Why a Personalized Approach Is Best

In this episode we discuss:

• Why standard vitamin D recommendations don’t work for most people
• The impact of obesity, aging, gut health, liver function, and kidney health
• Optimal vitamin D blood levels—and why more is not always better
• The U-shaped risk curve of vitamin D deficiency and excess
• Why testing and seasonal retesting are essential
• Magnesium’s newly discovered role as a “vitamin D thermostat”
• Sun exposure benefits beyond vitamin D production
• How to personalize supplementation based on labs and lifestyle
• Practical guidance on dosing, cofactors, and seasonal adjustments

Show notes:

• “Ambient ultraviolet-B radiation, supplements and other factors interact to impact vitamin D status differently depending on ethnicity: A cross-sectional study” by Brennan, M. M., et al.
  “Magnesium treatment increases gut microbiome synthesizing vitamin D and inhibiting colorectal cancer: Results from a double-blind precision-based randomized placebo-controlled trial” by Sun, E., et al.
• “Human serum 25-hydroxycholecalciferol response to extended oral dosing with cholecalciferol” by Heaney, R. P., et al.
• “25(OH)D2 half-life is shorter than 25(OH)D3 half-life and is influenced by DBP concentration and genotype” by Jones, K. S., et al.
• “Geographic location and vitamin D synthesis” by Kimlin, M. G.
• “Avoidance of sun exposure as a risk factor for major causes of death: A competing risk analysis of the Melanoma in Southern Sweden cohort” by Lindqvist, P. G., et al.
• “UVA irradiation of human skin vasodilates arterial vasculature and lowers blood pressure independently of nitric oxide synthase” by Liu, D., et al.
• “Traditionally living populations in East Africa have a mean serum 25-hydroxyvitamin D concentration of 115 nmol/l” by Luxwolda, M. F., et al.
• “Vitamin D toxicity redefined: Vitamin K and the molecular mechanism” by Masterjohn, C.
• “Drug-vitamin D interactions: a systematic review of the literature” by Robien, K., et al.
• “Vitamin D supplementation guidelines” by Pludowski, P., et al.
• “Vitamin D-binding protein and vitamin D status of black Americans and white Americans” by Powe, C. E., et al.
• “Seasonal vitamin D changes and the impact on health risk assessment” by Rosecrans, R., & Dohnal, J. C.
• “The synergistic interplay between vitamins D and K for bone and cardiovascular health: A narrative review” by van Ballegooijen, A. J., et al.
• “Colour counts: Sunlight and skin type as drivers of vitamin D deficiency at UK latitudes” by Webb, A. R., et al.
• “Decreased bioavailability of vitamin D in obesity” by Wortsman, J., et al.
• Learn more about Adapt Naturals Bio-Avail D3/K2, Core Plus bundle, or take our quiz to see which products best suit your needs
• If you’d like to ask a question for Chris to answer in a future episode, submit it here
• Follow Chris on Instagram or Facebook

Hey everybody, Chris Kresser here. Welcome to another episode of Revolution Health Radio. Researchers at Vanderbilt just published a fascinating study that could change how we think about vitamin D supplementation. They found that magnesium doesn’t just raise vitamin D levels across the board, it acts like a thermostat, raising vitamin D levels in people who are deficient while lowering them in those whose levels are already high.

This is the first clinical evidence suggesting that magnesium helps optimize vitamin D levels rather than simply increasing them. This finding is significant because vitamin D supplementation has become increasingly common. Millions of people are taking vitamin D supplements, often in doses of 2000, 5000, or even 10,000 IU per day. Many are doing this without ever testing their levels or considering the factors that affect how their body produces, absorbs, and uses vitamin D. The mainstream approach to vitamin D is essentially one size fits all. Take 400 IU if you’re following official guidelines, or maybe 2000 to 5000 IU if you’re following popular health advice. But here’s the problem with that approach. Your vitamin D needs are not the same as your neighbor’s needs or your spouse’s or what you read in a magazine article. They depend on your ethnicity, body weight, health status, where you live, how much time you spend outside, and the status of other nutrients in your body, like magnesium, vitamin K2, and vitamin A.

