Leah Higl
Welcome to the Ideal Nutrition Podcast. I’m Leah Higl, and I’m here with my cohost, Aidan Muir. Today’s episode, we are going to be addressing the question of does your metabolism slow as you age? So this is pretty complex topic, so let’s start with just defining what metabolism is and [00:00:30] differentiating that from your total daily energy expenditure.
So, there basically are two things we’re going to talk about. So A, total daily energy expenditure. That’s how much energy you burn in a day, taking into account your metabolism, your activity, the thermic effect of foods, how many calories you burn eating and your exercise. So it’s everything all together, how many calories do you burn?
Then we have our basal metabolic rate that is a part [00:01:00] of that equation, but that is what, I guess your “metabolism” is. So, that’s the amount of energy you burn, at rest, before accounting for any of those extra calories burnt through activity or anything like that. That’s kind of what your metabolism is.
So it’s important to differentiate these things. Because in this context of what we’re talking about, yes, your metabolism kind of does slow as you age, but [00:01:30] we’ve also got to look at the effects of your total daily energy expenditure from just general behavioral changes and differentiating between those two things. That’s going to be a huge part of this conversation. So, we’re actually going to start with diving into the argument against the metabolism changing and some research around that.
Aidan Muir
Yeah, I think it makes sense to start there, because a lot of people would automatically assume that it does drop, and that’s just a factual statement. [00:02:00] It is more complex in that there’s so many variables. Anytime you say something, you’ve got to understand all the variables and everything like that. But let’s propose the counter argument and then go with the more nuanced view.
So, probably the biggest study on this came out from Herman Pontzer and others, literally just last year, and what they found, which is an interesting finding that obviously media and everyone latched onto, was that total daily energy expenditure was stable between the ages of 20 and 60, [00:02:30] when fat-free mass was equated for. We have a slow decline after. That decline after was about 0.7% annually after the age of 60. And there’s a lot of things that go into that, which we’ll probably touch on later, as to why that potentially occurs. And prior to the age of 20, it was also a bit higher. But that’s just due to growth. As you’re growing, that obviously requires energy and stuff like that.
Obviously that’s a surprising finding for a lot of people, when they hear that [00:03:00] there’s no difference between age of 20 and 60. Because so many people just personally will have experiences where they’re like, “Oh, well, when I was 20, it was easy to stay skinny or stay lean or whatever, and then as I age, it got harder.” But there’s a lot of variables that go into this. The fat-free mass being credited for is obviously a key one.
They also found that men and women had similar total daily energy expenditure when variables such as fat-free mass were equated for. And that’s a really interesting finding. It’s super be useful to know. But that’s also the big issue with this [00:03:30] study in particular, in terms of you can’t just say metabolism doesn’t slow as we age, because when fat free mass is equated for there’s no difference. Because even using the men and women one, men on average have more lean mass than women. Therefore, on average, they have higher TDEEs. Both of these things are worth knowing. So you’re still knowing that when they’re equated… Yeah. And the same kind of thing goes for, as people age, does the average 20 year old have more muscle than the average 60 year old?
[00:04:00] Usually, yeah. And that’s a variable and there’s a lot of other variables that go into this, but that’s just pointing out why this study showed that there was no difference between ages of 20 and 60, because they equated for variables such as the fat-free mass being equated.
Leah Higl
So looking at some of the research around this is quite interesting. So, there is research looking at the average TDEE, so total daily energy expenditure, in different age groups and also compares this to the average, once they take into [00:04:30] account things like muscle mass and activity. And I think seeing those numbers side by side, it really adds a lot to this discussion.
So, there was one study that compared the RMR, resting metabolic rate, so similar to your basal metabolic rate, of three groups of people. Those were aged 20 to 34, 60 to 74 and over 90. So compared to the youngest group, people age 60 to 74 burned roughly 122 fewer [00:05:00] calories at rest, while people over 90 burned around 422 fewer calories.
