How do certain people who dont exercise stay remarkably thin?

Complicated question of course. Some people are on their feet a lot. Some people really don’t eat that much in relative terms. It is a lot easier to stay lean than to get lean of course.

there may be a link between obesity and gut bacteria; thin people have gut bacteria that helps to keep them that way.

scientificamerican.com

How Gut Bacteria Help Make Us Fat and Thin

Claudia Wallis

Intestinal bacteria may help determine whether we are lean or obese

Open link

Calories in vs calories out

Anyone who says otherwise has no idea what they’re talking about

Tapeworms.

Meth and cigs

exactly. Not sure why this has 3 dislikes.

Yall do realize that metabolism is part of the "Calories out" part, right?

I bet you are one of the thin people. People who are naturally lean always claim calories in vs. calories out.

I've witnessed too many scrawny people eat junk food and never put on weight. If I had a cheese burger, I gain 3 pounds.

I have two uncles. One was an athlete growing up, as was my father. Both are overweight today (in their early 70s) but certainly not obese, and both try to regularly exercise (light lifting at the gym plus walking). The other uncle never played a sport or did much activity during his life and still doesn't. He is still very thin at around 65. Guess which one had a heart issue? That's right, the one who never exercises. He may look the healthiest but you cannot see blood vessel blockages. Minor heart attack.

i'll bite - i downvoted because the guy's username is "BasicMath" so i don't think he accounts for metabolism as part of "calories out"

I was barely 18.5 BMI at 6’1” 137 lbs for many years since my early twenties. Always ate by hunger to satiety. I’d exercise regularly but that just meant going to the gym and doing random stuff for 20-30 mins like three times a week. Office job. All until a couple years back when I started running and quickly gained 5 lbs and then started weight training and gained 15 more lbs, now at just under 21 BMI. If I stop running and lifting, I’m positive I’d slowly drop weight over time.

It’s genetics and hormones and possibly microbiome, though I’m skeptical of the last one.

Cocaine, speed, or some other illicit drug is the most likely culprit.

It's called alien genetics. 😉

Just because you see someone eating junk food doesn't mean they are constantly eating more calories than they burn. I don't know how thick you would have to be to need to have this explained to you.

Because it's just stating the obvious in such overly simplified terms that it doesn't add anything to the discussion. If "calories in vs calories out" was the extent of the answer the OP was looking for, he presumably would not have even bothered to start the thread. The point of discussion is clearly to dig more deeply into those two terms and unpack why "calories in" and "calories out" differ from one person to another in ways that don't always seem readily apparent.

Imagine someone starts a thread asking how they can perform better at a particular race distance, and someone responds "Finishing time = pace X distance. Anyone who says otherwise doesn't know what they're talking about!" I mean, the statement is true, but it's also wholly unhelpful.

explain starvation mode then

If you eat lunch regularly with work colleagues, you will soon see that there are two types of people: (1) skinny people that order burgers/fries and (2) fat people that order salads.

My guess is their dietary habits are different the other 23 hours of the day.

When I turned 30, I realized all I needed to do was LOOK at a pizza to absorb its calories. Salivation gains before eating gains.

“Calories in, calories out” is just saying conservation of energy, which is a principle of physics that will always be satisfied, so as far as metabolism is concerned, it’s a tautology like 2=2 and not very useful in practice, not any more than saying the more you eat, the fatter you get. The calorie labels often mislead people, probably misleading more people than helping, and are not necessary for weight management.

For most people, the problem isn’t tracking how much they eat, but getting themselves to stop eating when their stomach is screaming in existential dread for more. Those signals are controlled by leptin and ghrelin, the primary hormones for satiety and hunger, but not the only ones. FYI, Ozempic like drugs increase secretion of a hormone called glucagon-like peptide 1 that also suppresses appetite.