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By E. Dylan Mayer

For the last century, scientists have been trying to understand the molecular underpinnings of human metabolism. At first glance, it makes perfect sense to say if we take in fewer calories than we expend, we will lose weight; and if we consume more calories than we expend, we will gain weight. However, this concept has been increasingly challenged. I recently came across a New York Times article which discusses recent research challenging the way we have looked at metabolism.

“Metabolism is the running total of how busy your cells are throughout the day.”

The majority of people who count their calories do so in order to ensure they consume a certain amount of calories to lose, maintain or gain weight. They are solely looking at their caloric intake as a means of losing fat. However, according to Herman Pontzer, an associate professor at Duke University, 55 to 70% of what we eat and drink actually goes toward fueling all the chemical reactions in our bodies, keeping us alive. He defines metabolism as the “running total of how busy your cells are throughout the day”.

Scientists have struggled to measure metabolism in real-world conditions with enough people and in a wide enough age range. It is generally thought that the larger you are, the more cells your body is made of, and therefore the more energy you expend. However, this doesn’t consider how variables such as age, sex, lifestyle, and illness influence our energy expenditure. This lack of real-world evidence has led to other assumptions such as that significant hormonal changes, such as puberty and menopause, cause our metabolism to change, or that as we age our metabolism slows indefinitely.

In August of 2021, an impressive paper published in Science by Pontzer and over 80 co-authors showed just how wrong we were about understanding metabolism. Looking at data from 6,400 subjects who ranged from 8 days to 95 years old, adjusting for body size and fat/muscle distribution, they found that our metabolism goes through four distinct phases in life.

“The authors found that total energy expenditure increased with fat-free mass in four distinct life stages.”

Metabolism increases rapidly in neonates during the first year of life to ~50% above adult values (equivalent to a grown-up burning 4,000 calories per day). Metabolism then gradually declines through adolescence until age 20, at what point it levels off and remains stable until age 60. Even during menopause, the study found that women burn calories as efficiently at 55 as a person at 25. However, once you reach age 60, energy expenditure begins to drop and continues to do so until the end of our lives. Another interesting point the authors found was that men do not have innately faster metabolisms than women (a common, but erroneous assumption), but rather they tend to burn more calories because they are generally made up of more muscle cells, which use more energy than fat.

By measuring how much CO2 we emit as a byproduct, we’ve long been able to calculate how many total calories we burn. However, when the “doubly labeled water” method was discovered in the 80s, measuring the total calories burned became much more viable. Essentially, using this method subjects are given water in which the hydrogen and oxygen are labeled with isotopes (different number of neutrons in their nuclei – chemistry flashbacks, ahh!). After drinking this radioactively labeled water, the subjects under study can carry on their normal lives for some time, taking samples of urine, blood, or saliva every week. Their energy expenditure is measured by the rate at which they eliminate the labeled hydrogen, which passes intact through the body, while the labeled oxygen is exhaled as CO2. The proportion of labeled oxygen that is missing allows researchers to figure out how much CO2 was emitted and thus, the caloric expenditure.

Amongst the many interesting questions arising from the new study results are the question of how nutritional recommendations should change as you age, as well as if there is a link between the decline of metabolism after 60 and the progressive increase in chronic disease. One may also speculate about the potential role that our gut microbiome and reported changes in the microbiome play in the observed age-related metabolism changes.

“We can almost be considered different organisms when we are 5 years old versus when we are 60 years old.”

The reason for the title being named “Calories in Doesn’t Equal Calories Out?” is that based on the new information, we all require different amounts of calories based on what phase of metabolism we are in. It is important to note that we can almost be considered different organisms when we are 5 years old versus when we are 60 years old.

If you’re looking to implement this information in your life to help with your weight (gaining, losing, or maintaining), you can begin to progressively reduce your caloric intake after 60 years of age. I didn’t go into detail about what you should be eating, but if you’re nearing 60 and plan on reducing your caloric intake, you should make sure to include an adequate amount of protein (recommend plant-based protein to maximize fiber intake as well), for muscle maintenance and growth, to prevent frailty in your later years! See my blog post, “What You Need to Know About Age-Related Muscle Wasting”, for more information of how to prevent frailty!

E. Dylan Mayer E. Dylan Mayer is a graduate from the University of Colorado at Boulder with both a major in Neuroscience and minor in Business. He is fascinated by the interactions of brain, gut and microbiome, and the role of nutrition in influencing the health of our microbiome, as well as our own well-being.