On the surface, replacing sugar with artificial sweeteners seems like a good idea. Consume less empty Calories, the better for your body weight. Unfortunately, studies have shown that drinking diet beverages in place of sugar-sweetened beverages doesn't help with weight loss. If you buy into the "Carbohydrate Hypothesis", which I find very interesting (and will address in another post), basically it states that managing insulin levels are key to keeping body fat off. Glucose (simple carbohydrate) is one of the most powerful stimulants of insulin release from the pancreas into the blood. However, insulin secretion may also be stimulated by artificial sweeteners. The sweet taste identified in the mouth will trigger a neurological stimulation of the pancreas to release insulin into the blood (study1, study2 study3 study4 ). The rise in blood insulin levels causes a decrease in fatty acid release from the adipose tissue and forces the rest of the body(muscles, brain, heart, etc.) to take up more blood glucose for its energy needs. The lowered fatty acid and glucose level in the blood stimulates the appetite center in the hypothalamus (part of the brain that helps regulate the internal environment of the body by monitoring and releasing hormonal and neurological signals). Thus, you may have a higher insulin level, decreased release of fatty acids from the adipose tissue, and a greater appetite from consuming artificially sweetened beverages. These physiological changes may cause you to consume additional Calories and/or store more fat- not what you want from your "Diet" drink.
Here is a quick overview to help you understand the mechanism:
1. Taste an artificial sweetener in your "Diet" drink
2. A neurological signal is sent from the hypothalamus to Insulin-releasing cells of the pancreas.
3. Blood insulin levels rise, causing a decreased release of fatty acids from the adipose tissue.
4. Increased uptake of blood glucose by other cells because there is less fat available for energy.
5. Lower blood glucose and fatty acid levels sensed by hypothalamus
6. Hypothalamus appetite center is stimulated.
7. You feel hungry, but have stored additional fat in adipose tissue instead of using it for energy.
Now, I have only reviewed a hypothesis about how artificially sweetened drinks could make gain more body fat (or at least not help you lose body fat). I am in no way recommending consuming real sugar-sweetened drinks in their place (that would be even worse). Also, there are other factors that come into play such as: whether the drink was by itself or with food, the type (Sacchrin seams to have a greater affect than Aspartame) and concentration of the artificial sweetener, individual differences in insulin responses, body fatness, baseline insulin levels, etc.
When choosing your beverages, don't assume a "Diet" drink has no effect on your body because it has zero Calories in it. It may be indirect, but the effects of drinking a "Diet" drink may be significant enough to cause you to gain body fat, or at least prevent you from losing body fat.