I get some version of "does X break a fast" more than almost any other question, and the honest answer is almost never a flat yes or no. It depends on what you're actually fasting for. There are two separate tests worth applying to anything you're considering during a fast: does it carry a meaningful caloric load, and does it trigger a real insulin response. Some things fail both. Some things fail one and not the other. A few things pass both tests technically and still work against the point of fasting in a way worth knowing about. Here's the honest breakdown, ingredient by ingredient.

Does Monk Fruit Break a Fast?

Monk fruit's sweetness comes from mogrosides, compounds your body doesn't process as sugar or carbohydrate. Research backs this up cleanly: a systematic review of randomized trials found monk fruit extract produced glucose and insulin responses no higher than baseline, and in some trials it reduced post-meal glucose and insulin compared to sucrose. On the technical test, monk fruit passes. Zero calories, no meaningful insulin response in the studies we have.

But here's something worth noting: the physiological test isn't the only thing that matters if your reason for fasting includes resetting your relationship with sweet taste. Sweetness, even from something metabolically inert, keeps a specific appetite pathway active. If your goal is strict metabolic benefits and you're comfortable with that distinction, monk fruit is a reasonable technical pass. If part of why you fast is to let sugar cravings actually settle, sweetened anything works against that regardless of the calorie count.

Does Allulose Break a Fast?

Allulose is a "rare sugar" that occurs naturally in small amounts in foods like figs and raisins. It's structurally similar to fructose but your body doesn't metabolize most of it for energy, which is why it carries roughly a tenth of sugar's calories per gram despite tasting close to identical.

The research on allulose and blood sugar is genuinely favorable: current evidence shows minimal impact on fasting glucose or insulin when consumed on its own, and several trials found it actually blunted the glucose and insulin response when added to a sugary drink. Most of that research looks at allulose alongside food rather than in isolation during a fasted window, so I'd call the fasting-specific evidence promising but not exhaustive. On the numbers we have, allulose is one of the more defensible sweeteners if you're determined to use one during a fast. The same sweetness-during-a-fast caveat from monk fruit still applies if cravings are part of what you're trying to manage.

Do Artificial Sweeteners and Diet Soda Break a Fast?

This is the least settled question on this list, and I'd rather tell you that than pretend the research is more decided than it is.

The concern is the cephalic phase insulin response, the idea that sweet taste alone can trigger a small insulin release in anticipation of calories that never arrive. The evidence here is mixed. Some studies find this effect is real but minor and doesn't apply equally to everyone. Other research, including a study following over 1,300 people, found no significant link between diet soda and fasting insulin or glucose. On the strict two-part test, artificial sweeteners and diet soda generally pass: no calories, and the insulin question is unsettled rather than clearly failing.

Where it gets more interesting is appetite. A 2023 USC study found sucralose reduced fullness hormones compared to sugar, and participants, especially women and people with obesity, ate more afterward at a snack buffet. Separately, newer research has found some artificial sweeteners reduce gut microbial diversity. Neither of those findings is about breaking a fast in the technical sense. Both are reasons I'd think twice before leaning on diet soda as a regular fasting tool, independent of the calorie count.

Does Diet Coke Break a Fast specifically? Same answer as artificial sweeteners generally: technically no on calories, unsettled on insulin, and worth being aware of the appetite research if you're drinking it daily.

Does Heavy Cream Break a Fast?

Yes, and this one isn't close. A tablespoon of heavy cream runs about 50 calories, almost entirely from fat, with close to zero carbohydrate. That means it won't spike insulin much on its own, but it absolutely fails the caloric test. A splash in coffee adds up fast if you're doing it more than once, and it's real, digestible fuel your body has to process.

This is where the distinction between a caloric fast and what some people call a metabolic fast actually matters. If your only goal is staying in a low-insulin, fat-burning state, a small amount of heavy cream might not move you out of that state much. If your goal is autophagy, a true caloric deficit, or the gut rest that comes from giving your digestive system nothing to do, heavy cream ends that the moment it goes in.

Does Coffee With Cream Break a Fast?

Following directly from that: yes. Black coffee on its own doesn't break a fast, running only 2 to 5 calories a cup with no meaningful insulin response. The cream is what changes the equation, not the coffee. If you're used to cream in your coffee and trying to fast, black is the version that actually counts, and it's worth giving your taste buds a week or two to adjust rather than assuming you can't drink coffee without it (I've had clients add cinnamon to their coffee and grow to love it in just a few days).

Does MCT Oil Break a Fast?

MCT oil runs about 120 calories per tablespoon, so on the caloric test, it fails clearly. That's true of bulletproof coffee too butter and MCT oil both carry real calories.

Where MCT oil gets defended as fasting-friendly is the metabolic side. Medium-chain triglycerides convert to ketones quickly, more quickly than most other fat sources, and they don't trigger a meaningful insulin response. Some people use MCT oil deliberately to extend what I'd call a modified fast: staying in ketosis and keeping insulin low while giving up strict caloric fasting for a specific stretch. That's a strategy for a specific goal, appetite control during a transition period, sustained energy during a modified protocol, but it is not the same thing as a true fast. If you're doing bulletproof coffee and calling it a fast, you're doing a modified fast. Nothing wrong with that as a tool. Just call it what it is.

The Practical Framework

Run anything you're considering through three questions, not two. First, does it carry real calories? Second, does it trigger a meaningful insulin response? Third, and this one doesn't show up in a lab test: does it keep a craving alive that you're specifically trying to let go of?

Monk fruit and allulose pass the first two tests convincingly. Artificial sweeteners mostly pass them too, with real open questions on appetite and gut effects that I wouldn't ignore just because the insulin data looks fine. Heavy cream, coffee with cream, and MCT oil all fail the caloric test outright, regardless of how quiet they keep insulin.

None of this is about finding the most technical way to bend the rules. It's about knowing exactly what you're doing so you can decide on purpose, rather than fasting eighteen hours and finding out afterward that a "zero calorie" habit was quietly working against the reason you started.

The Bottom Line

Everything above covers what you put in your body in terms of calories and sweeteners. Minerals are a separate question with their own rules, and if you want the full picture there, I've put together a complete guide to electrolytes for fasting here.

If you want a fast with nothing to second-guess, the simplest approach is also the cleanest: water, black coffee, plain tea, and electrolytes without any sweetener at all. That's the standard I built Optimize to meet: zero sugar, and no monk fruit, allulose, stevia, or artificial sweeteners of any kind, so there's nothing on this whole list to weigh against your fast in the first place.

If you're using one of the sweeteners above deliberately and understanding the tradeoff, that's a legitimate choice. I'd just rather you make it with the full picture than find out later what you were actually trading away.

Fuel your fast, feel your best.

Sources and Further Reading

Impact of Allulose on Blood Glucose: A Meta-Analysis of Clinical Trials
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11585728/

Mogroside-Rich Monk Fruit Extract Improves Glycemic Control
https://pmc.ncbi.nlm.nih.gov/articles/PMC13369284/

Do Electrolytes Break a Fast?, Fasted State Supplements
https://getfastedstate.com/blogs/news/do-electrolytes-break-a-fast

Does Coffee Break a Fast? The Real Answer, Fasted State Supplements
https://getfastedstate.com/blogs/news/does-coffee-break-a-fast

The Truth About Stevia and Fasting, Fasted State Supplements
https://getfastedstate.com/blogs/news/the-truth-about-stevia-and-fasting