I get why this is confusing. There’s conflicting advice everywhere. Some sources say 400 IU is plenty. Others say you need 5000 or 10,000 IU. Some claim vitamin D is a miracle nutrient that prevents everything from cancer to heart disease. Others warn about toxicity. The truth is more nuanced than any of these extreme positions. By the end of this episode, you’ll understand why a personalized approach to vitamin D makes sense, what your optimal range might be based on individual factors, why testing and retesting is crucial, which cofactors you need to pay attention to, and how to use sun exposure and supplementation strategically. Let’s dive in.

Why One Size Doesn’t Fit All

Let’s start with why the one size fits all approach doesn’t work. The standard recommendation you’ll see from most mainstream sources is 400 to 800 IU per day for adults. That comes from the Institute of Medicine, which set these levels based on skeletal health outcomes, specifically preventing rickets and osteomalacia. For some people, 400 IU might be adequate. For others, and I would say most people, it’s nowhere near enough. A large study published in the journal Clinical Nutrition in 2024 looked at vitamin D levels in nearly 440,000 people from different ethnic backgrounds in the UK. They found massive variation. Median vitamin D levels were about 25 nanomoles per liter in Asian participants, 31 in Black participants, and 48 in White participants. When you convert those to the units more commonly used in the US, that’s roughly 10 nanograms per milliliter for Asian participants, 12 for Black participants, and 19 for White participants. Nearly 50 percent of Asian participants and 35 percent of Black participants had frank vitamin D deficiency compared to 12 percent of White participants. These differences persisted even when researchers accounted for sun exposure, supplementation, diet, and other lifestyle factors.

Skin pigmentation matters enormously. Melanin blocks ultraviolet B radiation, the wavelength that triggers vitamin D production in your skin. If you have darker skin, you need significantly more sun exposure to produce the same amount of vitamin D as someone with lighter skin. But ethnicity is just one variable. Body weight is another huge factor. The same UK Biobank study found that the association between body mass index and vitamin D levels was consistent across all ethnic groups. Higher BMI meant lower vitamin D levels. And here’s the kicker – the negative effect of obesity on vitamin D status was actually stronger in people who were taking supplements. People with obesity convert less sunlight to vitamin D in their skin, and they absorb less vitamin D from supplements and diet. The mechanism isn’t entirely clear, but it probably involves sequestration of vitamin D in body fat compartments where it’s less bioavailable. Age also matters. As we get older, our skin’s capacity to produce vitamin D declines. A 70-year-old person produces about 25 percent less vitamin D from the same sun exposure compared to a 20-year-old. The UK study found that the weakening association between sun exposure and vitamin D levels started at relatively younger ages, not just in the elderly.

Health status is another critical variable that often gets overlooked. If you have inflammatory bowel disease like Crohn’s or ulcerative colitis, you’re likely not absorbing vitamin D as effectively from food or supplements. The inflammation damages the intestinal lining, which impairs fat absorption, and vitamin D is a fat soluble vitamin. Patients with IBD often have lower vitamin D levels than the general population, and they may need higher doses to reach adequate status. Liver disease and kidney disease can also affect vitamin D metabolism. Vitamin D gets converted to 25-hydroxyvitamin D in the liver, which is what we measure when we test vitamin D status. If liver function is compromised, that conversion might not happen efficiently. The final activation step happens in the kidneys, where 25-hydroxyvitamin D gets converted to the active form, 1,25-dihydroxyvitamin D. Chronic kidney disease can disrupt this process. When you add up all these variables, ethnicity, body weight, health status, sun exposure, diet, supplement use, it becomes clear that telling everyone to take the same dose of vitamin D makes no sense. What works for a 30-year-old White woman at a healthy weight living in Arizona is not going to work for a 65-year-old Black man with obesity and Crohn’s disease living in Seattle.

