Insulin Response: Why Doesn't Everyone Agree?

This post is all about the question:  does a sweet taste really cause an insulin release?  What does the science say?  I have some links at the end of this blog post that you can go to in order to draw your own conclusions.  Please do, in fact.  I will always believe that it is best to examine the science yourself rather than blindly believe what I say about it.

If this is the first you are hearing about this concept, and you aren't sure why it matters, check out my blog post about the importance of a clean fast.  It is located here.  Personally, I believe the key to long-term intermittent fasting success is the clean fast.  I've experienced it both ways, and the difference is night and day. 

 The “sweet taste and insulin” debate is one of the biggest sticking points for many people, and there are people out there who ridicule the concept completely.  I actually got a comment today on my coffee blog post from a guy who tried to prove to me that the science shows sucralose (Splenda) is actually fine during the fast, because it doesn’t raise insulin.  He had a study (his study is linked here) that “proved it”.  Well, I went to his link and read his study, and the sucralose was administered through something called “intragastric infusion.”

What does that mean?  The sucralose was inserted DIRECTLY INTO THE STOMACH.  That is what “intragastric infusion” means.

So, what did we learn from that study?  If you would like to insert sucralose directly into your stomach through intragastric infusion, this study shows it’s absolutely fine to do that.  Indeed, it does not appear that will cause you to secrete insulin.

The elephant in the room is that in real life, we are NOT inserting anything directly into our stomachs.  We drink beverages through our mouths, and we taste them.  In the insulin response theory, it’s the TASTE of the sweetness that is the problem.  All of the studies that I link in my books and blog posts about insulin release relate to the sweet TASTE of something you ingest tricking the brain into thinking that you need insulin to handle whatever sweet thing you are consuming.  According to the sweetness/insulin response theory, the body doesn’t understand that it’s actually a zero calorie sweet taste.  The body is ready for the calories it associates with sweetness, hence the insulin release.  Clearly, inserting something directly into the stomach bypasses the taste receptors, which is what the study about intragastric infusion illustrates.

This is such a confusing topic for many, and this is why:  as with MANY topics, you can find studies (and resulting opinions) that contradict one another.  That’s right!  You can find studies that show there IS an insulin response to sweet tastes, and you can also find studies that show there is NOT an insulin response to sweet tastes!  I could “prove” there is NOT an insulin response to you by referencing some studies that came to that conclusion, but I could also “prove” there IS an insulin response by selecting other studies that determined the opposite to be the case.  This is called “cherry picking” data:  only looking at information that agrees with what you believe to be true, and ignoring any that don’t match what you believe.

So, what do we do when faced with contradictory information?  Of course, I personally want to err on the side of caution.  If there is a possibility something is going to cause me to release insulin during the fast, I am going to avoid it.  Trust me.  NO ONE wanted to have Stevia during the fast more than I did.  I searched and searched for a rationale that would allow me to keep it in my coffee.  Once I decided to eliminate it, it changed the way I experienced intermittent fasting and made the process truly effortless. 
 
Here are some links if you want to dig in for yourself.  Note:  “CPIR” stands for “Cephalic Phase Insulin Response”.

1.  This one, from 2008, was performed on humans (not rats), and it is the one that finally convinced me to drop the stevia:  https://www.ncbi.nlm.nih.gov/pubmed/18556090
*Key takeaway:  “A significant increase of plasma insulin concentration was apparent after stimulation with sucrose and saccharin. In conclusion, the current data suggest that the sweeteners sucrose and saccharin activate a CPIR even when applied to the oral cavity only.”  Even if I read ten studies that had a different outcome, this one would make me stay away from anything sweet during the fast.  Better safe than sorry.

2.  https://www.ncbi.nlm.nih.gov/pubmed/17510492
*This is a rat study, but it is fascinating because:  “The non-nutritive sweetener saccharine elicited CPIR. However, starch, which is nutritive but non-sweet, did not elicit CPIR although rats showed a strong preference for starch which is a source of glucose. In addition, we studied whether CPIR was related to taste receptor cell activity. We carried out the experiment in rats with bilaterally cut chorda tympani nerves, one of the gustatory nerves. After sectioning, CPIR was not observed for sweet stimulation. From these results, we conclude that sweetness information conducted by this taste nerve provides essential information for eliciting CPIR.”  What that means is that when the scientists cut the nerves from the tongue to the brain, and the rats could not TASTE the sweetness, there was no insulin release.  It was related to the taste only.

3.  https://www.ncbi.nlm.nih.gov/pubmed/28899680
*Key takeaway:  "The results indicate the presence of a significant CPIR in a subset of individuals with overweight or obesity after oral exposure to sucralose, especially when present in solid food form." Don’t miss this important conclusion:  while the beverage form had a smaller insulin spike than the food form, there still was an insulin response to the sweet beverage.

4.  https://www.ncbi.nlm.nih.gov/pubmed/8246776
 *Key takeaway:  “obese subjects exhibited significantly greater CPIR than normal-weight subjects.”  This implies that when you are overweight, your body has MORE of an insulin response than someone of normal weight.

5.  https://www.ncbi.nlm.nih.gov/pubmed/3289998
*Another rat study.  Key takeaway:  “We conclude that saccharin (through taste) appears to elicit parasympathetic (insulin release) and sympathetic (HGP increase) reflexes in lean and obese rats. These taste-induced changes in plasma insulin and glucose turnover are exaggerated in the obese rats and may participate in obesity and in insulin resistance of the overall syndrome.”  Again, the obese animals had a HIGHER insulin response than the lean ones.

Those last 2 studies imply that if you are overweight, you need to be even more careful than others about what you ingest during your fast. I think that is important to understand. 

In conclusion:  you are an adult, and you are making your own decisions here.  I will continue to believe that if you are looking for the best possible results from an intermittent fasting lifestyle, you want to avoid all sweet tastes during the fast.  Yes, I cherry-picked the studies that support my conclusion and I didn't link to any that imply that the sweet tastes are a-okay.  But, if you want to err on the side of caution, that is what you would do.

Not everyone is going to agree with my conclusions, and I’m okay with that.  Remember, there are groups of people who still believe the earth is flat, and that scientists are lying to us with the whole “earth is round” nonsense.  Boy, do I wish I was kidding.  https://www.livescience.com/24310-flat-earth-belief.html  If we can’t come to 100% consensus on the shape of the earth, then I’m pretty sure the complex issue “does a sweet taste cause an insulin response” is never going to be “settled” for everyone.  (Spoiler alert:  I do believe the earth is round.  Thank goodness.)

And for everyone who remains unconvinced about sweet tastes and insulin:  I would like to issue a challenge to you.  Fast clean for at least two weeks, with nothing but black coffee and unflavored/unsweetened still and sparkling water.  Then, reintroduce whatever it is that you believe is not a problem.  Pay attention to how you feel.  I’ll be very surprised if you don’t notice that the fast is notably easier when you fast clean.  That’s what most people discover, and it is what I found personally.  Try it and see!  What do you have to lose?