Hi {{first name | there}},

I used to think most vitamin D supplements were a waste of money.

The sales pitch bothered me: one capsule for immunity, depression, testosterone, cancer, and a longer life, all built on observational studies and an elegant mechanism. Then large randomized trials found no broad benefit for cardiovascular disease, cancer incidence, fractures, or depression in generally healthy adults.

I filed vitamin D under “necessary in rare cases, overhyped for everyone else.”

I was too skeptical.

Vitamin D behaves more like a threshold than a performance drug: too little matters; once you have enough, more usually does not help.

If you have a vitamin D result sitting in your portal or a bottle in your cabinet, this issue is meant to help. I’m sharing the exact test I order, how I read the ranges, when I supplement, what I think about results above 50, and whether K2 or magnesium belongs in the plan. I also made a clear guide at the end you can save for your next lab review.

Long before wellness influencers found vitamin D, it had already produced one of nutrition’s great public-health victories.

Children with rickets had soft, poorly mineralized bones. Cod-liver oil, ultraviolet light, and later food fortification prevented or healed the disease. Severe deficiency was causal, and replacing the missing input worked.

The fear of excess is old, too. In 1950s Europe, concerns about infant hypercalcemia and vitamin D intoxication helped trigger restrictions on food fortification. Vitamin D has always had two real failure modes: too little and too much.

Decades later, low vitamin D levels were associated with almost every chronic disease imaginable. The biology sounded plausible: vitamin D receptors exist throughout the body.

But low 25-hydroxyvitamin D also travels with less time outdoors, winter, higher adiposity, frailty, chronic illness, inflammation, and poorer diet. It was easy to mistake a marker of poor health for the cause of all of it.

Testing exploded. Thresholds disagreed. In a same-specimen laboratory comparison, different laboratories classified anywhere from 12% to 41% of the samples as below 20 ng/mL.

Even the famous 20 ng/mL cutoff was widely misread. It was designed to cover the requirements of roughly 97.5% of the population, not to declare every individual below it diseased. A 2016 NEJM critique argued that treating that population boundary as a personal diagnostic line helped manufacture a “pandemic” of deficiency.

This is also why doctors have such conflicting opinions about vitamin D. “Does more help someone who already has enough?” and “Should we correct deficiency?” sound like one question, but they are not.

VITAL randomized more than 25,000 adults to vitamin D3 2,000 IU/day or placebo. Vitamin D did not significantly reduce cancer incidence or major cardiovascular events. Its ancillary studies were also neutral for fractures and depression.

What changed my read was the starting level: among participants with measurements, the average was 30.8 ng/mL.

VITAL mostly tested whether giving more vitamin D helped people who were already adequate. It did not test leaving people at 8 or 10 ng/mL untreated.

I still would not prescribe vitamin D to everyone as broad disease prevention. The trial simply did not answer whether deficiency should go untreated.

I realized I had been asking a drug trial to answer a nutrient question.

My mental model now is:

deficient → inadequate → adequate → no extra benefit → possible harm

Once someone is adequate, a higher number usually adds nothing. Deficiency remains a different problem.

My practice changed on the low end of the curve. I correct real deficiency. I am often comfortable with modest daily D3 when sun and dietary intake are predictably low.

I still do not recommend megadoses, high-number chasing, or vitamin D as a cure-all.

I also screen everyone I care for with a baseline total 25-hydroxyvitamin D, even though the 2024 Endocrine Society guideline recommends against routine testing in generally healthy adults.

The guideline has to answer a population question: does broad screening improve hard outcomes enough to recommend it for everyone? In clinic, 8, 28, and 78 ng/mL lead to very different conversations. The same test finds both a missing input and someone unknowingly stacking vitamin D across several products.

That second problem is becoming less hypothetical. In US data from 2007 through 2023, vitamin D inadequacy fell from 18.8% to 15.6%, while high 25(OH)D rose from 2.2% to 7.6%. Among people taking at least two vitamin-D-containing supplements, 30.6% had high levels in 2021–2023.

A test result is not an instruction to treat. It tells me which physiology I am dealing with. Guidelines help me interpret the evidence; the patient’s actual result still matters.

I look more closely with osteoporosis or fragility fracture, malabsorption or bariatric surgery, chronic kidney or liver disease, or abnormal calcium/PTH/phosphate physiology. Kidney stones, hypercalciuria, granulomatous disease, certain medications, and persistent nonresponse to a reasonable dose also change the read.

If I do test, I order total serum 25-hydroxyvitamin D [25(OH)D]. I do not use 1,25-dihydroxyvitamin D to screen for ordinary deficiency.

