"Take once daily" is doing a lot of quiet work on the back of most supplement labels. It implies timing is a non-issue — pick a time, stick to it, done. For a lot of supplements that's true. For a specific handful, it's not even close, and the difference isn't subtle.
The clear case: fat-soluble vitamins need fat
Vitamins A, D, E, and K are fat-soluble, which means your gut absorbs them by packaging them with dietary fat, not water. Take vitamin D on an empty stomach and studies have found absorption can drop substantially compared to taking it alongside a meal that actually contains fat — not a large one, just non-zero. This is the single most well-established timing effect in the entire category, and it's also the easiest to fix: take it with a meal, any meal, as long as something in it isn't fat-free.
The surprising case: magnesium and the evening window
Magnesium doesn't have a strict absorption window the way fat-soluble vitamins do, but it has a different kind of timing sensitivity: it has a mild relaxant effect on the nervous system, which is exactly why a lot of people take it in the evening rather than the morning. Taken earlier in the day, that same effect can just read as grogginess for no obvious reason — easy to blame on something else entirely if you're not tracking when you took it relative to how you felt.
Iron runs the opposite pattern: it absorbs best on an empty stomach, but calcium, coffee, and tea all block that absorption if taken close together — so the "right" time for iron is as much about what to avoid nearby as it is about morning versus evening.
What timing doesn't fix
None of this means timing perfection is the goal. Most of the supplement industry's advice implies precision that doesn't actually move the needle — "exactly 8am" versus "sometime in the morning" makes essentially no difference for the vast majority of what people take. The nutrients above are the real exceptions, not the rule. For everything else, the thing that actually predicts whether a supplement does anything for you is far less exciting: whether you take it consistently, at all, most days — not the seven-minute window you took it in.
That's the harder problem to see, because it's not about a single dose — it's about a pattern over weeks. It's also exactly what Re's reminders and Insights are built around: schedules that respect which supplements are actually timing-sensitive, and check-ins that let you see, from your own data, whether a change in energy or sleep tracks with what you're taking and when — instead of guessing from a label.
