Mendelian randomization analysis using genome-wide genetic variants as instrumental variables demonstrates that pescatarian dietary patterns causally reduce fasting insulin levels by 0.10 pmolL-1 per standard deviation increase in pescatarian diet score. This finding survived multiple sensitivity analyses designed to eliminate confounding factors that typically plague observational nutrition studies. The research represents a methodological breakthrough in nutritional epidemiology by leveraging genetic variants to establish true causality rather than mere association. The insulin-lowering effect of pescatarian eating—emphasizing fish over other animal proteins—likely stems from omega-3 fatty acids and reduced saturated fat intake, both established modulators of insulin sensitivity. However, attempts to use taste and smell receptor variants as alternative genetic instruments yielded null results, highlighting the complexity of genetic influences on dietary behavior. This preprint awaiting peer review provides some of the strongest causal evidence to date that specific dietary patterns directly influence metabolic health outcomes. The findings could inform precision nutrition approaches, though translation requires validation of the genetic instruments across diverse populations and consideration of individual genetic variation in taste perception.