Leisha For years, vitamin D has been presented as something remarkably simple. Get your blood tested, find out whether your number is low, take vitamin D3 and bring the number up. I don’t think the biology is nearly that simple, and the more I learn about vitamin D, the more cautious I become about treating it as another vitamin we can simply put into a bottle and take every day.
I have always been concerned about the idea that we can isolate one nutrient from nature, manufacture it in concentrated form and assume that taking more of it recreates what happens when that nutrient enters the body through food, sunlight and normal biological processes. I don’t think we know enough about nutrition yet to confidently believe that we understand every relationship involved. Nature supplies nutrients within incredibly complicated biological systems, many of which we are only beginning to understand.
Nutrition is a web of relationships. Minerals interact with vitamins, vitamins interact with enzymes, hormones communicate with organs, and the microbiome transforms compounds before and after we absorb them. The liver and kidneys continually convert molecules from one form into another, while our cells regulate what they use, store, transform and excrete. Vitamin D may be one of the clearest examples of why all of this matters.
“Vitamin D” Is Not Just One Thing
We commonly talk about “vitamin D” as though it were a single molecule, but it isn’t. Even the language can become confusing because several very different molecules are routinely discussed under the same broad name.
Vitamin D3, or cholecalciferol, is the form produced in the skin following exposure to ultraviolet B light. It is also the form commonly found in supplements. D3 then travels to the liver, where it is converted primarily into 25-hydroxyvitamin D [25(OH)D], or calcidiol.
This is generally what is being measured when you have a “vitamin D blood test.” When someone tells me that their vitamin D is 25, 35 or 50, they are usually talking about their circulating 25(OH)D level. They are not necessarily telling us what the biologically active hormone is doing within their tissues.
The kidneys and several other tissues can convert 25(OH)D into 1,25-dihydroxyvitamin D [1,25(OH)₂D], or calcitriol. Calcitriol is a potent secosteroid hormone, and this is where the conversation becomes much more interesting than simply asking whether someone’s vitamin D is “high” or “low.”
Calling vitamin D a vitamin makes it sound similar to taking vitamin C, yet its biology is much closer to a hormonal regulatory system. Once activated, calcitriol interacts with vitamin D receptors throughout the body and influences the expression of many genes. It participates in calcium and phosphorus regulation, bone metabolism, muscle physiology, immune signalling and numerous cellular processes.
The body regulates the conversion of one vitamin D metabolite into another very carefully. Taking D3 generally raises circulating 25(OH)D, but the body does not simply convert every available molecule into active calcitriol. The conversion is influenced by calcium, phosphorus, parathyroid hormone, kidney function and other physiological signals.
I think this gets lost when the entire conversation becomes focused on a laboratory number. We have created the idea that a low number automatically means we need to take more D3 until that number rises. I would much sooner ask why the number is low, what the rest of the mineral system is doing and whether the body has everything it requires to use these compounds appropriately.
Vitamin D and Calcium Cannot Be Separated
One of the major functions of vitamin D is its relationship with calcium absorption and calcium metabolism. Taking vitamin D therefore does more than change a vitamin D number on a laboratory report. It influences a much larger mineral-regulating system involving the intestines, kidneys, bones, thyroid and parathyroid glands.
The body keeps blood calcium within a remarkably narrow range because calcium is needed for far more than bones. Nerves, muscles, the heart and cellular signalling all depend upon tightly regulated calcium concentrations. When blood calcium begins to fall, the parathyroid glands respond by releasing parathyroid hormone, which influences the kidneys, bones and vitamin D metabolism to help restore circulating calcium.
The thyroid participates in this relationship as well. Specialized C cells within the thyroid produce calcitonin, another hormone involved in calcium regulation. Vitamin D increases intestinal calcium absorption, so adding supplemental vitamin D influences a system that is already being carefully managed by several organs and hormones.
Then there is magnesium, which I think deserves far more attention in the vitamin D conversation. Magnesium participates in many of the enzymatic reactions involved in vitamin D metabolism and is important for normal parathyroid function and mineral regulation. If we continually focus on raising vitamin D while overlooking magnesium and the rest of the mineral terrain, we may be looking at only one small part of a much larger picture.
This is why I don’t think we can sensibly discuss vitamin D without also considering calcium, magnesium, phosphorus, kidney function, thyroid and parathyroid physiology, healthy fats and the other fat-soluble vitamins. These nutrients and systems communicate continually. Raising one isolated compound does not tell us that the entire system is functioning better.
Having Calcium and Using Calcium Are Two Different Things
This distinction is particularly important to me. When I see signs that make me question calcium metabolism, my first thought is not necessarily that the person needs more calcium. I want to know whether the calcium they already have is being absorbed, regulated and directed appropriately.
