If you’re following a low-carb diet, or a diet with low-carb periods, like Carb Backloading or Intermittent Fasting, then you probably drink diet sodas. They’re a convenient, tasty alternative to water, coffee, and tea. And, more importantly, you can enjoy the sweet taste without worrying about the insulin spike that you’d get from a sugary drink. Or can you?
If you’ve done research on Carb Backloading, then you’ll be aware that Acesulfame K (Ace K) is forbidden because it spikes insulin. But does it really?
Sean: Going back to sweeteners, you make a point in the book about acesulfame potassium raising insulin levels. I noticed that Coke Zero has a small amount of it, but not Diet Coke. Based on that, would you say that Coke Zero could therefore possibly spike insulin and is a bad diet soda choice?
Kiefer: Yeah, I would go with the Diet Coke. Acesulfame potassium definitely causes an insulin response. So many factors can influence that so it’s hard to say how intense it is. I have noticed that people who drink a lot of energy drinks that have acesulfame potassium don’t fare nearly as well on Carb Nite until they cut those out. So I would say that the response is significant, even with a small amount.
In his Carb Backloading book, Kiefer references three studies. The first study found that infusions of Acesulfame K increased insulin secretion in rats . The second part of that study examined the effects of Acesulfame K on isolated rat pancreatic islets . It found that Acesulfame K had a dose and glucose-dependent effect on insulin secretion; but, most importantly, even without glucose it promoted insulin secretion. The third study found that artificial sweeteners with a bitter taste (including, but not limited to, Acesulfame K) augmented the insulin response from isolated rat pancreatic islets in the presence of glucose.
That appears to be strong evidence that you should avoid Acesulfame K. If you’re a rat.
Are You A Rat?
Let’s look at the first study:
Injection of Acesulfame K (150 mg/kg body weight) increased the plasma insulin concentration at 5 min from 27.3 +/- 3.0 microU/ml to 58.6 +/- 4.2 microU/ml without any significant change in the blood glucose. Infusion of Acesulfame K (20 mg/kg body weight/min) for one hour maintained the insulin concentration at a high level (about 85-100 microU/ml) during this period, and at the same time blood glucose was gradually reduced from 103.0 +/- 7.3 to 72.0 +/- 7.2 mg/dl. When using different amounts of Acesulfame K, the insulin secretion was stimulated in a dose-dependent fashion.
The human equivalent dose is calculated as follows:
Human equivalent dose (mg/kg) = rat dose (mg/kg) x human Km/rat Km
which gives, for adult humans,
Human equivalent dose (mg/kg) = 150mg/kg x 6/37 ~ 24mg/kg.
In particular, the equivalent dose for a 60kg adult human would be 1440mg. But how much diet soda would you have to consume to achieve that? Let’s make a reasonable estimate using the following information:
The ADI for acesulfame K is 15 mg/kg body weight per day for children and adults. This means that a 150 pound person can safely consume about 25 12-fluid ounce cans of diet soft drinks sweetened with a typical blend of acesulfame K and aspartame, or 20 tabletop sweetener packets nearly every day over his or her lifetime without any adverse health effect. The typical amount of acesulfame K in a 12 fl ounce beverage using an acesulfame K-aspartame blend is 40 mg; the average amount per sweetener packet is 50 mg.
Assume therefore that for every litre of diet soda there is approximately 140mg of Acesulfame K. That means that you’d have to consume 10 litres of diet soda to produce an insulin response equivalent to the one in the study. But even that might not be true, since the Acesulfame K was infused. We don’t know if it would have the same effect when taken orally. If you can imagine a 60kg person drinking 10 litres in one sitting, then perhaps this study is useful, but otherwise it seems slightly ridiculous!
What About Human Studies?
A review article  found that:
However, data from numerous publications on the effects of low-energy sweeteners on appetite, insulin and glucose levels, food intake and body weight have shown that there is no consistent evidence that low-energy sweeteners increase appetite or subsequent food intake, cause insulin release or affect blood pressure in normal subjects. Thus, the data from extensive in vivo studies in human subjects show that low-energy sweeteners do not have any of the adverse effects predicted byin vitro, in situ or knockout studies in animals.
And according to a study  investigating the effects of artificial sweeteners on glucagon-like peptide (GLP)-1 in humans:
The metabolic consequences of increased GLP-1 release after ingestion of both artiﬁcial sweeteners and glucose remain uncertain. In the present study, no signiﬁcant differences were observed in either plasma glucose or insulin after diet soda versus carbonated water ingestion, despite the signiﬁcant differences in GLP-1.
Therefore it seems doubtful that Acesulfame K produces an insulin response, and especially not without carbohydrates!
If you enjoy drinking diet sodas then you shouldn’t worry about it. If your goal is fat loss, then you will not derail your progress by including them.
Kiefer responded directly to the article recently, here are his thoughts:
I made the point distinctly in the book and in interviews that it’s not something to worry about unless things seem off. The direct studies on Ace-K (particularly on ketogenic diets in humans) is pretty much non-existent. There’s evidence to show it can increase insulin release and can shift the time-peak of insulin release in the presence of glucose, which may also happen with any insulinotropic substance (leucine, possibly ketones). So, it’s a possible culprit when things go wrong. To spend so much time on such minutiae when there’s not enough research to answer the question under the conditions being considered is akin to a religious debate that no one can win. I’m sticking with the fact that ace-k has been shown to cause cells of the pancreas to release insulin and the observation of hundreds of clients who, when having trouble, strip sugar-free sodas amazingly no longer have problems. Also, worth pointing out, for those with appreciable amounts of muscle, I’ve never seen sugar-free beverages have any effect whatsoever… it’s only “normal” people who seem to have an issue or, actually, stage athletes as they reach the insanely low levels of body fat.
So, in my summation: don’t worry about it unless stuff starts going wrong, then realize it’s a possible source of trouble.
There you have it. If you’re making progress, don’t worry about diet sodas. If you’re not, consider the possibility that you might have to remove them.