Let’s talk about the number that matters most when it comes to metabolism—and no, it’s not the number on the scale.
It’s your Basal Metabolic Rate (BMR).
What Is BMR?
Your BMR is the number of calories your body burns just to keep you alive. Breathing. Circulating blood. Producing hormones. Repairing cells. Growing hair. Digesting food. Maintaining your body temperature.
It’s the energy cost of existence before you add in any movement, exercise, or daily activity.
For most women, BMR accounts for about 60-75% of total daily energy expenditure. That means the majority of calories you burn each day aren’t from your workout—they’re from simply being alive.
Why BMR Decreases Over Time
Here’s what happens: Starting around age 30, we lose muscle mass unless we actively work to maintain it. This process, called sarcopenia, accelerates after menopause.
Muscle tissue is metabolically active—it burns calories even at rest. Fat tissue doesn’t burn nearly as much. So when you lose muscle and gain fat (even if the scale stays the same), your BMR drops.
Research shows that BMR decreases by about 1-2% per decade after age 20. Doesn’t sound like much, right? But a 50-year-old woman might burn 200-300 fewer calories per day than she did at 25—just from this natural decline.
Add hormonal changes to the mix—decreasing estrogen, progesterone, and testosterone—and you’ve got a recipe for metabolic slowdown.
But Here’s the Plot Twist
The biggest factor in BMR decline isn’t age. It’s muscle loss.
And muscle loss isn’t inevitable—it’s a consequence of becoming less active and not eating enough protein to maintain muscle mass.
Think about it: most of us move less as we age. We stop playing sports, we sit more at work, we skip the gym because we’re “too tired.” Meanwhile, we’ve been told to eat less to maintain our weight, so we cut calories without considering whether we’re getting enough protein to preserve muscle.
The result? We lose muscle, our BMR drops, and we blame our age.
The Diet Industry Made This Worse
Here’s where I need you to really hear me: restrictive diets tank your BMR.
When you drastically cut calories, your body adapts. It becomes more efficient—meaning it learns to function on less energy. Your metabolism literally slows down to match your intake.
This is adaptive thermogenesis, and it’s your body trying to keep you alive during what it perceives as a famine.
Women over 40 are especially vulnerable because we’ve often been dieting for decades. We’ve spent years under-eating, over-restricting, and losing muscle in the process. Then we wonder why we can’t lose weight eating 1200 calories a day.
What Actually Works
Supporting your BMR means:
- Building and maintaining muscle through strength training
- Eating enough—especially protein—to preserve that muscle
- Not chronically under-eating, which teaches your body to slow down
- Supporting your hormones through nutrition, sleep, and stress management
Your BMR isn’t fixed. It responds to what you do—or don’t do—for your body.
This isn’t about eating less and exercising more. It’s about eating enough, moving with purpose, and giving your body what it needs to thrive, not just survive.