Calcium has to be absorbed from the digestive tract, transported through the blood and used where it is needed. Its concentration in the blood has to remain tightly controlled. Bone acts as an enormous mineral reservoir, while the kidneys continually participate in deciding what is retained and what is excreted. Vitamin D, parathyroid hormone, calcitonin, magnesium and phosphorus are all part of this conversation.
This is one reason I am cautious about routinely adding isolated vitamin D3 without considering the mineral picture. If vitamin D increases calcium absorption, I want to know that the rest of the body’s calcium-regulating system has the nutritional resources it needs to manage that calcium appropriately.
A Higher Blood Level Does Not Necessarily Mean a Better Outcome
If someone’s 25(OH)D level rises after supplementation, we know something very specific: the supplement successfully raised 25(OH)D. What we don’t automatically know is whether the person’s overall health improved because the number increased.
This became particularly interesting in randomized trials examining very high-dose vitamin D. In one trial involving more than 2,000 older women, researchers gave 500,000 IU of vitamin D3 once annually. The women receiving this enormous annual dose experienced more falls and fractures than those receiving placebo.
Another randomized trial compared different monthly vitamin D doses in older adults. The higher-dose groups successfully achieved higher 25(OH)D concentrations, but they did not have better lower-extremity function and experienced more falls than the lower-dose group.
I don’t interpret these studies as evidence that every form of vitamin D supplementation is harmful. What I take from them is that successfully raising a laboratory value does not guarantee that we have improved the underlying physiology. Dose matters, frequency matters, starting nutritional status matters, and the person’s broader mineral and hormonal terrain matters.
Sunshine Does Something a Pill Cannot Completely Reproduce
There is a reason vitamin D became known as the sunshine vitamin. When ultraviolet B radiation reaches the skin, it interacts with 7-dehydrocholesterol, beginning the production of vitamin D3. But sunlight doesn’t simply manufacture the equivalent of a D3 capsule underneath our skin.
The photochemistry is considerably more complex. UV exposure can also produce compounds including lumisterol and tachysterol from vitamin D precursors in the skin. Continued ultraviolet exposure changes vitamin D-related compounds into additional photoproducts. We are looking at an entire biological process, not simply the manufacture of one isolated molecule.
The skin also has mechanisms that help regulate this process. Sunshine does not simply cause unlimited amounts of vitamin D to accumulate indefinitely. The chemistry changes as exposure continues, and some of the precursors and vitamin D-related molecules are converted into other compounds.
Sunlight also affects us in ways that have nothing to do with a vitamin D laboratory result. Natural light is involved in circadian rhythm, sleep-wake signalling and numerous physiological processes. Going outside also usually means movement, fresh air, contact with plants, soil and the microbial world around us. A capsule cannot reproduce all of these relationships.
Of course, sensible sun exposure matters. Burning the skin is not the goal. The amount of sunlight appropriate for one person may be completely different for another depending upon latitude, season, skin pigmentation, age, clothing, time of day and lifestyle.
The Microbiome Belongs in This Conversation
I find it difficult to talk about nutrition without talking about the microbiome, because humans do not function as isolated organisms. We live in partnership with enormous microbial communities in the intestinal tract, on the skin, in the mouth and throughout the surfaces where our bodies meet the outside world.
These microorganisms participate in nutrient transformation and metabolism. What we eat changes the microbial community, and the microbial community changes the chemistry that comes back to us. Our food therefore feeds far more than our human cells.
This relationship makes complete sense when we compare it with healthy soil. A plant doesn’t simply sit in dirt and absorb a list of isolated minerals. Around its roots exists an extraordinary soil food web. Bacteria, fungi, protozoa and many other organisms transform nutrients, exchange metabolites and influence which nutrients become available to the plant. Plants themselves feed microorganisms through their root exudates, creating a living exchange between plant and soil.
Our intestinal terrain has its own version of this relationship. Food feeds us, but it also feeds our microorganisms. Those microorganisms then transform components of that food into compounds that interact with our own physiology.
The more I learn about this, the less comfortable I become with the idea that human nutrition can ultimately be reduced to a shelf containing dozens of isolated pills.
Food Provides Relationships, Not Isolated Molecules
Think about oily fish, eggs, mushrooms, fermented foods, mineral-rich vegetables, herbs and foods grown in biologically active soil. None of these contains just one nutrient. They contain combinations of minerals, fats, proteins, vitamins, pigments and hundreds or thousands of other compounds arriving together.
We are still discovering what many of these compounds do and how they interact. Some may influence absorption, some may protect another compound during digestion, some may feed particular microorganisms and some may participate in metabolic pathways that we have barely begun to understand.
This is why I prefer whole-food nutritional sources whenever practical. It is not because an isolated nutrient can never have a place. There are circumstances where a documented deficiency or particular medical situation may require targeted supplementation. My concern is with the casual assumption that because a nutrient is biologically necessary, taking a concentrated amount of it every day must automatically be healthier.
What About Cod Liver Oil?
