Do We Have Less Energy as We Get Older — and If so, Why?

Unraveling the fascinating science behind the cliché

Photo: Lightfield Studios/Adobe Stock

It is a truth universally acknowledged that, once young kids fully master standing upright, they prefer to run instead of walking. As we adults scramble after them, we wonder aloud, “Why don’t I have that much energy?!”

The question is usually rhetorical, just something grownups say to each other as they roll their eyes. But really: why? Trying to answer this simple question will take us on a journey through some fascinating science experiments performed around the world.

You may remember learning about the Law of Conservation of Energy in high school, nicely summed up by this quote from the 2009 film Avatar: “[A]ll energy is only borrowed.” Basically, if kids have more energy, they had to have got it from somewhere. Where? Let’s take a look at suspect #1: Food.

Science Confirms That Kids Are Gluttons

Every few years, scientists around the world like to drop into random people’s homes, inventory their fridges and pantries, and grill the residents on what they ate during the past 24 hours. This is called a Health and Nutrition Survey, and many countries conduct one. [1,2] On the surface, the results of these surveys are pretty boring: fully grown adults in their early and middle years — in other words, humans who’ve reached their full height and attained peak muscle mass — consume the most calories. [1,3] That’s like saying it takes more electricity to power a high-rise building than a single-family bungalow. Duh.

A better way to look at calorie intake is relative to a person’s own body mass, and this reveals something remarkable: relative to their size, toddlers and preschoolers consume the most calories by far on a daily basis. For example, a boy between 2 and 5 years old eats an average of 100 kcal for every kilogram of his body weight every day. Compare that to a man in his twenties, who consumes only about 28 kcal for every kilo of his body weight — that’s almost a four-fold difference! [1,4,5] In fact, scientists suspect that kids’ intake may be even higher, since kids (or their parents) may easily forget about some of the kids’ numerous snacks during recalls. [6]

When it comes to the elderly, the decline in their energy intake tends to mirror their decreasing body mass. However, relative to their weight, people over 60 do consume slightly fewer calories than younger adults, partly because they tend to eat foods that are less energy-dense (such as those with less saturated fat) and that contain more fiber. [7]

Okay, so kids really do get more energy from food relative to their size. But what for? Is it just so they can burn it off in frenetic physical activity?

Kids Move More, but That’s Not the Whole Story

Even the most energy-efficient bus in the world will use up more gas than a small sedan to drive the same distance. That’s my clever way of saying that, to be able to compare energy use in kids and adults, we need to look at their energy spending relative to their mass.

Researchers in the UK found that 4-to-7-year-olds use up 87 kiloJoules a day for every kilogram of their body weight on physical activity. By contrast, a person between 71 and 91 years old expends less than half as much. Basically, the older we get, the less energy we use up on moving each kilo (or pound) of our bodies. [8]

Are grownups in the UK just particularly couch-bound? Probably not: studies in other developed countries have come up with similar numbers. When it comes to developing countries, adults there do engage in more physical activity but still can’t compete with kids in terms of energy used up on moving around. [8]

So is that the big reveal: kids eat more so they can move more? Not so fast.

What’s the Deal with Metabolism?

People like to bandy about the term “metabolism,” as in, “She is just lucky to have a fast metabolism.” But how do you know how fast your metabolism actually is? And how does the metabolism of adults compare to that of kids?

For years, scientists have tried to estimate the so-called basal metabolic rate (BMR, not to be confused with the BMI). The BMR is the amount of energy needed for a person just to stay alive and functional without performing any activity. If you’ve ever used an app to calculate how much to eat to maintain your current weight, you can be sure that BMR was behind the numbers. In fact, researchers estimate that BMR accounts for at least half of our total energy use every day. [9]

Measuring BMR is rather tricky. To get an accurate reading, the person being studied must be calm, well-rested, lying down in a comfortably warm room, and familiar with the lab environment. Any physical or emotional discomfort may speed up the heart rate and drive up calorie use. In addition, since digesting food takes energy, the study subject shouldn’t have eaten anything in the past 10–12 hours. [10] In other words, we want him or her doing absolutely nothing in a state of zen (which may be rather difficult with a rumbling stomach).

