Does Diet Soda Break A Fast? [Full Story + Personal Experience]

People love their diet soda these days, so it’s no surprise they want to keep drinking it while they fast. 

But does diet soda break a fast? 

Naturally, that depends on what you mean by “break”.

If you’re just trying to avoid calories, then perhaps you can get away with drinking diet soda.

But if you’re trying to improve your health or make fasting easier, drinking diet soda is probably counterproductive.

For the whole story, read on. 

My Experience With Diet Soda

I’ve been kind of addicted to sugar my whole life.

As a kid, I loved binging on candy and sweets whenever I could, from holidays like Halloween, Easter, or Christmas, to slice after slice of cake on my birthday.

Here’s me and one of my huge birthday cakes (I probably ate at least half of it over the next few days):

Sometimes, if I earned $10 by mowing a lawn or something, I would go to Walmart and spend it all on snack cakes, chips, and candy ($10 went a little farther back then), and eat most of it the same day. 

My mom loves to remind me of the time I “stole” a bag of brown sugar and hid it in the bathroom, where I would close myself in and eat spoonfuls at a time. 

I did this multiple times. 

In more recent years, this addiction has often manifested itself as late-night trips to 7-Eleven (for donuts, nachos, candy…you name it). 

Several years ago, around the time I was in med school, I decided to quit regular soda (because I learned it was “empty calories”…and it wasn’t my favorite treat anyway). 

Diet soda seemed like a natural replacement.

Once I got on the diet soda train, I quickly figured out that Diet Mountain Dew was my favorite, and I used to always drink it on road trips.

(Cue craving for Diet Mountain Dew.)

As a side note, the only time my sugar addiction really feels under control is when I’m eating low carb, or fasting. 

That’s probably because eating higher fat and protein controls hunger way better than eating mostly carbs. Ketones also help suppress my appetite, and decrease anxiety. 

Back to diet soda. 

Only recently did I realize there’s a long (and growing) list of harmful effects from the artificial sweeteners used in diet soda.

After that, I tried to cut back, and for a while I did.

About six months ago, however, diet soda started sneaking back into my home more and more often, sometimes even making appearances during extended fasts.

So I decided to do something about it.

I made a commitment to quit all artificial sweeteners–at least for a few months–and shared the commitment with my little brother (he’s been my accountability partner for health stuff over the past couple years).

As a result, since June 1st of this year, I have not had any diet soda or otherwise intentionally consumed any artificial sweeteners.

(Not that you can completely avoid them…they’re in the drinking water and groundwater, for crying out loud.) 

For now, I’m still using stevia (a “natural“ sweetener), but I’ll probably try cutting that out at some point as well. One step at a time. 🙂 

It’s still early, but I have the following to report after quitting diet soda:

• Fasting is easier without it
• My sleep has improved
• I don’t crave sweets as often
• I don’t get hungry as often

Some of this may be due to other lifestyle changes, but I think cutting out diet soda has been beneficial. 

So that’s my story, but what about you? Is it a good idea for you to drink diet soda when you fast?

Let’s break this topic down logically, and look at the science. 

Can You Drink Diet Soda While Fasting?

Well of course you can…but is it a good idea?

Diet soda is typically sweetened with aspartame (brand name Equal, or NutraSweet).

(Sometimes they mix in sucralose or stevia, but aspartame is still the king.)

So when you ask if diet soda breaks a fast, you’re mainly asking if aspartame will break your fast.

Let’s explore how diet soda (and aspartame) might impact your fasting goals.

(Note: I went a little more in-depth on some of these topics in my article on whether sucralose breaks a fast, so feel free to take a look at that as well.)

Will Diet Soda Make You Fatter While (Intermittent or Extended) Fasting?

Your body starts burning tons of body fat when you fast (because there’s basically no other energy available). 

Will diet soda interfere with this? 

There’s not a lot of research specifically about fasting with artificial sweeteners, but we can look at related info and try to draw some conclusions.

