Divergent remodeling of the skeletal muscle metabolome over 24 h between young, healthy men and older, metabolically compromised men

Circadian organization of lipid landscape is perturbed in type 2 diabetic patients

Lipid homeostasis in humans follows a diurnal pattern in muscle and pancreatic islets, altered upon metabolic dysregulation. We employ tandem and liquid-chromatography mass spectrometry to investigate daily regulation of lipid metabolism in subcutaneous white adipose tissue (SAT) and serum of type 2 diabetic (T2D) and non-diabetic (ND) human volunteers (n = 12). Around 8% of ≈440 lipid metabolites exhibit diurnal rhythmicity in serum and SAT from ND and T2D subjects. The spectrum of rhythmic lipids differs between ND and T2D individuals, with the most substantial changes observed early morning, as confirmed by lipidomics in an independent cohort of ND and T2D subjects (n = 32) conducted at a single morning time point. Strikingly, metabolites identified as daily rhythmic in both serum and SAT from T2D subjects exhibit phase differences. Our study reveals massive temporal and tissue-specific alterations of human lipid homeostasis in T2D, providing essential clues for the development of lipid biomarkers in a temporal manner.

Exercise training modifies skeletal muscle clock gene expression but not 24-hour rhythmicity in substrate metabolism of men with insulin resistance

Twenty-four hour rhythmicity in whole-body substrate metabolism, skeletal muscle clock gene expression and mitochondrial respiration is compromised upon insulin resistance. With exercise training known to ameliorate insulin resistance, our objective was to test if exercise training can reinforce diurnal variation in whole-body and skeletal muscle metabolism in men with insulin resistance. In a single-arm longitudinal design, 10 overweight and obese men with insulin resistance performed 12 weeks of high-intensity interval training recurrently in the afternoon (between 14.00 and 18.00 h) and were tested pre- and post-exercise training, while staying in a metabolic research unit for 2 days under free-living conditions with regular meals. On the second days, indirect calorimetry was performed at 08.00, 13.00, 18.00, 23.00 and 04.00 h, muscle biopsies were taken from the vastus lateralis at 08.30, 13.30 and 23.30 h, and blood was drawn at least bi-hourly over 24 h. Participants did not lose body weight over 12 weeks, but improved body composition and exercise capacity. Exercise training resulted in reduced 24-h plasma glucose levels, but did not modify free fatty acid and triacylglycerol levels. Diurnal variation of muscle clock gene expression was modified by exercise training with period genes showing an interaction (time × exercise) effect and reduced mRNA levels at 13.00 h. Exercise training increased mitochondrial respiration without inducing diurnal variation. Twenty-four-hour substrate metabolism and energy expenditure remained unchanged. Future studies should investigate alternative exercise strategies or types of interventions (e.g. diet or drugs aiming at improving insulin sensitivity) for their capacity to reinforce diurnal variation in substrate metabolism and mitochondrial respiration. KEY POINTS: Insulin resistance is associated with blunted 24-h flexibility in whole-body substrate metabolism and skeletal muscle mitochondrial respiration, and disruptions in the skeletal muscle molecular circadian clock. We hypothesized that exercise training modifies 24-h rhythmicity in whole-body substrate metabolism and diurnal variation in skeletal muscle molecular clock and mitochondrial respiration in men with insulin resistance. We found that metabolic inflexibility over 24 h persisted after exercise training, whereas mitochondrial respiration increased independent of time of day. Gene expression of Per1-3 and Rorα in skeletal muscle changed particularly close to the time of day at which exercise training was performed. These results provide the rationale to further investigate the differential metabolic impact of differently timed exercise to treat metabolic defects of insulin resistance that manifest at a particular time of day.

Disentangling the Complexity of Nutrition, Frailty and Gut Microbial Pathways during Aging: A Focus on Hippuric Acid

Hippuric acid (HA) is a metabolite resulting from the hepatic glycine conjugation of benzoic acid (BA) or from the gut bacterial metabolism of phenylalanine. BA is generally produced by gut microbial metabolic pathways after the ingestion of foods of vegetal origin rich in polyphenolic compounds, namely, chlorogenic acids or epicatechins. It can also be present in foods, either naturally or artificially added as a preservative. The plasma and urine HA levels have been used in nutritional research for estimating the habitual fruit and vegetable intake, especially in children and in patients with metabolic diseases. HA has also been proposed as a biomarker of aging, since its levels in the plasma and urine can be influenced by the presence of several age-related conditions, including frailty, sarcopenia and cognitive impairment. Subjects with physical frailty generally exhibit reduced plasma and urine levels of HA, despite the fact that HA excretion tends to increase with aging. Conversely, subjects with chronic kidney disease exhibit reduced HA clearance, with HA retention that may exert toxic effects on the circulation, brain and kidneys. With regard to older patients with frailty and multimorbidity, interpreting the HA levels in the plasma and urine may result particularly challenging because HA is at the crossroads between diet, gut microbiota, liver and kidney function. Although these considerations may not make HA the ideal biomarker of aging trajectories, the study of its metabolism and clearance in older subjects may provide valuable information for disentangling the complex interaction between diet, gut microbiota, frailty and multimorbidity.