I do not use one universal cutoff. This is the practical version:

There is no single “optimal” number for every person. The reason for testing and the rest of the mineral and bone picture still matter.

A level of 51 ng/mL is not the same as vitamin D toxicity. The NIH describes classic toxicity with hypercalcemia and levels typically above 150 ng/mL. It also recommends avoiding sustained levels around 50–60 ng/mL or higher. I do not intentionally target above 50 for general health or longevity. There is no demonstrated extra benefit.

I still remember one patient who landed in the cardiac ICU with heart block while taking very high-dose vitamin D as part of a sprawling supplement regimen. She had gotten the advice from someone calling himself the “Vitaminister.” It was an extreme case, not what happens with ordinary supplementation. But I think of her every time someone walks in with a 10,000 IU bottle.

For routine supplementation, D3 is the practical default. I prefer daily dosing to large intermittent boluses. In a randomized trial of older women, those who received 500,000 IU once a year had more falls and fractures.

A 500,000 IU bolus is not biologically equivalent to taking about 1,370 IU each day, even though the annual totals are similar.

Sunlight advice often mixes up two separate pathways.

UVB hitting your skin can make vitamin D. Visible light reaching your eyes helps set your circadian clock. Those are different wavelengths, tissues, and outcomes.

A D3 pill can replace the vitamin D input. It cannot replace bright daytime light for sleep-wake timing. Morning outdoor light can be useful for your clock even when the UVB is too weak to make much vitamin D.

How weak? Classic latitude experiments found no detectable winter vitamin D production in skin in Boston from November through February or Edmonton from October through March under the tested conditions. Winter sunlight simply did not supply enough UVB. And ordinary window glass blocks nearly all UVB, so a sunny window can be circadian light without being a vitamin D intervention.

Oral D3 was more reliable in one randomized trial of vitamin-D-deficient young adults: 20 to 30 minutes of noon sun raised vitamin D less than 500 IU/day of D3.

I still want people outside for daylight, movement, mood, and circadian biology. Look normally at the sky and your surroundings, not directly at the sun, and do not skip sunscreen to improve a lab result. Use food or a modest supplement when vitamin D input is predictably low.

UVB at the skin, visible light at the eye, and oral D3 enter through different pathways. This is a conceptual map, not a literal anatomical diagram.

People almost always ask whether D3 needs K2 and magnesium. Usually, no.

K2 and magnesium are not automatic add-ons.

Vitamin K activates proteins involved in bone and vascular tissue, and K2 reliably improves markers showing that those proteins are being carboxylated.

Supplement labels usually tell a stronger story: vitamin D absorbs calcium, K2 “directs” it into bone rather than arteries, and vitamin D without K2 is therefore dangerous.

Human trials have not shown it.

In one particularly useful three-year study, postmenopausal women with osteopenia were already receiving calcium and about 1,520 IU/day of D3. Adding MK-7 reduced undercarboxylated osteocalcin by 65%. It did not improve bone density, bone microarchitecture, or trabecular bone score.

In AVADEC, a much larger 720-mcg dose of MK-7 plus D3 dramatically improved a vitamin K biomarker. It did not slow aortic-valve or coronary calcification overall.

In both trials, K2 changed the lab marker without improving the clinical outcome.

The dose comparison is also slippery. Japan uses prescription MK-4 for osteoporosis at 45 mg/day, or 45,000 mcg/day. That is hundreds of times the dose in a typical 90–180 mcg MK-7 supplement. You cannot borrow evidence from the pharmacologic regimen and paste it onto an ordinary D3/K2 bottle. The broader vitamin K trial review becomes much less impressive when restricted to lower-risk-of-bias studies.

For most healthy adults taking ordinary D3, I consider K2 optional, not mandatory. It may be relevant in actual vitamin K deficiency, malabsorption, cholestatic disease, bariatric anatomy, prolonged broad-spectrum antibiotic exposure, or specialist-directed bone care. Even then, clinical deficiency is often treated with K1, not automatically K2.

Vitamin K can antagonize warfarin. Even low-dose MK-7 changed INR in some people taking a vitamin-K antagonist, so do not add it casually.

K2 also does not make excessive vitamin D safe. It has not been shown to prevent vitamin-D-related hypercalcemia, hypercalciuria, kidney stones, or kidney injury.

Magnesium is part of vitamin D metabolism, PTH secretion and action, and calcium handling. Severe magnesium deficiency can suppress PTH, create resistance to PTH, lower active vitamin D, and make hypocalcemia look resistant to calcium or vitamin D treatment.

