Intermittent fasting involves alternating periods of eating with periods of not eating. The benefits largely depend on maintaining a true fasted state. Many people beginning a fasting regimen are confused about which beverages are acceptable to consume during the fasting window. Since sodas are widely available, their consumption while fasting is a frequent concern. This article clarifies the rules regarding both regular and diet soda and explains the physiological reasons behind these guidelines.
What Constitutes a Fast
The metabolic goal of fasting is to shift the body’s primary fuel source from glucose to stored fat. This transition is triggered when the body exhausts its readily available glucose reserves. The body enters this fat-burning mode when insulin remains consistently low. Insulin is the hormone the pancreas releases in response to food intake, particularly carbohydrates. When insulin levels drop and stay low for an extended period, the body signals the breakdown of stored fat for energy. Therefore, any food or drink that causes a significant rise in insulin will interrupt this metabolic switch and break the fast.
Regular Soda: The Immediate Metabolic Impact
Regular soda is composed primarily of carbonated water, high-fructose corn syrup, or sucrose. A single 12-ounce can of typical sugary soda contains around 39 grams of sugar. This massive load of simple carbohydrates is rapidly absorbed into the bloodstream, causing an immediate and significant spike in blood glucose levels. In response, the pancreas releases a large surge of insulin. This high level of insulin instantly halts the fat-burning process and signals the body to store the incoming energy, ending the fasted state. Consuming regular soda is a clear violation of a fasting period because the sugar content directly counteracts the metabolic goal, making it incompatible with any form of fasting.
Diet Soda: Examining the Zero-Calorie Contention
Diet sodas are marketed as calorie-free and sugar-free, using artificial sweeteners such as aspartame, sucralose, or saccharin. While they technically do not contain calories to break a fast from a purely caloric standpoint, their consumption is a controversial topic among fasting experts. The concern stems from how these intense non-nutritive sweeteners may still affect the body’s metabolic signals.
Cephalic Phase Insulin Response (CPIR)
One theory revolves around the cephalic phase insulin response (CPIR). The sweet taste activates receptors in the mouth, signaling the brain to anticipate a sugar load. This anticipation could potentially cause the pancreas to release a small, preemptive amount of insulin. While some studies suggest non-nutritive sweeteners do not reliably trigger this response in all individuals, other research indicates that certain sweeteners, like sucralose, can increase insulin levels in specific contexts.
Impact on Gut Microbiome
A more established concern is the impact of artificial sweeteners on the gut microbiome. Studies suggest that regular consumption of sweeteners like sucralose and saccharin can alter the composition of gut bacteria. This disruption, known as dysbiosis, is linked to changes in metabolic health and glucose tolerance over time, potentially undermining the long-term benefits of fasting.
Because of the potential for an anticipatory insulin release and the known negative effects on the gut microbiome, many fasting practitioners advise against diet soda. While it may not technically “break” a fast in the same way as regular soda, it introduces ambiguity and potential interference with the metabolic and gut-health goals of fasting. If a person’s primary fasting goal is to maximize metabolic benefits or promote cellular cleanup (autophagy), avoiding all artificially sweetened beverages is the safer approach.
Safe Beverages During a Fast
To maintain a true fasted state, the best beverages are those that have zero calories and cause no measurable insulin response. Plain water, whether still or sparkling, is the foundation of hydration during any fast and can be consumed freely. Staying adequately hydrated is one of the most effective ways to manage hunger pangs during the fasting window.
Unsweetened black coffee is widely accepted as a fast-friendly beverage because it contains virtually no calories and typically does not raise insulin levels. Similarly, plain tea—including green, black, or herbal varieties—is also permissible, provided it is consumed without any added sweeteners, milk, or cream.
Adding anything to these beverages, such as sugar, honey, milk, or cream, will introduce calories and trigger an insulin response, thus breaking the fast. Even non-caloric flavor drops or artificial sweeteners should be approached with caution due to the potential for a cephalic insulin response. For a guaranteed fast-friendly option, stick to plain water, black coffee, or unsweetened tea.