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Do We Have Less Energy as We Get Older — and If so, Why?
Unraveling the fascinating science behind the cliché
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]
