Quick answers
Does diet soda break a fast?
On the day, it doesn't spike insulin. As a daily habit, the picture changes
On the day: no insulin spike
In a British trial, swallowing sucralose didn't raise insulin or gut hormones, and simply tasting it and spitting it out had no effect either 1. In Singapore, drinks sweetened with aspartame, stevia or monk fruit didn't cause the blood sugar and insulin spike that a sugary drink did 2.
Key study
Sweeteners didn't save calories in the end
- Randomized trial
- 30 healthy men
- 4 drinks compared
People who had a zero-calorie sweetened drink ate more at lunch, enough to make up for the calories they had saved. By the end of the day, total intake was the same as with the sugary drink.
Tey et al., Int J Obes 2017As a habit: a reason for caution
Things look different when the sweetener meets real sugar, or when it becomes a daily habit. In people with obesity who didn't usually consume sweeteners, a sucralose drink taken before a glucose drink raised the insulin response by 20% 5. And in two randomized trials with healthy adults, taking sucralose every day for two to four weeks lowered insulin sensitivity, meaning the body needed more insulin to do the same job 3,4.
These were small, short studies, and they don't prove that a diet soda will harm you. If one helps you on a hard afternoon, it won't undo your fast. If you find yourself needing several a day, sparkling water with lemon is a better habit.
- MYTH
- Anything under 50 calories doesn't count.
- FACT
- This popular rule has no study behind it. What matters is what the food does, not just its calories: protein foods release far more insulin than their calories would suggest 6. If you want a rule of thumb, stick to drinks with no calories at all.
Sources
- Ford HE, Peters V, Martin NM, et al. Effects of oral ingestion of sucralose on gut hormone response and appetite in healthy normal-weight subjects. Eur J Clin Nutr. 2011;65(4):508-13. doi.org/10.1038/ejcn.2010.291
- Tey SL, Salleh NB, Henry J, et al. Effects of aspartame-, monk fruit-, stevia- and sucrose-sweetened beverages on postprandial glucose, insulin and energy intake. Int J Obes (Lond). 2017;41(3):450-457. doi.org/10.1038/ijo.2016.225
- Romo-Romo A, Aguilar-Salinas CA, Brito-Córdova GX, et al. Sucralose decreases insulin sensitivity in healthy subjects: a randomized controlled trial. Am J Clin Nutr. 2018;108(3):485-491. doi.org/10.1093/ajcn/nqy152
- Lertrit A, Srimachai S, Saetung S, et al. Effects of sucralose on insulin and glucagon-like peptide-1 secretion in healthy subjects: a randomized, double-blind, placebo-controlled trial. Nutrition. 2018;55-56:125-130. doi.org/10.1016/j.nut.2018.04.001
- Pepino MY, Tiemann CD, Patterson BW, et al. Sucralose affects glycemic and hormonal responses to an oral glucose load. Diabetes Care. 2013;36(9):2530-5. doi.org/10.2337/dc12-2221
- Holt SH, Miller JC, Petocz P. An insulin index of foods: the insulin demand generated by 1000-kJ portions of common foods. Am J Clin Nutr. 1997;66(5):1264-76. doi.org/10.1093/ajcn/66.5.1264
Keep reading
- Does gum break a fast?An occasional piece of sugar-free gum is unlikely to matter. A whole pack, or sugary gum, is another matter
What breaks a fast (and what doesn't)Coffee, milk, sweeteners and supplements, judged by what they do to your body
Insulin and fastingThe hormone that decides whether you store or burn, how insulin resistance develops and what fasting can do about it
A note on health
Fasting Tracker is designed for general wellness and is not a medical device. Fasting is not right for everyone. If you are pregnant, diabetic, have a history of eating disorders or take medication, talk to your doctor first, and before any extended fast.