Does diet soda slow down your weight loss progress by slowing your metabolism? Does it make you pack on the pounds?

No, it doesn’t.

Let’s look at this 6 month study that compared sucrose sweetened soda (“regular soda”) with milk and diet coke given to obese people. Basically, they drank 1 liter per day of their drink of choice and then at the end the researchers compared the fat in their liver and muscle. End result? “Regular soda” showed an increase in liver and muscle fat over the other two groups. The study didn’t mention the rest of the diet of these individuals, much less the mean calories ingested by each person, however it does show that diet soda was no more likely to increase your fat than any other drink.

So, what if you replace regular soda with diet soda? Or juice with diet soda?I’ve got a study for that. As expected, when you replace regular soda (with calories) with diet soda (without calories) you lose weight. In fact, it had the same weight loss effect as switching from regular soda to water.

Fat gain comes down to ingestion of calories, no one single item slowing down your metabolism and sabotaging weight loss. Diet soda can be a helpful way for people to transition from “drinking their calories” in the form of regular soda.

When I was in undergrad I was told by a professor that diet soda makes you hungrier later. He said that aspartame binds to receptors and releases insulin and other hormones, and when your body goes to convert it to energy it finds no energy. This makes you hungry. I’ve also heard “empty calories” used to discuss a phenomenon in which you eat zero calorie food and, as a result, your hunger hormones flair and sabotage you.

None of this is true.

Let’s look at the first claim – your body sensing sweetness and releases enzymes in response to it. This is no phenomenon, this is the “Cephalic” or “Anticipatory” phase of digestion. Regardless of what you eat this phase is put into effect the minute you put anything into your mouth (and before). We’ve already shown in a previous study that insulin isn’t released by non-nutritive sweeteners, so we know that part is false. Additionally, it’s shown that carbohydrates decrease appetite in this phase, but sweetened carbohydrates do the opposite effect. Moral of the story? It’s too complex to chalk it up to just artificial sweeteners, so we’ll move on.

What about the hormones that control appetite?

There’s two major players examined in this study – CCK and GLP-1, the so-called “satiety hormones.” This study wanted to see if aspartame paired with a meal vs. aspartame alone vs. a control did anything to these hormones. They found that aspartame plus a meal actually increased desire to eat from 1 hour to 2 hours, but none of these hormones were effected. Basically, if you pair diet coke with amino acids in a meal the average joe took twice as long to get hungry again, but not because of the satiety hormones. Sorta on the same vein, this study found that aspartame doesn’t increase cortisol or growth hormone.

What about self reported hunger? Back in the day a man gave people as much as 10g of aspartame in a pill and checked out their hunger response.This reanalysis shows that aspartame at high levels may actually decrease your appetite, going against the idea that diet soda will make you ravenous.

So we started the series out by looking at some basic terminology, now we’re going to dive deep. This particular portion looks at how different types of sugar influence our insulin levels. Remember, insulin is the hormone that says “store this sugar in our liver, muscles or fat cells.” A lot of arguments against diet sodas, particularly aspartame sweetened ones, cite this as a cause of concern with weight gain.

Let’s look at aspartame in particular. This study and this study tried to see if ingesting aspartame caused an insulin spike. The first study compared just plain aspartame vs. aspartame plus albumin (a primary protein in the blood). They found that aspartame alone had no effect on insulin, but albumin + aspartame did. The second study looked at a common fear of aspartame – that if you drink a diet coke + carbohydrates your insulin spikes higher than normal causing you to store more sugar as fat. This turned out to be false as well. Diabetic? Turns out that non-nutritive sweeteners had no effect on insulin either.

It wasn’t just aspartame either, it was a wide range of low energy sweetenerswithout an effect on insulin.

So where did this myth come from? Probably this study, which showed that in certain cells in the rat pancreas when infused (not orally ingested) with aspartame AND glucose could cause a spike in insulin.

What about the others? Sucralose also didn’t cause a spike in insulin, nor did it effect appetite in those who ingested it. In fact, if we’re looking at sucralose vs. sucrose you’ll find that the “crash in blood sugar” x hours after ingestion only occurred with sucrose, and that sucrose was also the only sugar shown to cause a spike in insulin. If you’re familiar with table sugar this should come as no surprise to you. Stevia fairs about the same.

Long story short, current literature does not support the idea that no-calorie sweeteners cause a spike in insulin.

So, before we dive into the Sugar Series we have to start with some definitions. I can rattle off about fructose and its effect on ghrelin all I want, but if you don’t know what fructose is or what ghrelin does then this series is going to suck. So, call this our appendix (science joke)

The Sugars

Fructose – This is the sugar found in fruit. I remember this by calling it “fruitose” in my head. It’s also found in honey, flowers and roots.

Aspartame – Also known as “Nutrasweet” or “Aminosweet.” It’s generally what’s found inside diet sodas as a zero calorie sweetener.

Sucrose – This is table sugar.

Sucralose – An artificial sweetener that is zero calorie. It’s 600 times sweeter than sucrose and 3 times sweeter than aspartame. Splenda is a type of sucralose.

Stevia – A natural plant called the “Sweet leaf.” This is 300 times sweeter than sucrose and also is zero calorie.

The Hormones

Insulin – After you eat certain foods, glucose is introduced into your system. In response, insulin starts to pull that glucose from your blood, convert it to a storable form, and then store it in your liver, muscles and fat. An increase in insulin means an increase in storage of excess glucose.

Ghrelin – This is the “I’m hungry” hormone, which tells your brain it’s time to eat.