You hit a certain age, then the years and the decades just seem to roll by. And all of those years rolling by tends to come with another kind of roll: those stubborn ones right around your waistline. But what can you do? You’re not 25 anymore, and at this point, it’s all about your metabolism, right? Actually, you might be wrong about that! In fact, what many of us tend to think about metabolism and aging might be wrong, and blaming that “middle-age spread” on your metabolism might mean you’re doing yourself a disservice: you’re not getting to the real root of the issue and making the healthy changes you should be. So what does the latest science say about metabolism and aging, and why does it matter?
The Study
Remember the days when you felt like you could eat all the late night pizza you wanted and it didn’t matter? Then middle-age and/or menopause hit and you felt like you were packing on the pounds and you couldn’t help it? Or how about that old “eating for two” when you’re pregnant thing, or having a hard time losing the baby weight, and envying guys for being fat-burning machines? Turns out, all of those assumptions (like our metabolism changes as we hit milestones in our life like puberty or menopause, or that men have “better” metabolic rates than women do) are all pretty much wrong.
And how do we know this? A major, groundbreaking study published in the journal Science this year that suggests your metabolism, the rate at which you burn calories, actually peaks much earlier and starts its inevitable decline later in life than you might think.
It turns out that, before this study, we actually knew relatively little about how metabolism really works. According to the study’s principal investigator, Herman Pontzer, an evolutionary anthropologist at Duke University, “It was really clear that we didn’t have a good handle on how body size affects metabolism or how aging affects metabolism. These are basic fundamental things you’d think would have been answered 100 years ago.” So you’re not alone!
But now, after 80 study co-authors combined efforts from a half dozen labs collected over 40 years, there now seems to be sufficient information to ask general questions about changes in metabolism over a lifetime. The researchers analyzed the average calories burned by more than 6,600 people, ranging in age from 8 days old to 95, as they went about their daily lives. And, while most previous large-scale studies measured how much energy the body uses for basic vital functions like breathing, digesting, and pumping blood (which only account for 50-70% of the calories we burn each day), this massive, combined study had the money and resources to actually look at the energy we expend doing all those little everyday tasks, like washing the dishes, walking the dog, breaking a sweat at the gym, and even just thinking or fidgeting.
To get the most accurate info on how our bodies use energy, they used the gold standard “doubly labeled water” method. This involved measuring calories burned by tracking the amount of carbon dioxide a person exhales during daily activities. They also had participants’ heights and weights and body fat percentage, which allowed them to look at fundamental metabolic rates. They knew that a smaller person would burn fewer calories than a bigger person, but they wanted to find out, correcting for size and percent fat, were their metabolisms different?
And what they found blew them away, because it challenged assumptions about how our metabolisms work, especially when it comes to how they change as we age.
The Findings
According to one study co-author, Jennifer Rood, PhD, Associate Executive Director for Cores and Resources at Pennington Biomedical Research Center, “As we age, there are a lot of physiological changes that occur in the phases of our life such as during puberty and in menopause. What’s odd is that the timing of our ‘metabolic life stages’ doesn’t appear to match the markers we associate with growing up and getting older.”
In other words, the study found that the things that we would expect to change our metabolism, such as growth spurts during puberty, turning 30, 40, or even 50, pregnancy, and menopause, didn’t really do all that much. What they did find was that metabolism was different for everybody, but that there are four distinct stages of life when it comes to how much we’re individually burning:
- Infancy – While newborns actually have metabolisms similar to their mothers for the first month of their life, their metabolism shoots up after that, and by age 1, babies are burning 50% more energy than adults.
- Age 1 – Age 20 – Metabolic rate gradually starts to decline after age 1, decreasing by about 3% a year until age 20.
- Age 20 – Age 60 – Now here’s where things really get wild. This is when you’d guess that metabolic rate really drops off, right? Wrong. Between these ages, metabolism is actually at its most steady.
- Age 60 and over – The data suggest that our metabolisms don’t really start to decline again until after age 60, and that the slowdown is gradual, only 0.7% a year – and a person in their 90s needs 26% fewer calories each day than someone in midlife.
In addition, the study found that, controlling for body fat and muscle percentage, women’s metabolisms were essentially the same as men’s. They also found, maybe most unsurprisingly of anything in the study, that individual metabolic rates varied significantly: some subjects had rates 25% above average for their age, while others had rates 25% below average.
So what does this all mean? Well, it might leave you scratching your head, or maybe shaking it in disbelief, because your experience of slowdown and/or weight gain from your 40s on seems very real. Even Pontzer admits, “I’m in my 40s, so I expected to see some evidence to back up my subjective experience that my metabolism is slowing down. It feels that way to me! But it’s not really what’s happening.” So now what?
What This Means for You
Maybe calling middle-age spread a myth is going too far. After all, weight gain as we age is very real for a lot of people: in fact, research shows the average U.S. adult gains one to two pounds a year from early to middle adulthood. But this research shows us that there are contributing factors other than metabolism. According to Pontzer, “Your stress level, your schedule, your hormone levels, your energy levels are different in your 40s or 50s compared to your 20s. If you’re gaining weight it’s easy to say, ‘Oh, that’s my metabolism.’ It’s almost like a scapegoat. Now that we know it’s not metabolism, we can focus on some of those other factors.”
Research has also shown that metabolism and weight aren’t always as closely linked as you imagine. “It’s not about how many calories you burn, it’s about whether you’re burning more than you’re eating,” says Pontzer. “Just because you have a high metabolism doesn’t mean you’re better at matching your intake to your output.”
So that means sticking to the basics of maintaining a healthy weight:
- Eating a healthy, balanced diet consisting primarily of whole foods in the form of fruits, vegetables, lean protein, and whole grains.
- Maintaining an active lifestyle with a goal of at least 150 minutes of physical activity a week. This includes focusing on strength training to increase or maintain lean muscle mass. Lost muscle mass as we get older may be partly to blame for weight gain, the researchers say, since muscle burns more calories than fat.
- Getting enough sleep, which for most people means 7-8 hours a night.
- Managing stress through mindfulness, meditation, or other relaxing activities.
But beyond looking at the question of why we gain weight as we age (which maybe this study didn’t really help to answer!), the study’s findings could also be important for your health in other ways as you age. For example, knowing how the body’s metabolism works could help doctors treat cancer more effectively. The study also sheds new light on the aging process, specifically how cell activity changes as you grow older. “There’s an age-related decline that happens across the body’s systems,” says Pontzer. “One of the exciting things from the study is now we have a map of how this change is happening at the metabolic level — because metabolism is a reflection of how busy your body is.”
Knowing what’s happening in our cells as they age could help us figure out how to deal with all of the diseases that seem to come with aging. According to Rozalyn Anderson, professor at the University of Wisconsin School of Medicine and Public Health Anderson, who studies the biology of aging, “Around age 60 is when we start to see the emergence and increased risk for age-related conditions like cancer, cardiovascular disease or neurodegenerative diseases. When I saw this data, I was immediately struck by the fact that there’s also an intrinsic change in innate metabolism that begins at the same time.”
The bottom line is, researchers have brought us another step closer to the “how’s” of metabolism, but not really the “why’s.” It might be fascinating, or even hopeful, to know that, if you are gaining weight as you age, it’s not just an inevitable change in your metabolism – or it might be incredibly frustrating! If that’s not the reason, then what is?! Well, keep watching this space, as they say: researchers are hard at work. And, while you’re waiting, make any little changes to your lifestyle that could benefit your health (and your waistline), and know that they might just crack the code one of these days.
