Why You're Not Getting Enough Vitamin D

If you've been taking a vitamin D supplement for months but your blood levels still aren't where they should be, you're not alone. While most conversations about vitamin D focus on how much you need, far less attention is paid to a more fundamental question: Can your body actually absorb and use it?

The answer depends on much more than the dose printed on the bottle. Because vitamin D is a fat-soluble nutrient that relies on healthy digestion, liver function, and a network of supporting nutrients, a variety of medical conditions, medications, and nutritional deficiencies can prevent it from reaching your bloodstream — or from being activated once it gets there. Understanding these hidden barriers is essential for anyone trying to optimize their vitamin D status, especially if standard supplementation doesn't seem to be working.

Fat Malabsorption: Why Your Vitamin D Supplement May Not Be Working

One of the most clinically underappreciated factors in vitamin D status is the requirement for adequate fat digestion. Vitamin D3 — whether taken as a supplement or consumed in food — is a fat-soluble molecule. Before it can cross the intestinal wall and enter circulation, it must be incorporated into fat-digesting structures called micelles, which depend on the presence of bile salts secreted by the liver and stored in the gallbladder. From there, vitamin D is packaged into chylomicrons and transported through the lymphatic system into the bloodstream.

Any condition that disrupts fat digestion or absorption at any step in this process can significantly impair vitamin D absorption — sometimes profoundly. For individuals with these conditions, oral supplementation may raise blood levels far less than expected, or may fail to correct deficiency at all despite what would otherwise be adequate doses.

Conditions that impair vitamin D absorption

Inflammatory Bowel Disease (IBD). Both Crohn's disease and ulcerative colitis compromise vitamin D absorption through multiple mechanisms: intestinal inflammation reduces the absorptive surface area of the gut; damage to the ileum (where Crohn's disease most commonly occurs) impairs bile acid recycling, reducing the emulsification capacity needed for fat-soluble vitamin absorption; and systemic inflammation itself dysregulates vitamin D metabolism. Vitamin D deficiency is extremely prevalent in IBD populations and has been shown to influence disease activity in both directions — making it one of the conditions where monitoring and targeted supplementation are most important.

Celiac Disease. Untreated celiac disease damages the intestinal villi — the finger-like projections that dramatically increase the absorptive surface area of the small intestine. Without intact villi, the absorption of all fat-soluble vitamins (A, D, E, and K) is impaired. Even individuals on a gluten-free diet who have achieved mucosal healing may have residual absorption deficits. Supplemental vitamin D remains important in this population, and blood levels should be monitored rather than assumed.

Bariatric Surgery. Malabsorptive bariatric procedures — particularly Roux-en-Y gastric bypass — are among the most severe causes of impaired vitamin D absorption. The surgery bypasses the duodenum and proximal jejunum, which is where the majority of dietary fat-soluble vitamin absorption occurs. Post-bariatric patients frequently require substantially higher supplemental doses than the general population (often 3,000–6,000 IU/day or more) and require regular blood monitoring to maintain adequate levels. This is true even for purely restrictive procedures to some degree, as the reduction in dietary fat intake also reduces the vehicle for vitamin D absorption.

Liver Disease and Biliary Conditions. The liver is central to vitamin D metabolism in two ways: it produces the bile required for fat-soluble vitamin absorption, and it is the site of the first hydroxylation step that converts vitamin D3 into 25(OH)D (the form measured in blood tests). Chronic liver disease — whether from nonalcoholic fatty liver disease, cirrhosis, primary biliary cholangitis, or other causes — can impair both processes. 

Exocrine Pancreatic Insufficiency. The pancreas secretes digestive enzymes that are essential for breaking down dietary fats before bile can emulsify them. Conditions including chronic pancreatitis, cystic fibrosis, and pancreatic cancer can impair fat digestion upstream of the bile step, leading to fat malabsorption (steatorrhea) and subsequent vitamin D deficiency that is often resistant to standard supplemental doses.

Short Bowel Syndrome. Individuals who have had significant portions of the small intestine surgically removed — due to Crohn's disease, intestinal ischemia, trauma, or other causes — have reduced absorptive surface area and reduced transit time. Vitamin D deficiency is very common in this population, and intravenous or intramuscular vitamin D administration may be required in severe cases.

