Reader Request – Metabolic Age

Question: What is my metabolic age, and should I be concerned about it?

Response: Thank you for the question! This morning, I found a website that calculated my metabolic age for me after I answered some questions about my age/height/weight, and then a couple lifestyle questions (perceived sleep quality, stress, eating habits). Without any visibility into the calculation process, the website (after entering my email address, of course) told me my metabolic age is 37. Exactly 1 year less than my biological age.

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Hooray. I’m stoked. But, not really…

For the purposes of tracking body fat % trends, I use a scale from a company called Renpho. They claim the numbers are benchmarked against more exact measurements and calibrated before leaving the factory. So, the error compared against, say for instance, a BodPod analysis, should be relatively small. When it comes to stuff like that, I’m okay if the number is a little off from reality – just as long as its off by the same factor each time. If it is, then the TREND DATA will tell me all I need to know. Anyway, the Renpho app gives me a “metabolic age” number as well. As of my last measurement, it was 39. Exactly 1 year OLDER than my actual age.

Hmmm – two sources, two different numbers. One good, one bad.

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Bummer. I’m sad. But, not really.

After scouring the Internet for the better part of the morning, I couldn’t find an actual formula or algorithm (public anyway) that allows you to do the calculation on your own. I did find this “definition (source article linked)” of metabolic age from Patch.com (and it was corroborated by a couple other sites) – “Metabolic age is a comparison between a person’s basal metabolic rate (BMR) against the average BMR for an age. (Patch.com)”.

Okay – so that means if I calculate my personal BMR, and compare it against the average BMR for my age group, I’ll know how I stand. In other words, if my BMR says that I have the same metabolism as a 32 year old, then I’m metabolically 32, even though I’m biologically 38. Conversely, if the comparison says my metabolic rate is the same as a 44 year old, and I’m only biologically 38, then I’m metabolically older than I am biologically.

I guess the conclusion to draw is – if you are metabolically younger than your biological age, it’s a good thing. Easy enough – I’ll just make myself metabolically younger. Hold on – not so fast.

But what does that all actually mean, and is it useful, actionable data? If the bedrock of this analysis is the BMR, I’d say NO for three reasons.

First, the generally used formula for computing a person’s BMR is the Harris-Benedict formula. The formula is…

  • Women = 655 + (4.35 x weight in lbs) + (4.7 x height in inches) – (4.7 x age in years)
  • Men = 66 + (6.23 x weight in lbs) + (12.7 x height in inches) – (6.8 x age in years)

My problem with using this number to draw a conclusion about overall health is the fact that the only controllable variable in the equation for a person is their weight. As I’ve stated in other posts, weight can be a very false idol for evaluating your health. It is very possible to have a large amount of muscle, low to medium body fat, and be considered overweight according to the BMI (body mass index – ratio of weight to height) scale. My own BMI runs around 27.2. That puts me in the high/overweight category. But, with a body fat % between 10% and 15% (depending on the measuring tool used), I’m certainly not overweight. My point is that body weight alone isn’t a good metric to determine if you need to make a change. Body weight itself is a number based on a bunch of different variables – muscle/bone mass, water retention, and body fat to name a few. People at the extreme ends of the spectrum can probably make a blanket assumption about their condition (needing to gain weight or lose weight). But, the people in the middle really need to understand what numbers are driving their body weight. If a person has relatively low body fat and moderate to high muscle mass, there is probably no reason for them to try and lose weight, no matter what the scale tells them.

Second, the Harris-Benedict formula was shown to have some deficiencies in estimating/predicting energy requirements.

If the formula used to calculate the BMR, which is then used to calculate your metabolic age, has some limitations, then how reliable is the derivative number? How actionable is a number (based on a number) that may or may not be reliable/accurate?

Third, try to find a chart that maps out the BMR for age groups. Then, try to find a chart that maps out the IDEAL BMR for age groups. Go ahead – I’ll wait…

I couldn’t find one, and neither could some other websites that expressed a similar frustration. If you can’t find and scrutinize the benchmark, how good is your analysis going to be?

I agree that having metrics to measure elements of our health is a great thing, and people should do it as much as they can. Peter Drucker (a very famous management thinker) says, “You can’t manage what you can’t measure.” The corollary thought would be – if you measure it, you can manage it. However, if you track ambiguous metrics or even worse, incorrect metrics, you’ll have a “garbage in-garbage out” analysis situation. When deciding if a data point is worth tracking ask yourself if it is actionable.

In other words, does knowing that number allow you to zero in on specific action to affect it?

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Metrics like weight, BMI, and metabolic age have multiple inputs that affect the resulting number. You could reduce your weight by reducing muscle mass, reducing water mass, reducing bone mass, or reducing fat mass. Only one of those four is probably a desired option (assuming you have excess body fat). In that case your better bet is to track and try to affect your body fat % number. Sure, the weight, BMI, and metabolic age will come along for the ride, but it is the tracking and action on one variable that makes the impact.

So, in my opinion, metabolic age probably isn’t the best metric to help you formulate an action plan. All it is really telling you (based on its use of BMR) is that you might need to lose weight. I’d say your time is better spent trying to measure and track trends on something a little less ambiguous.

 

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2 comments

    1. My pleasure. With the ability to track so much data – I think it’s important for people to track data that actually matters, and is actionable.

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