If you’re fasting for the longevity benefits, put down the diet pop.
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? While diet soft drinks 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.
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.
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.
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).
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.
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:
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.
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:
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.
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 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).
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.
Artificially sweetened drinks are linked to an 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).
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.
“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.
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.
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