Medications. Several commonly used drugs impair fat and/or vitamin D absorption. Cholestyramine and other bile acid sequestrants (used to lower cholesterol) bind bile acids in the gut, reducing their availability for fat emulsification. Orlistat (a weight-loss drug that blocks fat absorption) predictably lowers fat-soluble vitamin levels. Some observational studies have reported lower vitamin D levels among long-term proton pump inhibitor (PPI) users, but evidence that PPIs directly impair vitamin D absorption remains limited.

When oral vitamin D doesn't work: what to try instead

For individuals with significant fat malabsorption, oral vitamin D supplementation has inherent limitations. The pill form still requires an intact digestive system to work. When fat absorption is severely compromised, alternative approaches that clinicians may consider include:

• Higher-dose supplementation, sometimes dramatically above standard doses, under medical supervision with frequent monitoring.

• Liquid or emulsified formulations of vitamin D, which may improve absorption in patients with fat malabsorption by pre-incorporating the vitamin D into an emulsified form that bypasses some of the bile-dependency.

• Intramuscular injection of vitamin D in severe or refractory cases, which completely bypasses the gastrointestinal tract.

• 25-hydroxyvitamin D (calcifediol), an already-hydroxylated form of vitamin D that is absorbed through a different pathway (passive diffusion rather than micellar transport), making it significantly less dependent on fat absorption. This form is available by prescription and is increasingly used in patients with malabsorption syndromes.

• Sun exposure, which remains an important route for these patients precisely because it entirely bypasses the gastrointestinal tract.

This last point is worth underscoring: Cutaneous vitamin D synthesis bypasses gastrointestinal absorption, although intentional UV exposure is not generally recommended as a primary treatment strategy because of the risks associated with UV radiation. As with the other alternatives above, this should be discussed with the physician managing the underlying condition.

The Nutrients That Make Vitamin D Actually Work

Vitamin D operates within a network of interdependent nutrients. Taking a vitamin D supplement without attending to this broader nutritional context can result in suboptimal outcomes, or in some cases can shift the balance of related nutrients in ways that are clinically relevant. Vitamin D interacts with several nutrients important to calcium and bone metabolism, particularly magnesium, calcium, and vitamin K.

Should you take vitamin D with food or on an empty stomach?

Because vitamin D is fat-soluble, timing your supplement with a meal that contains fat is not merely helpful; it is biochemically necessary for optimal absorption. Research has found that taking vitamin D with the largest fat-containing meal of the day can increase serum levels by approximately 50% compared to taking it on an empty stomach.

Not all fats appear to be equally effective. Studies suggest that monounsaturated fats — found in olive oil, avocados, and nuts — may be more favorable for vitamin D absorption than polyunsaturated fats from seed oils. A small amount of fat(about a tablespoon of olive oil, a quarter of an avocado, or a handful of almonds), appears sufficient to substantially enhance absorption.

Practical guidance: take your vitamin D supplement with breakfast or your largest meal of the day, particularly when that meal includes eggs, full-fat yogurt, nuts, avocado, olive oil, or similar fat sources.

Magnesium: the forgotten cofactor

Magnesium is required at multiple steps in vitamin D's metabolic pathway. Every enzyme involved in converting vitamin D to its active form — including the hepatic hydroxylation step (converting D3 to 25(OH)D in the liver) and the renal activation step (converting 25(OH)D to active calcitriol in the kidneys) — uses magnesium as a cofactor. Magnesium is also required for vitamin D to bind to its transporter protein in the blood and for vitamin D receptors to function at the cellular level.

The clinical implication is important: a person who is magnesium-deficient may appear to be a "non-responder" to vitamin D supplementation — their serum 25(OH)D levels may fail to rise as expected despite taking adequate doses, or may rise but not translate into the expected physiological effects. Restoring magnesium status can, in some patients, dramatically improve their response to vitamin D.

This is not a rare scenario. Inadequate magnesium intake is common in the United States; many Americans Americans consume less magnesium than the recommended amount. Older adults, people with type 2 diabetes, and those with gastrointestinal conditions are at particular risk.

The adult RDA for magnesium is approximately 310-420 mg per day from all sources, depending on age and sex; this should not be interpreted as a recommended supplement dose. Dietary sources include dark leafy greens, nuts (particularly almonds and cashews), seeds, legumes, and whole grains. Among supplemental forms, magnesium glycinate and magnesium malate tend to be better tolerated (less likely to cause gastrointestinal side effects) than magnesium oxide. When blood levels fail to respond to vitamin D supplementation as expected, checking magnesium status is a reasonable next clinical step.

