Metabolism changes with age- but not when you might think!

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Remember being a kid without calorie care in the world? Those were the days when we could enjoy food to our heart’s desire without the fear of weight gain. New research suggests our body’s ability to burn calories peaks earlier than once thought and declines later than originally believed.

Research published in the Journal Science was conducted at the Pennington Biomedical Research Center. According to co-author Jennifer Rood, Ph.D. and Associate Executive Director for Cores and Resources, there are many physiological changes that happen as we age with phases of life including puberty and menopause. The timing of “metabolic life stages” doesn’t coincide with markers typically linked with growing up and getting older.

The study of calories burned during normal life activities was evaluated in over 6.600 people by four Pennington Biomedical researchers as part of an international team of scientists. The ages of the subjects ranged from one week to 95 years of age.

Participants lived in 29 different countries. Pennington Biomedical scientists included Peter Katzmarzyk, Ph.D., Associate Executive Director for Population and Public Health Sciences; Corby Martin, Ph.D., Professor and Director, Ingestive Behavior Laboratory; and Eric Ravussin, Ph.D., Associate Executive Director for Clinical Science.

Research previously conducted on energy expenditure evaluated calories needed for basic, vital functions such as breathing, digestion, and heartbeat. These functions result in about 50 to 70 percent of calories used daily. This measurement doesn’t include calories used for other daily activities such as walking a dog, washing dishes, working out, or fidgeting. Fidgeting burns calories!

The doubly labeled water method was used to define a total daily calorie expenditure. This involves a urine test after a person has drunk water where the hydrogen and oxygen molecules in the water have been replaced with “heavy forms” that are naturally occurring. Measures are then done to see how quickly they’re flushed out. This technique has been used to measure human calorie expenditure since the 1980s and is considered the gold standard of its kind.

The cost was a limiting factor for study size in previous research. To reduce cost, multiple labs shared study data in one database in order to tease out the facts that were hidden in older studies.
After collecting and evaluating energy expenses across the lifespan, researchers discovered that infants had the highest metabolic rates of all.

Energy requirements rocket in the first year of life. Babies burn calories 50 percent faster than adults by their first birthdays according to Dr. Katzmarzyk. Babies grow very quickly, which accounts for much of the impact. Even after controlling for growth, their energy expenditure is high despite their small body size, according to Dr. Martin.
This extremely high metabolism may explain why kids that don’t get adequate food to eat in this developmental stage are less able to thrive and grow to be healthy adults.

Additional research is necessary to fully comprehend the metabolic rate of babies. Dr. Martin notes, “We need to know what is driving higher energy expenditures”. A person’s metabolic rate declines by roughly 3% each year after the initial surge as babies. It plateaus until we hit our 20’s, which becomes our new normal.

Scientists once believed growth spurts in teens increased daily calorie needs. Most people’s metabolisms were static in their 20’s until their 50’s. Energy needs during pregnancy didn’t change very much either.
This research suggests something else may be at play behind the “middle-age spread”. Data from the study finds that a decrease in metabolism doesn’t occur until after age 60. The decline is slow, about 0.7 percent each year. For example, someone in their 90’s needs 26% fewer calories per day than a person in midlife.

Loss of muscle mass as we age may be responsible for the metabolic drop, according to researchers. Muscle is more metabolic than fat. However, other factors are at play. Dr. Rayussin notes. “After 60, a person’s cells slow down”. Even when activity levels differed, metabolic patterns were unchanged. Multiple physiologic changes occur in the aging process, so it’s hard to figure out what results in the changes in calorie expenditure.

This new study supports the idea that body composition and changes in lifestyle are not the only things responsible. "This study shows that the work cells do changes over the course of the lifespan in ways we couldn't fully appreciate before. But massive data sets like the one we collaborated on allow us to answer questions we couldn't address," per Dr. Ravussin.

A faster metabolism is in your hands:
• Do some form of resistance training a few days per week to maintain muscle mass. Free weights, nautilus machines, or resistance bands are examples.
• Lift several cups of green tea daily. Catechins in green tea have metabolic effects. Studies show that 6 to 7 cups of green tea along with a calorie-controlled diet and exercise may aid in weight loss. Decaffeinated tea works. 2
• Mark your calendar to include cardio exercise most days of the week to help maintain metabolism. Walking, swimming, running, rowing or biking regularly keep your motor running.
• Put less food on your plate. Reducing calorie intake slightly over time helps prevent inevitable weight gain.
• Fluff up that pillow and hit the sack earlier. Poor sleep or chronic insomnia is a risk factor for weight gain. 3

Stay tuned for next week’s post on dealing with menopause.

1. Herman Pontzer et al. Daily energy expenditure through the human life course. Science, 2021 DOI: 10.1126/science.abe5017
2. J.D. Roberts, M.G. Roberts, M.D. Tarpey, et al “The effect of a decaffeinated green tea extract formula on fat oxidation, body composition, and exercise performance”
Journal of the International Society of Sports Nutrition 2015, 12:1, doi:10.1186/s12970-014-0062-7
3. St-Onge MP. Sleep-obesity relation: underlying mechanisms and consequences for
treatment. Obes Rev. 2017 Feb;18 Suppl 1:34-39. doi: 10.1111/obr.12499.

Lisa Andrews, MEd, RD, LD


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