You think it’s a “shaky” assumption that there was ever a food surplus sufficient to gain excess weight; and that factors other than just storing calories are relatively unimportant to survival - such as evading predators, fighting with human competitors?
It seems to me that you’re the one making shaky assumptions if you simply dismiss these possibilities and assume that our bodies have evolved to just continue eating and accumulate fat without limit.
Most people do not become obese despite the availability of virtually unlimited calories. How are you so sure that we did not evolve any mechanism to limit obesity? It may well exist, but just doesn’t work well in many people because of modern lifestyle and diet.
If you pee a lot and you are starting to lose weight even though your diet and lifestyle haven’t changed, that is actually a symptom of type II diabetes caused by being overweight. Your body will detect the higher sugar levels and your kidneys will attempt to filter the excess sugar off, making you pee a lot.
It doesn’t work well enough to stop you from getting overweight and developing type II diabetes, but there is a mechanism there.
This seems reasonable. If people lived the way they did many thousands of years ago (same kind of diet, same amount of movement, etc.), they might be limited in how much weight they could put on.
I suspect that science still has some things yet to be discovered about what causes (or prevents) obesity and why some people seem naturally so much more resistant to it than others.
Fifty years ago, even in countries like the US where almost everyone had access to enough calories to become obese, obesity was rare. There were several fat kids in my school and they really stood out.
Since then, at first slowly and then very rapidly starting in the 1990’s, obesity surged, to the point now where a lean fit person of any age is now the anomaly. There are some very obvious causes, and some less than obvious, like chemicals in plastics, fragrances, and pesticides.
Nothing with as long a generation as ours is going to adapt anything like that rapidly, even if those mutations were available.
Sure I can. For millennia we and our proto-human ancestors ran after our food, or scrounged for it out of the ground or off trees. Or spent long, brutal hours tending fields. Cakes and ice cream and Big Macs, etc, etc. didn’t come along until recently, in evolutionary terms.
An evolutionary trait does not emerge unless it confers an advantage.
Being so overweight that it affected one’s health, or ability to do the things one needed to prosper and pass on one’s genes was just not a problem that needed to be selected out until recently.
Yes, of course obesity can certainly become a problem for wild animals that are exposed to a long term surplus of food and become sedentary, like zoo animals or our two cats.
The U.S., according to the CDC, was at an over 40% rate of obesity as of 2017. While that makes your statement true by the barest of technicalities, obesity is a very serious and growing problem. Many people do not become obese because they have brains that tell them when they should stop eating, and mirrors that let them judge if they like what they see or not. So yeah, not everybody becomes obese in the presence of overwhelming plenty. But the OP question is, if we do choose to overeat, why has there not evolved a mechanism by which we do not store excess fat? The simple answer is because it never had to be selected for. You said yourself it would be a relatively simple process to evolve, the fact that it hasn’t is evidence that it wasn’t an evolutionary need until recently.
Lots of good points being made. But I think the one that only recently have seriously obese folk been able to: 1. Eat as much as their enablers can give them and not have to go out and do the hunter-gatherer thing. 2. Avoid being eaten by lions and such.
This makes it very hard to evolve a gene that caps serious obesity. You die at an earlier stage from something else before you get there.
Add in the point about short term weight gain is good (it means you have an increased chance of surviving the winter) vs. long term (decades) of excess weight being bad.
And then another point is that modern food is a whole different thing from the “good old” paleo days. Loading up on sugar is not the same as loading up on wildebeest.
There is a mechanism. Appetite and satiety are controlled by a feedback loop between hormones from the GI tract and the hypothalamus.
The GI tract hormone leptin is an appetite suppressor. Ghelin enhances appetite and plays a role in body weight. Leptin levels are lower when you’re thin and higher when you’re fat.
Right, because of course humans “in the wild” reproduced at an early age - teens and twenties. What kills you at 30 usually wouldn’t matter much from an evolutionary standpoint, particularly in a social species like humans where others were often available to take up the slack in childcare. What kills you at 12 does.
The premise is incorrect. The body DOES naturally just stop putting on weight at some point. That point is just at a heavier point than we’d like it to be.
If that weren’t true, then we’d have a lot of thousand-pound people running around.
