Can You Drink CELSIUS While Fasting? Here's What You Need to Know

January 11, 2026 •

4 min read

A study from Johns Hopkins neuroscientist Mark Mattson found that during intermittent fasting, the body can burn through its sugar stores and start burning fat for energy. So, can you drink CELSIUS while fasting without interrupting this metabolic process? The answer is more complex than a simple yes or no, depending largely on your specific health objectives.

Quick Summary

Whether a CELSIUS drink breaks a fast hinges on your specific health goals, as its zero-sugar, low-calorie profile and artificial sweeteners prompt different metabolic reactions depending on your regimen.

Key Points

Not a Simple Answer: Whether Celsius breaks a fast depends on your specific fasting goals, such as weight loss versus autophagy.
Calories Are Negligible for Weight Loss: For fasters focused on calorie restriction, the 10 calories in a Celsius drink are often considered too minimal to disrupt overall progress.
Artificial Sweeteners Pose a Risk: The sucralose in Celsius might cause a cephalic phase insulin response in some individuals, potentially interfering with deeper metabolic benefits like autophagy.
High Caffeine Content Can Help or Hinder: The 200mg of caffeine can suppress appetite and boost metabolism, but may cause digestive upset or anxiety on an empty stomach for sensitive people.
Strict Fasters Should Avoid It: Those pursuing strict fasts for autophagy or gut rest should avoid all calories and artificial sweeteners, opting for water, black coffee, or plain tea instead.
Individual Response Varies: The best way to know is to monitor your body's reaction, including hunger levels and cravings, to see how Celsius affects your unique fasting experience.

Understanding the Fundamentals of Fasting

To determine if a CELSIUS drink is compatible with fasting, you must first understand what fasting entails and what truly breaks a fast. For many, fasting involves a period of not consuming any calories to achieve a state of metabolic rest. This process allows the body to lower insulin levels, leading to several potential benefits, including weight management, improved insulin sensitivity, and cellular repair (autophagy).

There are two main interpretations of what constitutes 'breaking' a fast:

The Strict Calorie Rule: In this view, any caloric intake, no matter how small, technically ends the fasted state. A standard CELSIUS drink contains approximately 10 calories, meaning a strict interpretation would consider it a break.
The Metabolic Response Approach: This perspective focuses on what triggers an insulin spike. A small, negligible number of calories, particularly from fat or non-sugar sources, may not significantly disrupt the metabolic goals of fasting for everyone. Proponents of this view might tolerate trace calories.

Decoding CELSIUS's Impact on Your Fast

CELSIUS drinks contain several key ingredients that are relevant to the fasting debate: zero sugar, minimal calories, artificial sweeteners, and a significant dose of caffeine.

The Caloric and Sugar Content

CELSIUS is marketed as having zero sugar and very low calories, typically around 10 per can. For those fasting primarily for weight loss, where the focus is on a significant caloric deficit over time, these 10 calories are often considered negligible. An online discussion on Reddit showed that many individuals still achieve weight loss results while incorporating a Celsius into their routine. However, for someone pursuing a strict fast for maximum autophagy or gut rest, any calorie intake is a no-go.

The Artificial Sweetener Factor: Sucralose

CELSIUS uses sucralose as its primary sweetener. This is where the waters get murky for strict fasters. Here's a breakdown of the debate:

The Concern: Some experts and anecdotal reports suggest that the sweetness of artificial sweeteners like sucralose can trigger a cephalic phase insulin release. This is a physiological response where your body prepares for incoming sugar by releasing a small amount of insulin, potentially disrupting the metabolic benefits of fasting. Studies have also linked chronic use of sucralose to changes in gut microbiome and potentially negative effects on glucose metabolism.
The Counterargument: Other studies, along with manufacturer claims, assert that because sucralose is not metabolized for energy, it doesn't significantly impact blood glucose or insulin levels for most people. Many people who test their blood glucose with a continuous glucose monitor (CGM) find no spike after drinking a Celsius.

The Role of Caffeine

Each can of CELSIUS contains around 200mg of caffeine. Caffeine is known to have appetite-suppressing effects and can boost metabolism, both of which can be beneficial during a fast. However, consuming a large amount of caffeine on an empty stomach can cause side effects like jitters, increased heart rate, anxiety, or stomach discomfort, especially for caffeine-sensitive individuals.