Then, scientists use various clever methods to figure out exactly how much energy the person is using at each moment in time. For example, one way is to track the exact amount of carbon dioxide in the room where the person sits, alone, with the doors and windows closed. This works because, much like a burning fire, we produce carbon dioxide when we “burn” calories.

Numerous studies have measured BMR in a grand total of over 10,000 people of various ages. And — are you ready for this? Kids burn more energy per kilo even when they’re doing nothing! [10,11]

Estimates for kids under six years old are hard to find, probably because young kids can’t sit still long enough for the test, but for those six and older, the trend is very clear: Kids have a “fast metabolism,” and the older we get, the more our metabolic rate decreases.

Of course, the million-dollar question is why. The term “metabolism” captures all the tiny chemical reactions happening in our bodies every second. These include a constant turnover of molecules as our bodies break down injured cells, build new tissues, and reroute connections in our brains. Since kids are still growing and developing, it’s a safe bet that their bodies are furiously building even when they’re sitting still. And have you ever noticed how quickly a scratch heals in a 2-year-old compared to an adult? Fast metabolism strikes again!

As we reach our senior years, not only does out metabolism continue to slow down, but something else strange happens as well: ordinary activities begin to require more energy.

The Tortoise and the Hare

One group of researchers measured the amount of energy needed for adults to walk at a comfortable pace for a few minutes. What they saw was unexpected: Even for people who have no particular health complaints, the energy “cost” of walking a given distance increases sharply after age 65. It rises even more after age 80. The researchers found that, to make up for this increase in strenuousness, the elderly slow down accordingly. [12] In other words, believe it or not, the briskly striding 30-year-old and the shuffling 80-year-old on the same sidewalk are both burning about the same amount of calories at any given time!

By now, you may be feeling a bit discouraged. The youngsters seem to hold all the cards: they get to eat constantly and still burn it all up, they‘re always in motion, and yet they don’t get tired as fast as grownups do either. [13] At the other end of the spectrum, the elderly seem to have got the short end of the stick. But you know the moral of the fable about the tortoise and the hare. The elderly may have less energy, but they also don’t waste nearly as much of it.

In fact, two recent surveys found that seniors tend to feel less tired and more satisfied with their lives than younger adults. [14,15] Older adults report having more energy for social activities. Scientists suspect that this is because older people tend to restrict their social interactions to a small circle of meaningful connections. When it comes to physical activity, though seniors have less energy for exercise in general, they feel just as energized as younger adults do for their chosen pursuits. [14]

Of course, this rosy picture of the golden years might also have something to do with where the research was conducted: Sweden and Switzerland. According to the Economist Intelligence Unit’s “Where-to-Be-Born Index,” Switzerland ranks first and Sweden fourth among countries where one can expect to live a long, healthy, and prosperous life.

So what if we want to be energetic in our senior years, but moving to Switzerland isn’t an option? The answer is annoyingly banal: eat well and move your body. Research suggests that seniors tend to fall short in their intake of protein, vitamins B12 and D, iron, and calcium, so pay special attention to those.[17] Fortunately, biology seems to help us out by making us less fond of sweets and more tolerant of bitter-tasting foods as we get older, increasing the chances that we will choose healthy meals. [7]

Another thing to watch out for is sitting too much, as those who spend a lot of time sitting tend to feel the most fatigued. [15] While this can be a chicken-or-egg problem (are you tired because you’re sitting, or are you sitting because you’re tired?), one study found that a period of sitting predictably brings about a drop in mood and energy, resulting in a vicious cycle. [16]

So the next time a youngster zooms past you, take heart. While you may no longer have the energy to run endless laps around the house, with care and a bit of luck, you can have the energy for the activities that matter.

