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Glossary

What is the Tolerable Upper Intake Level (UL)?

Written by the Full Measure editorial team, which sells supplements. Approved for publication by the Full Measure owners. Updated .

Drafted with AI assistance from the sources listed below. How we write these

Quick answer

The Tolerable Upper Intake Level (UL) is the most of a nutrient you can take each day with adverse health effects unlikely. It is a ceiling, not a target. For adults the vitamin D UL is 100 mcg (4,000 IU) a day, and the UL for magnesium from supplements is 350 mg.1,2

Key facts

  • Some ULs count every source of a nutrient; others cover only supplements or only certain forms.2,3,4
  • The magnesium UL counts only supplements and medicines, which is why its 350 mg is below the adult RDA of 400 to 420 mg for men.2
  • The adult folate UL of 1,000 mcg covers folic acid and other synthetic forms, not folate in food.4
  • The adult vitamin A UL of 3,000 mcg covers preformed vitamin A, such as retinyl palmitate, not beta-carotene.3
  • Some nutrients have no UL; for biotin, there is no evidence in humans of toxicity at high intakes.5
  • ULs do not apply to people taking higher amounts under medical supervision.4,6

Adult ULs for common supplement nutrients

Tolerable Upper Intake Levels for adults 19 and older, from NIH fact sheets1,2,3,4,6,7,8,9,10,11
NutrientAdult UL
Vitamin D100 mcg (4,000 IU)
Vitamin A, preformed3,000 mcg
Vitamin E, supplemental alpha-tocopherol1,000 mg
Vitamin C2,000 mg
Niacin, supplemental35 mg
Folic acid and other synthetic folate1,000 mcg
Calcium2,500 mg to age 50, then 2,000 mg
Iron45 mg
Zinc40 mg
Magnesium, supplemental350 mg

Children have lower ULs, which step up with age: for zinc, 7 mg at ages 1 to 3 and 23 mg at ages 9 to 13.11

Check what each UL covers

  • Magnesium: only supplements and medicines count toward the 350 mg UL, not magnesium found naturally in food and drinks.2
  • Folate: the UL is for synthetic forms in supplements and fortified foods, and it is stated in plain mcg, not mcg DFE. 1,000 mcg of folic acid equals about 1,667 mcg DFE.4
  • Niacin: the 35 mg UL applies to supplemental niacin and was set from the amount linked to skin flushing.6
  • Vitamin A: the UL covers preformed vitamin A only. A product with 3,000 mcg RAE, 60 percent of it as beta-carotene, has 1,200 mcg of preformed vitamin A.3

Why ULs matter when you take more than one product

NIH advises people who eat fortified foods or take several supplements to check that the total does not push any vitamin or mineral above its upper limit. Vitamins and minerals are added to many foods, especially breakfast cereals and drinks, so intake can be higher than people expect.12,13

A UL is not the same as a Daily Value. For magnesium from supplements the 350 mg UL is below the 420 mg Daily Value, so a supplement providing 100% of the Daily Value as supplemental magnesium is above the adult UL.2,14

Going over a UL

Long term intakes above a UL raise the chance of adverse effects, and the effects depend on the nutrient. NIH lists diarrhea, nausea and abdominal cramping from high doses of supplemental magnesium, and birth defects from preformed vitamin A above the UL in pregnancy.2,3,7

Higher doses given under medical supervision, such as prescribed niacin or folic acid, fall outside the ULs. Do not go above a UL on your own; ask your doctor or pharmacist first.4,6

Questions people ask

Is the UL a recommended amount?

No. It is a ceiling for daily intake, not a goal. The amounts to aim for are the RDA or Adequate Intake; for vitamin D, the adult RDA is 15 mcg and the UL is 100 mcg.1

If a nutrient has no UL, can I take any amount?

No UL does not mean no risk. Biotin has none because toxicity has not been shown in humans, yet NIH notes high biotin intakes can give falsely high or low results on some lab tests. No UL is set for beta-carotene either, but the board advises against beta-carotene supplements for the general population except as a vitamin A source.3,5

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Sources

  1. NIH Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals. Updated June 27, 2025. Opened 2026-10-07.
  2. NIH Office of Dietary Supplements. Magnesium: Fact Sheet for Health Professionals. Updated January 6, 2026. Opened 2026-10-07.
  3. NIH Office of Dietary Supplements. Vitamin A and Carotenoids: Fact Sheet for Health Professionals. Updated March 10, 2025. Opened 2026-10-07.
  4. NIH Office of Dietary Supplements. Folate: Fact Sheet for Health Professionals. Updated November 30, 2022. Opened 2026-10-07.
  5. NIH Office of Dietary Supplements. Biotin: Fact Sheet for Health Professionals. Updated January 10, 2022. Opened 2026-10-07.
  6. NIH Office of Dietary Supplements. Niacin: Fact Sheet for Health Professionals. Updated November 18, 2022. Opened 2026-10-07.
  7. NIH Office of Dietary Supplements. Vitamin E: Fact Sheet for Health Professionals. Updated March 26, 2021. Opened 2026-10-07.
  8. NIH Office of Dietary Supplements. Vitamin C: Fact Sheet for Health Professionals. Updated July 31, 2025. Opened 2026-10-07.
  9. NIH Office of Dietary Supplements. Calcium: Fact Sheet for Health Professionals. Updated June 22, 2026. Opened 2026-10-07.
  10. NIH Office of Dietary Supplements. Iron: Fact Sheet for Health Professionals. Updated September 4, 2025. Opened 2026-10-07.
  11. NIH Office of Dietary Supplements. Zinc: Fact Sheet for Health Professionals. Updated January 6, 2026. Opened 2026-10-07.
  12. NIH Office of Dietary Supplements. Multivitamin/mineral Supplements: Fact Sheet for Consumers. Updated February 15, 2024. Opened 2026-10-07.
  13. NIH Office of Dietary Supplements. Dietary Supplements: What You Need to Know. Fact Sheet for Consumers. Updated January 4, 2023. Opened 2026-10-07.
  14. U.S. Food and Drug Administration. Daily Value on the Nutrition and Supplement Facts Labels. Content current as of March 5, 2024. Opened 2026-10-07.

This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.