That’s really significant numbers, 122 to 422. So literally 300 calories difference. However, when accounting for differences in gender, muscle and fat, they found that people age 60 to 74 only burned 24 fewer calories, while those over 90 burned 53 fewer calories, on [00:05:30] average, daily. So that’s a lot less significant, once all those things have been taken into consideration. When we’re talking about metabolism changing as we age, there’s a little bit of change that does happen, but we’re really referring to that drop, usually, in muscle mass and what we’ll talk about in terms of activity.
Those are the things that drop, that yes, have an effect on metabolism, but [00:06:00] it’s not just that your metabolism inherently drops significantly over time.
Aidan Muir
Yeah. Another interesting thing that comes up is, why when lean mass is accounted for, is there still a drop after the age of 60? It’s only a tiny drop, but it’s interesting to think about that question.
Leah Higl
That’s true.
Aidan Muir
And people have proposed explanations to that. And the most common, is that there’s a lot of explanations. But the most common one that seems to account for most of [00:06:30] the change, is a decrease in calories burned through internal organs and mitochondrial efficiency. For example, if your organs are no longer functioning as well, they probably are burning less calories to undertake the functions that they’re doing, is the main explanation. And some people have lept onto that and said some statements, which, there’s flaws, but it still kind of makes sense. They’re like, as we get older, the body starts breaking down. Certain health conditions start arising, kidneys not functioning as well. Heart’s not functioning as well. All these [00:07:00] things are not functioning as well. That’s probably part of why the calories are slightly less.
Leah Higl
Yeah. But that big difference is coming from that muscle mass difference. So, things like that.
Aidan Muir
Yeah. It’s after creating for all that, yeah. It’s only tiny thing.
Leah Higl
Which is- [crosstalk 00:07:14]
Aidan Muir
And that’s why people over the edge of 90, it was only 53 fewer calories on average, after equating for muscle mass.
Leah Higl
Yeah. Just insane to see those numbers together. So talking a little bit, we touched on it before, but energy expenditure and activity as we age. So, talking about another study, [00:07:30] they followed 516 older adults, aged 60 plus, for 12 years, to see how much their metabolism fell per decade. So after accounting for muscle and fat differences, per decade, women burned 20 fewer calories at rest, while men burned 70 fewer calories. So kind of similar to the previous study in its findings. What is really interesting about this study, is that both men and women were also less active, and burned 115 [00:08:00] fewer calories through activity per decade. So that really adds to the importance of maintaining activity as you age. That’s important if you want to maintain those higher calorie demands or that higher calorie budget.
Aidan Muir
And what’s really cool is that these numbers, they do stack up. As you can see, the numbers between these studies are actually, when you count for all these variables, are coming out kind of similarly. But over a 40 year timeframe, that’s like 400 and something fewer calories, [00:08:30] which does make sense. Like what if you have a knee replacement and like you struggle to get back to walking? Or there’s so many variables that go into it. And, I don’t know, outside of those are very specific examples they use, but most people as they age, do become less active for a variety of reasons. And it adds to the importance of, if you do want to keep calorie burn higher, the importance of staying active and also building muscle and stuff like that, which comes alongside staying active too.
Aidan Muir
Going into some other thoughts, because [00:09:00] that really sums up a lot of it. But then, there’s heaps more that I wanted to touch on as well. So how does menopause affect things?
Because that’s a common thing that a lot of people who read the Herman Pontzer study… If you just checked Instagram comments and stuff like that, and people were talking about it. That was the most common thing I saw. People were like, “How does menopause affect this?”
Just speaking from my thoughts, there’s clearly changes in body fat distribution. So somebody [00:09:30] could be X percent body fat the entire time pre-menopause and post-menopause, but it was previously more distributed throughout their body. And now it’s not as distributed because of the changes in estrogen, and it’s more around their stomach or whatever. So now they don’t feel as lean, even though they might have similar amounts of body fat. But then also premenopause and post-menopause activity can change. So many variables in life can change. What if you get worse sleep because you’re getting hot flashes and stuff like that? And then that carries over into food choices. What if your cravings changed because of that? [00:10:00] What if you are exercising less, or whatever, because you don’t feel was good? There’s a whole bunch of factors that go into this. That’s how I’d say menopause affects things. It’s heaps of variables from that, yeah?
