Diet Soda in a cup with a lime and a straw on a blue background.

Does Diet Soda Break a Fast?

If you’re fasting for the longevity benefits, put down the diet pop.

30-Second Takeaway

  • Since diet soda doesn’t have any calories, it technically doesn’t break a fast. 
  • However, the artificial sugars found in most diet sodas may interfere with a fast by triggering an insulin response and increasing hunger levels.
  • The body’s response to artificial sugars may negate some of the potential benefits of fasting like autophagy, weight loss, and improved metabolic health.
  • More research is needed, but if you’re fasting for health benefits you might want to skip drinking diet soda while fasting. 

If gulping down a diet soda makes your day a little bubblier, you’re not alone. According to market research the global diet soft drinks market reached $5.8 billion in 2022 and is forecasted to hit $9.3 billion by 2030 (1). 

 But if you’re practicing IF will a diet soda break your fast? They tend to contain zero (or very few) calories—the big factor that usually determines when a fast is broken. But experts say it’s a bit more complicated than that.

About the Experts

Jess Cording, M.S., R.D., C.D.N., is a registered dietitian, health coach, Pilates instructor, and creator of the Drama-Free Healthy Living Podcast

Erin Palinski-Wade, R.D., C.D.C.E.S., L.D.N., C.P.T., is a registered dietitian nutritionist, certified diabetes educator, certified personal trainer, TEDX speaker, and author of the 2-Day Diabetes Diet.

Does Diet Soda Break a Fast?

Consuming any food or beverage that provides calories will break a fast. Since diet soda doesn’t have calories, it technically won’t break a fast, according to registered dietitian Jess Cording, M.S., R.D., C.D.N.

However, the artificial sweeteners in diet drinks may interfere with a fast, negating some of the potential benefits of fasting like autophagy, weight loss, and improved metabolic health (2, 3, 4). Artificial sweeteners, particularly in high amounts, may trigger insulin secretion and impact hunger levels, says registered dietitian nutritionist Erin Palinski-Wade, R.D. 

How Diet Soda Might Impact a Fast

When you eat, blood glucose levels rise and insulin swoops in to pull that glucose from your bloodstream to use or store for energy. Although diet soda is typically calorie- and sugar-free, a recent small study found that drinking diet soda might increase insulin levels (5). One review also suggests that the hyper-sweet taste of non-nutritive (zero-calorie) sweeteners alone may lead to an anticipatory insulin response (6). 

Triggering insulin may derail your fast in a few ways. First, one of the biggest potential perks of fasting is an increase in fat oxidation (breakdown) for energy (7). However, insulin inhibits fat oxidation (8). A spike in insulin could also further reduce blood glucose levels, which may increase appetite and potentially lead to weight gain (5). 

Palinski-Wade adds that the body’s insulin response to artificial sweeteners could also affect autophagy—your body’s natural cellular recycling process that begins when cells are deprived of nutrients (9). One review suggests that artificial sweeteners and sugar might induce a similar metabolic response (6), which would disrupt autophagy. “But more research is needed,” says Palinski-Wade.

Diet soda may also increase insulin resistance—which has been linked to metabolic diseases like diabetes, hypertension, and non-alcoholic fatty liver disease (10, 11). One study found that adults with type 2 diabetes who consumed artificially sweetened beverages had higher insulin resistance than those who avoided them (12). 

What about caffeine?

There is no specific amount of caffeine reported to disrupt a fast. As a general rule, it’s recommended to consume no more than 400 milligrams of caffeine a day for overall health. Still, pay attention to the way your body responds to caffeine since it can spike cortisol levels, AKA the stress hormone. “A spike in cortisol can cause your blood sugar levels to go up a little bit, depending on how high it gets,” Cording explains. A spike in blood sugar would trigger insulin, which may cancel out many of the benefits of fasting. 

Which Diet Sodas Might Break a Fast?

Since artificial sweeteners—like aspartame, Acesulfame K, sucralose, and saccharin—may be associated with a glycemic response—you might want to limit or avoid these sweeteners when fasting to maximize the potential benefits of your fast.

Diet sodas that contain aspartame include:

  • A&W Cream Zero Sugar Soda
  • Mello Yello Zero
  • Coke Zero Sugar
  • Diet Pepsi
  • Diet Coke

Which Diet Sodas Won’t Break a Fast?

Full disclosure: There’s not enough scientific evidence at this time to state which low-calorie sweetener is best for fasting, stresses Palinski-Wade. “Natural options—such as monk fruit or allulose—may be the best choice when fasted, but more research is needed,” she says. 

Better alternatives include Zevia Caffeine Free Cola or Humm Probiotic Berry Cream Soda

Is Diet Soda Healthy?

The occasional diet soda may fit into an overall healthy and balanced diet, but experts agree it’s not considered a health-promoting food. “It doesn’t offer any nutritional benefits,” says Cording.

The fact that diet soda may cancel out the benefits of fasting might be reason enough to skip. But there are several other health-related reasons why you might want to stay away:

Sugar substitutes may not aid in weight loss

In 2023, the World Health Organization (WHO) released a new guideline on non-sugar sweeteners based on the findings of a systematic review which found that artificial sweeteners may not have any significant long-term effect on weight loss (11). The same review also uncovered potential serious harmful effects of artificial sweeteners, including an increased risk of type 2 diabetes, cardiovascular diseases, and mortality in adults.

