Does yogurt break a fast?
Yogurt breaks a fast. It contains protein, carbohydrate and fat, and like all dairy it raises insulin considerably more than its glycaemic index predicts. Greek yogurt is higher in protein than regular, which makes it a better food and a more complete break of an autophagy fast.
| Fasting for | Verdict | Why |
|---|---|---|
| Weight loss | Breaks it | Approximately 100 to 150 kcal per 170 g pot. It is a meal. |
| Autophagy | Breaks it | Dairy protein is leucine-rich and activates mTORC1 directly. |
| Blood work | Breaks it | Not permitted before a fasting test. |
| Ketosis | Depends | Plain Greek yogurt is relatively low in carbohydrate and may not end ketosis. Flavoured yogurt certainly will. |
| Gut rest | Breaks it | Protein, fat and lactose all require digestion. |
Why
Yogurt is the cleanest illustration of why the glycaemic index is the wrong instrument for fasting questions. Measured directly, yogurt is a low-glycaemic-index food — and it is simultaneously strongly insulinotropic, part of the documented inconsistency between the glycaemic and insulinaemic responses to regular and fermented milk products. The protein fraction, not the lactose, drives that insulin response, and the same protein supplies the leucine that activates mTORC1 and suppresses autophagy. A reader who has learned that low-GI foods are gentle will reach the wrong conclusion about yogurt every time, which is why this entry states the mismatch rather than the index.
Approximately 100 to 150 kcal per 170 g pot, plain.
The exceptions
This is usually where the real question lives.
- Flavoured yogurts commonly contain 15 to 20 g of added sugar per pot.
- The live cultures are a reason to eat yogurt, not a reason to consider it fasting-compatible.
- Kefir behaves the same way — fermented dairy is still insulinotropic.
- Greek yogurt is an excellent food for breaking a fast, precisely because the protein does what protein does.
Sources
- Yogurt Is a Low-Glycemic Index Food — J Nutr, 2017PMID 28615381
- Inconsistency between glycemic and insulinemic responses to regular and fermented milk products — Am J Clin Nutr, 2001PMID 11451723
- Nutritional and contractile regulation of human skeletal muscle protein synthesis and mTORC1 signaling — J Appl Physiol (1985), 2009PMID 19150856
- FoodData Central — USDA Agricultural Research Service, 2026
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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