The problem is that while there are lots of observational or cross sectional studies on soda and weight gain, there are very few studies which, by their design, can show cause and effect.

FORBES: ABC News Attacks Scientist Who Exposed Bias In Obesity Research

An international team of researchers carried out simultaneous cross-sectional studies for three student populations (roughly grade 4, grade 8, and students in final year of secondary school) and assessed nearly half a million students in 41 countries.

UNESCO: II Part Analytic Section