Is aging to blame for mid-life weight gain? A new study says think again.

 “Science is not truth. Science is finding the truth. When science changes its opinion, it didn’t lie to you. It learned more.”

Many folks read headlines and think scientists never get it right. A new study published in Science changes what we know about aging and weight gain. Some people won’t’ like the bottom-line of the research. A collaborative team of researchers looked at energy expenditure…. or how many calories we burn each day… throughout the life course. The new study looked at energy expenditure through the life cycle…from over 6400 people from 29 countries, ranging from 8 days to 95 years.

 First a quick refresher. Humans burn calories mainly in three ways:

  • Basal metabolism is the energy we burn for life-sustaining processes like breathing, keeping our heart beating, and repairing our body’s tissues. It is the largest portion of total energy expenditure, about 50-70%.
  • A smaller percentage, about 10%, is called the thermic effect of food. It is the amount of energy needed to digest food…. digestion, absorption, transport, and assimilation of the food we eat.
  • Exercise or physical activity is another way of expending energy, and it is highly variable. For an inactive person who spends most of the day in front of a computer screen, the percentage is quite low. For an ultra-endurance athlete, it is high. It runs the gambit between the two. A sub-component of exercise energy expenditure is called NEAT for nonexercise activity thermogenesis…a fancy name for energy expended outside of regular physical activity, such as fidgeting. We all know someone who can’t sit still…the leg shaker, the hair twirler…. they burn more calories than those of us who are good at sitting still!

The researchers found four distinct phases for energy expenditure:

  • From birth to 1 year of age
  • Juveniles, from 1 to 20 years
  • Adulthood, from 20 to 60 years
  • Older adults, over the age of 60 years
Rhoads & Anderson, Science, August 13, 2021

In the first year of life, energy expenditure is high, as might be expected with the rapid growth and development that occurs in neonates. “Between 9 and 15 months of age, energy expenditure is nearly 50% elevated compared with adults.”

Energy expenditure continues through the teen years and plateaus around age 20. In adulthood, energy expenditure remains relatively constant and stable from 20 to 60 years. That is new information. Prior to this research, it was commonly accepted that basal metabolism, that energy needed for everyday existence, declined about 2% per decade, leading to a slower metabolism and contributing to weight gain. However, this new research says, not so fast.

This study did not assess hormone levels, like estrogen and testosterone, which can affect body composition as we age, but blaming middle age spread on our metabolism, doesn’t seem to be the case.

After the age of 60, there are declines in energy expenditure, fat-free mass (like muscle), as well as fat mass. There is also a decline in the energy used by specific organs, such as the brain. The declines could increase the risk of weight gain after the age of 60.

So where does that leave us? From this paper, it is clear that physical activity is a key driver of energy expenditure. Only 1 in 5 adults meet the minimum guidelines for physical activity, so moving more is a good place to start. It would be helpful if it there was a social and community commitment to help people move more…sidewalks, safe neighborhoods and playgrounds, accessible public transportation, bike and walking trails, could help us move more. Individual effort is important, but not everyone can afford a gym membership (both in time and money).

The paper only addressed physical activity, but we can assume that excess energy from the food we eat plays a role. Instead of labeling food as “good” or “bad” for you, why not enjoy all foods but be aware of the portions and the frequency of eating? And, in middle-age, alcohol intake might increase which contributes to excess calories.

If science doesn’t evolve, it’s no longer science, it’s history.”

Chris Rosenbloom is a registered dietitian, nutrition professor emerita at Georgia State University, and co-author of Food & Fitness After 50. Follow her blog by clicking here.