People with low BMI aren't more active, they are just less hungry, 'run hotter'

What I mean is, the fact that you have muscle mass and stop lifting is not what's going to lead to "a quick onset of obesity." What leads to weight gain is still eating like you're lifting when you're not.

If you adjust your caloric intake to account for your new level of caloric expenditure, you will not gain weight, although you will likely see unfavorable body composition changes.

Graph your weight. After 6 weeks of HIIT, you'll see weight really shed because you're not burning calories during exercise but 24/7, and because of that metabolic shift, the weight will continue to shed for months even if you stop HIIT.

* HIIT is High Intensity Interval Training. It's a technical term, doesn't mean "oooh, that was intense". It means exercise at level that forces your body into using different metabolic mechanisms to fuel your effort. Initially it will probably make you feel ill. If you aren't used to it, and are doing it right, you will probably feel nauseous after the effort.

Your aim is expending energy about 4 minutes above your VO2Max, doing anaerobic exercise.

In a 4 - 8 minute routine called Tabata intervals** you can do:

- 2 minute warmup to hit your second highest heart rate band (probably 125 - 135), then

- 4 one minute intervals at max heart rate, generally 30 - 45 seconds "on" and 15 - 30 seconds recovery, four times in a row, followed by

- 2 minutes cooldown.

You can probably not hit these heart rates on foot or on normal exercise machines, you probably need a climber, rower, or elliptical (in that order of 'kicks your ass').

** For iOS, try the app "Seconds Pro" which offers brilliant timer routines for loops within loops.

EDIT TO ADD ANECDATA GRAPH:

Here is my personal graph in 2015 and 2016 when I tried this in my mid-40s to get rid of the 30 lbs gain that happened after I hit 40 with a corresponding metabolic slowdown:

https://i.imgur.com/pvbvoMy.jpg

I bought a Nokia scale about a month into HIIT in late spring, and stopped HIIT a couple weeks later.

After stopping, dropped 10 lbs from early summer through the fall. Over winter, began to gain.

Did HIIT again in spring of 2016 for six more weeks.

Stopped when weight started shedding, dropped another 10 lbs.

Finally, did one last cycle in 2017, dropped another 10 lbs.

Overall graph from 2015 to 2021 (stopped bothering with the scale because was steady state):

https://i.imgur.com/U6U7HrH.jpg

Haven't done HIIT since 2017, no purposeful exercise except for a few weeks of swimming in 2018. The three rounds of HIIT seem to have adjusted me back to my high school weight/metabolism balance.

WARNING: These heart rate numbers are examples, see your doctor. Anerobic exercise and heart rate in your max heart rate band are potentially dangerous.

If literally a majority of people in wealthy nations are what we define as overweight, and we keep finding new ways that is at least somewhat out of our control or difficult to perfectly control, why do we keep framing it as solely a matter of individual discipline or virtue? I have my own guesses about why of course, but pretty much all of the possibilities are unsettling.

Using this study for general weight loss advice doesn't make sense because they are looking at outliers.