Metabolism is the chemical conversion of food into usable energy by the body. To maintain a healthy balance, the fuel we consume is meant to supply our daily energy needs. Changes in metabolism from birth to elderhood are complex. It is a popular area of research because these shifts aren’t fully understood. We do know that how fast we use energy – called our metabolic rate – depends on age, sex, height, and weight.
Energy needs increase in one-year-olds to over 50% of adults.
Energy expenditures fall soon after the first year of life and reach adult levels around age 20.
Between ages 20 and 60, energy demands are stable, even during pregnancy and perimenopause.
After age 60, metabolism declines in response to shifts in body composition, physical activity, and organ metabolism.
Food has thermic effects and impacts metabolism.
Factors that control our resting or basal metabolism are:
- Organ metabolism – e.g., the energy needs of our brain and liver.
- Body size – larger bodies require more fuel.
- Body makeup – fat vs. fat-free mass.
A 2021 study looked at the daily energy expenditures of humans ages 8 days to 95 years old. The first key term to understand from these results is that “fat-free” mass represents the body’s bone, muscle, organ, and water content; essentially all tissue other than fat. Secondly, basal and active metabolic rates make up our total energy expenditure (TEE).
Researchers presented the following major findings in four life stages.
Newborn to one year old
Researchers discovered that neonates, who are less than a month old, have the same TEE as adults. Between months nine and 15 of life, however, this measure increases rapidly to 50% more than adult levels. This is reflective of the intense growth of organs like the brain as well as the changes in body composition during this developmental period.
Ages 1-20 years
Energy expenditures increase substantially during the first year of life, but soon after, the metabolic rate begins to decline slightly every year until about age 20. Despite this overall decline in TEE compared to infancy, the energy needed to fuel the body increases alongside fat-free mass between years one and 20. After this age, TEE begins to stabilize.
Not surprisingly, males have slightly greater metabolic rates than females during this life stage. And yet, sex has little to do with the rate of decline in TEE during this time. There is no significant spike during puberty (the development of secondary sex characteristics and the ability to reproduce) in either males or females.
Ages 20-60 years
Metabolism is stable through this life stage including during reproductive changes for women. Data suggests that energy expenditures not only remain stable for both men and women but also, in the latter group, despite a pregnant, post-partum, or menopausal state. These findings challenge popular beliefs that:
- a pregnant woman needs to consume significantly more calories to nourish themselves and a growing fetus.
- gaining weight, particularly around the belly, is an expected part of menopause that cannot be helped.
Other changes in women like hormones may increase the likelihood of weight gain, but study results do not point to metabolic shifts as a direct cause.
Over 60 years old
Several changes in the body result in a decline in TEE after age 60. Generally speaking, these include a reduction in physical activity and lean muscle mass. This is why strength training is highly recommended for older adults. Organs, too, factor into a fat-free tissue mass and therefore energy needs. With advancing age, organs like the brain and liver have fewer demands. For example, after the age of 90, energy expenditures are 26% less than middle-aged adults.
What role does food play?
The types of food we eat at any age have a direct impact on our metabolism. To further explore this concept, Julia Zakrzewski, registered dietitian, writer, and podcast host explains: “Your food choices are one of many factors that contribute to your metabolic rate. We refer to this impact as the thermic effect of food (TEF). It refers to the amount of energy your body uses to digest food and absorb essential nutrients. Typically, a greater amount of energy used to burn food means your metabolism is stimulated and prompted to do work, which is desirable for most people!”
Zakrzewski points to protein-rich foods as the kind of fuel most likely to elicit the greatest TEF response. This is compared to the other macronutrients such as carbohydrates and fat. Therefore, to see an improvement in metabolism, people might consider including a form of protein with every meal. Protein sources to try:
- Lean cuts of meat
- Fresh or tinned fish
- Pulses and legumes (chickpeas, lentils, kidney beans)
- Unsalted nuts
- Greek yogurt
An important consideration when making this change is to “choose whole foods whenever possible over processed protein powders and bars,” encourages Zakrzewski. This ensures you are getting the best quality nutrition as it comes from the original nutrients in the food itself. “Changing it up and including a variety of protein sources in the diet will help you meet your vitamin and mineral requirements as well,” she says.
Throughout life, many factors contribute to our overall energy needs. The stages of development that include periods of greater growth require more energy than those later in life. We are born with the same energy requirements as we have as adults. As we grow past a month old, this quickly changes to 50% over what we’ll need as adults. After we turn a year-old, our metabolic needs decline slowly, leveling off to adult levels by age 20. Until we turn 60, data suggests our energy expenditures are stable. In later life, changes in activity, body composition, and organ demands result in a lower metabolism. Adding protein to our diet is one intervention to improve our metabolic rate.