What is the primary effect of decreased physical activity in relation to obesity?

The primary effect of decreased physical activity in relation to obesity is that it contributes to weight gain. When physical activity levels drop, the total energy expenditure also decreases, leading to a lower number of calories burned throughout the day. If an individual's caloric intake does not decrease correspondingly, or if it remains the same or increases, the surplus calories can lead to an increase in body weight and fat accumulation.

Moreover, physical activity plays a crucial role in maintaining a healthy weight by regulating energy balance, enhancing metabolic health, and influencing body composition. Regular exercise helps to increase muscle mass, which in turn can elevate metabolic rate and improve the body’s ability to use energy effectively. When this activity level declines, not only is the immediate energy expenditure reduced, but there can also be a negative impact on body composition, further exacerbating the risk of becoming overweight or obese as sedentary behaviors become more prevalent. Therefore, decreased physical activity is closely linked to weight gain, making this the correct answer.