How Many Calories Break a Fast?
Quick answer: Strictly speaking, any calories break a fast. But for intermittent fasting for weight loss, there’s no research-backed cutoff, and a few calories (black coffee, a squeeze of lemon, a stick of sugar-free gum) won’t meaningfully change your results. The popular “under 50 calories” rule is a rule of thumb, not a scientific limit. For a fasting blood test, the answer is zero: follow your lab’s instructions.
Try it yourself: Say you eat between 12 PM and 8 PM on a 16:8 plan. Black coffee at 8 AM (about 2 calories) fits your fast. A coffee with 1 tablespoon of heavy cream (about 52 calories) is a small “dirty fast” snack, and a latte made with a cup of 2% milk (about 125 calories) is breakfast. Map your window in the intermittent fasting calculator.
Calories in Common Fasting Add-Ins
Most “does this break my fast?” questions come down to a handful of add-ins. Here’s how many calories each one has:
| Item | Serving | Calories (approx.) |
|---|---|---|
| Creatine monohydrate powder | 5 g | 0 |
| Black coffee | 8 oz cup | about 2 |
| Apple cider vinegar | 1 tbsp | about 3 |
| Lemon juice | 1 tbsp | about 3 |
| Sugar-free gum | 1 stick | about 5 (label) |
| 2% milk | 1 tbsp splash | about 8 |
| Regular sugared gum | 1 stick | about 10 (label) |
| Lemon juice | juice of 1 lemon | about 11 |
| Sugar | 1 tsp | about 16 |
| Honey | 1 tsp | about 21 |
| Heavy cream | 1 tbsp | about 52 |
| 2% milk | 1 cup | about 122 |
Food values from USDA FoodData Central; gum and creatine values from typical product labels. Brands vary, so check your own label.
It Depends on Why You’re Fasting
There isn’t one magic number because people fast for different reasons. Here’s how a small number of calories plays out for each goal:
| Calories during the fast | Weight-loss intermittent fasting | Strict “clean” fast | Autophagy goal | Fasting blood test |
|---|---|---|---|---|
| 0 (water, plain tea, black coffee*) | Fine | Fine | Fine | Water only, per lab |
| About 1 to 10 | Effectively fine | Breaks it | Unknown | Breaks it |
| About 50 | Small dent, many people accept it | Breaks it | Unknown, likely interrupts | Breaks it |
| 100 or more | Breaks it (that’s a snack) | Breaks it | Likely interrupts | Breaks it |
Black coffee has about 2 calories, and many labs ask you to skip it before blood work.
Weight-loss intermittent fasting
If your goal is fat loss, fasting mostly works by helping you eat less overall. Seen that way, 10 or 20 calories won’t move the needle (it’s why black coffee is fine), while a large sweetened coffee drink every morning will. In a one-year trial of 100 adults with obesity, alternate-day fasting (eating about 25% of energy needs on fast days) led to about the same weight loss as simply cutting calories every day (Trepanowski et al., 2017). The total matters more than perfect purity.
Some fasting styles even build in calories. On the 5:2 plan, women typically eat around 500 calories on two fasting days a week.
Strict or “clean” fasting
A clean fast allows only water, plain tea and black coffee. By that standard, any flavoring, sweetener or splash of milk counts. Some people do better with a clean fast because even a small taste can spark hunger.
Autophagy
Autophagy is the cell’s built-in recycling system. Fasting is thought to switch it on, but research on how long it takes in humans is limited, and much of the work has been done in animals. No one has shown that 5 or 50 calories turns it off in people. Anything you read promising an exact autophagy calorie limit is guessing.
Fasting blood tests
Medical fasting has a strict definition. For diabetes testing, the American Diabetes Association defines fasting as no caloric intake for at least 8 hours. Many labs ask for only plain water for 8 to 12 hours and also rule out coffee, tea and gum. Always follow your lab’s instructions.
Will 50 Calories Break a Fast?
Technically, yes. For weight loss, though, 50 calories is a small amount, about a tablespoon of heavy cream. If it helps you stick with your plan, many people find it worthwhile. If it makes you hungrier, cut it.
Will 5 or 10 Calories Break a Fast?
For intermittent fasting, no in practice. That’s a stick of gum or a squeeze of lemon. See does gum break a fast and does lemon water break a fast.
What About Protein, Fat and Sugar?
Not all calories feel the same. Sugar raises blood sugar, and some people find it makes them hungrier. Pure fats, like MCT oil or butter in coffee, add calories fast: a tablespoon of heavy cream is already around 52 calories. Broths contain some protein and calories too, which is why bone broth technically breaks a fast even though it’s light.
Women Over 40: When the Small Stuff Adds Up
Many women find that calorie needs feel tighter in their 40s and 50s, which makes “invisible” calories more important. Two lattes during your fasting window can add 200 or more calories a day without feeling like food. If your scale is stuck, read why intermittent fasting may not be working in menopause and check your numbers with the calorie deficit calculator. Getting enough protein in your eating window also helps protect muscle; the protein calculator gives you a daily target.
Frequently Asked Questions
Will 50 calories break a fast?
Strictly, yes. For weight-loss intermittent fasting, 50 calories is a small amount that many people accept, but there’s no proof it’s a safe “limit.”
Will 5 calories break a fast?
Not in any meaningful way for intermittent fasting. It does break a strict water-only fast or a fast before blood work.
Does 100 calories break a fast?
Yes. At 100 calories you’re having a small snack, so most people count that as ending the fast.
What is the 50-calorie rule?
It’s a popular guideline that says staying under about 50 calories keeps you “fasted.” It isn’t based on research, so treat it as a personal choice.
What breaks a fast the most?
Sugar and larger amounts of food or milk. A sweet coffee drink or juice breaks a fast far more than a splash of milk.
Does creatine break a fast?
Plain creatine monohydrate has no calories. See does creatine break a fast.
Do zero-calorie sweeteners break a fast?
They add no calories, but some people find they increase cravings. See does stevia break a fast and does diet soda break a fast.
Related Tools
This article is for general information only and is not medical advice. Read our Medical Disclaimer.
Sources
- USDA FoodData Central (milk, 2%; cream, heavy whipping; sugars, granulated; honey; lemon juice, raw) — https://fdc.nal.usda.gov/
- Trepanowski JF, et al. Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection Among Metabolically Healthy Obese Adults. JAMA Internal Medicine, 2017 — https://pubmed.ncbi.nlm.nih.gov/28459931/
- American Diabetes Association. 2. Classification and Diagnosis of Diabetes: Standards of Medical Care in Diabetes—2018. Diabetes Care, 2018 — https://pubmed.ncbi.nlm.nih.gov/29222373/
- Cleveland Clinic: Autophagy — https://my.clevelandclinic.org/health/body/24058-autophagy
- Patient.info: 5:2 Diet — https://patient.info/health/52-diet
Keep reading
More evidence-based guides for staying strong after 40.
