Excellent question!  Theoretically speaking, diet beverages and foods shouldn't cause weight gain-you can't make something (weight gain) out of nothing (zero calories, no sugar), right?  But yet, more and more research is showing a link between diet beverages (and other 'diet' foods) and weight gain.  At the very least, these products clearly aren't helping to reverse the obesity pandemic.  But why?   The causes of obesity are multi-factoral, just as are the theories behind weight gain caused by diet beverages.  Some of the theories discuss an indirect cause and effect, while other theories attempt to pinpoint direct causes of weight gain by looking at what these artificial sweeteners have on taste and satiety. 

Here are a few explanations of why artificially-sweetened ('diet') foods and  beverages may cause weight gain:

• We rationalize our food choices:  When choosing a food that is marketed as "low-calorie", "fat-free", "sugar-free", "diet", many people tend to rationalize this as a 'healthier' option, so they don't pay attention to the amount or frequency of their food consumption.  We think, "Oh, it's low fat so it's not going to matter if I have another..." or "I got the large value meal at the drive-thru but at least I got diet soda!" As a result, we may end up talking ourselves into eating more as a justification for making a 'healthier' choice. 

• We're not saving THAT many calories:  As several studies have shown, we may actually overestimate the benefit of eating low calorie foods and beverages. Sure, regular soda typically has 150 calories/can while diet soda has 0 calories, but what about diet foods such as sugar-free ice cream? Dreyers Grand regular vanilla ice cream has 140 calories for ½ cup (about the size of ½ of your fist) but the sugar free version still has 100 calories. And if we start to use the rationalization process mentioned above, we may wind up with 3 or 4 servings, or 300-400 calories. A large fry and a Big Mac still have a 1,000 calories even if you choose diet soda!

• Diet foods only create a "short-term" calorie deficit:  A few studies have shown that artificially sweetened foods and beverages may help with reducing caloric intake, which can be beneficial in the short term, but then later results in long term weight gain as the individual's metabolic rate has now slowed. (1)  Several studies have looked at the relationship between dietary intake and metabolic syndrome. One study discovered that a Western diet (fried foods, processed meats, refined grains, soda) is associated with a greater risk with metabolic syndrome (4). This study also found that those who consume diet soda are at greater risk for metabolic syndrome. Those who consumed the most soda had a 34% greater risk for metabolic syndrome compared to those who consumed little to no diet sodas.  Of course, it is possible that those who consumed the most diet soda are individuals who are already overweight and turn to diet sodas to reduce calorie intake.

• Foods with sugar alcohols aren't sugar free:  Many of the sugar-free candy and ice cream products use some type of sugar alcohol (also called polyols). Sugar alcohols are carbohydrates that have been hydrogenated (have added hydrogen atoms) to decrease some of the sugar content.  As a result, sugar alcohols have fewer calories per gram (1-3 cal/gram) than sugar (4cal/gram).  However, they are still metabolized by the body; you're just getting a bit fewer calories and carbohydrates than you would if you ate the regular version.  For example, Malitol has 2.1 cal/gram, which is about ½ of what regular sugar has.   To identify sugar alcohols in a product, look for an ingredient that ends in -tol, such as malitol, sorbitol, or xylitol (the exceptions to this are hydrogenated starch hydrolysates  and isolmalt).  Sometimes, sugar alcohols can cause a laxative effect, with symptoms such as bloating, cramping and diarrhea.  And since sugar alcohols still contain some sugar and carbohydrates, they can raise blood glucose levels if they are overeaten.   

• Macronutrient composition changes:  Sugar free foods can still have calories, carbohydrates and fat.  When sugar is taken out of a product, something else needs to be added back in to give the product taste. When additional fat is added to make up for the missing sugar, it usually means the calorie content is higher than the original product, as fat has 9 calories/gram compared to 4 calories/gram in sugar (carbohydrates).  Moreover, sugar produces a feeling of satiety.  If we don't feel satisfied with our food and are still hungry, we're going to eat more.  We may only have 1 or 2 sugar-free cookies, but then go overboard with crackers (which can contain a surprising amount of sugar!)

