Analysis of nearly 200,000 healthcare professionals tracked for three decades reveals that macronutrient source, not just carbohydrate restriction, determines diabetes outcomes on low-carb diets. Plant-focused low-carb patterns emphasizing vegetable proteins and fats reduced type 2 diabetes risk by 16%, while animal-heavy versions increased risk by 39-44%. The protective effect appears strongest when refined carbohydrates are specifically targeted for reduction while preserving whole grains and high-quality plant carbohydrates. This finding challenges the prevailing narrative that all low-carbohydrate approaches are metabolically equivalent. The divergent outcomes likely reflect differences in inflammatory profiles, fiber content, and metabolic effects of saturated versus unsaturated fats. For health-conscious adults, the research suggests that low-carb success depends heavily on implementation strategy. Those pursuing carbohydrate restriction for metabolic benefits should emphasize plant proteins like legumes and nuts over processed meats, and focus reduction efforts on refined sugars and starches rather than wholesale carbohydrate elimination. The 30-year follow-up strengthens causal inference, though the observational design cannot definitively establish causation. These results align with broader evidence favoring plant-forward eating patterns for chronic disease prevention.