Okay, looked it up. Bottom line is nobody really knows why people lose weight on the drug. Here is the text cut and pasted. It’ll be formatted weird, but you’ll figure it out.
3.3.4.G Weight decreased
1) Summary
a) Weight loss with topiramate has occurred in 6% to 21% during manufacturer trials (Prod Info TOPAMAX® oral tablets, oral sprinkle capsules, 2008) but incidences up to 90% have also been reported. This effect may be dose-related; a decrease in body weight of up to 7% has been reported in higher dose ranges. Weight loss (12.6% versus 2.3% placebo) occurred at doses of 600 to 1,000 milligrams/day. A diuretic effect has been suggested to explain weight loss with topiramate. Weight increase has also been reported (Anon, 1995; Britton & So, 1995a; Jack, 1992).
2) Incidence: 6% to 21% (Prod Info TOPAMAX® oral tablets, oral sprinkle capsules, 2008)
3) Adults
a) Weight decrease has been reported in 16% of adult patients on topiramate 400 mg/day (n=159) compared with 6% of patients on topiramate 50 mg/day (n=160), in a monotherapy epilepsy trial (Prod Info TOPAMAX® oral tablets, oral sprinkle capsules, 2008).
b) Weight decrease has been reported in 13% of adult patients on topiramate 600 to 1000 mg/day (n=414) and 9% on 200 to 400 mg/day (n=183) compared with 3% of patients on placebo (n=291), in placebo-controlled, add-on epilepsy trials (Prod Info TOPAMAX® oral tablets, oral sprinkle capsules, 2008).
c) Weight decrease has been reported in 11% of patients on topiramate 200 mg/day (n=514), 9% on 100 mg/day (n=386), and 6% on 50 mg/day (n=235) compared with 1% of patients on placebo (n=445), in placebo-controlled, migraine trials (Prod Info TOPAMAX® oral tablets, oral sprinkle capsules, 2008).
d) In a prospective, open-label study aimed at identifying factors associated with topiramate-related weight loss, patients lost an average of 5.9 kg (7.3% of baseline body weight) after 1 year. The mean topiramate dose after 1 year was 129 mg/day (range 45 to 262 mg/day). Of 38 patients, 58% lost at least 5% of their baseline body weight and 32% lost 10% or more after 1 year. Mean weight loss was greatest in patients with a baseline body mass index (BMI) equal to or greater than 30 kg/m(2). By 3 months, 88% of these patients had lost weight and by 1 year, 100% had lost a mean of 10.9 kg (11% of baseline body weight). Patients were found to have lost more body fat than lean mass. Although patients were instructed not to change their diet or exercise regimen, mean caloric intake at 3 months was reduced from 2009 kcal/day to 1823 kcal/d. However, by 1 year caloric intake had returned to baseline (1964 kcal/day). The authors concluded that mechanisms other than loss of appetite are involved in the long- term weight loss effects of topiramate (Ben-Menachem et al, 2003).
4) Children
a) Weight decrease has been reported in 21% of pediatric patients 10 to 16 years of age on topiramate 400 mg/day (n=57) compared with 7% of patients on topiramate 50 mg/day (n=57), in a monotherapy epilepsy trial (Prod Info TOPAMAX® oral tablets, oral sprinkle capsules, 2008).
b) Weight decrease has been reported in 9% of pediatric patients 2 to 16 years of age on topiramate (n=98) compared with 1% of patients on placebo (n=101), in placebo-controlled, add-on epilepsy trials (Prod Info TOPAMAX® oral tablets, oral sprinkle capsules, 2008).