A recent meta-analysis concluded that the sweeteners used in diet soda appear to cause weight gain / obesity over time. 

This may be because people tend to feel less satisfied after consuming artificial sweeteners (including aspartame), and may end up eating more in the long run as a result.

Artificial sweeteners are also associated with higher rates of metabolic syndrome (of which abdominal fat is one part). 

Bottom line?

I’m sure you’ll still burn some fat if you drink diet soda when you fast. 

But in the long run, drinking diet soda may make you fatter and sicker. 

Does Diet Soda Raise Blood Sugar While Fasting?

A lot of people (especially diabetics and pre-diabetics) fast to help lower their blood sugar. 

This works great, because their blood sugar goes down steadily as long as they keep fasting.

But what if you drink diet soda while you fast?

I don’t think there’s any evidence that diet soda directly raises blood sugar. 

But it may do so indirectly, at least over time.

How could diet soda increase your blood sugar, if it doesn’t do so directly?

One way is by changing the bacteria in your intestines (which has a surprising number of indirect effects).

For example, aspartame and other artificial sweeteners alter gut bacteria, which leads to glucose “intolerance” in rats, and also in mice. 

There seems to be a similar connection between aspartame and glucose intolerance in humans

What does glucose intolerance mean? 

It means the body can’t remove sugar (glucose) from the bloodstream as quickly, so blood sugar stays higher for longer.

That’s probably not what we want.

Bottom line?

Diet soda may not raise your blood sugar directly, but if you drink enough of it there’s a good chance it will raise your blood sugar indirectly.

Again, not ideal. 

Does Diet Soda Break Ketosis While Fasting?

When you fast, your liver makes ketones out of body fat. 

Why does it do this, you ask?

Ketones serve as an alternative energy source, especially for your brain (which normally runs on large quantities of glucose). 

When it comes to diet soda and ketones, we don’t really have any research data to look at.

In my experience, diet soda doesn’t seem to have much of an effect on my ketone levels.

For example, before I quit artificial sweeteners, I decided to use up my last several cans of diet soda during a four-day extended fast. I drank about four or five cans per day (sometimes I’m a little crazy like that). 

I checked my ketone levels regularly, and they didn’t seem any lower than expected.

That’s my experience, but you could be different.

Bottom line?

At least for me, diet soda doesn’t seem to have much effect on ketone levels.

Will Diet Soda Make You Crave Sweets While Fasting?

Yes, most likely it will.

How do I know this?

Multiple studies (here and here, for example) have shown increased preference for sweet foods after using artificial sweeteners.

I’ve also experienced this myself – whenever I eat anything sweet (artificial or not), it makes me crave more sweetness.

That may be at least partially because I’m a sugar addict…but as the research shows, I’m not the only one who craves more sweets after using aspartame.

Bottom line?

Diet soda will probably make you crave other sweet things, which is not ideal while fasting.

Will Diet Soda Make You Hungrier While Fasting?

Diet soda may or may not “break” your fast in terms of how it affects your blood sugar or ketones.

But it might break your fast…by making you stop fasting.

That’s because aspartame and other artificial sweeteners can make people feel hungrier. 

For example, Megan Ramos (a clinical educator who has coached many people through fasting) advises against using artificial sweeteners, because she’s noticed it makes her patients feel hungrier.

I’ve noticed the same thing myself. 

For example, when I drank a bunch of diet soda during an extended fast (as mentioned above), I felt hungry more often than usual, and my energy levels seemed lower as well.

Bottom line?

If you’re drinking diet soda and having trouble fasting, the artificial sweeteners may be the problem.

What’s The Bottom Line – Will Diet Soda Break Your Fast?

Diet soda may not raise your blood sugar or make you fatter in the short-term, but there is mounting evidence that it could do these things over the long-term.

You may also find it harder to fast if you drink diet soda, because it often stimulates increased hunger and cravings.

Bottom line?

