I think a lot of the magical thinking surrounding weight loss is because there's no one-size-fits-all solution, so people make up patterns in their heads and believe them to be true.
Does he keep a detailed food log? I have power lifted and worked out for over a decade now. Every single skinny person who has asked for my help to gain weight has told me exactly what you said above, "I eat 3-4k day and just can't seem to get any bigger." As soon as I put them on a food log we find out that nope, they are barely crossing 2k calories and are like most people who severely under/over estimate how much they eat.
In my experience it's very hard to compare the amount of food I eat with how much someone else at the table eats.
If inputs > outputs, there is net energy storage (as fat or protein)
If inputs < outputs, there is net energy deficit (lost from fat and protein)
Inputs = food and drink consumed
Outputs = Basal metabolic rate (BMR) + energy used for activity + energy lost in urine and faeces
Faeces caloric content varies between 50-350 kcal / day, urine 91–117 kcal / day [1]
Basal metabolic rate is primarily decided by fat-free mass, mass, and age, 26% of BMR is not defined by these parameters. Mean BMR is 1500 kcal/day, and 26% of this represents a variability of 1305-1695 kcal per day.[2]
(350-50) + (117-91) + (1695-1305) = 716 kcal
Therefore two people of identical age, fat-free mass and mass who consume identical caloric intake and perform identical exercise will have, at most, a variance of 716 kcal / day in their energy expenditure from the variance in urinary and faecal caloric loss and BMR
[1]: Rose C, Parker A, Jefferson B, Cartmell E. The Characterization of Feces and Urine: A Review of the Literature to Inform Advanced Treatment Technology. Critical Reviews in Environmental Science and Technology. 2015;45(17):1827-1879. doi:10.1080/10643389.2014.1000761. Section 3.2.5 and Section 3.6.4
[2] Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine. Johnstone AM, Murison SD, Duncan JS, Rance KA, Speakman JR. Am J Clin Nutr. 2005 Nov;82(5):941-8.
Thermodynamics provides a lower bound on the amount of energy taken from food, but but couldn't it cost me 2 kcal to put 1 kcal into a stationary bike, while it costs someone else only 1.5?
All conservation of energy says here is that it has to cost at least 1.
His friends may be the same age, but have wildly different amounts of fat and muscle despite similar exercise regimens. I think that's what his point was, not that thermodynamics don't apply.
That kind of variance is enough to mean that if we trust the study, you could raise two different twenty-year-olds and one will die of starvation and the other will be clinically obese despite the same routine and diet.
Obese people usually display the opposite: they sit very still.
I often wonder how much obesity is a result of telling young children to keep still and stop fidgeting.
its likely the couch potatoe is unhealthy inside even though he looks fit on the outside.
on the other side, if someone is overweight (albeit not very obese) but overall eats a healthy diet, it shouldnt be that worrisome.
People just need help to find what works for them and for a majority of people who don't have hormone imbalances starting with energy in = energy used personally is going to get them very far in their weight control and just generally understanding what happens to their bodies in relation to food intake. We focus too much on anything but energy intake (this food is a super food, and this is healthy and that isn't). I'm not saying they are not important but if you want to make quick health improvements to a large group of people, starting from energy input to output is more effective.
I used to not be able to put on mass (hard gainer). What I found was that I wasn't eating as much as I thought and really to gain muscle mass and not just mass you then need to not only be in a caloric surplus you need to look a level deeper at your macros and increase your protein in take. If it fits your macros diet would get you close to a body that you are happy with as long as you tweak your macros to what you want to accomplish.
Also, healthy foods do have merit with their micros but again if you don't have hormone issues you can eat what is untraditional considered unhealthy foods, such as McDonalds, and not put on weight or even lose weight as long as you control for caloric intake. The hard part with unhealthy foods is controlling your calories. It is really easy to eat your daily total calories in one or two meals eating unhealthy foods (McDonalds in this example) than eating vegetables and healthier proteins and carbs (lean meats and whole grains). The Supersize me guy didn't get fat because he ate McDonalds he got fat because he ate sooo many calories. Fat Head was a bit of a rebuttal showing what happens when you try to control for calories more. Athletes are known to eat a lot of calories and don't become obese but again it's about getting your energy equation to balance or off balanced in the way you'd like. (caloric surplus to gain weight and a caloric deficit to loss weight)
There will be a lot of nuance in how everyone's bodies will respond and yes watching some metrics other than weight, like blood pressure and cholesterol , will be important in our health. Starting from a high level of just understand OUR personal energy needs and then working from there will give people a lot of benefit and make discussions easier. Because I hear so often from people that what I'm eating is unhealthy when in reality I'm within my caloric intake and within my macro makeup. Just because you eat a homemade cookie or a homemade cake doesn't make it healthier than my Snickers Bar (ooh I love Snickers). And it is so much easier to keep track of my calories when eating a Snickers bar than something homemade. It really is so easy to overeat on those treats especially during the holidays because you have no idea what the person used to bake those.