Association of physical activity, sedentary behaviour and habitual diet with resting fat oxidation in overweight and obese women

Metabolic flexibility, defined as the ability of metabolism to regulate the oxidation of energy substrates, is impaired in obesity and is known to have a significant impact on the development of chronic diseases. Considering the increasing trend in obesity, it becomes critical to understand the factors associated with metabolic flexibility as well as the identification of such factors and the planning of interventions to increase metabolic flexibility will provide significant benefits in the prevention of obesity. Therefore, the aim of this thesis was to determine the relationship between physical activity levels, sedentary behaviors, and habitual dietary patterns with resting fat oxidation, an important parameter of metabolic flexibility, in normal-weight, overweight, and obese women. A total of 110 women (age: 35,7±7,5 years, height: 161,8±6,2 cm), 55 normal weight (body mass index [BMI]: 18,5-24,9 kg/m2), 30 overweight (BMI: 25-29,9 kg/m2) and 25 obese (BMI: 30-34,9 kg/m2), aged between 25-50 years were included in this study. Initially, body composition and resting metabolic rate (RMR) measurements were performed for all participants. Resting fat and carbohydrate (CHO) oxidation values were calculated from RMR data using the Frayn (1983) equation. Subsequently, dietary patterns, physical activity levels, and sedentary behaviours were evaluated by 4-day food consumption records and 7-day physical activity records with accelerometer. After determining the normal distribution of the data, one-way analysis of variance (ANOVA) was used to compare the groups in regard to the variables examined, and multiple linear regression analysis was used to determine the effect of independent variables on resting fat oxidation. Resting fat oxidation of normal weight women (1,84±0,57 mg/kg/min) was significantly higher than that of women with obesity (1,38±0,52 mg/kg/min) (p<0,05). No significant difference was found between fat oxidation and CHO oxidation values of women with overweight and obesity (p>0,05). Multiple linear regression analyses showed BMI, visceral adipose tissue (g), daily intake of protein, carbohydrate, fat, dietary fibre, cholesterol, daily duration of moderate-high intensity physical activity and daily average duration of sedentary bout predicted 14,2% of the change in fat oxidation (p=0,007; adjusted R2=0,142). In addition, being obese according to BMI classification (β:-0,333; p=0,036) and the average duration of daily sedentary bout (β:-0,261; p=0,011) significantly associated with resting fat oxidation. Furthermore, the duration of moderate-high intensity physical activity and the average duration of daily sedentary bout accounted for 30% of the change in resting fat oxidation in women with obesity (F=4,250; p=0,018; R2=0,389; adjusted R2=0,298). Moreover, a 1-min increase in the mean duration of daily sedentary bout was found to lead to a 0,216 mg/kg/min decrease in resting fat oxidation (p=0,022). However, this relationship was not significant in normal weight women or women with overweight (p>0,05). In conclusion, having a BMI ≥30 kg/m² and increase in the average duration of sedentary periods significantly reduce resting fat oxidation. Particularly in individuals with obesity, disrupting sedentary periods with short bouts of physical activity during the day can significantly improve resting fat oxidation, an important indicator of metabolic flexibility. Keywords: Metabolic flexibility, resting fat oxidation, obesity, sedentary behavior, physical inactivity