Vitamin K2: directing calcium to the right places

When vitamin D intake increases — whether from sun or supplements — the body absorbs more calcium from the diet. This is the intended effect for bone health. But calcium that circulates in the blood must be directed into bone and teeth rather than being deposited in soft tissues, arterial walls, or kidneys. This is where vitamin K2 plays a crucial role.

Vitamin K2 activates two key proteins:

• Osteocalcin, a protein produced by osteoblasts (bone-building cells) that requires K2 activation to bind calcium into the bone matrix.

• Matrix Gla Protein (MGP), the most potent known inhibitor of soft-tissue and arterial calcification. Without adequate K2, MGP remains inactive and cannot prevent calcium from depositing in arteries and other soft tissues.

Vitamin K-dependent proteins participate in bone mineralization and regulation of soft-tissue calcification. Without sufficient K2, elevated calcium absorption from high vitamin D intake could theoretically contribute to arterial calcification — a concern that has been discussed in the scientific literature, though not conclusively proven.

Vitamin K2 is distinct from vitamin K1 (found in leafy green vegetables, and involved primarily in blood clotting). K2 is found in fermented foods - most abundantly in natto (a Japanese fermented soybean product), as well as in certain aged cheeses, and to a lesser extent in grass-fed dairy and egg yolks. It is not present in meaningful amounts in most Western diets.

The most researched form is MK-7 (menaquinone-7), which has a longer half-life and greater bioavailability than MK-4. Supplemental doses of 90–200 mcg/day of MK-7 are commonly recommended alongside vitamin D3.

An important note for anyone on anticoagulant therapy: Vitamin K (in any form) interacts with warfarin and related blood-thinning medications. Anyone on anticoagulants should consult their physician before adding vitamin K2 supplementation.

Calcium: How much, and when?

Vitamin D's most well-established function is facilitating calcium absorption in the intestine - roughly 30–40% of dietary calcium is absorbed when vitamin D status is adequate, compared to 10–15% when it is deficient. This is why vitamin D supplementation alone, without adequate dietary calcium, is insufficient to protect bone health.

Calcium requirements for adults are approximately 1,000–1,200 mg/day from all dietary sources. Most people who eat dairy products, leafy greens, and fortified foods reach this level without supplementation. Dedicated calcium supplementation in the absence of diagnosed deficiency or documented dietary inadequacy has become more cautiously regarded in recent years, as some evidence has raised questions about high supplemental calcium intake and cardiovascular risk (a concern that does not apply to dietary calcium). When calcium supplementation is indicated, calcium citrate tends to be better absorbed than calcium carbonate (which requires stomach acid), particularly in older adults who may have reduced gastric acid secretion.

How to take vitamin D correctly: a practical summary

For most healthy adults who do not have underlying malabsorption conditions, the practical guidance simplifies as follows:

1. Take vitamin D3 with a fat-containing meal.

2. Ensure adequate magnesium intake — ideally from food, and supplementally if dietary intake is poor.

3. Include dietary sources of vitamin K2 (fermented foods, grass-fed dairy), or consider a K2-MK7 supplement if you are taking meaningful doses of vitamin D. Although vitamin K is important for bone metabolism, evidence is insufficient to recommend routine K2 supplementation specifically for people taking vitamin D.

4. Ensure calcium needs are met through diet rather than high-dose supplementation where possible.

5. If vitamin D blood levels are not responding to supplementation as expected, consider whether magnesium status or fat absorption may be limiting the response.

For those with any of the malabsorption conditions described in the previous section, these issues are amplified and monitoring under medical supervision is essential.

Measuring Vitamin D: What the Blood Test Tells You (and What It Doesn't)

What is the 25(OH)D vitamin D test?

When a clinician orders a "vitamin D test," they are almost always measuring serum 25-hydroxyvitamin D [25(OH)D] — the form of vitamin D stored and circulated in the blood before the kidneys convert it to the active hormone. This is the accepted clinical standard for assessing vitamin D status because it reflects total input from all sources — sun, supplements, light therapy, and food — and has a relatively long half-life of approximately 2–3 weeks, making it a stable indicator of status over time.

The active form, 1,25(OH)₂D (calcitriol), is rarely measured in clinical practice because it is tightly regulated by the kidneys and does not reflect overall vitamin D status reliably — it may be normal or even elevated in states of deficiency.

