Most of us remember a time when we could eat anything we wanted and not gain weight. But a new study suggests your metabolism, the rate at which you burn calories, actually peaks much earlier and starts its inevitable decline later than you might think.
“As we age, there are a lot of physiological changes that occur in the phases of our life such as during puberty and in menopause. . What’s odd is that the timing of our ‘metabolic life stages’ doesn’t appear to match the markers we associate with growing up and getting older,” said study co-author Jennifer Rood, PhD, Associate Executive Director for Cores and Resources at Pennington Biomedical Research Center.
Most previous large-scale studies measured how much energy the body uses for basic vital functions — breathing, digesting, and pumping blood — the calories you need just to stay alive. But basic functions account for just 50-70% of the calories we burn each day. This study used pooled data from previous research to account for energy we spend doing everything else: washing the dishes, walking the dog, breaking a sweat at the gym, even just thinking or fidgeting.
“Some people think of their teens and 20s as the age when their calorie-burning potential hits its peak,” Dr. Katzmarzyk said. “But the study shows that, pound for pound, infants had the highest metabolic rates of all.”
Energy needs shoot up during the first 12 months of life. By their first birthdays, babies burn calories 50% faster for their body size than adults. And that’s not just because infants are busy tripling their birth weight in their first year.
“The babies grow rapidly, which accounts for much of the effect. However, after you control for this, their energy expenditures tend to be higher than what you would expect for their body size,” Dr. Martin said. “More research is needed to better understand the metabolism of babies. We need to know what is driving higher energy expenditures,”
An infant’s explosive metabolism may help explain why children who don’t get enough to eat during this developmental stage are less likely to survive and grow up to be healthy adults. After the initial surge in infancy, a person’s metabolism slows by about 3% each year until our 20s, when it levels off into a new normal.
Surprisingly, the growth spurts of adolescence didn’t generate an increase in daily calorie needs after researchers took body size into account. Another surprise? People’s metabolisms were most stable from their 20s through their 50s. Calorie needs during pregnancy grew no more than expected.
“We took dwindling muscle mass into account. After 60, a person’s cells slow down,” Dr. Ravussin said.
The data suggest that our metabolisms don’t really start to decline again until after age 60. The slowdown is gradual, only 0.7% a year. But a person in their 90s needs 26% fewer calories each day than someone in midlife.
Lost muscle mass as we get older may be partly to blame, the researchers say, since muscle burns more calories than fat. But it’s not the whole picture. The patterns held even when differing activity levels were taken into account.
Aging goes hand in hand with so many other physiological changes that it has been difficult to parse what drives the shifts in energy expenditure. But the new research supports the idea that it’s more than age-related changes in lifestyle or body composition.
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4 Random facts about metabolism
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- On a vegan diet, the nutrients that might need the most supplementing are Vitamin D, Vitamin A, Vitamin B12.
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- Sleep deprivation triggers hormonal responses altering glucose metabolism, in some cases producing blood glucose levels similar to Diabetes, it can also dampen the feeling of fullness and might cause fats from food to be metabolised differently – stored rather than broken down.