Metabolic Flexibility to Lipid Is Not Correlated With Metabolic Health in Humans Without Obesity

• Published in: Obesity (Silver Spring, Md.) Md.), 2023-11, Vol.31, p.151-151
• Main Authors: Verdejo, Rodrigo Esteban Fernandez, Galgani, Jose, Gutierrez-Pino, Juan, Hayes-Ortiz, Thomas, Zbinden-Foncea, Hermann, Santiago, Mauro Tunon Suarez, Vargas-Foitzick, Ronald
• Format: Article
• Language: English
• Subjects: Lipids; Metabolic syndrome; Musculoskeletal system; Obesity; Oxidation; Standard scores
• ISSN: 1930-7381; 1930-739X

Background: Metabolic flexibility to lipid (MetF-lip) is the capacity to adapt lipid oxidation to lipid availability. A low MetF-lip in skeletal muscle may lead to ectopic lipid accumulation, thus underlying some metabolic disturbances in obesity. If so, MetF-lip should influence metabolic health independently of obesity. We aimed to determine the correlation between MetF-lip in skeletal muscle and metabolic health in humans without obesity. Methods: Eight men and nine women aged (median [IQR]) 55.4 [48.6-58.5] y, with a BMI of 24.4 [22.6-26.0] kg/m2 were included. They exercised in a cyclo-ergometer at 48.2% [45.6-49.2] VO2max for 2 h. MetF-lip was assessed as the increase in relative lipid oxidation during exercise, quantified as the drop in respiratory quotient (dRQ = RQfinal-RQmax). HOMA-IR, a metabolic syndrome z-score, fat percent, trunk-to-appendicular fat ratio, and VO2max were used as metabolic health markers. Results: Exercise increased lipid availability, as shown by a progressive increase in circulating non-esterified fatty acids concentration (p < 0.001; final-minimum = 0.67 [0.46-0.88] mM). This response was accompanied by increases in the relative lipid oxidation, as shown by a progressive decrease in RQ driven by muscle activity (p = 0.009; dRQ = -0.04 [-0.08 to 0.02]). MetF-lip (dRQ) was unrelated to HOMA-IR (rho = -0.17, p = 0.514), metabolic syndrome z-score (rho = -0.08, p = 0.757), fat percent (rho = -0.31, p = 0.222), trunk-to-appendicular fat ratio (rho = 0.00, p = 1.00), and VO2max (rho = 0.46, p = 0.065). Conclusions: MetF-lip measured during an exercise challenge appears unrelated to metabolic health in middle-aged humans. This suggests that a low MetF-lip in skeletal muscle is not a major driver of insulin resistance, metabolic syndrome, unhealthy fat accumulation and distribution, and reduced cardiorespiratory fitness in this population.