Citric acid and aspertame
In all likelihood, problems with diet soda relate to one of these two compounds, both of which are used as artifical sweeteners in diet producs. Citric acid may inhibit ketosis and diet sodas containing citric acid or apertame are not allowed for epileptic children on a ketogenic diet(14). However, there is some debate over this point(15). Possibly citric acid might affect ketosis by effecting liver motabolism, primarily the Krebs cycle. Some individuals report that citric acid prevents them from entering ketosis but does not affect ketosis once it has been established. One study, examining very low-calorie diets, found that the consumption of citric acid inhibited ketosis and increased appetite in many individuals(16). Ultimately, individuals will have to determine for themselves whether citric acid or aspertame has any effect on ketosis, appetite or fatloss on a ketogenic diet.
References....
14. Gash At. Use of the traditional ketogenic diet for treatment of intractable epilepsy. JAm Diet Assoc (1990) 90:1433-1434.
15. Brunett A. Should diet soft driniks be restricted on a ketogenic diet[Letter]. J Am Diet Assoc (1991) 91:776.
16. Krietman S. Factors influencing body composition during very low-calorie diets. Am J Clin Nutr (1992) 56 (suppl):217S-223S.