Analysis of NHANES data covering 286 million U.S. adults revealed that higher diet quality significantly reduced HbA1c levels (a marker of long-term blood sugar control) only among low-income individuals, with no protective benefit observed in middle-income groups. The interaction effect was statistically significant across income tiers, despite similar diet quality scores between groups. This challenges fundamental assumptions in nutrition science about universal metabolic responses to healthy eating. The finding suggests that structural factors—perhaps including chronic stress, housing instability, healthcare access, or environmental toxins disproportionately affecting lower-income communities—may either amplify the benefits of good nutrition or create ceiling effects in higher-income populations already accessing other health-promoting resources. Importantly, this interaction disappeared when analyzing only non-diabetic participants, indicating the effect is most pronounced in disease states. As a preprint awaiting peer review, these results require validation, but they point toward a paradigm shift suggesting personalized nutrition recommendations based on socioeconomic context may be more effective than one-size-fits-all dietary guidance.