There are plenty of posts asking whether or not diet drinks break a fast, and I’m not here to ask that once again. What I do see though is when people fall on the side of saying that yes, they do break a fast, the reasoning is the drinks trigger an insulin response. Now I have looked a some studies on this and even still this isn’t abundantly clear. It might even come down to different people’s bodies responding differently.
But my question is this: why does initiating an insulin response necessarily break a fast? It seems what would matter is your actual blood glucose levels, not how insulin responds. If anything, initiating an insulin response without a rise in blood sugar would put you more at risk of low BG, since insulin works to move sugar from the blood into cells for use. But I haven’t seen any studies that seem to indicate this as a major risk either. So as long as you are not introducing sugar or other calories into your system, I have a hard time understanding how a diet drink breaks a fast from this perspective.
I can’t help thinking that maybe this is more of an psychological “purity” response. The argument of “you can only have water, tea, or black coffee” sounds similar to a vegan who won’t eat an impossible burger from Burger King because it is cooked on the same grill as a regular burger. They have done nothing to lessen the harm to animals by doing this, so it’s clearly just ideological for them. Similarly, if my unsweetened iced tea and my Diet Pepsi have the same amount of sugar and calories in them, it seems like there is little reason to view them differently as far as how they affect my fast.
Note: I’m not saying that the tea isn’t probably better for other reasons. Intuitively I think we all would say the tea is healthier for other reasons. I do also notice that diet drinks will sometimes make me want to eat. Conversely though, sometimes I really want to eat and a diet drink gives me the satisfaction to hold me over until I can eat, so that goes both ways. But while there may be other good reasons to avoid them, I have yet to be convinced diet drinks break a fast.
TL;DR: insulin inhibits the oxidation of fatty-acids by preventing them from entering the mitochondria and moves the fatty-acids back into storage locations.
Additionally, needing more insulin to shove those sources of energy into the mitochondria is called insulin resistance which leads to needing more insulin than your pancreas can release is called Type-II Diabetes.
But ultimately, insulin inhibits fatty-acid oxidation even if you’ve been reducing your calories to induce fatty-acid liberation.
Liberation by way of exercise or diet causes the fat cells to release their fatty-acids into the blood, Oxidation is the actual burning of those stores of energy by the mitochondria in all your cells. The longer you fast without an insulin response, the more of those fatty-acids are consumed by oxidation.
I will say for me personally, I kept in zero calorie drinks for the first month or so of IF, I have noticed a significant difference since cutting them out. Slightly more weight loss per week and maybe that’s a coincidence, but I have noticed a significant shift in body recomp without them, particularly in my lower abdomen which is usually the last place I lose from. I’m wearing a smaller pants size now then when I weight 30lbs less.
If using them works for you and you’re getting the results you want, keep going. You can also try a month or two with none and see if you feel any different. If you are fasting to fix health issues, like insulin resistance or a fatty liver, I think the artificial sweeteners are a bigger issue. In my opinion it doesn’t hurt to try cutting them out to just to see what your own body’s response to that would be, after all how your body reacts is more relevant to you then a generalized study.
Insulin can inhibit autophagy.
Here is a study that explains it quite well: https://www.frontiersin.org/articles/10.3389/fcell.2021.708431/full (tl;dr: read only paragraph “Insulin Signaling and Autophagy—A Balance of Anabolic and Catabolic Processes”)
There was one Israeli study on mice that indicated fake sugar impacted blood sugar levels, but through the gut microbiome…so indirectly…however this was never replicated in humans…and of course they were feeding mice a boatload of it….
In terms of humans, the jury is out and its certainly dependent on the type of fake sugar…
Sucralose caused a slight increase about 20%…
Studies have not linked aspartame with raised insulin level…
Studies of saccharin are mixed, one study found that mouth washing with a saccharin solution caused insulin levels to rise while other studies have found no effects….
Acesulfame potassium can increase insulin levels in rats, but the effect of acesulfame-K on insulin levels in humans is unknown…..