Optimal Vitamin D Range

All right, let’s talk about optimal vitamin D range. The common reference range you’ll see on lab tests in the US is 30 to 74 nanograms per milliliter for serum 25-hydroxyvitamin D. Some advocacy groups like the Vitamin D Council recommend a higher target of 40 to 80, with an ideal of around 50. When you look at the research, there’s little to no evidence showing benefit to levels above 70, and increasing evidence suggesting that very high levels may cause harm. Vitamin D follows what’s called a U-shaped curve for health outcomes. Both low levels and very high levels are associated with increased risk. Consequences of vitamin D toxicity include cardiovascular events, kidney stones, hypercalcemia, nausea, vomiting, bone loss, and in severe cases, organ damage. Most cases of toxicity come from over supplementation, not sun exposure, although there’s at least one study on Israeli lifeguards suggesting that toxicity from sun exposure alone is possible in extreme circumstances. Based on my assessment of the literature and my clinical experience, I think the functional range for most people is around 40 to 60 nanograms per milliliter. For people with non-white ancestry, the optimal range may be somewhat lower. Black people, for example, typically have lower vitamin D levels than white people, yet they also have higher bone mineral density. There’s evidence suggesting that people with non-white ancestry may be adapted to lower optimal vitamin D levels. For people with autoimmune disease, the optimal range might extend up to 70 nanograms per milliliter to maximize the immune regulating benefits of vitamin D. Again, I haven’t seen clinical benefits or research supporting additional benefits above 70, and the risk of vitamin D toxicity begins to increase at those higher levels. This needs to be individualized based on testing and clinical response.

In rare cases where someone is having trouble getting their vitamin D levels above 35 nanograms per milliliter despite supplementation, I might test parathyroid hormone to see if they’re chronically deficient or just have a lower optimal vitamin D level. But for most people, this level of complexity isn’t necessary. The basic framework of testing your 25-hydroxyvitamin D level, and adjusting based on your individual factors is sufficient. The importance of testing and retesting can’t be overstated. Test, don’t guess, as we say in functional medicine. This phrase applies to so many things in functional medicine, and it definitely applies to vitamin D. Your vitamin D level is not static. It changes throughout the year based on sun exposure, it changes when you modify your supplement dose, and it changes based on all the individual factors we’ve been discussing. Seasonal variation of vitamin D levels is significant. A study looking at vitamin D testing patterns found that levels follow a sinusoidal pattern that closely tracks hours of sunlight. Winter levels are substantially lower than summer levels for most people who aren’t supplementing heavily. If you test in late summer, when your levels are naturally at their peak from sun exposure, and you look adequate, that doesn’t mean you’ll stay adequate through the winter, when UV radiation is insufficient for vitamin D production. Conversely, if you test in late winter or early spring when your levels are naturally at their lowest and your doctor prescribes a supplement based on that single test, you might end up with levels that are too high by late summer, when sun exposure is adding to your vitamin D production. This is why I recommend testing at least twice a year, once in late winter or early spring and once in late summer or early fall. That gives you a sense of your year round status and allows you to adjust your supplement dose seasonally if needed. You should also retest three to four months after changing your supplement dose. Vitamin D has a half-life of about two to three weeks, so it takes a few months for levels to stabilize after you change your intake. Don’t test two weeks after starting a supplement, and assume you know where you’ll end up. Give it time to reach a steady state.

Vitamin D is one of the most misunderstood—and misused—supplements in modern health. In this episode, we break down why standardized dosing recommendations often fail and how a personalized approach leads to better outcomes and fewer risks. #vitaminD #ChrisKresser

