Artificial sweeteners (product names such as Nutrasweet, Sweet 'n' Low, Equal) have been linked to obesity in a study conducted at Purdue University. Susan Swithers found that rats fed a diet of sugar substitutes ate more calories and gained more weight than rats that ate sugary foods. This finding could explain why people who consume a lot of diet soda have higher rates of obesity. It seems that the artificial sweeteners fail the body's ability to regulate caloric intake and slow the metabolism, providing a formula that will lead to weight gain and health problems.
Aspartame is an artificial sweetener that has been in use in the United States since the 1970's, when it was first approved by the FDA. "It is found in more than 6,000 products including carbonated and powdered soft drinks, hot chocolate, chewing gum, candy, desserts, yogurt, and tabletop sweeteners, as well as some pharmaceutical products like vitamins and sugar-free cough drops." Studies from 2004 indicate that rats treated with daily intake of aspartame, at simulated dosages equal to the current level approved for humans, had significantly higher increases in the occurrence of malignant carcinogenic tumors.
Sources:
"A Pavlovian approach to the problem of obesity," International Journal of Obesity (2004) 28, 933-935."Aspartame causes cancer in rats at levels currently approved for humans," www.medicalnewstoday.com