[The energetics of obesity]

Area of Science:

• Human physiology
• Metabolic health
• Obesity research

Context:

• Rising obesity rates in the USA despite decreased energy intake suggest reduced energy expenditure.
• The roles of genetics, diet, and physical activity in obesity are debated.
• Sedentary behavior's impact is often inferred from exercise benefits, lacking direct inactivity models.

Purpose:

• To review the contribution of resting metabolic rate, thermogenesis, and activity energy expenditure to lower energy output.
• To investigate the role of physical inactivity as a major contributor to obesity.
• To provide evidence for a causal link between inactivity and obesity using a bed rest model.

Summary:

• Physical inactivity is identified as the primary driver of decreased energy expenditure contributing to obesity.
• A hypothesis suggests impaired body mass regulation mechanisms below a critical physical activity threshold.
• Long-term bed rest demonstrates that inactivity disrupts fuel homeostasis, favoring fat storage and promoting weight gain.

Impact:

• Provides evidence that inactivity per se promotes weight gain by altering fat metabolism.
• Highlights the need for further research into the mechanisms linking inactivity and obesity.
• Emphasizes determining the evolutionary-determined minimal physical activity requirements for metabolic health.