One of the engineers I worked with was skinny as a rail, but every morning he drank a big plastic bottle of Mountain Dew. I don't think he even touched anything nutritious. He sat around coding all day and never exercised. Last year he was diagnosed with prediabetes and had an absurd amount of body fat considering his size. He's now on paleo +exercised and has normalized his blood sugar and reduced his body fat.
I think the same lifestyle is responsible for "Hipster belly." Beer + no exercise + little nutrition + carbage.
One way can be continual yo-yoing of weight. Exercise and dietary composition can have some impact on preservation of lean mass during a loss phase, but some loss of lean mass is almost a given. Folks tend to stop the exercise regime as part of the regaining of the yo-yo and gain more fat mass than lean. Rinse, repeat ... that's one way.
Eating disorders - restrict your calories, do too much cardio and lose too much weight (including muscle mass). Then eventually, when you regain, it comes back as mostly fat. Do this a few times in your life and by age 35 you'll be a skinny fat like me.
Age is the easiest way and the most difficult to avoid/change.
Also, regardless of diet if you don't use the muscles they will atrophy and excess energy will be even more likely to go to fat since the muscles don't need it.
Thin-fat bodies and the Barker hypothesis: http://questioning-answers.blogspot.com/2011/08/barker-hypothesis-and-thin-fat-bodies.html
Not eating enough protein, at least for me I think.
When I was really skinny fat I lifted weights plenty, and... nothing happened. I just got tired, and continued to have puny-pre-pubescent-girl-arms and a little pouchy belly (which would have been fine, had I any boobs to speak of, damn you Asian genes!).
Now I eat much, much more of it and have a noticeably (to me) different body composition, without actually lifting that much anymore, at least not on purpose/at the gym/with a trainer. Plus, I grew a butt! Awesome!
I've wondered this too. I think it's a combination of many factors. Age is a huge one, many naturally thin people who used to be lean enough end up with high body fat in their 30s or later. But I have met many a skinny-fat young teenage girl.
My father, sisters and I are all naturally very thin, but lean and with hard little muscles, despite often very poor habits (along the lines of no exercise whatsoever and lots of calories from crap foods). I've often wondered what the difference is between us and the other very slender women I have known with very little muscle mass and visibly high body fat. Chronic cardio and a diet low in calories, protein and nutrients seem to be an issue for most of them. But genetics have to be at play here as well.
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