“Our paper provides the first road map of metabolism across the life span,” says study co-researcher Herman Pontzer, professor of evolutionary anthropology at Duke University in Durham, N.C., and author of “Burn.” “Metabolism is incredibly steady from 20 to 60 years old, despite the widespread perception of our metabolisms slowing as we age,” Pontzer says.
Let’s look at how the researchers came to their conclusions and what it means for efforts to maintain a healthy weight throughout life.
Why are these results so significant?
Metabolism is the process your body uses to break down food and convert it to energy. Past studies of metabolism and aging weren’t able to provide a clear understanding of how the two are related; their sample sizes weren’t large enough because of the expense of the best method for measuring energy expenditure, known as the doubly labeled water technique.
To overcome the hurdle of conducting costly research, more than 80 co-authors pooled 40 years of studies for this report. Data from more than 6,400 participants, ranging in age from 8 days to 95 years, provided a wide-angle view of metabolism over many years, and the researchers found that the changes in metabolic rate across the life span were larger than expected.
According to the new study, metabolism can be viewed as four distinct life stages: Energy expenditure accelerates rapidly to 50 percent above adult values at 1 year of age; declines slowly to adult levels by age 20; remains stable between ages 20 and 60; and declines steadily after age 60.
“We knew going into the study that body size, particularly lean mass, would be the biggest factor affecting the calories burned each day: Larger people burn more energy,” Pontzer says. “Our study was able to show the relationship with size very clearly and then ask new questions: Once we account for size and fat percent, how do age and sex affect metabolism?”
Learning that metabolism slows down after age 60, rather than at 30 or 40, was a surprise to the researchers, but so was the difference found between males and females. It’s long been assumed that men have a “faster metabolism” than women, but Pontzer says that theory was not really supported by the research.
“Men do burn more calories per day than women, on average, but only because men tend to be larger and carry a bit less body fat,” Pontzer says. “Once you control for those differences, their metabolism is the same. A man and woman of the same body weight and same fat percent will have the same expected metabolic rate.”
But I did gain weight in my 40s
At this point, many of you may be recalling weight you put on in midlife and thinking: “This study is wrong!” And perhaps you did gain weight after age 40. But this study suggests it was for reasons beyond metabolism, perhaps a change in lifestyle, diet, exercise levels or hormones, or a medical condition.
Or, you may be an outlier to the population-based results. The researchers’ data showed that some people had metabolic rates 25 percent below or above the average for their age. But these outliers did not change the overall pattern of metabolic rates over the life span.
Can't I just boost my metabolism?
If you’ve spent years buying metabolism-boosting supplements such as cayenne, caffeine and green tea, you’re wasting your money. “Our paper provides more support for the view that our metabolism is hard to budge: Our bodies follow a programmed course throughout our lives, and there’s not a lot we can do to change the energy burned per day,” Pontzer says.
Catherine M. Champagne, a professor of nutritional epidemiology at the Pennington Biomedical Research Center at Louisiana State University in Baton Rouge, says there is “inadequate research on supplements used to boost metabolism. The evidence for supplements such as green tea or cayenne stems from anecdotal promotion — not from evidence-based research.”
Champagne explains that products such as caffeine and ephedrine — touted for metabolism-boosting and weight loss — may negatively affect metabolism, and are potentially dangerous for some.
Does exercise help boost metabolism? It may help a bit. Yoni Freedhoff, associate professor of family medicine at the University of Ottawa and medical director of the Bariatric Medical Institute in Ottawa, says resistance training that increases muscle mass can help boost metabolism, but he adds that the increase isn’t dramatic. Furthermore, he says, “we decondition quickly following injury or life getting in our ways.”
Then what causes weight gain?
I wasn’t surprised to learn that a slow metabolism isn’t a leading cause of weight gain at midlife. In my 22 years as a dietitian, I’ve learned how complex weight control can be, and I know that obesity can be caused by many different factors. The calories-in-calories-out theory (you lose weight when you consume fewer calories than you burn) is always important, but it’s not the whole story for weight control. It doesn’t account for how different foods affect your hormones, nutrient needs and satiety levels.
“Studies have shown that the onset of obesity is frequently triggered by behavioral, medical and socioeconomic factors,” says Mary L. Rosser, director of integrated women’s health in the Department of Obstetrics and Gynecology at Columbia University Irving Medical Center in New York City. “Sedentary lifestyle, poor diets, stress, mental health, sleep deprivation, medical issues such as hypothyroidism, PCOS [polycystic ovary syndrome], diabetes and others contribute.”
For those other factors, I’d add genetics, medications, muscle loss (sarcopenia), hormones, the gut microbiome and all of the social determinants of health. The last category includes access to and quality of education, health care, neighborhood supports, economic stability, gender, race and more.
Freedhoff points to unhealthy food environments as another troublesome factor, saying: “There is no event too small to be celebrated with food, where ultra-processed foods and their advertising is ubiquitous.”
Rosser also believes weight changes in women between age 20 and 60 are tied to pregnancy and menopause. Weight gained during pregnancy isn’t always lost, and multiple pregnancies often account for weight increasing at mid-age.
And menopause, a normal part of aging marked by a decrease in estrogen and progesterone, also may be related to changes in body composition and fat distribution. “Changes related to fluctuations in these hormones begin to occur in the mid- to late 40s and may last for four to 10 years,” Rosser says. She adds that estrogen therapy does not prevent weight gain in postmenopausal women, although it may minimize fat redistribution.
“Several studies have shown that perimenopause, independent of age, is associated with increased fat in the abdomen as well as decreased lean body mass,” Rosser says. This explains the transition from a pear-shaped body, with more weight below the waist, to an apple-shaped body, with more weight above the waist.
How can I control my weight?
There are as many methods to lose weight as there are reasons we gain weight in the first place. But first, consider whether that excess weight is putting your health at risk. Remember: A number on the scale alone is not an adequate determinant of your overall health. Maybe your body size is exactly right for your age, and you can learn to love it, even if you’re a few pounds heavier than you were at age 30.
If your weight gain is unhealthy or troubles you for other reasons, the key to losing it will be figuring out why you gained it. You can rule out a slow metabolism, but it’s worth investigating your diet, exercise patterns, sleep schedule, stress levels, hormones and overall health. Consider keeping a diet, exercise and sleep journal to get a clear picture of your current habits, then find a sustainable plan that works long-term. (So, not a fad diet.)
Although it may be frustrating to learn that you can’t blame extra pounds on your metabolism, it also offers an opportunity to dig deeper and learn more about your weight. A health-care professional can help you determine whether lifestyle changes are necessary or whether you require a medical consultation for a more complex weight-related issue involving poor food environments, hormone issues or unresolved past traumas that affect your eating habits. Look for a practitioner who does not have weight stigma and who practices with unbiased, intuitive eating principles in mind.