Why does diet soda promote obesity?

According to a recent study published in the Journal of the American Geriatrics Society people over age 65 who drink diet soda daily are much more likely to gain abdominal fat than those who drink other beverages.  This prospective be-ethnic cohort study followed 749 Mexican-American and European-American peoples aged 65 and older for nine years.  During the cumulative study duration, people who reported drinking diet soda gained an average of 3.16 inches in waist circumference, compared to 1.83 inches for occasional diet soda drinkers, and 0,80 inches for people who reported no intake of diet soda.  This positive dose-response relationship between diet soda intake and worsening abdominal obesity raises concerns about the safety of chronic diet soda intake by older individuals, especially those already at high cardio-metabolic risk.  Previous studies have linked frequent intake of diet sodas with increased BMI, obesity, hypertension, metabolic syndrome, diabetes mellitus, and cardiovascular events.

Fowler SP, Williams K, Hazuda HP. Diet soda intake is associated with long-termincreases in waist circumference in a biethnic cohort of older adults: the SanAntonio Longitudinal Study of Aging. J Am Geriatr Soc. 2015;63(4):708-15. doi:10.1111/jgs.13376)