Calories in, calories out is NOT the way our bodies work. (Read Gary Taubes, Why We Get Fat). And diet soda proves this to be true.
The following article does a great job of explaining why:
Diet Coke is a Joke (originally found here)
Countless numbers of dieters consume Diet Coke thinking that it is inert to their diet efforts. After all, it’s called Diet Coke, right? Wrong.
Diet Coke, regardless of how many calories it has, wreaks havoc on your fat loss efforts and will ultimately cause you to gain weight. There are two main reasons for the Diet Coke fallacy. First, the sweet taste from Diet Coke elicits an insulin spike, which blocks your ability to burn fat. Second, artificial sweeteners found in Diet Coke disrupt satiety, the feeling of being full. Combined, the actions of Diet Coke go against a healthy lifestyle. Understanding why ensures that we think twice before consuming it.
When it comes to losing fat, it is more about how much sugar (or sugar substitute) you consume rather than calories or dietary fat intake. Hence, the goal is to consume as little sugar or sugar substitute as possible (including fruits and their juices). Why? The sweet flavor elicits the release of insulin from the pancreas to enhance the uptake of sugar by the cells so that it doesn’t linger in the bloodstream. Once insulin is released it inhibits your fat burning hormone called HSL (hormone sensitive lipase). This hormone is responsible for releasing fat into the bloodstream to be utilized as fuel. If inhibited, your body is unable to burn fat and will then begin utilizing amino acids (from muscle) and carbohydrates as fuel. This will leave you feeling tired, grumpy, and sloth-like toward the end of the day. Not to mention, you will become abnormally hungry. Those with large amounts of HSL burn fat all day and look thin and slim. Those who inhibit it by eating or drinking the wrong substances grow fat throughout their adult years.
Second, as discovered by Professor Terry Davidson and associate professor Susan Withers at Purdue University, artificial sweeteners disrupt satiety, the feeling of being full. Their results, published in International Journal of Obesity showed that “mouth feel” plays a crucial role in the body’s ability to count calories and that when we consume artificial sweeteners we disrupt the body’s ability to count calories based on sweetness. Not able to use mouth feel to count calories, those who drink diet coke will overeat without conscious awareness. In other words, you think you’re not eating like a pig, but in reality you are.
Diet Coke is not the only substance having these abilities. Makers of health food bars and protein supplements are either not aware or ignore the ill effects of sugar alternatives and sugar. This can be seen by the fact that most every health food bar and protein supplement is loaded with sugar or artificial sweeteners. The belief that these bars and supplements are healthy for you is a perfect example of how marketing strategies can supersede medical science and common sense.
In closing, Diet Coke is a joke. Stay away from it and other sugar sources if you’re serious about losing fat and keeping it off forever. Regardless of your diet and/or training efforts, the aforementioned ill effects of sugar alternatives and sugar will greatly hinder them.