Nocturnal fat oxidation is lower in older individuals with overweight/obesity, including those with type 2 diabetes, and is associated with fasting triglyceride levels.

aims/hypothesisWe previously showed that nocturnal fat oxidation is reduced in older individuals with overweight/obesity and impaired glucose tolerance and insulin sensitivity compared with young lean individuals. Here, we compared nocturnal energy expenditure and substrate oxidation across groups varying in age, body composition and metabolic status to unravel factors underlying variations in nocturnal substrate metabolism.

methodsData were collected from 18 previously conducted human clinical studies (N=187), all performed under conditions of energy balance with similar diet composition and meal timing. Individuals were categorised into four groups: young lean (YL); older lean (OL); older with overweight/obesity (OBE); and older with overweight/obesity and type 2 diabetes (T2D). Nocturnal energy expenditure and substrate oxidation were determined by whole-room indirect calorimetry, body composition was assessed by air-displacement plethysmography or dual energy x-ray absorptiometry, and glucose, insulin, HOMA-IR, NEFA and triglycerides were measured from fasted blood samples. Group comparisons for nocturnal energy expenditure and substrate oxidation were performed using Kruskal-Wallis tests, and over time using linear mixed models including group × time interactions, with Bonferroni correction applied to both analyses. Multivariate linear regression analysis was applied to identify whether age, sex, HOMA-IR, fasting NEFA, fasting triglycerides, fat mass and fat-free mass were independent factors of nocturnal energy expenditure and substrate oxidation.

resultsNocturnal energy expenditure, adjusted for fat-free mass, was higher in OBE compared with YL and OL (p<0.01 for both); it was also higher in T2D compared with OL (p<0.01). Nocturnal fat oxidation, expressed as a percentage of energy expenditure, was lower in OBE (median: 46.28%, IQR: 37.74-53.05) and T2D (median: 46.48%, IQR: 41.05-53.65) compared with YL (median: 52.95%, IQR: 47.82-57.61; p<0.01 for both) and OL (median: 55.21%, IQR: 54.15-58.89; p<0.01 for both). Standardised linear regression models revealed that fasting triglycerides were positively associated with nocturnal respiratory exchange ratio (β=0.337; 95% CI 0.165, 0.508) and nocturnal carbohydrate oxidation (% of energy expenditure; β=0.337; 95% CI 0.166, 0.509), and inversely associated with nocturnal fat oxidation (% of energy expenditure; β=-0.352; 95% CI -0.520, -0.813). CONCLUSIONS/

interpretationNocturnal fat oxidation (% of energy expenditure) is diminished in older individuals with overweight/obesity, irrespective of diabetes status. No differences in nocturnal energy expenditure (adjusted for fat-free mass) or substrate oxidation were observed between young and older lean individuals, suggesting that age per se may not strongly influence nocturnal substrate metabolism. Fasting triglyceride level was the strongest associated factor of nocturnal substrate oxidation.