I Took 40,000 IU of Vitamin D. Bloodwork Saved Me.

Mike HartnettEvidence-Based

7+ years researching longevity protocols. Runs a 20+ supplement daily stack. Built CoreStacks after losing his father to heart disease.

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I was taking 40,000 IU of Vitamin D every single day. I felt fine. No symptoms at all — no nausea, no fatigue, no warning signs. If I hadn’t gotten bloodwork done through my doctor, I would have kept taking it for months. Maybe years. My levels came back dangerously high. I immediately cut my dose to 10,000 IU. That single blood test probably prevented real damage. It’s the reason I tell everyone to get tested before they supplement.

This content is for informational purposes only. Consult your healthcare provider before starting any supplement protocol.

How I Got to 40,000 IU

It didn’t happen overnight. Nobody wakes up and decides to take 40,000 IU of anything.

I started listening to longevity podcasts. Huberman discussed vitamin D. Attia mentioned it as foundational. Every expert seemed to agree: most people are deficient, higher doses are often needed, and the typical 600-1000 IU recommendation is probably too low. I agreed with all of that. I still do.

So I started supplementing. D3 is cheap — $10-12 per month. I was already taking a multivitamin that contained some D3. Then I added a standalone D3 capsule. Then I switched to a higher-dose capsule because I figured I wasn’t getting enough sun (I work indoors, live in the northeast). The doses stacked. I wasn’t tracking the total across all sources.

It crept up over time. I wasn’t being reckless. I was doing what I’d heard on podcasts and read in articles. Nobody told me to get a blood test first. That was the missing step.

The Bloodwork That Changed Everything

I got a standard blood panel through my doctor. Nothing special — just the annual checkup I should have been getting all along. The panel included a 25-hydroxy vitamin D test, which is the standard way to measure your D status.

My results came back significantly above the optimal 40-60 ng/mL range that most longevity experts recommend. Well above it. My doctor flagged it immediately.

I remember the conversation clearly. He asked what supplements I was taking. When I added up the vitamin D from all sources, the number was around 40,000 IU daily. His reaction told me everything I needed to know.

I cut to 10,000 IU that day. Not gradually — immediately. I also started paying much closer attention to what was in every supplement I took, checking labels for overlapping ingredients. That habit stuck.

For anyone who thinks bloodwork is optional, this is my answer. I felt perfectly fine at dangerously high levels. My body gave me zero signals. The only thing that caught it was a $30 blood test. If you’re supplementing without testing, you’re flying blind. My blood testing guide covers exactly which panels to request.

Why Vitamin D Toxicity Is Real

I’m not trying to scare you away from vitamin D. Most people taking normal doses (2,000-5,000 IU daily) are completely fine. Vitamin D deficiency is a real problem and supplementation helps millions of people.

But vitamin D is fat-soluble. That’s the key distinction. Water-soluble vitamins like C and B-complex — your body flushes the excess through urine. Fat-soluble vitamins (A, D, E, K) get stored in your fat tissue and liver. They accumulate over time. You can’t just pee out the extra.

When vitamin D levels climb too high, it causes your body to absorb too much calcium from food. That excess calcium — hypercalcemia — can cause nausea, vomiting, weakness, and kidney problems. Over time, calcium deposits can build up in your kidneys and blood vessels. In severe cases, it leads to kidney stones, kidney failure, or cardiac issues.

The people most at risk are the ones doing what I did: stacking D3 from multiple sources without tracking the total, megadosing because they heard higher is better, and never getting a blood test to check where they actually stand. The D3+K2 guide I wrote covers safe dosing ranges based on what the research shows.

The Optimal Vitamin D Range

Most longevity experts target a blood level of 40-60 ng/mL for 25-hydroxy vitamin D. Some push toward 50-80 ng/mL. Huberman has discussed targeting the 40-60 range on his podcast. Attia aims for a similar window. Stanfield emphasizes evidence-based ranges.

The dose you need to reach that range depends on your starting level, body weight, skin color, sun exposure, and genetics. That’s exactly why a blood test matters — there’s no universal dose. Someone in Miami who works outside might only need 1,000 IU. Someone in Minnesota who works indoors might need 5,000-10,000 IU.