How Nutrient Co-Factors Shape Individual Needs

Now let’s talk about co-factors, because this is where things get really interesting. The Vanderbilt study I mentioned at the beginning found that magnesium essentially regulates vitamin D levels. When magnesium was given to people with low vitamin D, it increased their levels. When it was given to people with high vitamin D, it decreased their levels. This regulatory effect is important. The researchers noted that magnesium deficiency shuts down the vitamin D synthesis and metabolism pathway. So if you don’t have adequate magnesium, your body can’t convert vitamin D into its active forms efficiently. This could explain why some people who take high doses of vitamin D and their levels barely budge, while others take moderate doses and their levels skyrocket. Magnesium status may be the missing variable. The study authors pointed out that up to 80 percent of Americans don’t consume enough magnesium to meet the recommended dietary allowance, and that’s based on the outdated RDA. Newer studies that have adjusted the magnesium RDA upwards suggest that over 90, maybe 95, percent of Americans are not consuming enough magnesium. That’s a staggering number. Magnesium deficiency is an under-recognized public health problem, and it may be quietly undermining vitamin D status in a huge portion of the population. Foods rich in magnesium include dark leafy greens, beans, whole grains, dark chocolate, fatty fish like salmon, nuts, and avocados. But even with a good diet, it’s very difficult to get adequate magnesium from food alone because of soil depletion. This is one of the few nutrients where I think supplementation over the long term makes sense for most people. I typically recommend 400 to 600 milligrams of magnesium glycinate per day.

Vitamin K2 is another critical cofactor that works synergistically with vitamin D. Vitamin D promotes the production of certain proteins, including osteocalcin and matrix Gla protein. These are vitamin K-dependent proteins, meaning they require vitamin K to become activated through a process called carboxylation. When these proteins are properly carboxylated, they help direct calcium into your bones and teeth where you want it, and keep it out of soft tissues like your arteries where you definitely don’t want it. Research published in the International Journal of Endocrinology found that vitamin D and K mutually enhance each other. Animal studies showed that giving both vitamins together increased bone formation markers in a time-dependent manner, while giving them separately didn’t produce the same effect. Clinical trials in postmenopausal women found that combined vitamin D and K2 supplementation improve bone mineral density more than either vitamin alone.

There’s also emerging evidence for cardiovascular benefits. A study in the journal Hypertension found that low vitamin D and low vitamin K status together were synergistically associated with higher blood pressure and increased hypertension risk. When vitamin K-dependent proteins like matrix Gla protein remain undercarboxylated because of inadequate vitamin K, they can’t inhibit vascular calcification effectively. Vitamin D increases the expression of these proteins, but without adequate vitamin K to activate them, you may be setting the stage for arterial calcification. Vitamin K2 exists in multiple forms. You’ll often hear that MK-7 is superior because it has a longer half-life in plasma, but that argument misses something important. MK-4 doesn’t do its work in plasma. It works inside cells. Plasma half-life tells you nothing about whole-body half-life or tissue distribution. In fact, research suggests that MK-4 and MK-7 distribute to different tissues based on how they’re packaged in lipoproteins. MK-4 tends to be preferentially delivered to tissues like the heart, brain, kidneys, and skeletal muscle, while MK-7 goes more to liver and bone. The Rotterdam Study, which is observational but quite large, found that dietary vitamin K2, much of which is MK-4, was associated with reduced risk of coronary calcification and heart disease, while vitamin K1 was not. Similar findings emerge for prostate cancer risk. These are nutritional doses we’re talking about, not megadoses. The idea that MK-4 has no relevance at low doses doesn’t hold up when you look at the totality of the evidence.

Vitamin A is the third key co-factor. Research going back to the 1930s has suggested that vitamin A and vitamin D have an antagonistic relationship at very high doses, but at physiologic doses they work together. A 2007 paper in Medical Hypotheses proposed that vitamin D toxicity may work by inducing a functional vitamin K deficiency, and that vitamin A protects against this by supporting vitamin K-dependent processes. What this means practically is that if you’re taking high doses of vitamin D, you want to make sure you’re getting adequate vitamin A and K2 to protect against potential toxicity. This isn’t about megadosing these vitamins. It’s about maintaining balance. Good sources of vitamin A include liver, egg yolks, butter, and dairy products from pastured animals. The form matters. You want retinol or retinyl palmitate from animal sources, not just beta-carotene from plants, because conversion of beta-carotene to active vitamin A is highly variable and often inefficient.

One thing that’s become clear from the research is these nutrients don’t work in isolation. They work as a system. When one is out of balance, it affects the others. The Vanderbilt magnesium study is a perfect example of this. Magnesium status influences vitamin D status. Vitamin D status influences calcium metabolism. Vitamin K status influences whether that calcium goes into bone or soft tissue. It’s interconnected.

