Newsletter: Lies my fitbit told me

Nutrition and sports are back to the Newsletter today. Can we really lose weight continuously over time? Does it always “taper off”?

Today’s Newsletter comes from a Scientific American article written by Herman Pontzer, from Hunter College; Dave Raichlen, from University of Arizona; Brain Wood, from Yale University.

<< (…) We had spent the past month living with the Hadza, traditional hunter-gatherers who live off of wild plants and animals in the dry savanna wilderness of northern Tanzania.

We were conducting the first direct measurements of daily energy expenditure in a hunter-gatherer population. We enlisted a couple dozen Hadza women and men to drink small, incredibly expensive bottles of water enriched in two rare isotopes, deuterium and oxygen-18. Analyzing the concentration of those two isotopes in urine samples from each participant would allow us to calculate their body’s daily rate of carbon dioxide production and thus their daily energy expenditure. (…)

Life for the Hadza is physically demanding. Each morning the women leave the grass huts of camp in small groups, some carrying infants on their back in a wrap, foraging for wild berries or other edibles. (…) Men cover miles each day hunting with bows and arrows they make themselves. When game is scarce, they use simple hatchets to chop into tree limbs, often 40 feet up in the canopy, to harvest wild honey. Even the children contribute, hauling buckets of water back from the nearest watering hole, sometimes a mile or more from camp. In the late afternoon, folks sit on the ground and talk around small cooking fires, sharing the day’s returns and tending to the kids. But forget any romantic notions of some lost Eden. Hunting and gathering is cerebral and risky, a high-stakes game in which the currency is calories and going bust means death. (…)

One of our motivations for measuring Hadza metabolism was to determine the size of the daily energy shortfall between us Westerners and the hunter-gatherers of the past, as most researchers have long assumed that our hunter-gatherer ancestors burned more calories than people in cities and towns do today. (Many went so far as to argue that this reduction in daily energy expenditure is behind the modern obesity pandemic in the developed world.) (…) However, when the analyses came back, the Hadza looked like everyone else. Hadza men ate and burned about 2,600 calories a day; Hadza women about 1,900 calories a day – the same as adults in the US or Europe. We looked at the data every way imaginable, accounting for effects of body size, fat percentage, age, and sex. Still no difference.

It seems so obvious and inescapable that physically active people burn more calories that we accept this paradigm without much critical reflection or experimental evidence. However, the Hadza result, strange as it seemed, was not some thunderbolt out from the blue (…) humans are not the only species with a fixed rate of energy expenditure: captive primates, sheep, kangaroos, and giant pandas, have all been also shown to expend the same number of calories each day as their counterparts in the wild, despite obvious differences in physical activity.

(…) On average, couch potatoes tended to spend about 200 fewer calories each day than people who were moderately active: the kind of folks who get some exercise during the week and make a point to take the stairs. Energy expenditure plateaus at higher activity levels: people with the most intensely active lives burned the same number of calories each day as those with moderately active lives.*

(…) How could this be? We aren’t really sure. We do know that the cost of the activity per se is not changing: Hadza adults burn the same number of calories to walk a mile as Westerners do. An intriguing possibility is that the body makes room for the cost of additional activity by reducing the calories spent on the many unseen tasks that take up most of our daily energy budget: the housekeeping work that our cells and organs do to keep us alive. For example, exercise often reduces inflammatory activity that the immune system mounts as well as levels of reproductive hormones such as estrogen.

(…) All of this points toward obesity being a disease of gluttony rather than sloth. This should not be news; the old adage in public health is that you can’t outrun a bad diet, and experts know that just hitting the gym to lose weight is frustratingly ineffective. It is not that we are not trying hard enough; it’s that our bodies have been plotting against us from the start.

You still have to exercise. This is not a note from your mom excusing you from gym class. Exercise has tons of well-documented benefits, from increased heart and immune system health to improved brain function and healthier aging. In fact, (…) metabolic adaptation to activity is one of the reasons exercise keeps us healthy, diverting energy away from activities such as inflammation that have negative consequences if they go on for too long (e.g., chronic inflammation has been linked to cardiovascular disease and autoimmune disorders). Exercise to stay healthy and vital; focus on diet to look after your weight.

(…) We have also evolved to be fat. Humans burn more calories than any of our great ape relatives. Even after accounting for body size, activity level, etc., humans expend about 400 more calories a day than chimpanzees, and the differences are larger still with gorillas and orangutans. Those extra calories represent the extra work our bodies do to support larger brains, produce more babies, and maintain our bodies so we live longer. Many of the energy-rich foods we need to fuel our faster metabolism are inherently difficult to obtain in the wild, increasing the energy cost of finding food and heightening the risk of starvation for hunter-gatherer populations. Even Hadza adults, lean by any human standard, carry twice as much fat as chimpanzees idling away in zoos [taking into account obvious differences in size, etc.]. Problematic though it may be, our propensity to store fat most likely coevolved with our faster metabolism as a critical energy buffer to survive lean times. >>

[* – Kike: However, note that this comes from populations fully adapted to their activity levels. For someone untrained or for someone who exercises from time to time, strategically using exercise to boost calorie consumption does work and will lead to a reduction in body weight. The issue at hand is that the human body is extremely adaptable, and that while exercise can be used to lose weight very effectively… running 1h per day is simply not the way to do it, at least not for the long term.]

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