A common starting dose for adults is 2,000-5,000 IU daily. If you’re currently taking more than 5,000 IU without having tested your levels, get tested. Not next month. Now. My analysis of 5,000 IU dosing covers whether that amount is safe for most people.

How to Get Your Vitamin D Tested

This is simpler than most people think.

Option 1: Ask your doctor. Request a 25-hydroxy vitamin D test at your next appointment. Most doctors will include it in a standard blood panel without pushback. Insurance usually covers it as part of annual bloodwork, especially if you mention fatigue or bone health concerns.

Option 2: Direct-to-consumer testing. Services like InsideTracker, Marek Health, and Quest Direct let you order your own blood tests without a doctor’s visit. You pay out of pocket ($30-100 for a vitamin D test) and go to a local lab for the blood draw. I reviewed the best options in my blood test services comparison.

What to test: The specific test is “25-hydroxy vitamin D” (also written as 25(OH)D). This measures the circulating form of vitamin D in your blood. Do not accept a 1,25-dihydroxy vitamin D test — that’s a different measurement and doesn’t tell you what you need to know about your supplementation status.

How often: Test once before you start supplementing. Test again 3 months after starting. Once you’ve dialed in your dose and your levels are stable, annual testing is sufficient unless you change your dose or sun exposure significantly.

My Current D3 Protocol

After the scare, here’s where I landed:

• Dose: 10,000 IU vitamin D3 daily
• With K2: Always. K2 (MK-7 form) helps direct calcium to your bones and teeth instead of your arteries and soft tissue. Taking high-dose D3 without K2 is a mistake. I wrote about why D3 without K2 can be problematic.
• Brand: I use Sports Research D3+K2 softgels. Affordable, combined formula, one capsule per day.
• Timing: Morning, with food containing fat (D3 is fat-soluble and absorbs better with dietary fat)
• Cost: ~$10-12 per month
• Monitoring: I retest every 6 months to make sure my levels stay in the 40-60 ng/mL range

10,000 IU is still higher than the standard RDA of 600-800 IU. But my bloodwork supports it at this level. That’s the point — the right dose is the one that puts YOUR blood levels in the optimal range. Not the dose a podcast recommended. Not the dose your friend takes. Your dose, verified by your blood test.

The Bigger Lesson: Supplements Aren’t “More Is Better”

My vitamin D experience changed how I think about every supplement I take. The “more is better” mindset is dangerous with fat-soluble compounds, and it’s misleading even with water-soluble ones.

Magnesium at very high doses causes GI distress — diarrhea, cramping, nausea. I’ve seen people take 800mg+ of magnesium oxide and wonder why their stomach is wrecked. Taking the right form at the right dose matters more than taking more. I use magnesium glycinate specifically because the absorption is better and it doesn’t wreck your gut.

Too much zinc depletes copper over time. They compete for the same absorption pathways. I dropped zinc from my own stack because my levels were already adequate — I didn’t need more.

Even NAC, which I take daily, has concerns at very high doses. The body has a balance, and shoving more of any single compound into it isn’t a strategy. It’s gambling.

Blood testing is the only way to optimize instead of guess. The experts I follow all agree on this, even when they disagree on nearly everything else. Attia’s entire medical practice is built on biomarker tracking. Huberman routinely tells his audience to test before supplementing. Sinclair monitors his own biomarkers obsessively.

I spent 7 years learning this lesson the hard way. You can learn it in the time it takes to schedule a blood draw. I document all of my supplement-related bloodwork trends in my 3-year supplement bloodwork results. A basic panel through your doctor or a direct-to-consumer service costs less than one month of supplements. That’s the best investment in your stack that you’ll ever make.

If you want to see what my full supplement protocol looks like after everything I’ve learned, I break it down in my complete 8-supplement stack. And the 5 biggest mistakes I made covers the other hard lessons beyond vitamin D.

Get tested. Seriously. Your body won’t always tell you something is wrong.