Chronic Health Condition Concerns

This brings us to an important point about how health conditions affect your vitamin D needs. We touched on inflammatory bowel disease earlier, but the impact of GI conditions on vitamin D absorption deserves more attention. Celiac disease, short bowel syndrome, chronic pancreatitis, cystic fibrosis, and small intestinal bacterial overgrowth, or SIBO, impair fat absorption, which means they impair absorption of fat-soluble vitamins, including vitamin D. SIBO has become quite common, and it interferes with vitamin D and mineral absorption in ways that often go unrecognized. Even beyond absorption issues, active inflammation itself can affect vitamin D status. Some research suggests that 25-hydroxyvitamin D may act as a negative acute phase reactant, meaning levels drop during active inflammation. This could explain why people with active Crohn’s disease or ulcerative colitis often have lower vitamin D levels than people in remission, even when dietary intake is similar. Obesity, which we already discussed, is another condition that increases vitamin D requirements. Studies suggest that people with obesity may need two to three times more than a standard dose to achieve the same vitamin D levels as someone at a healthy weight. A paper in the Journal of Nutrition Science has argued for weight based vitamin D dosing strategies specifically because of this differential response.

What I want you to take away from this section is, if you have any chronic health condition, particularly one that affects your gut, liver, kidneys, metabolism, or inflammation, you need to be especially thoughtful about vitamin D. The standard recommendations were not developed with these conditions in mind. You need to test, you may need higher doses than the average person, and you need to monitor your response over time.

Benefits of Sun Exposure

Let’s shift gears and talk about sun exposure, because vitamin D is really just the tip of the iceberg when it comes to the benefits of sunlight. When your skin is exposed to UVB radiation, you produce several important compounds beyond just vitamin D. These include beta-endorphin, a natural opiate that promotes relaxation and pain tolerance. Calcitonin gene-related peptide, which dilates blood vessels and protects against hypertension and vascular inflammation. Substance P, which regulates immune function. And melanocyte-stimulating hormone, which affects appetite and metabolism. UVA radiation, which is a different wavelength from UVB, triggers the release of nitric oxide from storage in your skin. Nitric oxide is a potent vasodilator that reduces blood pressure. A 20-year study following nearly 30,000 people found that those who avoided sun exposure were twice as likely to die from all causes compared to those who got regular sun exposure. That mortality benefit can’t be explained by vitamin D alone. Something else is happening with sunlight that’s profoundly beneficial. Sunlight also entrains your circadian rhythm. Bright light exposure during the day signals your suprachiasmatic nucleus in the hypothalamus, which regulates melatonin production from your pineal gland. Disrupted circadian rhythms are associated with mood disorders, cognitive deficits, and metabolic syndrome. Getting morning sunlight is one of the most important things you can do for your sleep quality and metabolic health.

So when we talk about vitamin D optimization, we need to think beyond just the vitamin D number. Sun exposure provides benefits that supplements can’t replicate. That doesn’t mean supplements don’t have a place. They do, especially during winter months or for people who can’t get adequate sun exposure. But sunlight should be your primary source when possible. My recommendation for sun exposure is simple. Spend about half as much time in direct sunlight as it takes your skin to turn pink. This automatically accounts for variation in skin tone, latitude, time of year, and solar angle. Someone with very fair skin in summer might start to pink in 20 minutes, so they should get only about 10 minutes of sun exposure without protection. Someone with darker skin might not pink for an hour, so they could safely get 30 minutes. This guideline is practical because it’s self-regulating. You don’t need to calculate UV index or consult charts. Your skin tells you. And importantly, you want to do this without sunscreen, at least initially. Sunscreen blocks not only vitamin D production, but also all the other beneficial photoproducts your skin produces in response to UVB. You can apply sunscreen after you’ve gotten your baseline exposure, or you can cover up with a shirt if you’re going to be out longer. Expose as much skin as is reasonable and socially appropriate. The more surface area you expose, the more vitamin D you’ll produce. Arms and legs are good, but if you can safely expose your torso, that’s even better. The goal is to get regular, moderate sun exposure, not to burn. Sunburns are damaging and should be avoided if possible.

