People often say “muscle weighs more than fat” when a muscular person reveals his or her body weight and it’s heavier than expected. For example, a fit, muscularly-built man of 5 feet 10 inches can easily weigh over 180 pounds, while an out-of-shape man of the same height who sports extra body fat may weigh less.
The surprising disparity in these men’s weights is attributable to the difference between the densities of muscle (or lean tissue) and body fat. Lean tissue weighs more per unit volume than body fat.
More specifically, muscle and other non-fat tissues in the body (such as bone) have a combined density of 1.1 grams per cubic centimeter, while body fat has a density of 0.9 g/cc.
Laboratories use this difference to determine people’s body compositions, using a technique that involves weighing a person submerged in water and comparing that to their weight out of water. This process, called hydrostatic weighing, estimates body fat percentages by computing the overall density of the body.
Simply put, the less you weigh when you’re submerged, the more body fat you have — fat floats because it is less dense than water (water has a density of 1.0 g/cc). Conversely, you’ll weigh more underwater if you have a higher percentage of lean tissue, which sinks because it’s more dense than water.
Since muscle weighs more than fat per unit volume, the obesity measure known as body mass index (BMI) gives erroneous values for some folks. BMI is simply a weight-to-height ratio (weight in kilograms divided by height in meters squared) — it does not take body composition into account.
If your BMI is over 25, you are considered overweight, and if it’s over 30, you’re classified as obese. Back to our fit and trim man at 5’10″ and 180 pounds: His BMI comes out to 26 and the guideline says he’s overweight.
But his chubby counterpart could easily weigh less, which might make his BMI less than 25. He would not be considered overweight according to the BMI guidelines, when in fact the opposite is true. So BMI has a limitation — body composition is not considered — that relates directly to the issues raised in your question.
Now, why is fat so hard to lose if it weighs less than muscle? Because body fat provides the richest energy storage in the body — 9 calories in every gram. (Stored carbohydrate packs only 4 calories per gram, while protein is not stored for fuel but instead makes up your muscles and other tissues.) To lose a pound of fat, you need to burn off an extra 3,500 calories by eating less, exercising more, or a combination of the two.
Let’s carry these calorie densities over to food. A teaspoon of butter or other fat has about 50 calories, while a teaspoon of sugar contains just under 20, even though the two weigh about the same. As in your body, dietary fat is more calorie-dense than carbohydrate or protein. Yes, fat is lighter — but it packs a lot more energy.