Sitting all day can have a negative impact on your body composition in a number of ways. For one, sitting for prolonged periods of time can lead to an increase in the amount of fat around your midsection. This is because when you’re inactive, your muscles don’t burn as many calories as they would if you were active. As a result, your body starts to store more fat as a way of conserving energy. Additionally, sitting for long periods of time can lead to a decrease in muscle mass. This is because when you’re inactive, your muscles don’t get the same level of stimulation as they would when you’re active. As a result, your muscles start to shrink, and you lose muscle mass.