Food · lesson 4 of 12
RDA vs optimal vs upper limit
Why 'enough to avoid disease' isn't the whole story — and why more isn't better.
6 min read · reviewed October 2026
When you see a "% Daily Value" on a label, that's anchored to the RDA (Recommended Dietary Allowance). It's a useful floor — but it answers a narrower question than most people assume.
RDAs prevent deficiency — not necessarily optimize
An RDA is set to meet the needs of ~97–98% of healthy people and prevent deficiency disease (scurvy, rickets, anemia). That's not the same as the intake that best supports long-term health or performance. For some nutrients those numbers are close; for others (vitamin D is the classic debate), many researchers argue the optimal range sits above the bare-minimum RDA.
What's the adult RDA for vitamin C?
Somewhere between 10 and 200 mg.
Reveal the answer →
90 mg (men) / 75 mg (women)
The adult RDA for vitamin C is 90 mg/day for men and 75 mg for women (smokers need ~35 mg more). That's the amount that comfortably prevents scurvy and keeps tissues saturated — reachable from a couple of servings of fruit and vegetables. Megadoses don't raise blood levels much further because absorption is tightly regulated and the excess is excreted.
The U-shaped curve: too little AND too much both harm
For most nutrients there's a sweet spot. Below it you get deficiency problems; above it, especially with supplements, you can hit toxicity or other harm. More is not automatically better.
How it works →
Plot health outcome against intake and you often get a U (or J) shape: risk is high at low intake, drops to a broad minimum in the adequate range, then climbs again at very high intake. The rising right-hand arm reflects toxicity (vitamin A, D), pro-oxidant or interfering effects (very high iron, zinc displacing copper), or downstream imbalance. The flat bottom is why 'topping up a gap' helps but 'loading way past your needs' usually doesn't.
What the studies show →
Several large trials cooled early enthusiasm for high-dose single nutrients. Beta-carotene supplements raised lung cancer risk in smokers (CARET, ATBC trials). High-dose vitamin E and 'antioxidant cocktail' trials largely failed to show benefit and sometimes hinted at harm. The lesson across this literature: correcting a real deficiency is valuable, but pushing a nutrient far above need in already-replete people rarely helps and can backfire.
The Upper Limit (UL) is the ceiling
The Tolerable Upper Intake Level is the most you can take daily with little risk of harm. It mostly matters for supplements (food rarely gets you there). Watch it with vitamin A, vitamin D, iron, zinc, niacin, and B6 — all have documented problems above their ULs. Between the RDA (floor) and the UL (ceiling) is your safe operating range.
Your vitamin C intake already meets the RDA. What does taking a 1,000 mg supplement on top mostly do?
- Dramatically boosts blood and tissue levels
- Is largely excreted — absorption is regulated and tissues are already saturated
- Cures the common cold reliably
- Builds up to dangerous levels over time
Show the answer →
B.Is largely excreted — absorption is regulated and tissues are already saturated
Vitamin C absorption is tightly controlled and tissue stores saturate at modest intakes, so most of a megadose is simply excreted. It won't reliably prevent colds, and while it's not stored to toxic levels, very large doses can cause GI upset and, in susceptible people, kidney stones.