The "Trick" Theory
A common argument against artificial sweeteners is the theory of Cephalic Phase Insulin Release (CPIR). The logic goes like this: tongue tastes sweet ➜ brain signals pancreas ➜ insulin spikes ➜ fat storage occurs.
If this were true, drinking a Diet Coke would be metabolically similar to drinking a sugary soda, even without the calories. But biology is rarely that simple.
The Study: Head-to-Head
In 2016, researchers conducted a randomized, double-blind crossover study to test this exact theory. They recruited healthy men and had them consume 20 ounces (approx. 600ml) of four different beverages on separate days:
- Regular Soda (Sweetened with Sugar/High Fructose Corn Syrup)
- Diet Soda (Sweetened with Aspartame)
- Diet Soda (Sweetened with Sucralose/Splenda)
- Carbonated Water (Control)
They then measured blood glucose and insulin levels at 30, 60, 90, and 120 minutes.
The Results: No Calories, No Spike
The results were definitive.
- Regular Soda: Caused a significant, rapid rise in both blood glucose and insulin levels within 30 minutes7.
- Diet Sodas: Both the Aspartame and Sucralose drinks resulted in zero significant rise in insulin or glucose8. Their metabolic impact was statistically identical to plain water9.
The WellFact Verdict
The "Cephalic Phase" insulin response to artificial sweeteners is functionally non-existent in healthy humans10.
While water is always the healthiest choice for hydration, diet soda is a neutral tool. It allows you to satisfy a craving without the metabolic damage of liquid sugar. The idea that it "pauses fat burning" by spiking insulin is a myth unsupported by clinical data11.