I’ll begin this summary by saying that Herman Pontzer, in his book Burn, repeatedly remarks how important exercise is to maintain health into old age. The book is a new way of looking at how we gain, lose or maintain weight. He illuminates how all primates use their metabolic energy for life’s essential tasks, including growth or reproduction, maintenance (e.g., immune function, tissue repair), and physical activity.
Humans, unlike other primates or other mammals, share surplus food with other members of their group. “Sharing increases the energy available for all tasks including reproduction and maintenance, leading to longer lives, larger families, larger brains, and increased activity. Humans burn more energy each day than other apes to fuel these traits.” (P.135) The greater energy available also directs, however, extra calories to fat to survive periods of energy shortage which we don’t see in other primates – even sedentary zoo inhabitants.
Pontzer used a doubly labeled water protocol developed for his study, to measure energy expenditures in a hunter-gatherer society (i.e., the Hadza). He found, to his surprise that “Hadza men and women were burning the same amount of energy each day as men and women in the United States, England, the Netherlands, Japan, Russia. Somehow the Hadza, who get more physical activity in a day than the typical American gets in a week, were nonetheless burning the same number of calories as everyone else.” He even controlled for age, sex, fat mass, and height, but none of it mattered. He saw a new way of understanding the body. Daily energy expenditure was being maintained within some narrow window, regardless of lifestyle. He calls this view of metabolism “constrained daily energy expenditure.” As we increase daily physical activity, daily energy expenditure does not increase with activity but plateaus.
After years of research, he found that “our metabolic engines shift and change to make room for increased activity costs, ultimately keeping daily energy expenditure within a narrow window.” (p.166) As a result, he concludes that physically active people burn the same amount of energy as people who are much more sedentary.
Our brains manipulate both our hunger and metabolic rate in ways that make it hard to maintain weight loss because our metabolic engines are exquisitely tuned to match the energy we burn each day with the energy we eat. “When we burn more, we eat more.” (p. 168) Our hypothalamus that controls body temperature, hunger, thirst, fatigue, sleep, and circadian cycles adjusts where our energy expenditures occurs on life’s essential tasks, including growth or reproduction, maintenance (e.g., immune function, tissue repair), and physical activity. Eat less and our body uses less on functions that have less immediate needs – we are built to have long lives so we can avoid reproduction expenditures when we don’t have enough food unlike rodents who have to reproduce quickly because of short lives.
Although at times I felt like I was reading an anthropologist writing a physiology book, Pontzer makes it easy to understand by grounding the science in real life activities.