As for why the equilibrium point is heavier than we’d like, well, everything in evolution involves tradeoffs. The equilibrium point isn’t set in a vacuum; it’s a result of a number of other traits. And people with traits that would lead to a lower equilibrium point wouldn’t be as good at saving up fat for hungry times.
Even if there had been previous times in history where there was so much abundance for so long that people were suffering ill effects from obesity, just the fact that abundance is more common now than it was then means that our current equilibrium point won’t be optimal.
There wasn’t really much reproductive benefit for limiting weight. By the time someone would have a weight-related health issue, they are likely past the age of reproduction. Weight-related diabetes is something that could hit early, but occurrences of it would be rare since it wouldn’t have been that common to have so much food that a person could get overweight enough to get weight-related diabetes. And for women, lack of fat can slow or stop their menstrual cycle. That would obviously have a negative effect on the reproduction rate of the tribe.
One reason primitive people weren’t overweight is that primitive foods were not as delicious and did not produce the same food-reward feedback loop that modern foods do. Like, even if there was endless amounts of brown rice, so what? No one wants to eat endless brown rice, and it’s a lot of work to harvest and prepare it anyway. If modern humans only ate things like natural grains, legumes, plain meat, fruit, etc., there would be less obesity. With primitive foods, it’s easy to stop when you’ve had enough. But modern foods are crispy, crunchy, sugary, salty, creamy, and have all the characteristics that cause our reward centers to seek them out like a drug. People don’t eat potato chips for the nutritional value or to satisfy their hunger–they eat them because the taste and crunch light up the reward centers in our brain. Ancient humans didn’t face the same food-related challenges we do. If ancient humans were in a similar situation of having large quantities of easily acquired, super-tasty foods, then likely obesity would have been much more common and evolution would have worked out a solution.
There have been several 1000+ lb people. I think most people could readily achieve incredible weight if they were determined to and had access to the food and enablers. The body would not generally stop such attempts.
The reason why this doesn’t happen more is that most people know that this is a Bad Idea and/or are unable to get into the situation where they could do it and other things that having nothing to do with a person’s body self-limiting the weight gain to “only” 300lbs (for example).
Such a mechanism does exist. The more obese you get, the higher your leptin/ghrelin ratio becomes. Ghrelin tells you you’re hungry and leptin tells you you’re full. The problem is research has shown us that eating is tied more to habit than actual hunger.
One very important factor in evolution mechanisms often seems to be forgotten.
Mutations do lead to evolutionary selections, but mutations do no occur whenever they are needed. Mutations occur because of random mistakes in DNA replication. Most are harmful, some are of neutral benefit, and a few are beneficial and advantageous to the organism/species, and are inherited, ultimately leading to change, or evolution.
But mutations do not occur because of some existing need. They are accidentally the raw material that can lead to advantages, and are not the result of any type of arising to solve a problem. So just because it would be nice to have a certain trait is of no help in actually getting a mutation that leads to that trait.
“Several”, yes. But compare that to the number of people who say “I just can’t control my weight”, or “I just keep on putting on pounds”. And yes, people can put on that much weight if they’re determined to, but most people aren’t determined to: That’s one of the feedback mechanisms that prevents it.
Any given person has some amount of food that they have an appetite for. Give them unlimited access to food and no conscious attempts at dieting, and that’s how much they’ll eat. If that amount is more than their rate of Calorie burning, they’ll gain weight. As weight increases, so does the base rate of Calorie burning just from sitting there and being alive. At some point, the person will gain enough weight that those two curves cross, and they’ll reach equilibrium. For a very few people, that equilibrium point is over a thousand pounds, but for the vast majority, it’s much lighter (probably somewhere around 300 pounds).
Chronos: I am interpreting the OP’s question in a fairly straightforward way. There does not seem to be any mechanism within a person’s body that limits ones weight to being 300+lbs, for example. That there are people who are 1000+lbs strongly reinforces this.
The “equilibrium” you seem to believe in is not really a limit at all, esp. when you consider only internal factors such as genes and such.
Sure, people can choose to limit their food, can decide when they are satiated, etc. But that’s not a natural internal bodily affect, as in the OP, where the body stops adding more fat.