Use it (only) if you must, but diet soda is definitely not ideal for fasting, or for health in general. 

Does Aspartame Break a Fast?

That’s kind of a trick question. 🙂

If you’ve been paying attention so far, you know that aspartame (Equal, NutraSweet) is the main sweetener in diet soda. (It’s also in many other products, including energy drinks, most brands of chewing gum, and more.)

And while it may not cause weight gain or high blood sugar directly, it appears to make people fatter and sicker in the long run.

Bottom line? 

You can try using aspartame while you fast, but it’s not good for your long term health, and it may cause more hunger or cravings during a fast.

What About Specific Types of Diet Soda?

Does Diet Coke Break a Fast?

A lot of people ask specifically about Diet Coke because it’s the most popular diet soda in the US. 

Of course, the answer is the same as it was for diet soda in general (and anything else made with aspartame): 

You may do okay fasting with Diet Coke in the short-term, but it might give you more hunger and cravings, and it’s not good for your health in the long run. 

Does Zevia Soda Break a Fast? 

Zevia is a trendy new “diet” soda sweetened with stevia (instead of the usual aspartame or sucralose).

So the real question here is whether stevia breaks a fast.

I’ve written about this elsewhere, but here’s the short answer:

For most people, stevia is okay while fasting, and it’s probably worth using if it makes fasting easier for you.

There is one main caveat though:

It’s not as common as with regular diet soda, but stevia makes some people hungrier when they fast.

So keep an eye on things, and pay attention to how your body reacts.

What Else Should I Know About Diet Soda?

Diet Soda and Depression

In this study, people consuming aspartame had higher rates of irritable moods and depression, as well as worse spatial orientation.

The people in that study were using medium doses of aspartame, so it could be even worse for someone drinking diet soda by the gallon.

If you needed another reason to avoid that soda…that’s a pretty good one!

Diet Soda and The Gut Microbiome

I’ve gone into this in more detail before, so I’ll be brief here.

Suffice it to say, artificial sweeteners change our intestinal bacteria, which can have a million indirect effects on our health.

Want to know more?

Check out this podcast episode.

Diet Soda and Pregnancy

Did you know moms who eat junk food while they’re pregnant are more likely to have obese babies?

And it’s not just regular junk food. The same thing appears to be true of artificial sweeteners.

This topic reminded me of a podcast episode I listened to a few years ago (Rhonda Patrick on Stem-talk). If you want to get scared into becoming healthy before you have kids, give it a listen!

Here’s the short version:

The health of both parents before pregnancy has a dramatic impact on a child’s future health, largely through epigenetic effects (i.e. which genes become activated or suppressed).

What Could You Drink Instead?

I wrote a separate, more detailed post about what to drink while fasting. But here are a few quick suggestions:

Drink plenty of water. Make it your default, and keep some handy throughout the day.

Get your salt! In other words, stuff with lots of sodium. Getting enough salt helps you sleep better and keep your energy up, especially during long fasts.

Coffee and tea are both reasonable (without any sugar).

Apple cider vinegar has various health benefits, and is fine to use when fasting.

Lemons water is ok as well, as long as you don’t overdo it.

It’s not necessarily ideal, but if you enjoy the carbonation in particular, you may want to try some seltzer water while you fast.

Lastly, if you really want something sweet, stevia (including Zevia soda) is among the better options. But be cautious – any sweetener can potentially cause cravings.

Conclusion – Does Diet Soda Break a Fast?

Diet soda is definitely not ideal for fasting, or for health in general.

But even if you’re hooked, don’t let that stop you from fasting. 

If you can’t give up your diet soda right away, and you want to try fasting anyway, give it a whirl.

It’s better to fast with diet soda than not to fast at all.

Everybody’s different, so maybe you won’t have as many issues with hunger, cravings, etc. as other people do. 

Over time, fasting might make it easier for you to cut back on diet soda. 

And who knows, eventually you may even be able to quit. Like I recently did.