“Clean fasting” means avoiding sweet flavors during your fast, and intermittent fasters are divided over whether it actually matters. There are some pretty strong opinions on both sides, and that’s ok because we’re all different.
Personally, though, these three scientific studies have me convinced that clean fasting matters quite a bit.
Experiment #1–What does a sweet flavor do to rats’ insulin levels?
In one experiment, scientists had rats taste something sweet and tested their insulin levels. When these rats got a taste of sweetness, their insulin levels went up.
Then (poor rats), they did the same experiment in rats whose taste-nerves had been severed so they could no longer taste anything. The same sweet substances did not raise insulin in these rats who couldn’t taste it.
In this study, it’s pretty clear that a sweet flavor (not calories) is what made insulin spike.
Experiment #2–What does a sweet flavor do to humans’ insulin levels?
Ok, those are rats. What about humans? Another study on insulin release in humans asks this very question. They had humans rinse their mouths with a flavored liquid for 45 seconds, then measured their insulin levels. Guess which flavor resulted in elevated insulin? If you guessed sweet, you’d be right.
This study concludes that “the sweeteners sucrose and saccharin activate a CPIR (Cephalic Phase Insulin Release) even when applied to the oral cavity only.”
So getting something sweet in your mouth causes an insulin release, whether you swallow or not.
Experiment #3–What if humans don’t taste the sweet flavor?
Since cutting tongue-to-brain nerves for a study on humans is generally frowned upon, scientists had to find another way to get sucrose and saccharin into human stomachs without engaging their taste buds. They got around taste buds by using intragastric infusion–which means inserting stuff directly into people’s stomachs. There were tubes involved.
So they inserted sucrose and saccharin straight into the stomachs of several people and measured their insulin responses. When people were not able to taste the sweet things going into their stomachs, they did not have an insulin response.
This study is why you might see claims that artificial sweeteners do not raise insulin. If you can get them to your stomach without tasting them, they don’t. (But isn’t tasting them kind of the point?)
Here’s the thing: I am absolutely not about to insert anything directly into my stomach via intragastric infusion. It sounds…unpleasant.
Fasting Should Not Be a Delicious Experience
The only other way to avoid spiking insulin during a fast is to avoid sweet tastes–whether they involve calories or not.
That means no raspberry sparkling water. No flavored coffee. No stevia-sweetened herbal teas. No calorie-free water enhancers. Nothing with an artificial fruit flavor in the ingredient list.
Fasting should not be a terrible experience, but it shouldn’t be a delicious one, either.
It also means that while insulin stays low, your body does a cool cellular-recycling thing called autophagy. It means that you adapt to burning stored fat for fuel and increase your metabolism at the same time. I could go on—a lot of good things happen during a clean fast.
It may take a few tries for some people to get used to clean fasting, but the rewards are kind of a big deal. It’s worth it. You are worth it. Don’t give up.