Energy fuels our growth and maintenance. How exactly that changes, and the relative contributions of exercise and metabolism over time, is difficult to ascertain. A new study provides some answers. It is both dynamic and, as is often the case, nuanced.
Scientists have developed multiple techniques to measure our energy expenditure and assign portions to our metabolism, our body's maintenance, and the energy necessary to go from the couch to the refrigerator and back. Most measurements require us to be in a laboratory, hooked up to a measuring device; we are “caged.” A new study makes use of a measurement technique that allows us to be “cage-free” to assess our free-range energetics better. First, let’s consider the underlying measurement, then we will have the context to understand what the research found.
“The method allows the measurement of CO2 production over a period when the subject is free-living and unencumbered by any apparatus. …the measurement gives a reflection of free-living energy expenditure. It is regarded as the gold standard assessment method.”
Carbon dioxide forms as we create energy in our bodies from food. If we were to measure the amount of carbon dioxide we produce, we could measure the energy we create. But food only contributes one of the two necessary oxygen molecules present in carbon dioxide. The other oxygen molecule comes from the 60% of us that is water. We can label water by changing the number of neutrons in the oxygen, 18O, and hydrogen atoms, 2H, making it slightly radioactive so it can be traced. This double-labeled water (DLW) continues to act metabolically, just like unlabeled water.
While the oxygen molecules in our body water are lost primarily through metabolism, some of those molecules are converted into bicarbonate, making the correlation between oxygen created and energy inaccurate. The labeled hydrogen, deuterium, in the DLW is lost only with body water, from our urine and evaporative losses. Knowing the amount of deuterium lost allows us to compensate for the non-energetic losses of oxygen and calculate how much of the carbon dioxide we produce comes from energy formation. 
The DLW is our most reliable way of determining our energy expenditures, but for a long time was quite cumbersome and costly. The current study is the largest one utilizing this technique and reflects a collaboration by the International Atomic Energy Agency of the United Nations.
The amount of energy we expend daily reflects the energy needs to maintain our body’s form and function (basal metabolic expenditure) and account for the energy we expend in motion, walking about. The problem is further confounded by our body type, age, and gender. This study was designed to tease out and correct for those variations. Our body type, i.e., weight, consists of the fat present along with the fat-free mass – our bones, muscles, and organs. Fat is not significantly metabolically active, so fat-free mass is a good way of correcting our body weights.
The study involved 6,421 participants with these DLW measurements, across 29 countries, from eight to 90 years.
- During our first year of life our energy expenditure is very similar to adults. But quickly, both the basal and total energy expended Ithe difference between the two as a reminder, reflects our “walking-about” activity rapidly accelerates. By the age of 9 months or more, energy expenditure is 150% of that of adults.
- Between ages 1 and 20, energy expenditure continues to rise, but at a slower rate, as we “coast” up to what we become our adult energy expenditure. Two fun facts, puberty did not seem to alter energy expenditure, although any parent can tell you it changes attitude. And while males had higher energy expenditures, there was no gender difference in the changing rates of energy expended.
- For adults, those between 20 and 60, energy expenditure seemed to plateau. Pregnancy had no effect, nor did gender in the stability of energy expended.
- For those of us over 60, energy expenditure began to decline. Part of this is due to the loss of muscle mass with aging. Still, even with that taken into consideration, an additional percentage point or two of energy expenditure is lost with every birthday. At 90 years of age, our energy expenditure is 80% of our adult values.
The researchers also modeled the relative contributions of activity and metabolism to these observations. They found their model was more accurate when it took both factors into account, inferring two important takeaways. First, our energy expenditures due to exercise and exertion peak in our adolescence. We, that is, the adults in the room, could all benefit from more physical activity. Second, the decline in energy expenditure at age 60+ is, in part, related to the diminishing needs of our organs – they are no longer as energy productive as they had been; they, along with “us,” are in decline.
As an accompanying commentary points out, we now can begin to quantify our changing energetics. Children and juveniles are not small adults; they have very different energetic and metabolic profiles – factors we need to better understand and account for if we are to achieve the goal of “precision medicine." Same for adults and those of us who are “seniors.” Our maintenance energy needs, predicated upon the needs of our organs and muscles, decline as with age, as they write:
“It cannot be a coincidence that the increase in incidence of non-communicable diseases and disorders begins in the same time frame.”
Older adults are not the same as those in “their prime.” What may or may not be beneficial for adults may differ for the seniors amongst us. Understanding disease and treatments based solely upon young mice and rats may differ from the findings in more elderly research animals. As always, the deeper we search, the more we learn how complex our biology is.
 If that explanation leaves you a bit lost, you are not alone. Even in medical school, it takes a bit of time to wrap your head around the biology and calculations. You can do a deeper dive here or here.
Source: Daily energy expenditure through the human life course Science DOI:10.1126/science.abe5017
Taking the long view on metabolism Science DOI: 10.1126/science.abl4537