Background/objectives <p>Animal studies support that diet affects metabolic fuel utilization and metabolic flexibility. We hypothesized that individuals with contrasting dietary patterns would have different metabolic responses. Differences in metabolic fuel utilization, metabolic flexibility, and gastric emptying time to carbohydrate challenges (rapidly vs slowly digestible carbohydrates [RDC/SDC]) were assessed between US and Kenyan cohorts consuming diets characteristic of each population.</p> Subjects/methods <p>We assessed metabolic fuel utilization using a portable breath CO₂ measuring device and gastric emptying in two cohorts (Kenya, <i>n</i> = 23; US, <i>n</i> = 13) for 2 h following RDC and SDC challenges. Study meals, matched in energy content (732 kJ), consisted of test carbohydrates (30 g) mixed into applesauce (200 g). An estimated respiratory exchange ratio (RER_est) was calculated from the CO₂ values. Metabolic flexibility (MF) was assessed using Percent Relative Cumulative Frequency followed by modeling with the Weibull Cumulative Distribution function. We collected dietary data using three 24-h dietary recalls and used multivariate mixed effect models to assess dietary influences on RER_est/MF to carbohydrate challenges.</p> Results <p>Kenyan participants had higher RER_est and greater MF compared to US participants regardless of the carbohydrate challenge (<i>P</i> &lt; 0.0001), and had improved MF response with SDC vs RDC. Multivariate Model 1 (macronutrient composition) showed that carbohydrate (<i>P</i> = 0.02) and protein (<i>P</i> &lt; 0.001) were predictive of RER_est; and for Model 2 (carbohydrate quality), total fiber (<i>P</i> = 0.026), starch (<i>P</i> = 0.001) and added sugars (<i>P</i> &lt; 0.001) were predictive of RER_est.</p> Conclusion <p>The Kenyan cohort consuming a diet of high carbohydrate quality and low in fat showed greater carbohydrate oxidation and improved MF.</p>