Do Sweeteners and Diet Soda Break a Fast?

Most zero-calorie sweeteners, and diet soda, won’t break a fast for weight loss because they have no meaningful calories. For a strict fast (water only, or fasting for autophagy), skip them all. The research on insulin is mixed, so the safest choices are pure stevia, monk fruit, erythritol and allulose, and the most debated are sucralose, aspartame and saccharin.

Sweeteners are a common way to make black coffee, tea or sparkling water easier to drink through a fasting window. Below is each one, what the research says, and how it fits your reason for fasting.

Do Sweeteners Break a Fast? Quick Answer by Sweetener

These match the ratings in our what breaks a fast tool, where you can check any brand or product not listed here. To plan your eating window, use our intermittent fasting calculator.

What Breaks a Fast?

It depends on why you fast:

• For weight loss, a fast is broken by calories. Zero-calorie sweeteners don’t add any, so they don’t break it in that sense.
• For blood sugar and insulin, the question is whether a sweetener raises insulin. For most, the answer on its own is no or very little, but a few studies suggest some can change how your body handles the next meal.
• For a strict fast or autophagy, the safest rule is water, black coffee and plain tea only. There is no good human research on sweeteners and autophagy, so strict fasters skip them.

New to fasting? Start with our guide to intermittent fasting for beginners.

Natural Sweeteners

Stevia

Stevia is a zero-calorie sweetener made from the leaves of the stevia plant. Its sweetness comes from compounds called steviol glycosides, which are 200 to 300 times sweeter than sugar.

In a small 8-week trial in 34 people with type 2 diabetes, tea sweetened with stevia had no effect on blood sugar, HbA1c, insulin or blood fats. (1)

Many stevia products also contain dextrose, maltodextrin or sugar alcohols, which add calories or affect blood sugar. Choose pure stevia extract.

Will it break your fast? Pure stevia won’t break a fast for weight loss or insulin control. For a strict fast, skip it.

Related: Best Stevia Products From Top-Notch Brands Reviewed

Monk Fruit

Monk fruit (luo han guo) has been used in southern China for centuries. The sweetener is made from compounds called mogrosides, which are about 150 to 200 times sweeter than sugar and have no calories. The FDA accepted it as generally recognized as safe in 2010.

It doesn’t appear to raise blood sugar. Lab research suggests mogrosides may even help insulin secretion, though human data are limited. (2) Many monk fruit products are blended with erythritol or sugar, so check the label.

Will it break your fast? Pure monk fruit won’t break a fast for weight loss or insulin control. For a strict fast, skip it.

Artificial Sweeteners

Sucralose (Splenda)

Sucralose is about 600 times sweeter than sugar and has no calories on its own. Splenda packets add dextrose and maltodextrin as fillers, which add a few calories per packet.

Its effect on insulin is debated:

• In 17 people with obesity who didn’t normally use sweeteners, sucralose taken 10 minutes before a sugary glucose drink led to a 20% bigger insulin response to that drink. (3) Note that this was sucralose plus sugar, not sucralose alone.
• In a 10-week trial in healthy young adults, daily sucralose raised insulin levels during a glucose test, though the effect didn’t increase with the higher dose. (4)
• Other studies found no change in blood sugar or insulin, including one where healthy adults took high doses for a week. (5)

Will it break your fast? On its own, sucralose has no calories, so it won’t break a fast for weight loss. If you fast for blood sugar control or a strict fast, skip it.

Aspartame and Acesulfame K

Aspartame (NutraSweet, Equal) and acesulfame K (Sunett, Sweet One) are both about 200 times sweeter than sugar, and they are often used together in diet drinks. Aspartame technically has calories, but you use so little that a serving adds almost none. It contains phenylalanine, so people with the genetic condition phenylketonuria (PKU) must avoid it.

Neither has a clear effect on blood sugar at normal doses. Claims that aspartame stops autophagy aren’t backed by human studies.

Will it break your fast? It adds no meaningful calories, but because the evidence is thin, our tool rates it as “might break” for every goal. A strict fast skips it.

Does Diet Soda Break a Fast?

Diet soda (Diet Coke, Coke Zero, Pepsi Zero Sugar, Sprite Zero and others) has no calories. Most are sweetened with aspartame, acesulfame K or sucralose, so everything above applies.

For weight loss, a diet soda won’t break your fast in calorie terms. The Mayo Clinic says a moderate amount is fine for most adults. (6) For a strict fast, skip it. Our tool rates diet soda as “might break” for every goal because of its sweeteners.

