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Health Claims Head Check: Does Metabolism Really Slow Down As You Age?

It’s no secret that the media publishes a lot of misleading health articles.  Are eggs good or bad for you this month?

In our inaugural column, we looked at recent articles claiming that the glycemic index of food doesn’t matter for weight loss (mostly true) and that having people to listen to you can slow the progression of Alzheimer’s (questionable, highly open to interpretation).  

Today we’ll look at two more popular media articles about recent health studies.  Does your metabolism really start slowing down as soon as you turn 25?  And do you need to go plant-based to prevent heart disease?

Metabolism in adulthood does not slow as commonly believed, study finds

The Background

It’s commonly believed that human metabolism slows down during adulthood.  When exactly this starts depends on who you ask– generally somewhere between 18 and 30 years old– but it’s routinely blamed for weight gain during young adulthood.  

The Claim

As reported by NBC News, metabolism in adulthood does not necessarily slow down as is commonly believed.  

According to a study, by a massive team led by Pontzer et al, metabolic rate, controlled for body weight and composition, stays stable from about ages 20 to 60, before declining at an average rate of 1% per year.  Metabolic rate doesn’t (automatically at least) slow during adulthood before reaching old age.

Now, this isn’t actually new: it’s been confirmed via multiple studies, mostly using a “bod pod”– a chamber which measures how much heat the body of the person inside gives off as a measure of metabolic rate.  In fact, this study was essentially a review of, or a synthesis of the data from, those other studies.  

However, most people “know” that their metabolism slows as they get older, and well before sixty.  What gives?

Digging Deeper

The confusion here stems from “metabolism” actually being three different things.  

First, there’s the rate at which body cells burn energy just to maintain themselves.  This doesn’t change for a given cell type, which is what this study shows.  

In fact even in theory this “cellular metabolic rate” can’t vary much; your metabolism couldn’t slow down much without your body failing to do what it takes to maintain itself and keep you awake and active.  And on the flip side, since all metabolism produces waste heat as a byproduct, your metabolism can’t speed up much without overheating your body.

Second, there’s body composition.  Many people do lose muscle and gain fat in adulthood as they become less active, especially if working a desk job.  This can slow their “metabolism” in as much as lean mass burns more calories than fat mass.  

The reverse can also be true; your metabolism can speed up in adulthood if you take up weight lifting later in life, and this is in fact quite common among the physically active.  

Because men are generally leaner than women, they usually have a higher metabolic rate on a pound for pound basis.  For the same reason, overweight people have a lower metabolic rate per kilogram of body mass. 

Third, there’s your activity level.  Again, this often declines in adulthood, but burning calories through activity is different from burning them doing nothing, as in the other two definitions of metabolism.  This is often unnoticed, as it can be as simple as walking less when you stop living on a college campus and start driving everywhere.  

While “exercise” burns a decent number of calories, simple lifestyle activities like walking or dancing actually burn more calories over the long run if you’re doing them for 10-20 hours a week.    

The Verdict

All in all, human metabolic rate– that is, the total number of calories burned– doesn’t vary a whole lot.  Studies find most people have very similar metabolic rates even without controlling for body size or composition.

In fact, most people’s daily resting metabolic rates (energy expenditure excluding activity) are within 300 calories of each other, and even a 5th and a 95th percentile individual would only be 600 calories apart.  Again, that’s without controlling for other factors: a large and physically active male and a small and mostly sedentary woman are still closer in resting energy usage than you’d think.  

On the other hand, a whole day spent hiking or even just walking around can burn a lot of calories– but how regularly do you do that?  

Finally, two other things are important to remember.  First, the 60 years old cutoff cited in the study is only an average.  People who stay active may not begin declining until 70-80, while the morbidly obese and inactive may decline as early as 40.  

Second, the reason for that 60-year figure is primarily because people start losing muscle around sixty years old.  But that too is an average; many healthy people stave off muscle loss into their eighties.  

In other words, 60 can be late middle age or well into old age, depending on how well you take care of yourself.  

A plant-based diet is the best way to avoid heart disease, according to a new report

The Background

Everyone knows that meat is bad for you and plants are good, right?  And after all, rates of obesity and heart disease have skyrocketed in the past few decades, just as people began eating more and more meat and fewer fresh fruits and vegetables.  

It’s pretty well-known that people should be eating a plant-based diet and/or going vegan– and not everyone understands the difference between the two.  But we all know the gist– eat more plants and less (or no) meat.    

The Claim

As reported by The Washington Post, yet another study has found that a plant-based diet is most effective for preventing heart disease.  

Big kudos to Cara Rosenbloom at The Washington Post for actually linking to the study, and even correctly describing it as a meta-analysis; popular media hardly ever does either of those things– linking to the study, or clearly describing the methodology used.   

So, plant-based diets for heart disease and overall health?  Numerous studies have found this already, so again, it’s not news.  However, why they work so well is often misrepresented.

Digging Deeper

Plant-based diets differ from other diets in more than one way.  They have more plants, less meat, less processed food, and fewer calories overall.  So which of those is important?  Calories obviously are, but what about food types?  

Studies which make an effort to separate these factors out generally find that adding plants is key; cutting back on meat isn’t necessary, and cutting it out altogether is probably counterproductive.  

That said, take note that the term “plant-based” was used rather than “vegan.”  The authors may not be totally explicit about it, but they are trying to differentiate between eating more plants and eating fewer animal products.  In general, “plant-based” is usually taken to mean something like 80% of one’s diet consisting of plant foods.  

In fact, it’s entirely possible to eat an unhealthy plant-based diet which is worse than the standard American diet.

The Verdict

Highly processed foods are terrible for you, and plant foods certainly can be highly processed.  Plant-based diets tend to be minimally processed because the people who follow plant-based diets usually also understand the importance of avoiding processed foods, not because plant-based foods inherently avoid processing.

My recommendation: focused on eating unprocessed foods and more fruits and vegetables.  Don’t go out of your way to cut out meat, but do cut out highly processed meat.  Try to make your diet about 60-90% plant foods– and that mena mostly produce, not bread and cereal.  

Doing this tends to also handle the calorie issue without needing to specifically focus on calories, as long as you avoid obvious junk food.  

About the author

John Fawkes is the Managing Editor of The Unwinder. John is an NSCA-certified personal trainer and Precision Nutrition-certified nutritional counselor who has been featured on over two dozen websites and podcasts. He works with clients in Los Angeles and online, and can be reached on Instagram and Twitter.

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