If you're among those who enjoy the bubbly lift of a diet soda, you might wonder, "Will a diet soda break my fast?" This is a common question, especially among those practicing intermittent fasting (IF). While diet soft drinks contain very few or zero calories, which is usually the deciding factor in breaking a fast, experts suggest the situation is more complex.
The Calorie Question: Does Diet Soda Break a Fast?
Consuming any food or beverage that provides calories will technically break a fast. Registered dietitian Jess Cording, M.S., R.D., C.D.N., notes that since diet soda typically has no calories, it technically won’t break a fast. From a strictly caloric perspective, diet sodas contain virtually no calories, so they don’t break your fast in the traditional sense.
The Artificial Sweetener Controversy
However, the artificial sweeteners in diet drinks may interfere with a fast, negating some of the potential benefits of fasting, such as autophagy, weight loss, and improved metabolic health. The artificial sweeteners in these beverages might impact your fasting state in other ways.
When you eat, blood glucose levels rise, and insulin is released to pull glucose from your bloodstream for energy use or storage. A recent small study found that drinking diet soda might increase insulin levels, even though diet soda is typically calorie- and sugar-free.
The Insulin Response and Its Impact
Triggering insulin may derail your fast in a few ways. One of the biggest potential perks of fasting is an increase in fat oxidation (breakdown) for energy. Insulin inhibits fat oxidation. 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. One review suggests that artificial sweeteners and sugar might induce a similar metabolic response, which would disrupt autophagy.
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For strict fasting purists focused on autophagy and maximum metabolic benefits, even this potential insulin response might be concerning enough to avoid diet sodas during fasting periods. For those primarily fasting for calorie control, the impact might be negligible.
Insulin Resistance and Metabolic Diseases
Diet soda may also increase insulin resistance, which has been linked to metabolic diseases like diabetes, hypertension, and non-alcoholic fatty liver disease. Research published in the American Journal of Clinical Nutrition suggests some sweeteners might influence glucose metabolism and appetite regulation. However, results vary widely across studies and individuals. This suggests that while diet sodas don’t add calories, they might subtly interfere with some fasting mechanisms.
Caffeine's Role in Fasting
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.
Expert Opinions and Guidelines
The occasional diet soda may fit into an overall healthy and balanced diet, but experts agree it’s not considered a health-promoting food. The fact that diet soda may cancel out the benefits of fasting might be reason enough to skip.
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. Also in 2023, the WHO declared that aspartame could be “possibly carcinogenic.”
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Potential Health Risks of Diet Soda
One study found artificially sweetened soft drinks were associated with an elevated risk in kidney cancer among American postmenopausal women. 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.
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.
Drinks that contain sucralose may increase food cravings and appetite in women, as well as in people who are obese. 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.
Better Alternatives for Hydration During Fasting
“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. “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.
Plain sparkling water contains only water and carbonation, making it virtually identical to regular water metabolically. The bubbles add a refreshing sensation that can help curb cravings during fasting periods without triggering insulin responses or other metabolic effects. If you find plain sparkling water too bland, consider options with natural essences (not sweetened versions). Varieties with just a hint of citrus or berry essence can provide flavor without added sweeteners or calories that might compromise your fast.
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The Benefits of Tea Over Diet Soda
When comparing tea and diet soda for fasting periods, unsweetened tea offers clear advantages. Tea contains natural bioactive compounds like catechins and theanine that may actually enhance some fasting benefits rather than potentially interfering with them. Research from the National Institutes of Health indicates certain tea compounds might support autophagy and improve insulin sensitivity-the very processes many seek to enhance through fasting. Diet sodas, by contrast, contain artificial ingredients that lack these beneficial properties. The caffeine in tea tends to be released more gradually than in diet sodas, providing a more sustained energy boost without dramatic spikes and crashes. This gentler stimulant effect may better support the stable energy levels that make fasting sustainable.
Diet Pop Popularity: Why Fasters Reach for Zero-Calorie Drinks
Diet coke fasting has gained popularity for several practical reasons. Many intermittent fasters struggle with hunger during fasting windows, and the sweet taste and carbonation of diet sodas can provide psychological satisfaction without adding calories. The caffeine content in many diet sodas also offers an energy boost during fasting periods when energy levels might naturally dip. This stimulant effect can make fasting more manageable and productive, especially for those new to the practice. Additionally, transitioning from regular sodas to diet varieties during fasting represents a significant calorie reduction for many people. While purists might avoid all sweetened beverages during fasts, for many, diet sodas serve as a helpful intermediate step toward developing sustainable fasting habits.
The Importance of Individual Responses
Individual responses to diet drinks during fasting vary considerably. Some people report increased hunger after consuming artificially sweetened beverages, while others experience reduced cravings and find fasting easier with these zero-calorie options. Research from the National Center for Biotechnology Information suggests these individual differences might relate to gut microbiome composition, metabolic health, and prior dietary patterns. Some people may experience more pronounced insulin responses to artificial sweeteners than others. The psychological component shouldn’t be underestimated either. For those transitioning to intermittent fasting from a diet high in sweet beverages, diet sodas can provide familiar comfort that makes fasting sustainable. This psychological benefit might outweigh potential minor metabolic concerns for some individuals.
Not All Diet Drinks Are Created Equal: What to Know
The fasting impact of diet beverages varies based on their ingredients. Different artificial sweeteners have distinct metabolic effects-aspartame (in Diet Coke), sucralose (in some diet drinks), and stevia (in natural diet beverages) may affect insulin and blood sugar differently. Additives beyond sweeteners also matter. Some diet drinks contain phosphoric acid, caramel color, and other ingredients that might affect digestive processes or trigger inflammatory responses in sensitive individuals. These reactions could potentially influence fasting benefits. Diet beverages with natural sweeteners like stevia might theoretically have less impact on insulin than those with artificial options, though research remains limited. Similarly, carbonated flavored waters with no sweeteners likely have minimal impact on fasting compared to traditionally sweetened diet sodas.
Side Effects of Coke Zero During a Fast
Consuming Coke Zero while fasting may cause several side effects worth considering. The acidity can potentially erode dental enamel, especially when consumed frequently during fasting periods when saliva production may already be reduced. Research published in the Journal of Oral Rehabilitation indicates carbonated beverages, even sugar-free ones, can significantly impact dental health due to their acidic nature. This concern may be heightened during fasting when protective food consumption is absent.