New Study Finds the Human Metabolism Has Four Stages

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A slower metabolism is not why you put on weight as you age — at least not until you hit age 60. As first reported by the New York Times (NYT) on August 12, 2021, a new paper published in Science shared research of energy expenditure through the course of human life.

The study, comprised of 80 co-authors sharing their efforts across half a dozen labs over a timeframe of 40 years, followed 6,500 subjects spanning from eight days in age to 95 years old to “measure their total and basal energy” during their lifetimes. It found that there are four distinct stages over a human’s lifetime when their metabolism shifts. (1)

  1. Infancy — up until age one, metabolic rate accelerates to 50 percent above the adult rate.
  2. From Age One to 20 — metabolic rate slows by approximately three percent each year.
  3. Age 20 to 60 — metabolic rate holds steady.
  4. Age 60 and Over — metabolic rate declines 0.67 percent each year across that period of three and a half decades.

There were no noticeable differences in metabolic rates between women and men after controlling for overall body size and total muscle mass. Furthermore, a study in the American Journal for Clinical Nutrition found race does not alter metabolic adaptation either. (2)

Of note, in Science‘s paper, participants’ heights, weights, and body fat percentages were tabulated so “fundamental metabolic rates” could be tracked. The study is so “pivotal…it is likely to show up in textbooks,” Leanna Redman, an energy balance physiologist at Pennington Biomedical Research Institute, told the NYT.

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There were, of course, participants who fell upwards of 25 percent either over and under the average metabolic rates, but the general trend was clear. Dr. Redman continued to say, “there isn’t a constant rate of energy expenditure per pound.”

That means that weight is not the deterministic factor for metabolic rate — the metabolism does not speed up or slow down based on how much someone weighs. The participants whose numbers fell outside the general trend will lay the path for future questions as research continues. 

The mystery of why people put on weight during adulthood is not much of a mystery after all. The most likely reason for unwanted weight gain before age 60 is simply consuming more calories than calories burned. An active lifestyle will naturally “promote more favorable health effects” than a sedentary one. (3)

However, there is no one most effective diet to promote weight loss. (4) The only requirement to drop the pounds is to remain consistent with a diet that allows for a manageable negative energy balance (calorie deficit) that supports habits that allow that diet to be sustainable long term.

References

  1. Herman Pontzer, et al. Daily energy expenditure through the human life course. 2021. Science. doi: 10.1126/science.abe5017.
  2. Catia Martins, et al. Metabolic adaptation is not a major barrier to weight-loss maintenance. 2020. American Journal of Clinical Nutrition. doi: 10.1093/ajcn/nqaa086.
  3. Karimé González, et al. Physical Inactivity, Sedentary Behavior and Chronic Diseases. 2017. Korean Journal of Family Medicine. doi: 10.4082/kjfm.2017.38.3.111.
  4. Rachel Freire. Scientific evidence of diets for weight loss: Different macronutrient composition, intermittent fasting, and popular diets. 2020. Nutrition. doi: 10.1016/j.nut.2019.07.001.

Featured image via Shutterstock/pathdoc

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