Vegetarian dietary patterns likely reduce hemoglobin A1c [MD (95% CI): –0.40% (–0.59, –0.21)] and body mass index [MD (95% CI): –0.96 kg/m2 (–1.58, –0.34)] (moderate certainty evidence); may allow for reduced diabetes medication (in 2 of 3 included studies) (low certainty); and may improve metabolic clearance of glucose (insulin sensitivity) [MD (95% CI): 10% (1.86, 18.14)] (very low certainty), compared with nonvegetarian dietary patterns. This evidence concerns the gene INS and diabetes mellitus.