Do multivitamins break a fast?
A plain multivitamin tablet does not meaningfully break a fast — the vitamins themselves contain negligible energy. Gummy vitamins do break a fast: they are sugar-based and typically contain 2 to 4 grams of sugar per serving. The real issue is absorption rather than fasting: fat-soluble vitamins are absorbed poorly without dietary fat, so taking them fasted mostly wastes them.
| Fasting for | Verdict | Why |
|---|---|---|
| Weight loss | Depends | Tablets and capsules are effectively 0 kcal. Gummies are roughly 15 to 30 kcal per serving. |
| Autophagy | Depends | Micronutrients are not mTOR signals. Gummy versions contain sugar and do break it. |
| Blood work | Depends | Biotin in particular interferes with several common immunoassays, including thyroid and troponin tests. Tell the clinic what you take. |
| Ketosis | Depends | A multivitamin tablet or capsule contains no meaningful carbohydrate and does not interrupt ketosis. Gummy vitamins carry 2 to 4 g of sugar per serving and do. |
| Gut rest | Depends | Iron and zinc on an empty stomach commonly cause nausea. This is the most frequent practical complaint. |
Why
Vitamins are cofactors, not fuel: they are required in milligram and microgram quantities and supply no usable energy, so a tablet cannot break a fast on the energy or insulin test. The more useful information is that fasting is a bad time to take half of them. Vitamins A, D, E and K are fat-soluble and depend on dietary fat for micelle formation and intestinal uptake — the mechanisms of vitamin E intestinal absorption and the determinants of vitamin D bioavailability are both well characterised, and both require lipid to be present. Broader work on fat-soluble vitamin and phytochemical metabolites reaches the same conclusion about gastrointestinal absorption generally. So a fasted multivitamin does not break your fast and does not do much else either. Take it with the meal that ends the fast.
Effectively 0 kcal for tablets and capsules. Approximately 15 to 30 kcal for gummies.
The exceptions
This is usually where the real question lives.
- Gummy vitamins are sugar confectionery with vitamins in them. They break a fast.
- Fat-soluble vitamins (A, D, E, K) are poorly absorbed without fat. Taking them fasted is largely wasted.
- Biotin interferes with common lab immunoassays — a laboratory assessment found the effect across a range of assays at supplement-level concentrations. Tell the clinic what you take before blood work.
- Iron on an empty stomach causes nausea in many people, though it is absorbed better that way.
Sources
- Comprehensive assessment of biotin interference in immunoassays — Clin Chim Acta, 2018PMID 30296442
What it does not show: Tests which assays are affected and at what biotin concentrations. It does not estimate how often supplement users are actually misdiagnosed.
- Vitamin E Bioavailability: Mechanisms of Intestinal Absorption in the Spotlight — Antioxidants (Basel), 2017PMID 29165370
- Vitamin D bioavailability: state of the art — Crit Rev Food Sci Nutr, 2015PMID 24915331
- Fat-soluble vitamin and phytochemical metabolites: Production, gastrointestinal absorption, and health effects — Prog Lipid Res, 2023PMID 36657621
No clinician reviews this site. Sources on this entry were verified by the author, who is not a doctor or a dietitian — what that means.
Last updated 2026-08-15.
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