References

  1. Centers for Disease Control and Prevention (CDC). National Center for Health Statistics (NCHS). National Health and Nutrition Examination Survey Data. Hyattsville, MD: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, 2018. https://www.ars.usda.gov/northeast-area/beltsville-md-bhnrc/beltsville-human-nutrition-research-center/food-surveys-research-group/docs/wweia-data-tables/
  2. Pan, K., Smith, L.P. et al. Increased energy intake and a shift towards high-fat, non-staple high-carbohydrate foods amongst China’s older adults, 1991–2009. J Aging Res Clin Pract 3(2): 107–115 (2014). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4315239/
  3. Wakimoto, P. and Block, G. Dietary intake, dietary patterns, and changes with age: an epidemiological perspective. The Journals of Gerontology Series A: Biological Sciences and Medical Sciences 56(suppl 2): 65–80 (2001). https://pubmed.ncbi.nlm.nih.gov/11730239/
  4. Centers for Disease Control and Prevention (CDC). National Center for Health Statistics (NCHS). CDC Growth Charts. Hyattsville, MD: U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, 2016. https://www.cdc.gov/growthcharts/cdc_charts.htm
  5. I calculated average daily intake from NHANES data on daily food consumption [1] and CDC data on average weight [4]. For those under age 20, “average daily intake” for each age group corresponds to the mean daily intake divided by the median weight for that age group. For those 20 and older, “average daily intake” corresponds to the mean daily intake divided by the mean weight for that age group.
  6. Zhou, Q., Wang, D. et al. The use of wearable cameras in assessing children’s dietary intake and behaviours in China. Appetite
    139: 1–7 (2019). https://pubmed.ncbi.nlm.nih.gov/30946865/
  7. Drewnowski, A. Sensory control of energy density at different life stages. Proc Nutr Soc 59(2): 239–44 (2000). https://pubmed.ncbi.nlm.nih.gov/10946792/
  8. Brage, S., Lindsay, T. et al. Descriptive epidemiology of energy expenditure
    in the UK: findings from the National Diet and
    Nutrition Survey 2008–15. International Journal of Epidemiology 49(3): 1007–1021 (2020). https://pubmed.ncbi.nlm.nih.gov/32191299
  9. Wong, W.W., Butte, N.F. et al. Are basal metabolic rate prediction equations appropriate for female children and adolescents? J Appl Physiol 81(6): 2407–14 (1996). https://journals.physiology.org/doi/full/10.1152/jappl.1996.81.6.2407
  10. Henry, C.J.K. Basal metabolic rate studies in humans: measurement
    and development of new equations. Public Health Nutrition 8(7A): 1133–1152 (2005). https://pubmed.ncbi.nlm.nih.gov/16277825/
  11. Maffeis, C., Schutz, Y. et al. Resting metabolic rate in six- to ten-year-old obese and nonobese children. The Journal of Pediatrics
    122
    (4): 556–562 (1993). https://www.sciencedirect.com/science/article/abs/pii/S0022347605835358
  12. Schrack, J.A., Simonsick, E.M. et al. The role of energetic cost in the age-related slowing of gait speed. Am Geriatr Soc 60(10): 1811–1816 (2012). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3470763/
  13. Ratel, S., Lazaar, N. et al. Age differences in human skeletal muscle fatigue during high-intensity intermittent exercise. Acta Pediatrica 92(11): 1248–1254 (2003). https://pubmed.ncbi.nlm.nih.gov/14696843/
  14. Cardini, B.B. and Freund, A.M. More or less energy with age? A motivational life-span
    perspective on subjective energy, exhaustion, and
    opportunity costs. Psychol Aging 35(3): 369–384 (2020). https://pubmed.ncbi.nlm.nih.gov/32077733/
  15. Yannakoulia, M., Mamalaki, E. et al. Eating habits and behaviors of older people: Where are we now and where
    should we go? Maturitas 114: 14–21 (2018). https://pubmed.ncbi.nlm.nih.gov/29907241/
  16. Engberg, I., Segerstedt, J. et al. Fatigue in the general population — associations to age, sex, socioeconomic status, physical activity, sitting time and self-rated health: the northern Sweden MONICA study 2014. BMC Public Health 17:654 (2017). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5557471/
  17. Giurgiu, M., Koch, E.D. et al. Sedentary behavior in everyday life relates negatively to mood: An ambulatory assessment study. Scand J Med Sci Sports 29(9): 1340–1351 (2019). https://onlinelibrary.wiley.com/doi/abs/10.1111/sms.13448

Maria has a Ph.D. in Neuroscience and writes depth pieces about the biology of humans and other animals. Follow her on Twitter @MariaTerScience

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