During winter months at latitudes above about 35 degrees north or south, the sun angle is too low for meaningful vitamin D production, even on clear days. In Boston, for example, no previtamin D is produced in skin from November through February, even on cloudless days. If you live in these latitudes, you’ll need to rely more on supplementation or dietary sources during winter, which is why testing in late winter makes sense to see where you actually land.

Personalizing Your Supplementation

When it comes to supplementation, the dose you need depends on all the factors we’ve discussed – your current vitamin D level, ethnicity, body weight, health status, age, sun exposure patterns, and your intake of co-factors. This is why I can’t give you a universal number. What I can give you is a framework. If your vitamin D level is below 20 nanograms per milliliter, you likely need some combination of sun exposure, dietary sources like fatty fish or cod liver oil, and a vitamin D3 supplement. The dose might be anywhere from 2000 to 10,000 IU per day, depending on your individual factors, with higher doses for people with obesity or malabsorption issues. The key is to retest in three to four months, and adjust accordingly. If your level is between 20 and 40 nanograms per milliliter, I recommend supplementing with 2000 to 5000 IU per day, depending on your individual factors, until your level is in the 40 to 60 range. People with obesity and other health conditions may need 10,000 IU per day or even more. There’s virtually no risk in taking vitamin D supplements to bring your levels into this range, and there’s huge potential benefit. If your level is between 40 and 60, continue what you’re doing. That’s a solid functional range for most people. If your level is above 60, consider reducing your supplement dose and make sure you’re getting adequate vitamin K2, [vitamin] A, and magnesium to protect against potential toxicity.

The form of vitamin D matters too. Vitamin D3, or cholecalciferol, is more effective at raising 25-hydroxyvitamin D levels than vitamin D2, or ergocalciferol. Some prescription vitamin D formulations use vitamin D2, and cod liver oil also tends to contain vitamin D2. But D3 is the form you generally want. It’s also the form your skin produces naturally from sun exposure. This is one of the reasons I developed Bio-Avail D3/K2 for Adapt Naturals. I wanted a clean vitamin D3 supplement that included vitamin K2 and allowed for precise dose titration. It’s a liquid formula where one drop delivers 1000 IU of vitamin D3 and 120 micrograms of vitamin K2. I chose MK-4 because of its preferential distribution to heart, brain, and kidneys, and skeletal muscle – tissues where vitamin K-dependent processes are critical. The only other ingredient is MCT oil, which serves as a carrier and enhances absorption. The liquid format makes it easy to personalize your dose. If you need 2000 IU, you just take two drops. If you need 5000 you take five drops. If testing shows you need to adjust up or down, you can do that precisely without having to switch between different capsule strengths. And because it includes vitamin K2, you’re getting that synergistic benefit for bone and cardiovascular health without having to take a separate supplement. I formulated this because I saw the need clinically. Too many patients were either taking vitamin D without K2, which concerned me from a calcium metabolism standpoint, or they were taking capsules in fixed doses that didn’t allow them to fine-tune based on their test results. Having a liquid format with K2 included solved both problems.

All right, let’s review what we’ve covered. Vitamin D optimization requires a personalized approach. Your optimal level depends on your ethnicity, body weight, age, health status, and co-factor status. For most people, aim for 40 to 60 nanograms per milliliter, with levels up to 70 for those with autoimmune conditions. Test your levels, don’t guess. Retest seasonally and after changing your dose. Pay attention to co-factors like magnesium, vitamin K2, and vitamin A that work together with vitamin D. Prioritize sun exposure when possible and supplement strategically based on your individual needs. The mainstream approach is based on population averages, designed to prevent deficiency disease. It’s not designed to optimize your health based on your individual biology. The one size fits all approach doesn’t work, and you now have the tools to personalize your vitamin D strategy.

Thanks for listening, everyone. You can find show notes and links to all the studies I mentioned at ChrisKresser.com. If you have questions about vitamin D or any other topic you’d like me to cover, head over to ChrisKresser.com/podcastquestion and submit your question there. I read all of them, and I use them to guide what topics I cover on the show. I’ll talk to you next time.