How do you know if your metabolism has slowed down?

Battery on low: Imagine tape player running on low battery: things s-l-o-w down. You may feel cold, tired, gain weight, be constipated.

Check metabolic rate: The formal medical method to calculate one's metabolic rate is to use a breath test that can measure the amount of calories that one is using up in a resting state. (known as the resting metabolic rate). This may determine whether your metabolism has changed over time. The results of the test can predict how many calories used at rest, at moderate exercise, intense exercises, etc.