• Physiological adaptations & "taste distortion":  Since artificial sweeteners are anywhere from 180 to 7,000 times sweeter than regular sugar, researchers are questioning if they have altered our ability to detect how sweet a food really is.  Moreover, it is possible that these artificial sweeteners cause an increase in appetite for extremely sweet, high calorie foods (1, 5).  Animal studies in rats have shown that excessive consumption of aspartame can damage parts of the hypothalamus, which controls levels of leptin, a hormone that helps regulate energy (calorie) intake and energy expenditure (1).  A similar study from Purdue University noticed similar trends when rats were given saccarhin (i.e. Sweet 'n Low) (2,3).  In this study, rats who had regular sugar showed an increase in core body temperature at meals, meaning their bodies were getting ready to burn calories from the upcoming meal.  The rats on the saccharin diet did not show a rise in body temperature, perhaps due to their bodies know that they weren't going to receive as many calories, thus prompting their body to not 'burn' as many calories and creating a slower metabolism (3). 

So what's a person to do? Have regular soda and gain weight, or have diet soda...and possibly gain weight?  It's unrealistic to think that you'll have to avoid all sweets and soda, diet or regular, but cutting back on both is a smart move.  If you're looking to cut back on artificial sweeteners (and regular sugar too!), here are several good suggestions:

• Read the ingredients:  If you can't pronounce, you probably don't need it in your diet.   Remember, there's a reason why they discontinued Olestra! If you see aspartame, sucralose,  or acesulfamate potassium on the label, the product has been artificially sweetened.

• Gradually reduce your intake:  If you use three packets of Splenda in each cup of coffee, try reducing it to two packets, then one, until can . Yes, people do drink their coffee black!

• Make your own flavored water:  Slice up citrus fruits (or berries) and place them in a pitcher of water. When you pour the water into a cup, the berries stay in the pitcher, giving your water a hint of flavor with any artificial ingredients.

• Give Stevia a try, but use sparingly:  Stevia (brand names: Truvia, Stevia in the Raw) is derived from a plant. Some studies have found that steevia may not create a sugar craving like artificial sugar substitutes are believed to do.  However, there is some degree of processing to turn it into fine little sugar-like granules, so it's best to use it small amounts. 

• Make a dessert a real treat:  As they say,absence makes the heart grow fonder. If you're used to having several sugar-free items a day, you'll appreciate them a lot more if you don't have them as often. You'll learn to savor the experience and truly enjoy it. Same goes with regular (non-sugar -free food and beverages).

References

1)  Fowler SP, et. al.  Fueling the Obesity Epidemic? Artificially Sweetened Beverage Use and Long-Term Weight Gain.  Obesity.  2008; 16: 1894-1900.

2)  Swithers SE, Davidson TL.  A role for sweet taste:  Calorie predictive relations in energy regulation by rats.  Behavioral Neuroscience.  2008; 122 (1):  161-173.

3)  Park, Alice.  "Can Sugar Substitutes Make You Fat?"  TIME on-line.  February 10, 2008. Accessed on April 26, 2012.

4)  Lutsey PL, Steffen LM, Stevens J.  Dietary Intake and the Development of the Metabolic Syndrome.  The Atherosclerosis Risk in Communities Studies.  Circulation. 2008; 117: 754-761

5)  Dhingra R., et. al.  Soft Drink Consumption and Risk of Developing Cardiometabolic Risk Factors and the Metabolic Syndrome in Middle-Aged Adults in the Community.  Circulation. 2007; 116: 480-488.

Other Relevant Studies

Gardener H, et. al . Diet Soft Drink Consumption is Associated with an Increased Risk of Vascular Events in the Northern Manhattan Study.  Journal of General Internal Medicine. Published online 27January2012.

 Stellman SD, Garfinkel L.  Patterns of artificial sweetener use and weight change in an American Cancer prospective study.  Appettite.  1998; 11 (1):  8591.