What breaks a fast — a decision tree

The honest answer is: it depends on what you're fasting for.

The question "does X break my fast?" is asked constantly, and answered badly. The answer depends entirely on which mechanism of fasting you're trying to preserve. There are three big ones, and they break at different thresholds.

Three mechanisms, three thresholds

1. Caloric restriction. Anything with calories contributes to caloric intake. If your goal is a caloric deficit for weight loss, any calorie breaks the fast in that sense — but the impact is proportional. Ten calories is not zero, but it's close.

2. Insulin suppression. If your goal is to keep insulin low (for metabolic reasons, for fat mobilization, for β-HB production), then anything that triggers an insulin response breaks the fast. Protein and carbohydrate both do. Fat barely does.

3. Autophagy / mTOR suppression. If your goal is to maximize autophagy, then anything that spikes mTOR breaks the fast. The main mTOR trigger is amino acids — particularly leucine. Small amounts of fat or carbohydrate don't suppress autophagy directly.

Notice how these three aren't the same. A splash of cream in coffee technically adds calories (breaks #1 mildly), doesn't meaningfully raise insulin (preserves #2), and doesn't activate mTOR (preserves #3). So "does cream break a fast?" has no single answer.

The decision tree

Here's a rough guide. Start by identifying which mechanism matters most to you.

If you're fasting for metabolic health / insulin

These are probably fine:

• Water, plain
• Black coffee
• Plain tea (any type)
• Electrolytes without sugar (sodium, potassium, magnesium)

These are probably fine in small amounts:

• A splash of cream or unsweetened plant milk in coffee (<1g protein, <1g carb)
• A small pinch of salt
• Vitamins (most)

These break the fast:

• Anything sweet, artificial or not, if it triggers a cephalic-phase insulin response (varies by person)
• Any meaningful protein (>3g or so)
• Any meaningful carbohydrate (>2g or so)
• Bone broth (high in amino acids)

If you're fasting for autophagy

Stricter. These are fine:

• Water
• Black coffee (moderate)
• Plain tea

These likely break autophagy:

• Any protein, period — leucine is the main mTOR trigger and even small doses count
• BCAAs (explicitly designed to activate mTOR)
• Bone broth

Fat alone may not activate mTOR significantly, but pure extended fasts (water only) are the cleanest way to maximize autophagy.

If you're fasting for weight loss

The mechanism that matters most is caloric balance over time. A few calories during the fasting window don't meaningfully move weekly calorie totals. What matters is that the eating window stays bounded and you're not overcompensating when you do eat.

Common questions, short answers

• Black coffee? Fine for all three goals.
• Coffee with cream? Fine for metabolic; borderline for autophagy.
• Coffee with sugar or sweetener? Breaks the fast for most goals (sweetener is person-dependent).
• Lemon water? Fine for all three.
• Apple cider vinegar? Fine.
• Gum? Most sugar-free gum has a few calories from polyols. Trivial for caloric balance, possibly stimulates a cephalic-phase response.
• Vitamins? Fat-soluble vitamins benefit from being taken with food; water-soluble are fine.
• Toothpaste? Fine — you're not swallowing it.
• Bone broth? Breaks every fast in any meaningful sense. It's a food.

The pragmatic rule

If you have to think about whether something breaks your fast, it's probably not the right side of the line for you. Err clean. The discipline of the fast — the commitment to a defined window — is part of the benefit. Protecting the edges is easier than litigating them.

Sources

1. de Cabo R, Mattson MP. "Effects of intermittent fasting on health, aging, and disease." N Engl J Med, 2019. doi:10.1056/NEJMra1905136
2. Anton SD, Moehl K, Donahoo WT, et al. "Flipping the metabolic switch." Obesity (Silver Spring), 2018. doi:10.1002/oby.22065
3. Stote KS, Baer DJ, Spears K, et al. "A controlled trial of reduced meal frequency without caloric restriction in healthy adults." Am J Clin Nutr, 2007. doi:10.1093/ajcn/85.4.981
4. Suez J, Korem T, Zeevi D, et al. "Artificial sweeteners induce glucose intolerance by altering the gut microbiota." Nature, 2014. doi:10.1038/nature13793
5. Related reading: drinks while fasting, the electrolytes guide, and the ghrelin wave.