You’re staring at that brightly colored can of Celsius in the fridge, your stomach is growling, and you’re wondering if crackin’ it open is going to ruin your sixteen-hour fast. It’s a common dilemma. We want the caffeine hit, the metabolic boost, and frankly, something that tastes like something other than plain tap water. But the world of intermittent fasting is filled with self-appointed gatekeepers who swear that even a single calorie will "break" your fast and destroy your autophagy.
Honestly? The answer to can you drink celsius while fasting isn't a simple yes or no. It depends entirely on why you are fasting in the first place.
If you’re fasting for weight loss, you’ve got a lot more wiggle room than someone fasting for deep cellular repair or religious reasons. Celsius is marketed as a fitness drink that burns calories, but when you look at the back of the label, things get a bit murky for the fasting purists.
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What's Actually Inside a Can of Celsius?
To understand if this drink fits into your fasting window, we have to look at the ingredients list, not just the "zero sugar" claim on the front. A standard Celsius contains a blend of ginger, guarana, green tea extract, and several B-vitamins. It also uses sucralose as a sweetener.
Here is the kicker: most cans technically list 10 calories.
Now, in the world of strict "water fasting," 10 calories is 10 calories too many. If your goal is 100% gut rest, those calories—minimal as they are—technically require digestion. However, most practitioners of intermittent fasting (IF), like Dr. Jason Fung, author of The Obesity Code, suggest that staying under 50 calories generally won't kick you out of ketosis or significantly spike your insulin.
But Celsius isn't just flavored water.
The caffeine content is massive. We are talking 200mg per can. While caffeine itself is a known metabolic stimulant that can actually enhance fat burning during a fast, the sheer volume of stimulants in Celsius can be tough on an empty stomach. Some people find that drinking it while fasting leads to jitters, nausea, or a massive energy crash later because there is no food to buffer the absorption.
Can You Drink Celsius While Fasting for Weight Loss?
If your primary goal is dropping pounds, the short answer is usually yes. You can likely get away with a Celsius during your fasting window. The 10 calories are negligible in the grand scheme of a daily caloric deficit. In fact, many people use the "MetaPlus" blend in Celsius specifically to bridge the gap between their morning wake-up call and their first meal at noon.
The green tea extract (EGCG) and guarana are designed to increase your metabolic rate. When you're in a fasted state, your insulin levels are already low. Adding a thermogenic drink might theoretically push your body to tap into fat stores a bit faster.
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However, you have to watch out for the sucralose.
Sucralose is an artificial sweetener. While it doesn't contain calories, some research—including a notable 2020 study published in Cell Metabolism—suggests that certain artificial sweeteners can still trigger a cephalic phase insulin response. Basically, your brain tastes "sweet," thinks sugar is coming, and tells your pancreas to release a little insulin. If your insulin spikes, your fat-burning (lipolysis) slows down.
It's a "maybe" situation. For some, it has zero effect. For others, it stalls their progress. You have to listen to your body. If you drink a Celsius and suddenly feel ravenous 20 minutes later, that’s a sign your insulin spiked and your blood sugar dropped. That's a fast-breaker.
The Autophagy Argument
Maybe you aren't doing this for the scale. Maybe you’re fasting for autophagy—the "self-eating" process where your cells clean out damaged components. This is where the can you drink celsius while fasting question gets a lot stickier.
Autophagy is highly sensitive to nutrient sensing. The primary regulators are mTOR (which stops autophagy when it senses protein or energy) and AMPK (which starts it when energy is low).
Even though Celsius is low-calorie, the vitamins and the proprietary blend might be enough to signal to your body that "food" has arrived. Dr. Rhonda Patrick, a well-known expert in biomedical science, often notes that anything other than water, black coffee, or plain tea might technically interfere with the maximum benefits of autophagy. The liver has to process the compounds in the drink, which means your digestive system isn't truly "at rest."
If you are a purist fasting for longevity, skip the Celsius. Stick to black coffee. It gives you the caffeine without the extra "fluff" of preservatives and sweeteners.
Is the "On-the-Go" Powder Different?
Celsius also sells powder sticks. People often think these are "cleaner" because they mix them into their own water.
Check the labels carefully. Some of the powder versions actually have slightly different sweetener profiles or fillers like maltodextrin. Maltodextrin has a higher glycemic index than table sugar. If you see that on the label, stay far away during your fast. It will spike your insulin faster than a donut.
Real World Side Effects to Watch For
Let's be real: drinking 200mg of caffeine plus ginger and green tea extract on an empty stomach is a recipe for a "sour stomach" for about 30% of the population.
- Heartburn: The carbonation and citric acid in Celsius can trigger acid reflux when there’s no food to soak it up.
- The Jitters: Without fiber or fats to slow down the absorption, that caffeine hits your bloodstream like a freight train.
- Dehydration: Caffeine is a diuretic. If you’re fasting, you’re already losing electrolytes faster than usual. If you don't chase that Celsius with twice as much plain water, you're headed for a headache.
Practical Steps for Your Next Fast
So, how do you actually handle this? Don't overcomplicate it. Fasting is supposed to make life easier, not give you a math headache.
First, identify your "Why." If you just want to look better in jeans, a Celsius isn't going to ruin your life. If you are trying to heal a chronic gut issue or maximize longevity, keep it to water and black coffee.
Second, do a "Sweetener Test." Drink a Celsius one morning during your fast. Pay close attention to how you feel two hours later. Are you shaking? Are you suddenly so hungry you want to eat the drywall? If yes, the sucralose is messing with your blood sugar. Switch to a naturally sweetened alternative or plain sparkling water.
Third, timing matters. If you must have one, drink it right before a fasted workout. This way, your body immediately uses the energy and the caffeine to power your movement, which can help mitigate any minor insulin response.
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Finally, prioritize minerals. If you choose to drink Celsius while fasting, make sure you are supplementing with high-quality electrolytes—specifically magnesium, sodium, and potassium. The stimulants in these drinks can deplete your mineral stores quickly, leading to that "fasting flu" feeling that makes people quit after three days.
Stick to the basics. Use Celsius as a tool, not a crutch. If you find yourself needing three cans just to make it to lunch, the problem isn't the drink—it's your sleep or your overall caloric intake during your eating window. Fasting works best when you're working with your body, not trying to trick it with chemistry.