  Artificially sweetened beverages like Diet Coke are often considered
healthier alternatives, but the debate over their impact on obesity persists.
Previous research has predominantly relied on observational data or randomized
controlled trials (RCTs), which may not accurately capture the causal
relationship between Diet Coke consumption and obesity. This study uses causal
inference methods, employing data from the National Health and Nutrition
Examination Survey (NHANES) to examine this relationship across diverse
demographics. Instead of relying on RCT data, we constructed a causal graph and
applied the back-door criterion with its adjustment formula to estimate the RCT
distributions. We then calculated the counterfactual quantity, the Probability
of Necessity and Sufficiency (PNS), using both NHANES data and estimated RCT
data. We propose that PNS is the essential metric for assessing the impact of
Diet Coke on obesity. Our results indicate that between 20% to 50% of
individuals, especially those with poor dietary habits, are more likely to gain
weight from Diet Coke. Conversely, in groups like young females with healthier
diets, only a small proportion experience weight gain due to Diet Coke. These
findings highlight the influence of individual lifestyle and potential hormonal
factors on the varied effects of Diet Coke, providing a new framework for
understanding its nutritional impacts on health.