Cod liver oil is particularly interesting because it naturally provides vitamin D together with vitamin A and omega-3 fatty acids. It is still concentrated nutrition and needs to be used appropriately, particularly because fat-soluble vitamins can accumulate, but nutritionally it is very different from continually adding isolated D3 drops.
I prefer looking at these nutrients as relationships. If someone is already eating nutrient-dense foods, taking cod liver oil and receiving appropriate sunlight exposure, I want to understand why we would automatically add isolated vitamin D3 on top of that.
What are we trying to accomplish? If the answer is simply that we want to make one laboratory number higher, I don’t think that is enough information.
What Does a “Low Vitamin D” Test Actually Tell Us?
A standard vitamin D test generally measures 25(OH)D. If that level is low, then we know that circulating 25(OH)D is low. It doesn’t necessarily tell us why it is low or give us a complete picture of the entire vitamin D system.
I want to know how much sunlight that person receives, what time of year the test was performed, what they eat, what their magnesium status looks like and what is happening with calcium and phosphorus. I want to consider kidney function, parathyroid function, digestion, absorption, medications and the person’s overall metabolic terrain.
I also want to know how the person actually feels. A laboratory value is useful information, but I don’t like allowing one laboratory number to become the person.
True Deficiency Is Real
None of this means that vitamin D is unimportant. True vitamin D deficiency is real, and severe deficiency can impair calcium absorption and normal mineralization of bone. Children can develop rickets and adults can develop osteomalacia. Vitamin D clearly has important physiological functions.
My concern is not vitamin D itself. My concern is the idea that everyone needs to achieve the same blood level and that the answer to a lower number is automatically more supplementation.
There is a great deal of space between ignoring a true deficiency and taking high-dose D3 simply because we have been told that higher vitamin D numbers equal better health. Individual biology matters.
Why I Prefer Sunshine
For someone capable of safely obtaining regular sun exposure, I prefer working with the system our physiology developed with: sunlight, whole food, minerals, movement, healthy fats, a diverse microbiome and healthy soil. These pieces belong together.
The amount of sunlight needed varies enormously between individuals and environments, so I don’t think prescribing a universal number of minutes makes sense. The objective is not to burn the skin or chase vitamin D through excessive UV exposure. It is to recognize that regular, sensible exposure to natural light is biologically different from swallowing a concentrated dose of one isolated molecule.
Every time science identifies another vitamin D metabolite, enzyme, receptor, microbial interaction or regulatory pathway, we discover another layer of complexity. I find that exciting because it reminds us how much biology is happening beneath the simple labels we give things.
We Are Still Learning the Language
Vitamin D3 is only the beginning of the story. There is 25(OH)D, calcitriol, multiple vitamin D metabolites and alternative metabolic pathways. There are vitamin D receptors throughout the body and relationships involving calcium, phosphorus, magnesium, kidneys, parathyroid glands, thyroid physiology, bone and immune signalling. Sunlight generates photoproducts that don’t appear simply because we swallowed D3.
There are also many questions we have not yet learned to ask. That should create some humility around supplementation, particularly when supplementation is being used indefinitely without looking at the larger nutritional picture.
I don’t believe we need to become frightened of vitamin D. I think we need to become more biologically curious about it and stop treating the words “vitamin D” as though they describe one simple substance with one simple job.
Before automatically reaching for a bottle because a laboratory number falls outside someone’s preferred range, I want to look at the person. I want to look at their food, minerals, sunlight exposure, digestion, microbiome, thyroid and parathyroid relationship, kidneys and the rest of their terrain.
We are living ecosystems, just as healthy soil is a living ecosystem. In healthy soil, nutrients become available through relationships between minerals, roots, microorganisms, organic matter, water and the environment around them. Human nutrition is also built upon relationships, many of which science is still discovering.
Nature rarely delivers nutrients to us in isolation. I think we should be cautious about assuming that we understand enough to continually do so ourselves.
NIH Office of Dietary Supplements: Vitamin D – Health Professional Fact Sheet. This is a very good reference for the basic physiology: D3, 25(OH)D, calcitriol, calcium absorption, deficiency, excess and current reference ranges.
NIH Vitamin D – Health Professional Fact Sheet
Sanders et al., JAMA, 2010 – Annual High-Dose Oral Vitamin D and Falls and Fractures in Older Women. This is the study involving 500,000 IU once yearly in women aged 70+, and it did find increased falls and fractures in the vitamin D group.
JAMA – Annual High-Dose Vitamin D Trial
Bischoff-Ferrari et al., JAMA Internal Medicine, 2016 – Monthly High-Dose Vitamin D Treatment for the Prevention of Functional Decline. This compared 24,000 IU monthly with higher-dose regimens. The higher-dose regimens raised 25(OH)D but did not improve function and were associated with more falls.
PubMed – Monthly High-Dose Vitamin D Trial
