I just read an article at ABC news online on how artificial sweeteners in diet soda can actually cause people to gain weight (from a Purdue University study)! Though not fully researched and proven, your clients' metabolic rates could be being altered by diet foods, including the low-calorie sweetener saccharin, which increases the chance of weight gain.
Don't think it's relevant to you? Fifty-nine percent (59%!) of Americans consume soft drinks, and artificial sweeteners aren't even limited to diet sodas - they're found in yogurt and even water!