I bought a vitamin D supplement because of a claim you’ve probably heard: most Americans are low on it, and if you’re tired, that’s likely why.

I took it. I did feel more energy for a while — though I couldn’t tell you if that was the vitamin D, the two protein shakes I drink every day, the immunity booster I take most mornings, my regular multivitamin, or just believing something would work. All four of those already contain vitamin D. So does a fair amount of what I eat on a normal day.

That’s five separate sources of the same nutrient, and I only found out by reading labels I already owned instead of the one I was about to buy.

Which raises the actual question nobody selling vitamin D wants asked: how much am I actually supposed to be taking? The label lists a percentage of the “Daily Value” — a number built for the average adult. Am I the average adult? I don’t know. Neither does the label.

Here’s the mechanic, and it works on any nutrient, not just this one: before you buy a supplement, add up every source of that ingredient already in your cabinet, your shakes, and your regular food. Check your multivitamin first — that’s where it usually hides. If the total’s already near the daily amount, the new bottle isn’t correcting a shortage. It’s paying twice for the same ingredient.

If you take one supplement and you’ve never stacked five products that touch the same nutrient, none of this is costing you anything. Keep doing what you’re doing.

For everyone else: the “Daily Value” on every label was written for a person who doesn’t exist. Guessing at your version of that number, one duplicate bottle at a time, is the current default — because until recently, there wasn’t another option. There is now: a formula built from your own numbers instead of an average adult’s.

The label tells you what’s in the bottle. It never tells you how many other bottles are already giving you the same thing.