What do vitamin D blood test numbers mean?

Thresholds vary among organizations, and there is no universally accepted definition of vitamin D sufficiency. Some experts recommend the following diagnostic categories for 25(OH)D:

• Severe deficiency: below 12 ng/mL (30 nmol/L)

• Deficiency: below 20 ng/mL (50 nmol/L) — the threshold used many clinical labs and guidelines

• Insufficiency: 20-29 ng/mL (50-72 nmol/L)

• Sufficiency: There is no universally accepted cutoff for vitamin D sufficiency. The National Academies considers ≥20 ng/mL adequate for most healthy people, while the 2024 Endocrine Society guideline no longer endorses a specific sufficiency threshold for generally healthy adults.

• GrassrootsHealth recommendation: 40–60 ng/mL (100-150 nmol/L), based on the findings of their scientist panel and cohort data

Vitamin D toxicity generally occurs at very high 25(OH)D concentrations, often >150 ng/mL, but the National Academies recommends avoiding sustained serum concentrations above approximately 50–60 ng/mL because of potential adverse effects.

Where genuine disagreement begins is what counts as sufficient for optimal health, as opposed to simply avoiding deficiency-related bone disease. A number of researchers argue that 30 ng/mL, while adequate to prevent rickets and osteomalacia, is lower than what's biologically typical for humans living with regular sun exposure. GrassrootsHealth, a public health research organization that has tracked vitamin D levels across large cohorts, recommends a target range of 40-60 ng/mL (100-150 nmol/L) based on its scientist panel's review of the data.

This isn't a fringe position. Dr. Michael F. Holick, the Boston University endocrinologist who led the committee that wrote the original Endocrine Society guideline, has pointed to populations with ancestral, high-sun-exposure lifestyles as evidence that target levels should sit above his own guideline's 30 ng/mL cutoff:

“Masai herders and the Hadza were evaluated and showed that their circulating concentrations of 25-hydroxy vitamin D were around 40 up to 60 nanograms per ml. To get there you would need to be ingesting a minimum of about 4,000 international units of vitamin D a day. I think that’s where you should be focusing — our hunter-gatherers outside every day, even with the darkest of skin. Mother nature wanted you to be in that range.”

— Dr. Michael F. Holick, Ph.D., M.D., a leading endocrinologist and professor at Boston University, Modern Healthspan, 2025

The gap between these recommendations reflects a genuine scientific debate — not error on anyone's part - about whether the thresholds established primarily from bone-health endpoints are appropriate for evaluating cardiovascular, immune, and metabolic outcomes.

How common is vitamin D deficiency?

By the widely adopted 20 ng/mL threshold, vitamin D deficiency is widespread. Global analyses of data from nearly 8 million participants confirm that deficiency is a genuine public health concern across virtually all regions, including developed nations. In the United States, approximately 35% of the general adult population has serum levels below 20 ng/mL. Higher rates of deficiency are found in those with darker skin pigmentation (who require more sun exposure to produce equivalent vitamin D), elderly individuals (whose skin produces vitamin D less efficiently), those who spend most of their time indoors, and people living at high latitudes.

“We estimate 40% of the world’s population is vitamin D deficient. It’s because you essentially have no vitamin D in your diet. Mother nature had designed us early in evolution to get this essential nutrient by making it in your skin. Our hunter-gatherers were all the time outside making plenty of vitamin D.”

— Dr. Michael F. Holick, Ph.D., M.D., a leading endocrinologist and professor at Boston University, Modern Healthspan, 2025

What Does the Clinical Evidence Actually Show?

This is where the conversation becomes genuinely complex — and where a fair, honest reading of the evidence matters enormously.

What observational studies show about vitamin D

Epidemiological studies have consistently found associations between lower vitamin D levels and higher rates of a striking array of diseases — cardiovascular disease, multiple sclerosis, type 2 diabetes, colorectal cancer, breast cancer, respiratory infections, depression, dementia, and others. These associations are often dose-responsive: as serum 25(OH)D levels rise, risk of these conditions appears to fall.

A meta-analysis of 70 observational studies found a clear relationship between rising vitamin D levels and reduced breast cancer risk. Ecologic studies comparing multiple sclerosis rates across latitudinal gradients — where sun exposure and vitamin D levels differ systematically - show striking differences that are considered one of the most powerful natural experiments in epidemiology.