In that situation, restoring magnesium can be essential.

Online, “magnesium is required for vitamin D metabolism” often becomes “vitamin D will not work unless everyone takes magnesium.”

We do not have evidence for that jump.

The trials we have are small and mostly measure biomarkers. One 180-person analysis found that magnesium changed vitamin D metabolites differently depending on the starting vitamin D level. But only two participants were profoundly deficient, and the analysis excluded nearly one-third of the randomized sample.

Another trial in 95 adults with overweight or obesity was often described as using 360 mg of magnesium glycinate, but the compound supplied only 50.76 mg of elemental magnesium. The combination had the largest within-group vitamin D rise, yet the incremental difference from D3 alone was not statistically significant. It did not improve PTH or inflammatory markers either.

We do not have a large randomized trial showing that adding magnesium to D3 prevents fractures, falls, cardiovascular events, or other patient-important outcomes.

I correct magnesium deficiency. I do not add magnesium automatically to every D3 regimen.

Magnesium deserves more attention with chronic diarrhea or malabsorption, bariatric surgery, alcohol dependence, type 2 diabetes with urinary losses, older age with low intake, long-term PPI use, diuretics, certain chemotherapy or transplant drugs, hypocalcemia/hypokalemia, or an unexpectedly poor response to calcium and vitamin D.

Serum magnesium is imperfect because less than 1% of body magnesium is in serum. RBC magnesium is not a definitive wellness gold standard either. We do not have a universal “magnesium optimization” test.

For everyone else, I would start with food: seeds, nuts, beans, leafy greens, whole grains. If intake is predictably low, a modest supplement can be reasonable for magnesium adequacy, not because D3 requires a pill partner.

There is no validated vitamin-D-to-magnesium ratio. Routine D3 has not been shown to deplete magnesium, and magnesium does not make a megadose safe.

For most adults:

If your vitamin D is genuinely low, treat it with a defined plan. If it does not rise, do not escalate forever. Check adherence, meal timing, malabsorption, bariatric history, adiposity, medication effects, liver/kidney physiology, assay variability, and magnesium risk when the rest of the mineral pattern supports it.

I put the detailed testing thresholds, forms, dose ranges, nonresponse checklist, sunlight questions, and evidence on K2 and magnesium into a separate guide:

Read the full vitamin D guide: sunlight, testing, dosing, K2, and magnesium

If you are low, correct it. If your intake is predictably poor, a modest daily dose may be reasonable. Once you are adequate, stop trying to turn vitamin D into a longevity drug.

Hillary Lin, MD

I made a video about why a supposedly “normal” vitamin D result can still deserve a closer look. I would frame it a little differently now: measure the actual level, decide whether vitamin D is a bottleneck, and stop treating “optimized” as a reason to keep pushing the number higher.

Why You Still Feel Terrible With 'Normal' Vitamin D Levels

Watch on YouTube | Subscribe to The Longevity Show

🫘 SGLT2 inhibitors are not just diabetes drugs anymore. In adults 65 and older with CKD, they cut kidney events and cardiovascular death or heart-failure hospitalization, with the familiar tradeoff of more genital infections and volume depletion.

🧠 More movement keeps showing up in dementia data. Across two long-running US cohorts, the most active adults had 28% lower dementia risk than the least active, although this is still observational.

📞 An AI voice agent handled the pre-cath calls; nearly nine in ten patients completed them. In a 1,606-call implementation study, system errors fell from 6.0% to 2.6%.

🐟 Six grams of omega-3s changed muscle fatty-acid composition, but not strength or growth. That was the result of a 14-week randomized trial in trained men already eating enough protein.

🍵 L-theanine looked more convincing for reaction time than stress. A meta-analysis of 31 randomized trials found that the stress evidence leaned heavily on high-risk-of-bias studies.

🔬 Pancreatic tumors build a support network around themselves. A Science study mapped the immune-cell and fibroblast niche that expands around them. Fascinating biology, but not a treatment result yet.

Science of Skin Summit, September 17–20, Austin, TX. I’ll be speaking on AI in dermatology and skin and hair as windows into biological age.

MVMNT Longevity Summit, September 22–23, Coronado, CA. I’m slated for a lead session and the closing keynote panel; final timing is still being worked out.

Livelong Women’s Health Summit NYC, September 25–26, New York. Women’s health and longevity; my role is still being finalized.

No vitamin D affiliate in this issue. If you use any of these, the links help support the newsletter:

If you want physician-guided biological age testing, start on the testing page. It includes epigenetic age testing with consultation, plus organ-specific proteomic and SystemAge assessments when they fit the clinical question.

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