There's no shortage of misinformation out there when it comes to health and fitness. False information can often be dangerous as it was last year when the then director of the Centers for Disease Control and Prevention (CDC) called vaccine misinformation one of the biggest threats to public health. But even when misinformation is less threatening, it can still be troublesome − especially when certain myths never seem to die. 

These include ones like catching a cold if you go outside with wet hair; that you need to wait 30 minutes after eating to swim; or that stretching is necessary before going for a run. Another common one that has lingered for too long is that a pound of muscle weighs more than a pound of fat. 

Why do muscles weigh so much?

Of course, muscles do weigh a lot, and for good reason. "Muscles are largely protein and protein is heavy and dense," explains Loren Fishman, MD, a professor of physical medicine and rehabilitation at Columbia University. He adds that muscles are also "extremely vascular" and that the blood that circulates through them weighs a lot as well. So do the large quantities of water and collagen stored in and around each group of muscle. And Fishman says that muscles are unique because everything contained within them "is more tightly packed" than things are in other areas of the body.

Does muscle weigh more than fat?

Because of such composition, muscle weighs more than it often appears to. But where people get hung up when thinking about fat and muscle is the difference between weight and density. A pound of muscle and a pound of fat both, of course, weigh the same: 1 pound. But as with a led ball and a bowl of Jell-O, two things that weigh the same can be very different in size, and that's where density comes into play. "Because muscles are approximately 20% denser than fat, a pound of muscle takes up about 20% less space than a pound of fat," explains Fishman. This helps explain why 10 pounds of muscle on a person looks very different than 10 pounds of fat − even though both amounts of weight are the same. 

One of the reasons fat is less dense than muscle is because "fatty acids and triglycerides are less dense than proteins, and they repel water," explains Anthony Beutler, MD, an associate medical director of sports medicine at Intermountain Health in Salt Lake City. This affects both fat volume and distribution. As such, and because muscle is denser and more compact than fat, people often appear thinner as we gain muscle mass − regardless of our true physical weight. 

Does muscle burn more calories or does fat? 

In addition to affecting one's body composition this way, muscles also have the advantage of boosting one’s metabolism and burning more calories at rest than a pound of fat does. This means that even if someone is sitting around and doing nothing, their body is burning more calories simply because it has more muscle. And muscles, of course, come with a host of other health advantages including improved bone and cardiovascular health, enhanced athletic performance and improved emotional well-being. 

These are some of the reasons why the CDC recommends including muscle strengthening activities that work all major muscle groups in at least two workouts every week. "To gain health benefits, you need to do muscle-strengthening activities to the point where it’s hard for you to do another repetition (rep) without help," the public health agency suggests.

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