However, observational studies have a fundamental limitation: they cannot establish causation. Low vitamin D may cause disease, or low vitamin D may be a marker of circumstances that independently cause disease — limited outdoor activity, chronic illness (which depresses vitamin D levels), obesity (which sequesters vitamin D in fat tissue), or other factors correlated with both indoor living and poor health.

“Vitamin D cannot treat osteoporosis. What vitamin D does is critically important for increasing the efficiency of the absorption of calcium from your diet. When you’re vitamin D deficient and calcium absorption is decreasing, your parathyroid glands instantly get turned on — and parathyroid hormone will increase numbers of osteoclasts, leaving a hole in your bone. That collagen gets hydrated like Jell-O, and patients complain of throbbing, aching bone pain, especially in the winter time — often given the diagnosis of fibromyalgia or chronic fatigue syndrome. I would treat them for vitamin D deficiency and they’d have a dramatic improvement in their symptoms.”

— Dr. Michael F. Holick, Ph.D., M.D., a leading endocrinologist and professor at Boston University, Modern Healthspan, 2025

What vitamin D clinical trials actually show

Randomized controlled trials (RCTs) — where participants are randomly assigned to receive a vitamin D supplement or a placebo and outcomes are tracked prospectively — are considered the gold standard for establishing causation. And the RCT evidence has, by and large, been more sobering than the observational data suggested.

The most important RCT to date is VITAL (VITamin D and OmegA-3 TriaL), a large trial involving 25,871 U.S. adults who received 2,000 IU of vitamin D3 daily or a placebo and were followed for a median of 5.3 years. The primary findings showed no significant reduction in cancer incidence or cardiovascular events - the two outcomes the trial was designed to test.

Secondary and ancillary analyses from VITAL have reported some positive signals including reductions in advanced cancer mortality among participants who developed cancer, and reductions in autoimmune disease incidence. But the main headline — that supplementing with vitamin D prevents heart attacks and cancer in people who are not severely deficient — has not been supported by the largest and best-controlled trials.

How can vitamin D studies contradict each other?

Several scientific explanations have been proposed for why observational studies show strong associations that RCTs fail to confirm:

1. Reverse causation. Illness depresses vitamin D levels. People with cancer, heart disease, or other chronic conditions may have low vitamin D not because low vitamin D caused their disease, but because their disease reduced their vitamin D levels. Treating the low vitamin D after disease is established may not undo the underlying process.

2. Correcting severe deficiency matters; optimizing moderate insufficiency may not. Some evidence suggests the biggest gains come from moving people out of genuine deficiency (below 20 ng/mL). Supplementing people who are already moderately replete may add little marginal benefit.

3. Vitamin D is a marker of sunshine, not its replacement. This is perhaps the most provocative and intellectually honest explanation — and one that science journalist Rowan Jacobsen explores in detail in his 2025 book In Defense of Sunlight. If the health benefits attributed to high vitamin D in observational studies are actually driven by the sun exposure that produced the vitamin D — along with all the other biological effects of sunlight — then it follows that a pill providing vitamin D in isolation would not replicate those benefits. Vitamin D, may be a biomarker of overall sun exposure, not the mechanism of benefit.

This hypothesis is supported by studies showing that sun exposure has measurable effects on immune function, blood pressure, and metabolic health that appear to be independent of vitamin D levels — effects not replicated by supplementation alone.

4. Dose timing and form. Most RCTs use daily bolus oral dosing, which produces a rapid spike in circulation followed by a sharp decline — a very different pharmacokinetic profile from the gradual, sustained release of vitamin D3 produced by sun exposure.

The honest summary: The observational evidence is suggestive that vitamin D status, as measured by a 25(OH)D blood test, matters for health across many domains. But the RCT evidence has not confirmed that supplementation alone reliably prevents the diseases associated with low levels. That gap doesn't mean vitamin D is unimportant, it means the blood test is measuring something real, but a pill may not be the most complete way to deliver it. A 25(OH)D result reports a number, not how that number got there. Sun exposure and supplementation can both raise it. 

One hypothesis is that some observational associations attributed to vitamin D may partly reflect other biological or behavioral effects associated with sunlight exposure. Whether these effects translate into clinically meaningful benefits independent of vitamin D remains uncertain. 

The takeaway isn't to dismiss the blood test; it's to treat it as a meaningful marker of status rather than a dial you can turn in isolation and expect the same results sunlight itself would produce.