efficiently.
If your goal is to fix a vitamin D deficiency, you’ll get far better results with D3 than D2, even at the same dose. Whether you’re restoring low levels after a long winter or maintaining your levels year-round, D3 gives your body what it already knows how to use.
The Hidden Step Most People Miss – Why Vitamin D3 Still Needs Activation
Even if you’re taking vitamin D3 and your blood test shows healthy levels of 25-hydroxyvitamin D, there’s a critical step that determines whether your cells can actually use it. Before vitamin D becomes biologically active, your body must add two hydroxyl groups to the molecule in a two-stage process that many people’s bodies struggle to complete.
• The first hydroxylation happens in your liver. This converts vitamin D3 into 25-hydroxyvitamin D – the form measured in standard blood tests. But this intermediate form still can’t activate your cells.
The real work begins when 25-hydroxyvitamin D enters your cells and encounters an enzyme called CYP27B1, which adds the second hydroxyl group to create 1,25-dihydroxyvitamin D – the only form that can bind to vitamin D receptors and trigger the genetic changes that support immunity, calcium regulation and metabolic health.
• Here’s the problem: research suggests that up to 50% of people may have impaired CYP27B1 activity. This means their cells struggle to complete this final activation step. You could have perfectly normal 25-hydroxyvitamin D levels on paper, yet still experience symptoms of vitamin D deficiency because your cells can’t convert it into the active form that does the actual work.
• Currently, no commercial test measures intracellular 1,25-dihydroxyvitamin D levels. This leaves this conversion problem invisible to most doctors and patients. Emerging research is exploring ways to deliver the active form directly into cells, bypassing the need for oral vitamin D3 entirely and eliminating overdose risk through natural cellular feedback mechanisms that shut down production when adequate levels are reached.
How to Restore and Maintain Healthy Vitamin D3 Levels
If you’ve been taking vitamin D2 thinking it would fix your deficiency, it’s time to take a different approach. Your body depends on vitamin D3 to regulate calcium, support immunity and sustain energy production. The problem isn’t that you’re low on “vitamin D” in general; it’s that the wrong form of it has disrupted your body’s ability to maintain balance. The goal now is to restore that balance by working with your biology, not against it. Here’s how to do it safely and effectively.
1. Get your vitamin D from the sun whenever possible. Your skin is designed to make vitamin D3 when exposed to direct sunlight. Aim for regular exposure on large areas of skin – such as your arms and legs – without sunscreen daily. It’s important to monitor your skin when you’re out in the sun, keeping an eye out for redness or burning – a simple “sunburn test” that accounts for the season and your skin type. If your skin remains free of pinkness, your sun exposure is likely safe.
2. Cut out seed oils before getting sun exposure during peak hours. If you’re still using canola, soyabean, sunflower or other types of “vegetable” oil, you’re flooding your cells with linoleic acid (“LA”) – a polyunsaturated fat that oxidises under UV light and damages your skin from within. This buildup makes you more prone to sunburn and skin damage, especially during peak hours of 10 am to 4 pm.
Switch to stable animal fats like tallow, ghee or grass-fed butter instead of LA. Avoiding seed oils for at least six months gives your skin time to shed stored LA, helping you tolerate midday sun safely and produce vitamin D naturally again.
3. If you supplement, use vitamin D3 – not D2 – and pair it with magnesium and K2. Vitamin D3 is identical to the form your skin makes from sunlight, while D2 interferes with your natural balance and lowers your D3 levels. When you take D3, include magnesium and vitamin K2 for better absorption and safer calcium regulation.
These three nutrients work together – magnesium activates vitamin D in your body, while K2 directs calcium into your bones instead of your arteries. Research shows that people not taking these co-factors need more than twice as much vitamin D to maintain healthy blood levels, so combining them saves effort and reduces risk.5
4. Test your vitamin D levels twice a year to personalise your plan. Testing your blood levels is the only way to know where you stand. Aim for a range between 60 and 80 ng/ml (150 to 200 nmol/l). If your levels are low, focus on daily sunlight and consistent D3 supplementation to restore them.
Tracking your levels every six months helps you see how your efforts are working, so you can adjust dosage, diet or lifestyle before problems develop. Treat it like a progress check – a simple way to measure how well your body is converting your efforts into results.
5. Use exercise to activate and protect your vitamin D during the winter. When sunlight fades in the colder months, your body’s ability to make vitamin D drops sharply – especially if you live in northern regions or carry extra body fat. That doesn’t mean you’re powerless. A 10-week study found that regular exercise alone, without supplements or dieting, helped overweight adults maintain healthy vitamin D activity even when their sun exposure was minimal.6
Physical movement changes the way your fat tissue handles vitamin D. It stimulates key enzymes that convert stored, inactive vitamin D into its usable form while slowing down the rate at which it’s broken apart. The people with the most fat tissue saw the biggest improvements, suggesting your body can train itself to use vitamin D more efficiently through consistent activity.
If you struggle to get enough winter sun, make exercise your built-in vitamin D activator – walk briskly outdoors, do strength training or take a daily movement break to keep your energy, mood and immune defences strong all season long.
FAQs About Vitamin D2
Q: Why does taking vitamin D2 lower your vitamin D3 levels?
A: Vitamin D2 triggers enzymes in your liver that speed up the breakdown of vitamin D3, your body’s active and usable form. Instead of improving your vitamin D status, D2 actually causes D3 levels to drop, leaving you more vulnerable to fatigue, weak immunity and calcium imbalance.
Q: What’s the main difference between vitamin D2 and D3?
A: Vitamin D3 is identical to the type your skin makes from sunlight, while D2 comes from UV-exposed plants and fungi. Your body easily converts D3 into its active form, but it struggles to process D2, resulting in lower and less stable blood levels.
Q: Which form of vitamin D supplement should I take?
A: Choose vitamin D3 whenever possible. It raises your levels faster and maintains them longer than D2. D3 also supports stronger immune, hormonal and metabolic function and is the form your body naturally recognises.
Q: How can I naturally improve my vitamin D levels without supplements?
A: Get regular sun exposure on bare skin daily and avoid seed oils that make your skin more prone to burning. Exercise, even without weight loss, helps your body activate and protect stored vitamin D – especially during the winter when sunlight is limited.
Q: How much vitamin D should I aim for and how do I know if I’m low?
A: Have your blood tested twice a year and aim for a level between 60 and 80 ng/ml (150 to 200 nmol/l). If your levels are low, combine safe sunlight exposure with vitamin D3 supplementation and co-factors like magnesium and vitamin K2 for the best results.
Sources and References
- 1 Nutrition Reviews September 18, 2025
- 2 Front Immunol. 2022 Feb 24:13:790444
- 3 Ibnosina Journal of Medicine and Biomedical Sciences 2019;11:57-61
- 4 Cleveland Clinic, Vitamin Deficiency
- 5 GrassrootsHealth March 10, 2020
- 6 Advanced Science June 12, 2025, Volume 12, Issue 22
About the Author
Dr. Joseph Mercola is the founder and owner of Mercola.com, a Board-Certified Family Medicine Osteopathic Physician, a Fellow of the American College of Nutrition and a New York Times bestselling author. He publishes multiple articles a day covering a wide range of topics on his website, Mercola.com.
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And if you read how vit. D is made, you will go out in the sun and have it made by your body like it should. Vit. D should be only given to very sick people and not used for everyone! Another bad generalization by the medical world. Also, when you compare American numbers to European ones, you wonder who set the standard, and why all of a sudden, after hundreds of thousands of years without supplements, everyone needs one.