Below are three claims that are popular on social media and often treated as articles of faith, even though the data behind them may be flimsy, outdated, or biased by selection effects:
1. People’s metabolisms do not slow until the age of 60.
2. Drug dealers effectively only earn minimum wage.
3. The link between IQ and income or wealth drops off around 125-130 or so.
All three claims despite being popular and parroted as literal fact by the media, there is also so much counter-evidence anecdotally that it should call into doubt the veracity of such studies. Yes, as it’s commonly said, the plural of anecdotes does not mean data, but at some point there must be a threshold where it does, especially when the data is old and not reproduced. And we’re not talking physics or math, but rather social phenomena, which by definition are not static.
In the first case, on social media, whenever someone attributes their middle-aged or post-college weight gain to a slowing metabolism, someone will inevitably respond with a link to a widely cited 2021 study by H. Pontzer, “Daily Energy Expenditure Through the Human Life Course,” as if that alone settles the matter. Usually the response links to media coverage summarizing the study, though occasionally, the study itself is cited. For example, on Reddit:
The authority conveyed by the study (with 679 citations and counting) is invoked to end the debate, even though science historically arose to challenge orthodoxy and unquestioned authority. By its nature, scientific knowledge is provisional and open to debate. It’s like:
“You are wrong because of science.”
“What if the science is wrong?”
“Impossible.”
From the abstract:
Measurements of total and basal energy in a large cohort of subjects at ages spanning from before birth to old age document distinct changes that occur during a human lifetime. Pontzer et al. report that energy expenditure (adjusted for weight) in neonates was like that of adults but increased substantially in the first year of life (see the Perspective by Rhoads and Anderson). It then gradually declined until young individuals reached adult characteristics, which were maintained from age 20 to 60 years. Older individuals showed reduced energy expenditure. Tissue metabolism thus appears not to be constant but rather to undergo transitions at critical junctures. —LBR
Visually, metabolism (adjusted for body mass) appears stable from 20 to 60, as claimed:
So that settles it? Not so fast. When so many people report the same problem of sudden midlife weight gain despite not being less active or appreciably eating more, maybe there is something else at play, or there is something missed by the study. I don’t think the experiences of so many people can be just swept under the rug as calorie miscounting or being sedentary. There is so much anecdotal evidence, and when combined with the study having never been reproduced, should at the very least merit skepticism.
It is possible there were methodical or other flaws with the data or other aspects of the study. Or the data does not map well to the population despite the large sample size (e.g. there are still large populations that DO experience metabolic slowing after 30-40). One obvious problem with the study is that it’s not tracking the same people over time, but just compares metabolism at different ages versus predicted values relative to weight, like a snapshot.
Maybe many people DO experience significant metabolic slowing after 20-30 and hence become obese as adults, but because they are excluded from the study owing to their present obese status, we wouldn’t know. A longitudinal study would be more useful, because then we’d be able to see if there is a transition of abnormal metabolic slowing among some participants that is possibly causal of developing obesity. Or if obese people have abnormally slow metabolisms relative to weight.
Of course, a longitudinal study would be far more difficult than simply taking a cross-sectional snapshot of participants’ metabolism at different ages, but would also provide much more information, which could shed light on this widespread mystery of unexplained weight gain.
To be continued…