I got back from Greece a few days ago and somewhere between the laundry and wading through my inbox, I found about a dozen versions of the same headline waiting for me. Your supplements might be harming your brain.

One of you even forwarded me an AI summary that named names. Vitamin D, fish oil, glucosamine, all supposedly linked to a faster-aging brain. It read as very confident and very tidy.

So I did what I always do, I went and read the actual studies. The real story is more interesting than the headline, so let's chat about it.

Quick reminder my filter...a claim has to clear a few questions before it moves me. Is this human data or a mouse in a lab? Is it a randomized trial or just an observed association? And the big one, is there other good evidence pointing a different way that got left out. Most scary headlines don't pass the last one.

Here's what I found.

Vitamin D first. There's a real study behind this one, a BMJ Open cohort of just over 5,000 older adults followed for six years and vitamin D users did show faster cognitive decline. But this is the part the headline skipped; it's an observational study, so it can only show a link, not a cause. The decline only showed up in people who already had normal vitamin D levels. In the people who were actually deficient, there was no such signal.

If that sounds familiar, it should. It's the exact thing I said in the previous supplement newsletter. If you're not deficient, you're not getting a benefit. Now there's also a hint that topping up when you don't need it may not be perfectly free either. The study authors themselves say their results don't support giving vitamin D to people who already have enough. That's not a reason to panic. It's a reason to test before you top up.

Fish oil is the messy one,and it's where that AI summary really showed its hand. Yes, there's a 2026 analysis linking omega-3 supplements to faster cognitive decline. It's real. But a much larger review, pulling together 48 studies and over 100,000 people, found that people getting more omega-3, mostly from their diet, had around 20 percent lower risk of dementia or decline. And in one group tracked within that review, long-term supplement users did best of all. Same nutrient, serious studies, opposite conclusions.

That contradiction is not something to hide. It is the answer. When careful studies disagree this much, the honest read is that it is not settled yet. One alarming headline is not a reason to change a routine. It's a reason to wait for the weight of the evidence to tip.

There is one other fish oil finding I found interesting and it has nothing to do with your brain. Randomized trials covering more than 80,000 people found that marine omega-3 slightly raised the risk of atrial fibrillation, a heart rhythm problem and it was dose-related. Above one gram a day the signal was clearly bigger than at or below one gram. That's randomized-trial evidence, not a scary correlation, so it's the one I noticed more. For what it's worth, the big trial I mentioned in the supplement letter used one gram a day, and that's about what I take. If you're on a mega-dose for no particular reason, that's worth a conversation with your doctor.

Glucosamine got swept into the scare too and this is the thinnest evidence of all of them. The alarming part rests on a mouse study and a couple of papers that haven't even been peer-reviewed yet. One of those papers already pokes holes in the other. The real, well-established caution on glucosamine isn't your brain, it's that it can interact with blood thinners like warfarin. If that's you, ask your doctor. Otherwise this one does not earn much worry.

So what actually protects an aging brain? The least clickable answer, which is usually the true one. Eat fish you enjoy a couple of times a week, because food beats a capsule nearly every time. Keep moving and keep lifting, because better blood flow and lower inflammation do more for your brain than anything in a bottle. If you do want to supplement, test first, so you're correcting something real instead of guessing. That's why I'm a big fan of services like Superpower and Hundred Health, that let you take testing into your own hands.

I still take my vitamin D and my fish oil, at that one gram. I'm not going to tell you they're proven to save your brain or hurt it and I'm not going to throw them out because a tidy summary left out half the research. The whole point is to not be fearless or fearful based on one headline. Now more than ever it's important to go read the actual studies and not rely on what the media and AI are putting out in the headlines.

If there's one thing to take away from all of this, it's the same thing I said before. Test, don't guess. Get your vitamin D and your omega-3 measured before you add or drop anything, so you're working from your own numbers instead of someone's headline. If you want a simple place to start, here's the testing I pointed you to last time.

If your numbers say you do need something, what's in the bottle matters as much as what's on the label. Supplements aren't approved before they hit the shelf. The legal minimum is that they're made in a facility following good manufacturing practices (GMP) and that's the minimum not a guarantee. What I look for on top of that is third-party testing, meaning an outside lab has checked that what's in the capsule matches the label and doesn't carry anything it shouldn't.

Fish oil is where this matters most, because fish oil goes rancid. When researchers in New Zealand tested every fish oil capsule on their shelves, most were more oxidized than recommended limits, only 3 of 32 actually contained the omega-3 their labels promised and price and best-before date were poor clues to which ones were good. Rancid isn't just a waste of money. In a small randomized trial of 54 healthy adults, good-quality fish oil improved LDL cholesterol particles over seven weeks, while the same dose of oxidized fish oil didn't.

So my take is simple. I'd rather you take no fish oil than rancid fish oil. If yours smells or tastes strongly fishy, or the bottle has been sitting in a warm cabinet for a year, toss it.

As for where I buy mine, I don't have one loyal brand, but several trusted brands I have thoroughly vetted. I rotate between Puori, Kion, Momentous and UpNourish for my vitamin D and fish oil, mostly based on who has the better price that month.

Hit reply and tell me which headline got you, or which supplement you've been going back and forth on. I read every single one,and I'm happy to tell you what I think of what you're taking.

Have a great weekend!

P.S. Remember that final panel I ran right before Greece? The results are back and I'm pulling together what actually moved over the protocol and what didn't, with the variables held as steady as I could manage. That's coming next week, the honest version, numbers and all. If you want the background first, the running write-up is here.