A few practical points:

• Caffeine. Diet colas contain caffeine. Count it toward your daily total, especially if you also drink coffee.
• Teeth. The acids in soda, diet or not, can wear tooth enamel over time.
• The research on weight gain. Some large studies link diet soda with weight gain and metabolic syndrome. These studies observe habits and can’t show that diet soda causes the problem: people already gaining weight often switch to diet drinks.
• Blood tests. Before a fasting blood test, drink only water unless your lab says otherwise.

Better swaps during a fast: sparkling water (plain or naturally flavored with no sweetener), black coffee, and plain green or herbal tea. All three are fine for every type of fast.

Sugar Alcohols

Erythritol and Xylitol

Sugar alcohols such as erythritol, xylitol, maltitol and sorbitol are less sweet than artificial sweeteners. They differ a lot:

• Erythritol has almost no calories and doesn’t raise blood sugar or insulin. It fits a weight loss fast.
• Xylitol has about 2.4 calories per gram and a small effect on blood sugar. Keep it to small amounts.
• Maltitol and sorbitol have more calories and a bigger effect on blood sugar. Avoid them during a fast.

Large amounts of any sugar alcohol can cause bloating and diarrhea.

Will they break your fast? Erythritol won’t for weight loss; xylitol only in small amounts. Skip both for a strict fast.

Allulose

Allulose is a rare sugar found in small amounts in figs, raisins and jackfruit. It tastes and behaves like sugar but has about 70% of its sweetness and only 0.2 to 0.4 calories per gram, because the body absorbs it and passes it in urine without using it for energy. (7)

It doesn’t raise blood sugar or insulin, and early research suggests it may help blood sugar control. (8)

Will it break your fast? Allulose is one of the best-tolerated choices for any fast in small amounts.

FAQs

Can I drink Diet Coke while intermittent fasting?

For weight loss, yes: it has no calories. For a strict fast, no. Water, black coffee and plain tea are better choices.

Does stevia break a fast?

Pure stevia has no calories and doesn’t raise blood sugar, so it won’t break a fast for weight loss. Skip it on a strict fast.

Does Splenda break a fast?

Not for weight loss, though each packet has a few calories from its fillers. Some studies suggest sucralose can change the insulin response to food, so skip it if you fast for blood sugar control.

Which sweetener is best while fasting?

Pure stevia, monk fruit, erythritol or allulose, in small amounts. For a strict fast, none.

The Bottom Line

Zero-calorie sweeteners and diet soda don’t break a fast for weight loss, because they add no meaningful calories. The research on insulin is mixed, so if you fast for blood sugar control or follow a strict fast, stick to water, black coffee and plain tea. If you want something sweet, pure stevia, monk fruit, erythritol and allulose are the safest picks.

References

Fitness Volt is committed to providing our readers with science-based information. We use only credible and peer-reviewed sources to support the information we share in our articles.

1. Ajami M, Seyfi M, Abdollah Pouri Hosseini F, et al. Effects of stevia on glycemic and lipid profile of type 2 diabetic patients: A randomized controlled trial. Avicenna J Phytomed. 2020;10(2):118-127.
2. Zhou Y, Zheng Y, Ebersole J, et al. Insulin secretion stimulating effects of mogroside V and fruit extract of luo han kuo (Siraitia grosvenori Swingle) fruit extract. Yao Xue Xue Bao. 2009;44(11):1252-1257.
3. Pepino MY, Tiemann CD, Patterson BW, et al. Sucralose affects glycemic and hormonal responses to an oral glucose load. Diabetes Care. 2013;36(9):2530-2535.
4. Bueno-Hernández N, et al. Chronic sucralose consumption induces elevation of serum insulin in young healthy adults. Nutr J. 2020;19(1):32.
5. Thomson P, Santibañez R, Aguirre C, et al. Short-term impact of sucralose consumption on the metabolic response and gut microbiome of healthy adults. Br J Nutr. 2019;122(8):856-862.
6. Mayo Clinic. How much diet soda can you safely drink?
7. Iida T, Hayashi N, Yamada T, et al. Failure of d-psicose absorbed in the small intestine to metabolize into energy and its low large intestinal fermentability in humans. Metabolism. 2010;59(2):206-214.
8. Jiang S, Xiao W, Zhu X, et al. Review on D-Allulose: In vivo Metabolism, Catalytic Mechanism, Engineering Strain Construction, Bio-Production Technology. Front Bioeng Biotechnol. 2020;8:26.