Leah Higl
Yeah, 100%. And there’s also the fact that, just generally talking about women, that their total daily energy expenditure is lower than men typically. So those smaller changes in energy expenditure as we age, whether it’s behavioral or that little bit that comes from decrease in organ function and stuff, [00:10:30] probably more noticeable in women as they age anyway, because they’ve already starting with that lower calorie budget.
Aidan Muir
Yeah. It makes so much sense to me. I talk about that heaps with clients, like the classic husband and wife combo, where the wife’s like, “I’m eating the same as my husband, but he’s dropping a kilo per week and I’m barely changing.” But it’s just lower to start off with and if you’ve already got a kind of low budget to start off with in terms of calories and you take 100 away from it, it’s more noticeable than somebody who started at 3000.
Yeah. Something that we’ve been doing a little bit recently with clients is testing resting energy expenditure or resting metabolic rate. So, there’s a thing called indirect calorimetry and there’s businesses in most major cities, not in rural areas or anything like that usually, but in most major cities you can find somewhere that does test metabolism. And I am often big on testing this stuff for people who are convinced that they have a slow metabolism, because [00:11:30] I’m a big believer in confidence when it comes to nutrition and if you think the intervention you’re going to do is going to work, you’re more likely to execute on an intervention. And if you think that you have a slow metabolism or whatever, it’s hard to build that confidence. And testing can sometimes be useful there because two things can happen. One, the test could say that you don’t have a slow metabolism and it’s like, “Sweet.”
Leah Higl
Which is good to know.
Aidan Muir
That’s good to know. You’re on an even playing field and then you have to focus on all the other variables and everything that you would’ve had to focus on otherwise. [00:12:00] Or, it tells you that you do have a slower than expected metabolism and you have a lower total daily energy expenditure due to that. And now you know the barrier that needs to be overcome. For example, if your basal metabolic rate was going to be 1,700 calories for example, and what it actually is 80% of that, so it’s a little bit less than that, you now know that you might need to go 200 calories lower than previously to achieve the same outcome.
Or you could be [00:12:30] like, “Maybe I need to spend more time on higher calories to address that?” In some cases, people talk about reverse dieting or just spending time on maintenance calories or whatever. If you’d previously been on lower calories for an extended period of time, that could be something to think about. But I like that because it gives you the barrier that needs to be overcome, to a certain degree. It’s very hard to have confidence if you don’t know what you’re trying to overcome. And this kind of helps build that confidence.
Leah Higl
Yeah. Just having that, especially if it’s something you are questioning. You’re like, “Oh, I think I have a slow metabolism.” Just having [00:13:00] that yes or no answer, I think it helps a lot. So I’m glad we have access to something like that.
Aidan Muir
Yeah, for sure. And that’s why I mentioned the city thing. Because it’s like, I don’t know, you don’t need access to this. Because at the end of the day… [crosstalk 00:13:13]
Leah Higl
It’s not necessary to have.
Aidan Muir
It’s not necessary. It’s just nice to have. And then the biggest thing, and it’s really more empathy and everything like that comes into play, but I wanted to talk through this as well, is changes occur throughout life. That thing can’t really be debatable. But I think this is the biggest factor in metabolism [00:13:30] changes or whatever most people actually experience.
I don’t know how to go through this in a way that is like the easiest way to go through it. Like for example, when you are under 18 or whatever, or when you’re young, obviously there’s the growing variable, but you’re also not working. You’re not working. You probably have less stress. Then there’s activity. Like when you’re walking around school and stuff like that, you’re walking around versus driving everywhere, or now we’ve got to Uber [00:14:00] and everything like that. Another thing that I think is interesting, but as an adult, if you earn a decent income, you can eat whenever you want.
