Your body composition has a big impact on your metabolic rate. Your body is composed of fat mass and fat-free mass. Fat-free mass includes your muscle, organs, bones, skin, and so on. The proportion between the two can vary from person to person based on DNA, sex, age, and fitness level. The takeaway here is that muscle mass is metabolically active tissue that burns more calories than fat mass.
The exact amount of calories burned by muscle is under debate. However, the more lean muscle mass your body has, the more calories it burns, and therefore the more efficiently your metabolism works.
Muscle mass is denser than fat and can weigh up to three times more than the same volume of fat. My clients sometimes get frustrated when starting a workout program because they aren’t losing weight. I often have to remind them that they are probably losing fat but building muscle tissue, which will burn more calories at rest, even when they’re just sitting around watching TV.
Weight loss doesn’t always equal body fat loss. You can lose weight in the form of lean muscle mass if you’re following a restrictive diet or not fueling yourself properly for the exercise you’re doing. When you lose that desirable calorie-burning muscle mass, you’re not only causing your metabolism to slow down even further, but you’re setting yourself up for failure when it comes to weight maintenance.
Men have more muscle
When a husband-and-wife strive together for weight loss, 90 percent of the time, the male has more success than the female. Often he makes one change, such as cutting out soda or juice, or simply starting to exercise, and is able to lose weight. Meanwhile she struggles to do everything she can with her diet, activity, and lifestyle but her weight doesn’t budge for weeks.
One reason is that once men hit puberty, they typically develop 20–40 percent more muscle mass than women, making their metabolic rates higher. Why is this so? Men produce more of the hormone testosterone, which promotes muscle development, and women are actually genetically engineered to have more body fat in preparation for pregnancy and lactation.
On top of that, once women hit pre-menopause and menopause, their metabolism-boosting sex hormone, estrogen, drops, and a sluggish metabolism ensues. Typically, women gain about a pound or two per year during this time, and taking the weight off becomes more difficult.
You lose muscle as you age
Do you remember when you could eat anything you wanted without putting on a pound? And now you gain weight just looking at a plate of French fries? The truth is that as you get older, your muscle mass declines. After age 20, your metabolic rate slows down between 5–10 percent each decade due to less lean muscle mass and more fat mass.
If you aren’t doing any muscle-building activity, and you need 1,900 calories to maintain your weight at age 20, your calorie need could decrease to 1,750 calories at age 30, 1,650 at 40, 1,500 at 50, 1,350 at 60, and down to 1,300 by age 70. Over 50 years, that’s 600 fewer calories that your body needs daily to maintain the same height and weight.
Research also shows that it’s not just body composition that changes as you age. Your organs, such as your heart and lungs, also seem to require fewer calories as you get older. At each decade milestone, you need to re-evaluate how you eat and how you move to stay within a healthy weight range.