Respiratory quotient during waking (top) and sleeping (middle) periods and total fat oxidation during sleep in lean (n = 10), obese (n = 9), sedentary reduced-obese (Red-SED, n = 7) and exercising reduced-obese (Red-Ex, n = 12).

<p>Statistics are presented on the Figure.</p

Subjects characteristics and fasting hormone and metabolites concentration.

<p>One-Way ANOVA: ^a P<0.05 vs Lean group; ^b P<0.05 vs Obese group, ^c P<0.05 vs RED-SED group BMI, Body mass index; FFA, free fatty acids; FFM, Fat free mass: FM, Fat mass; RED-EX, Reduced-Exercise group; RED-SED, Reduced-Sedentary group; RMR, resting metabolic rate; RMR_adjFFM, resting metabolic rate adjusted for fat free mass; VO2_peak, maximal aerobic capacity; VO2peak/FFM, maximal aerobic capacity normalized by fat free mass. Values are mean ± SD</p

Cumulative responses of plasma triglycerides (top) and free fatty acids (FFA; bottom) concentrations expressed as the area under the curve (AUC) over 24 h in lean (n = 10), obese (n = 9), sedentary reduced-obese (Red-SED, n = 7) and exercising reduced-obese (Red-Ex, n = 12).

<p>Statistics are presented on the Figure.</p

Twenty four hour total fat (top), carbohydrate (middle) and protein (bottom) oxidation in lean (n = 10), obese (n = 9), sedentary reduced-obese (Red-SED, n = 7) and exercising reduced-obese (Red-Ex, n = 12) after adjustment for differences in fat-free mass (FFM).

<p>Statistics are presented on the Figure.</p

Room calorimeter results over 24

<p>Tukey post-hoc: ^a P<0.05 vs. Lean group, ^b P<0.05 vs. Obese group, ^c P<0.05 vs. RED-SED group.</p><p>RED-EX, Reduced-Exercise group; RED-SED, Reduced-Sedentary group; RQ, respiratory quotient.</p><p>Values are mean ± SD.</p

The Physiology of Sedentary Behavior

Sedentary behaviors (SB) are characterized by a low energy expenditure while in a sitting or reclining posture. Evidence relevant to understanding the physiology of SB can be derived from studies employing several experimental models: bed rest, immobilization, reduced step count, and reducing/interrupting prolonged SB. We examine the relevant physiological evidence relating to body weight and energy balance, intermediary metabolism, cardiovascular and respiratory systems, the musculoskeletal system, the central nervous system, and immunity and inflammatory responses. Excessive and prolonged SB can lead to insulin resistance, vascular dysfunction, shift in substrate use towards carbohydrate oxidation, shift in muscle fiber from oxidative to glycolytic type, reduced cardiorespiratory fitness, loss of muscle mass and strength, and bone mass, and increased total body fat mass and visceral fat depot, blood lipid concentrations, and inflammation. Despite marked differences across individual studies, longer-term interventions aimed at reducing/interrupting SB have resulted in small, albeit marginally clinically meaningful, benefits on body weight, waist circumference, percent body fat, fasting glucose, insulin, HbA1c and HDL concentrations, systolic blood pressure, and vascular function in adults and older adults. There is more-limited evidence on other health-related outcomes and physiological systems, and for children and adolescents. Future research should focus on the investigation of molecular and cellular mechanisms underpinning adaptations to increasing and reducing/ interrupting SB and the necessary changes in SB and physical activity to impact physiological systems and overall health in diverse population groups.</p

Plasma FFA (± SE; Left) and triglycerides (± SE; Right) during LF (Top) and HF (Middle) diets in LN (N = 10; dashed line), and OB (N = 9; solid line).

<p>24 h FFA and triglycerides areas under the curve (AUC) are presented in the bottom graphs.</p

Room calorimeter results (mean±SE).

1<p>OB > LN.</p>2<p>LF > HF.</p>3<p>LF < HF.</p>4<p>PAL  = 24 h EE/resting metabolic rate.</p><p>RQ = respiratory quotient.</p><p>FFM = fat free mass.</p><p>FM = fat free mass.</p

Mean (± SE) Protein (Top), carbohydrate (Middle), and fat (Bottom) oxidation (g/kg FFM/day) measured using room calorimetry during LF (black bars) and HF (grey bars) in LN (N = 10) and OB (N = 9).

<p>* LF significantly different from HF (P<0.05).</p

Mean (± SE) 24 h (g/day) protein (Top), carbohydrate (Middle), and fat (Bottom) balance measured using room calorimetry during LF (black bars) and HF (grey bars) in LN (N = 10) and OB (N = 9).

<p>* LF significantly different from HF (P<0.05).</p