Can You Drink Diet Soda While Intermittent Fasting?

Updated: Jan. 19, 2021

Most have 0 to 10 calories per serving, but drinking diet soda while fasting is tricky. Here's what registered dietitians want you to know.

Intermittent fasting and diet soda

There’s one simple rule when it comes to choosing your drinks when you’re fasting: Avoid anything with calories. The goal of intermittent fasting is to stay in a “fasted state,” eating or drinking no carbohydrates, fats, or protein for a certain period of time.

That allows your body to stop producing insulin, so it uses ketones stored in the fat for energy instead of glucose stored in the liver. In a 2019 article in The New England Journal of Medicine, some scientists believe that this process, called ketogenesis (which you may have heard of in relation to the keto diet), may slow aging, reduce chronic inflammation, and lead to better blood sugar control.

Because diet soda doesn’t have calories, it’s usually OK to drink while you’re fasting. But the scientific community is debating the impact of artificial sweeteners on two very key components in the intermittent fasting puzzle: insulin and hunger. (More on that later.)

What is intermittent fasting?

Intermittent fasting is not just one diet plan—it’s actually several. The basic concept is that this eating style restricts when you eat, not what you eat. Get all of the details about different styles in our complete guide on how to start intermittent fasting.

Here are some of the most common options:

  • The 16:8 or 14:10 Method: With these time-restricted methods, you eat only within a specific period. You might eat only in an eight-hour window (say, 10 a.m. to 6 p.m.) and fast for the next 16 hours (6 p.m. to 10 a.m.). Or eat within a 10-hour window (9 a.m. to 7 p.m.) and fast for the next 14 (7 p.m. to 9 a.m.).
  • Alternate-Day Fasting: Switch between days of eating as you normally would and days of fasting (consuming about 500 calories).

  • The 5:2 Method: Eat as you normally would five days per week, and choose two non-consecutive days of the week (like Monday and Thursday) to consume about 500 calories.

  • Eat-Stop-Eat Method: Fast for up to 24 hours once or twice a week. On the other days, eat as you normally would.

  • The Warrior Method: Quite possibly the toughest of all, this requires consuming all of your calories within a four-hour window (like 11 a.m. to 3 p.m.) and fasting for 20 hours (3 p.m. to 11 a.m.).

Can you drink diet soda while intermittent fasting?

“Technically speaking, most diet sodas that have zero calories will not break a fast,” explains Autumn Bates, a certified clinical nutritionist in Manhattan Beach, California. “However, if your goal is to lose weight, drinking diet sodas can still work against your weight loss goals by increasing sugar cravings and hunger throughout the day.”

Although most diet drinks come without calories, some humans might secrete insulin—a hormone that regulates fat storage—in reaction to the sweet taste even though it’s not sugar, Bates says. She points to an April study in the Journal of Family Medicine and Primary Care that suggests a link between artificial sweeteners and insulin resistance.

Bates adds that some people might also experience higher levels of hunger and more sugar cravings, especially later in the day, after consuming artificial sweeteners, according to a 2016 study in Cell Metabolism.

The challenging part, Bates notes, is that not all people secrete insulin after eating or drinking something that includes artificial sweeteners, just as not everyone that drinks diet soda experiences a case of the hangries after drinking a can.

“There is some evidence that artificial sweeteners such as sucralose and aspartame often contained in diet sodas may still cause a rise in insulin response due to a chemical reaction in the brain similar to when actually sugar is consumed,” says Leigh Merotto, RD, a Toronto-based registered dietitian with a focus on metabolic health, digestion/gut health, and sports nutrition.

“The jury is still out, however, we know now that in general diet sodas have not been shown to generally be supportive of weight loss goals,” she says.

soda in glasses with strawsrez-art/Getty Images

Potential health benefits and risks of drinking diet soda

Sugar-sweetened drinks like soda, juice, or sports drinks are the top source of added sugars in the American diet, reports the U.S. Centers for Disease Control and Prevention (CDC).

Those can lead to a whole host of health harms, including weight gain, heart disease, tooth decay, type 2 diabetes, and more.

Things can get a little tricky with calorie-free beverages, however, when you introduce artificial sweeteners. They’re consumed in minuscule quantities when you consider the safe upper limit.

Take the common diet soda sweetener, aspartame (aka Equal), for example. The U.S. Food and Drug Administration (FDA) has determined that the acceptable Daily Intake is 50 milligrams per kilogram of body weight.

If you weigh 150 pounds, that means the FDA says you can safely eat or drink up to about 3,400 milligrams per day. This works out to roughly 17 12-ounce cans of diet soda daily, according to the Kendall Reagan Nutrition Center at Colorado State University.

It’s doubtful you’ll drink that much.

But despite the fact that reasonably quantities of artificial sweeteners are considered safe by the FDA, the American Heart Association suggests that we all “limit low-calorie sodas” and “stick to water.” The Center for Science in the Public Interest says it’s “best to avoid” artificial sweeteners—intermittent fasting or not.

To date, studies about non-sugar sweeteners are generally fairly short-term, small in scale, and rely on (sometimes inaccurate) self reports, according to a 2019 large review published in the journal BMJ.

After analyzing 56 other studies, this review found no link between increased body weight and intake of artificial sweeteners.

There could be later-in-life health impacts we’re not aware of, though, hints a study of more than 100,000 people published in 2019 in  Circulation.

The researchers examined those who drank sugar-sweetened or artificially sweetened drinks. Compared to people who drank zero artificially sweetened beverages, those who drank six or more servings per day were 20 percent more likely to have cardiovascular disease during their lifetime.

It’s impossible to prove that these drinks caused the health problems, though—the scientists say they can only say there is a connection, and it’s possible other factors are involved.

What dietitians say about intermittent fasting

While diet soda is a fairly recent development (circa 1950s), “fasting is not a new idea,” says Alyssa Pike, RD, a manager for nutrition communications at the International Food Information Council (IFIC) in Washington, D.C. “People have been fasting since the dawn of time, sometimes out of scarcity but usually for spiritual reasons.

The act of fasting isn’t new, even if we’ve added the word ‘intermittent’ in front of it.”

You could argue that what’s new about intermittent fasting is the intention behind it. Fasting was previously primarily used as part of a spiritual practice of discipline, now it’s viewed as a trendy way to lose weight in the short term, Pike adds.

“Because intermittent fasting is a type of time-restricted eating, it can cause people to experience a slew of unhelpful eating consequences. Research shows restriction leads to heightened sense of available food, a more intense preoccupation with thoughts about food and overeating when food is available,” she says.

However, there are some intermittent fasting benefits, including increased weight loss, but it’s important to set up realistic goals and expectations. And remember: Your particular intermittent fasting plan will only be effective as long as you can stick with it.

It’s not safe to stop eating for 24 hours, or even 14 hours for some people. Those who are pregnant, breastfeeding, are on medication to control blood sugar, have diabetes, or have a history of eating disorders should skip it.