Fasting Goals and CELSIUS Consumption: A Comparison

To make an informed decision, align your choice with your personal fasting goals. The following table summarizes how CELSIUS aligns with different fasting approaches.


Fasting Goal	Will CELSIUS Break My Fast?	Rationale	Verdict
Weight Loss	Unlikely to hinder progress significantly.	The 10 negligible calories and appetite-suppressing caffeine can aid in overall caloric restriction. The metabolic benefits often outweigh the trace calories for this goal.	Generally Acceptable
Metabolic Health	A possibility, depending on individual sensitivity.	The potential for a cephalic phase insulin response from artificial sweeteners could interfere with desired insulin level reduction.	Proceed with Caution
Autophagy	Yes, it will.	The goal of autophagy (cellular cleansing) requires a complete metabolic rest. Any caloric or sweet stimulus, however small, interrupts this process.	Not Recommended
Ketosis	No, it will not.	Since CELSIUS is zero-sugar and the calories are minimal, it won't kick your body out of ketosis, the fat-burning state.	Generally Acceptable
Gut Rest	Yes, it will.	The artificial sweeteners can disrupt the gut microbiome over time and interfere with the digestive system's complete rest.	Not Recommended

The Final Word: Is It Worth the Risk?

For the average intermittent faster aiming for weight loss, a can of CELSIUS is probably fine and may even help curb appetite. The 10 calories are likely burned by the metabolic boost from the caffeine. However, for those with a stricter regimen, such as pursuing deep metabolic healing or autophagy, or those with known sensitivities to artificial sweeteners, it is best to avoid it. Plain water, black coffee, or herbal tea remain the safest, most neutral options for fasting.

Ultimately, every person's body responds differently. If you are unsure, the best course of action is to stick to pure, unsweetened beverages during your fasting window to ensure you reap the full benefits.

Recommended Fasting-Friendly Beverages

Water (Plain or Sparkling): The gold standard for hydration with zero calories.
Black Coffee: Minimal calories and a metabolism-boosting caffeine kick. Do not add sugar or cream.
Unsweetened Tea: Green, black, and herbal teas offer antioxidants without breaking your fast. Avoid fruit-infused options with sugar.

For more in-depth information on intermittent fasting and its benefits, consult authoritative sources on health and nutrition, such as Johns Hopkins Medicine on Intermittent Fasting.

Conclusion

In summary, whether you can drink CELSIUS while fasting depends entirely on the purpose and strictness of your fast. For those focused on weight management and general health, the low-calorie, zero-sugar drink likely poses minimal risk and may even assist with cravings due to its caffeine content. However, individuals aiming for the deeper metabolic benefits of a strict fast, like autophagy, should avoid CELSIUS and other artificially sweetened beverages to prevent any potential disruption. Paying attention to your body's individual response is the most reliable guide. If you experience increased cravings or stomach upset, it is a clear sign to stick to water, black coffee, or tea during your fasting window.

Frequently Asked Questions

For most people practicing intermittent fasting for weight loss, CELSIUS will not significantly break the fast. The 10 calories per can are considered negligible, and the caffeine can help suppress appetite and boost metabolism.

No, if your goal is autophagy (cellular repair), you should avoid CELSIUS. Autophagy requires a complete metabolic rest, and the calories and artificial sweeteners in Celsius could trigger a response that disrupts this process.

The effect of sucralose on insulin is debated. While it has zero sugar, some evidence suggests that its sweetness can trigger a cephalic phase insulin response in some individuals. This is not a significant spike but can be a concern for strict fasters.

The high caffeine content (200mg) can suppress your appetite and temporarily increase your metabolism, which can help you get through your fasting window. However, consuming it on an empty stomach can cause jitters or stomach upset for some.

The safest alternatives are pure, unsweetened beverages. This includes water (plain or sparkling), black coffee, and plain teas (green, black, or herbal).

Some studies suggest that regular consumption of artificial sweeteners like sucralose may alter the gut microbiome over time. This is a consideration for those fasting to improve gut health, and for whom it's better to avoid Celsius.

Pay attention to your body's signals. If you notice increased cravings, digestive discomfort, or feel irritable after drinking Celsius during your fasting window, it may be having a negative impact on your body's fasted state.

Because Celsius is zero sugar and low-calorie, it should not kick your body out of ketosis. For those on a ketogenic diet who also practice intermittent fasting, it is generally considered acceptable.