Does Diet Soda Break Your Fast?

If something has zero calories, it must be OK to consume during your fasting window, right? Unfortunately for diet-soda fans everywhere, that assumption is false. Calories aren’t the only fast-breaking culprits — there are other ingredients in these fizzy beverages that can derail your fasting goals. Depending on your fasting goals, diet soda may or may not break your fast.

Why Diet Soda Breaks Your Fast

Most diet sodas contain artificial sweeteners (e.g., aspartame, sucralose, saccharin, and Ace-K), which have mixed reviews when it comes to weight loss. In fact, artificial sweeteners can contribute to weight gain, or, at the very least, make weight loss harder. And who needs extra obstacles in their health-gain journey?

Diet soda also impacts your gut. There was a time when researchers thought artificial sweeteners could pass through your gut without any absorption, but a few studies have now shown that, in adults and children, some artificial sweeteners actually do get absorbed. These studies demonstrate gut absorption with blood tests that measure sucralose levels pre and post-beverage consumption. It’s now argued that artificial sweeteners interact with our gut microbiome and can alter the function of our microbiota — a big no-no if you’re fasting for true gut rest.

Artificial sweeteners don’t contain protein that would stimulate mTOR (stimulation of mTOR inhibits autophagy), however, research is limited on the actual impact of diet-soda drinking on longevity. Because of the possible metabolic and gut-health concerns other studies have cited, it’s better to play it safe and assume artificial sweeteners have more of a negative impact your long-term health than a positive one. Instead, stick to your body’s favorite beverage during a fast: good ol’ H2O.


  • If you’re fasting for weight loss or metabolic health, diet soda does break your fast.
  • If you’re fasting for gut rest, diet soda does break your fast.
  • If you’re fasting for longevity, more research is needed here, but it’s safer to avoid while fasting.
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