Biomarker #3: BMR

And now…back to biomarkers, those statistical snapshots that can help us figure out how old we are biologically and then can track our counterclockwise movement as we live healthier, more active, more engaged lives. I’ve already written about resting heart rate and blood pressure as important biomarkers. To review: Low resting heart rate = good. Low blood pressure = good. “Good” means fit and biologically youthful. With Basal Metabolic Rate (BMR), the opposite is true. A high BMR is associated with biological youthfulness.

So, what’s BMR (sometimes also referred to as Resting Metabolic Rate, which is not the same. but is enough the same for our purposes)? BMR is a measurement of the amount of energy we expend at rest. It’s the energy the body uses to maintain itself, the energy expended by heart, lungs, liver, kidneys, nervous system, all the internal organs that support life. If we lie in bed, absolutely still in twitchless slumber – or, less appealingly, if we found ourselves in a coma – we would still be expending energy. That’s our personal BMR. Although the mathematical calculations to get to BMR are mind boggling (at least my mind boggles), the measurement itself is easily understood. It is expressed in calories. Back to that in a second.

First, here’s something pretty interesting: About 70% of a human’s total energy expenditure is due to the basal life processes within the body’s organs. About 20% of our energy expenditure comes from physical activity and another 10% from thermogenesis, or digestion of food. Which kinda puts exercise in perspective, doesn’t it? And almost makes you want to do an over-eating, under-exercising experiment to see if you can make the math work in your favor. Spoiler alert: You can’t)

So why would a higher BMR be associated with biological youthfulness? It’s because BMR is strongly linked to muscle mass. The greater the muscle mass, the more felicitous the lean-to-fat ratio (as in more lean, less fat), the higher the BMR. Muscle is metabolically active. It uses more energy just to maintain itself than does fat. So an in-shape, leaner body burns more calories and registers a higher BMR. Men, by virtue of their generally bigger bodies and genetically determined greater muscle mass, have higher BMRs than women. Which is why your husband, most infuriatingly, can eat twice what you do and not gain a pound.

It is stated in the literature that “BMR decreases with age” and that, more specifically, our “metabolic rate goes down about 2 percent or more per decade after age 25.” Well, sort of. Yes, BMR decreases —  but not because years pass. It’s because muscle mass decreases. Here’s what Tufts university researchers say: “Older people’s reduced muscle mass is almost totally responsible for declining BMR.” In other words: You can have a major effect on your BMR by increasing and maintaining muscle mass. Thus BMR is yet another biomarker over which we have considerable control.

So, how to determine BMR:
1. Easy (and not terribly accurate) Here is link to a BMR calculator. Here’s another one. The problem: They both use your chronological age, making assumptions about muscle mass that may not be true for you.
2. Expensive (and accurate) Find a sports or health clinic that uses indirect calorimetry to measure BMR. It involves breathing into a mask/ apparatus attached to a computer with sophisticated software that measures the oxygen/ carbon dioxide mix. You have to be capable to truly relaxing and zoning out with a mask over your face in a clinical setting. And willing to pay $150 or so.
3. Easy, inexpensive, accurate. This assumes you already own a heart monitor, the kind with the watch and chest belt. Make sure you’ve recently/ accurately input your weight and height (and gender, of course). Strap it on before going to bed and press the start button when you are comfortably situated and just about to fall asleep. The next morning, see how many calories you burned while asleep in how many hours, divide by the hours, then multiply by 24 for your personal BMR. Do this several nights and take the average as you may have a particularly restless night that would skew the results. Try it!