Artificial sweeteners may increase risk of cancer

In 2023, the WHO declared that aspartame could be “possibly carcinogenic (12).” One study found artificially sweetened soft drinks were associated with an elevated risk in kidney cancer among American postmenopausal women (13). However, results from another study on healthy European women found that after adjusting for obesity, artificially sweetened soft drinks weren’t associated with kidney cancer (14).

Aspartame may affect the nervous system

Aspartame could interfere with compounds in the brain, setting off a chain of events that can inhibit the release of neurotransmitters (like dopamine and serotonin) and make the brain more vulnerable to oxidative stress (or molecular damage). This reaction could lead to possible neurophysiological symptoms, including learning problems, headaches, seizures, migraines, irritable moods, anxiety, depression, and insomnia (15).

Artificial sweeteners may cause hunger

Drinks that contain sucralose may increase food cravings and appetite in women, as well as in people who are obese (16). But more research is needed. 

Artificial sweeteners may increase risk of cardiovascular events

Artificially sweetened drinks are linked to increased risk of irregular heart rhythms. Adults who consumed two liters (about 67 ounces) or more of artificially sweetened beverages each week had a higher risk of atrial fibrillation (AFib), a quivering or irregular heartbeat (17).

Healthier Drinks That Won’t Break a Fast

Ditching Diet Coke? Quench your thirst during a fast with these registered dietitian-approved picks.


“Pure water is always the best choice for hydration during fasting as it contains no calories, sugars, or proteins that could interrupt the fast,” says Palinski-Wade. She suggests livening it up with a tiny splash of citrus, like lemon or orange.


“Thanks to carbonation, seltzer is a great alternative for someone who is not drinking soda,” says Cording.

Coffee and tea

“Unsweetened coffees and teas can be consumed during a fast, but avoid additives like milk, cream, and sweeteners,” says Palinski-Wade. Most additives contain calories and may break a fast.

The Bottom Line

Diet soda technically won’t break a fast. However, artificial sweeteners may trigger an insulin response which could negate the main benefits of fasting like autophagy, weight loss, and improved metabolic health. Look for diet sodas with natural sweeteners like monk fruit or allulose. Or stick to healthier drinks like water, seltzer, coffee, and tea—which are known to not interfere with a fast. 

1. DataM Intelligence (2024). Diet Soft Drinks Market Size, Share, Industry, Forecast and Outlook (2024-2031).
2. Shabkhizan, R. et al. (2023). The Beneficial and Adverse Effects of Autophagic Response to Caloric Restriction and Fasting.
3. Varady, K. et al. (2022). Clinical Application of Intermittent Fasting for Weight Loss: Progress and Future Directions.
4. Vasim, I. et al. (2022). Intermittent Fasting and Metabolic Health.
5. Finassi, Carolina Martins, et. al. (2023). Effect of Sweetened Beverages Intake on Salivary Aspartame, Insulin and Alpha-Amylase Levels: A Single-Blind Study.
6. Tucker, R. et al. (2017). Do Non-Nutritive Sweeteners Influence Acute Glucose Homeostasis in Humans? A Systematic Review.
7. Andriessen, C. et al. (2023). The Impact of Prolonged Fasting on 24 Hour Energy Metabolism and Its 24 Hour Rhythmicity in Healthy, Lean Males: A Randomized Cross-Over Trial.
8. Carpentier, A. et al. (2021). 100th Anniversary of the Discovery of Insulin Perspective: Insulin and Adipose Tissue Fatty Acid Metabolism.
9. Cleveland Clinic. Autophagy.
10. National Institute of Diabetes and Digestive and Kidney Diseases. Insulin Resistance and Prediabetes.
11. Zhao X. et al. (2023). The Crucial Role and Mechanism of Insulin Resistance in Metabolic Disease.
12. Mathur, Kushagra, (2020). Effects of Artificial Sweeteners on Insulin-Resistance Among Type 2 Diabetes Mellitus Patients.
13. World Health Organization. (2023). WHO Advises Not to Use Non-Sugar Sweeteners for Weight Control in Newly Released Guideline.
14. World Health Organization. (2023). Aspartame Hazard and Risk Assessment Results Released.
15. Ringel, Nancy, (2022). Association of Artificially Sweetened Beverage Consumption and Urinary Tract Cancers in the Women’s Health Initiative Observational Study.
16. Heath, Alicia, (2021). Soft Drink and Juice Consumption and Renal Cell Carcinoma Incidence and Mortality in the European Perspective Investigation Into Cancer and Nutrition.
17. Choudhary, Arbind, et al. (2018). Neurophysiological Symptoms and Aspartame: What Is the Connection?
18. Yunker, Alexandra, et al. (2021). Obesity and Sex-Related Associations With Differential Effects of Sucralose Versus Sucrose in Appetite and Reward Processing.
19. Koeth, Robert, et al. (2024). Artificial Sweeteners: A New Dietary Environmental Risk Factor for Atrial Fibrillation?