Investigating Predictors of the Transition from Fat to Carbohydrate Oxidation in Overweight Individuals during Submaximal Exercise

Abstract

ABSTRACT Purpose: This study is important for knowing the mechanisms behind fuel transitions from fat to carbohydrate oxidation, so that we can further understand what the main predictors are in this crossover point in a group of people who are currently overweight and are at risk of obesity. Understanding what factors are associated with the transition from fat to carbohydrate oxidation will help us better recognize who could be at risk for metabolic diseases later in life. Methods: Up to 60 subjects (29 ± 7 yr) men and women classified as overweight (26.4 ± 1.8 kg/m2) were recruited and participated in this observational study. Blood pressure and blood metabolites were measured after an overnight fast. Lactate was also measured before and after a three-day eucaloric high-fat (70 %) diet. The homeostatic model assessment for insulin resistance (HOMA-IR) was calculated as a measure of insulin resistance. Visceral adipose tissue mass was determined via dual X-ray absorptiometry. A 65% VO2 max test was then administered to assess the transition point from fat to carbohydrate oxidation. Prior to the test, age, height, weight, and resting heart rate was first collected from the participant. A 10-lead ekg was also attached to the participant for continuous monitoring which is essential throughout the test. The participant was then ordered to maintain a steady cadence of 50-70 revolutions per minute (RPM) throughout the exercise phase. The test duration is typically between six and twelve minutes, allowing the participant to reach a steady-state heart rate. Throughout the test, heart rate and their perceived exertion (RPE) were recorded at the end of each stage using the Borg scale to gauge the individuals level of intensity of each stage. Once the individual’s 65% of their VO2 max was achieved, the test then transitioned to a cool-down phase. During this stage the resistance was decreased on the ergometer and the participant was told to cycle at a low intensity for several minutes. This allowed for adequate time for blood pressure and heart rate to return to resting levels. Once the values returned to normal levels, the test was then terminated. Results: To investigate the associations between various physiological, metabolic characteristics and the primary variable of transition time, a series of Pearson correlations were performed. Each variable was analyzed in relation to the independent value of transition time. The level of significance of each variable was assessed using the corresponding P-value (significance level) and R-value. Among all the variables tested, carbohydrate oxidation (CHOX), fat oxidation (FAOX), age, and body mass index (BMI) emerged as the statistically significant predictors (p < 0.05) of transition time, each demonstrating a meaningful level of significance. CHOX displayed the strongest connection with a p-value of 0.01 and an r-value of -0.36. FAOX was also a statistically significant predictor with a p-value of 0.04. Another variable that was significant was age, which yielded a p-value of 0.02 with an r-value of 0.35. The last variable we tested that showed significance as predictor of transition time was BMI (p= 0.02, r-value= -0.33). Several variables (fat mass, lactic acid, % body fat, REE, among others) failed to demonstrate statistically significant relationships with the transition time from fat to carbohydrates, and their significance was minimal. Discussion: This study identified carbohydrate oxidation, fat oxidation, age, and BMI as significant predictors of the transition from fat to carbohydrate oxidation during submaximal exercise. These findings suggest that both substrate utilization patterns and basic physiological characteristics can have significant influence on an individual's metabolic flexibility. With substrate oxidation patterns, BMI, and age being significant indicators of transition time, traditional fitness markers such as VO₂ max, resting energy expenditure, and time to exhaustion did not show a significant enough correlation. These findings suggest that metabolic flexibility is influenced more by physiological factors such as body composition and substrate oxidative capacity rather than peak aerobic and exercise performance. Future research of this transition should aim to validate these findings in larger and more diverse populations. This includes individuals with obesity, different ethnic backgrounds, and varying levels of physical activity. This would be of great benefit for continuing research in the future.