Does Human Metabolism Decline With Age?

Study review on how the metabolism changes through the human life span.

Today I'm only going to be talking about a recent research paper that has been making the rounds over the last few months.

My reason is because it's one of those papers you've probably seen on social media or even in the New York Times where it was recently covered.

So I want to break it down and give some additional context that you may not get in a social media post.

Study covered: Daily energy expenditure through the human life course

This was a research paper that looked at the total energy expenditure of humans taken from a data base with a large and diverse population. 6421 subjects, spanning from infants that were 8 days old to adults that were 95 years old. Also with people from 29 different countries.

They measured energy expenditure by using a method called doubly labelled water. Basically they gave all the subjects a special kind of water. Then collected urine/saliva samples. Part of the doubly labelled water leaves the body as water and part leaves through your breath via CO2. They analyze the urine/saliva samples after taking it to calculate how much energy was burned by subtracting both parts that leave as water to calculate how much CO2 was produced. This gives an accurate calculation of energy expended.

The upside of this method is that it allows subjects to live freely. As opposed to being stuck in a controlled lab/ward to monitor energy expenditure. Which would obviously change their behaviour. This method allows for there to be over 6400 subjects all using the same method of assessment.

Now that that's out of the way, let's look at what his paper found.

Findings

This is where people were shocked. This paper basically showed that your metabolism does not decrease from age alone from about 20 years old to 60 years old. Key word there is "age alone." Some people will say "well, once you turn 40, your metabolism just falls off a cliff!"

This may be true, but it does not seem to be from age. More so, age is probably more correlative than causative here.

You will above that adjusted energy expenditure stays quite static from about year 20 to year 60 on average. With some clear outliers, of course. The circled dots represent the average for populations within that age group.

You may be wondering what "Adjusted TEE" means. It simply means what your total energy expenditure is in relation to what would be expected of someone with a certain amount of fat free mass (muscle, connective tissue, organ tissue etc.) and fat mass. The more fat free mass and fat mass you have, typically the more energy you expend.

100% would mean you burn about exactly what would be expected for someone with your amount of lean tissue and fat tissue.

You'll see here that age 0-5 is when you're burning more energy than would be expected. The most was actually between 9-15 months when you're burning about 150% of what would be expected from someone with that level of lean mass and fat mass.

Then this trends down to around 100% at age 20. That trend held remarkably tight until age 60.

This does NOT mean that people burn the same amount of calories per day from age 20 to age 60. It means that relative to lean mass and fat mass, they burn about the same rate.

What often happens is that when people get more advanced in their careers, adopt more responsibilities or adjust their lifestyle with age, they often move less. By proxy, they also lose more lean mass as a function of moving less/working out less. The reduction in lean mass and reduction in general activity is contributing to the decline in energy expenditure. Not necessarily age itself.

So your total energy expenditure may very well have dropped (and probably did from a practical perspective), but it doesn't seem like age is the reason why. More so, reductions in general activity and losses in fat free mass are more to blame for that.

As you'll see below, increases in fat free mass trend pretty tightly with increases in total energy expenditure. There is some obvious outliers. The random dots up near 30 MJ/D would signify that person was burning near 7000 calories per day. While if you draw a straight line down from that high blue dot at 30 MJ/D you'll see another adult with the same amount of fat free mass who only burns about 8 MJ/D which is equivalent to 1900 calories.

Because yes, human metabolism is just fucking unfair sometimes, just as everything is in life to some degree. With that being said, the averages are very tightly correlated with fat free mass here.

Was there a difference between males and females?

To the surprise of many, no. If you look at the first graph above, you will see adjusted energy expenditure is very similar between males and females. Pretty much hugging each other at the averages.

This may be confusing, since you may me a female and know a male around your same body weight who can eat far more than you and maintain or vice versa.

One thing to remember is that on average (not always, of course) males will have more fat free mass than females. So if a male and female are both 160lbs, on average, that male will have more lean body mass than the female within that same body mass. Making them burn more energy.

But if you had a male and a female who weighed the exact same amount, with the same amount of lean tissue and similar activity levels, they would most likely have a similar energy expenditure. Which is supported by the first graph.

So there does not seem to a be a difference theoretically between the sexes, but practically males tend to have higher energy expenditures for the reasons listed above.

Lastly, when does age lead to metabolic decline?

From the evidence here, around 60 and onward. The drop was very small at 60 but continued to trend down. At around 95 years of age, the subjects were expending about 75% of what would be expected for someone with their levels of fat free mass and fat mass. It also should be noted, that appetite and drive to eat tends to reduce with older age as well.

Takeaways

There is actually a lot to take away from this paper, so credit to Herman Pontzer and his colleagues for the awesome work here. Herman wrote a book about human metabolism called "burn" that I have only heard good things about too, so you may want to check that out.

Here are three take aways that I think can be quite valuable to know.

1. The idea that metabolism inherently declines with age seems to be a myth until around 60 years old.

2. Strength training, building/maintaining muscle and having a generally active lifestyle are crucial for maintaining a higher total metabolic rate (not adjusted). So please, continue to train and move plenty. Also, if you're afraid to build muscle, I hope this helps reduce that fear!

3. There are plenty of outliers when it comes to total energy expenditure. You can even see with those graphs. One person in their late 50's was burning 200% of their expected energy output while another at the same age was burning about 80%. One adult with a fat free mass of 60kg was burning about 7000 calories per day while another with the same amount of fat free mass was burning around 1900 calories. So just because mostly people will hover around the average, does not mean everyone does. Which is why relying on calorie calculators to tell you how many calories you need could be really demoralizing for some. My advice here is to use your own data of tracking food intake and and seeing how it impacts your weight over time. This will be more valuable to you than only relying on calorie calculators to tell you how much you need.

Cheers
-Coach Dylan 🍻