Like, do you remember when you’re a kid and you’d ask your mom or whatever, you’ll tell her you’re hungry?
Leah Higl
Yeah, like, “Can I have an afternoon snack?” And they’re like, “Oh, there’s apples in the fridge.” And you’re like, “Aw.” But as an adult now I’m like, “Yeah, I’m going to have that finger bun that I know is in the cupboard.”
Aidan Muir
Yeah, you’ve got access. You don’t have to wait until dinner. You can eat whenever you want. And that’s just one [00:14:30] of many variables. Alcohol, ideally most people aren’t drinking heaps of alcohol under the age of 18 or whatever. And then they add that variable in. Sleep, most people get better sleep when they’re younger as well. And then they get worse sleep because they have kids, or because they have jobs, or because once again, their parents aren’t making them sleep, or whole bunch of things. Caffeinated drinks, like coffees and stuff like that, usually people when they’re under the age of 18, they’re probably not having as much calories coming through that kind of stuff as they are over the age of [00:15:00] 18. Then body composition changes as we talked about with muscle and fat and everything like that. And then just activity, like when you’re in school, you might be playing sports and stuff like that. And then you might not, when you’re older and everything like that.
I’ve just touched on so many things, but the whole point I’m getting at is that variables change. You are not living the same life that you lived five years ago, no matter how close it feels or whatever. And even if it’s only like 200 calories different in terms of you eat 100 calories more and you move 100 calories less, because you get slightly [00:15:30] less steps or whatever it is, that’s enough to make a bit of a change. And what if you only gain a few kilos per year, for example. That’s a very small surplus separated out over a long period of time? Or what if you only gain a small amount over holidays and you just maintain for the rest of the year, for example? And then like that adds up over four years or something that. Is that a slow metabolism thing or is that another all these other variables adding up kind of thing? Because it only takes tiny changes for all of this to [00:16:00] happen very slowly over time.
Leah Higl
Yeah. And what I think is great and cool about that answer rather than, oh, our metabolism just inherently decreases as we age, is that we have control, to some degree, this stuff and over these variables. Sure, life is going to change and there’s sometimes you don’t have control over those things. But being active, trying to maintain muscle mass, trying to get sleep, trying to make good food choices. They are all somewhat in your control. It’s not like there’s this inherent thing that is really [00:16:30] significant that happens to you over time. You have some control over this stuff, which I think is a positive.
Aidan Muir
Yeah. I see it as empowering. Because there is a lot of people who I see who offhand will just say, “I know your metabolism slows down as you age.” And they just chuck it in, being like, “Oh, that’s out of my control.” And as you said, a lot of this stuff is within your control to a certain degree. There is stuff that’s out of your control, like it’s harder to build muscle post 40. There is stuff, but the thing that I think is super relevant, is [00:17:00] what percentage of 45 year old women start lifting weights for the first time and training hard, versus those who don’t. There’s a lot of easy wins that are still there, and I’m not saying that’s easy, that’s still very thing to do.
But staying active is an outlier activity. Not many people have a high level of activity. Getting good sleep is an outlier activity. Or even, I shouldn’t say it like that, I mean doing whatever is in your control to get good sleep. [00:17:30] We can’t control if kids are up all night screaming or whatever, but we can control little details, like whether we choose to stay up later or certain things like that. Trying to minimize stress and stuff like that. There’s some things we have a little bit of control. There’s stuff we can’t control, but if we focus on what we can control, I don’t know.
Leah Higl
Yeah, we have some control over this. So, you can do what you can, you’re not like at the grips of this thing that just happens to you.
Aidan Muir
Yeah, I think that’s a better description. I want to go with that one. I reckon we wrap it up after that.
Leah Higl
Yeah. Well [00:18:00] this is episode 50, which is a pretty cool milestone for us halfway to 100. So thank you for tuning in and we’ll be back week.