It seems to me that how people respond to the SAD depends on genetics. In my case, I think it would be incorrect to say "Obesity and diabetes run through my family," and correct to say "When my family eats the SAD, they become obese and diabetic." Luckily, I'm still young enough to change my ways with minimal pain!
My mother in law thinks I'm crazy and fad-ish for eating minimal carbs, and was quite vocal about it until I told her I'd figured out I'm a sub-clinical celiac. Apparently a disease need a clinical name before she'll take it seriously. But she's a skinny little thing who can eat the worst parts of the SAD (especially politically correct pseudo-vegetarianism and saturated fat-hating), with gallons of sweet soy milk, industrial oils, trans-fats, hardly any animal fat, almost no red meat, etc. At least she gets her leafy greens, but they probably lose all their goodness with the industrial salad dressing she pours on. She's not toned, but I couldn't call her skinny-fat either, by any means! But she has a lot of digestive and gall bladder issues, allergies and sinus headaches, and all her kids have narrow faces and desperately needed braces and glasses (they otherwise flourished and have always been smart and happy, something my siblings can't all say).
So while my family genetically responds mainly by getting larger and diabetic, her family genetically responds by mainly getting nasty, uncomfortable digestive issues. A family like theirs gets non-visible issues, non-obviously-diet-related, but they're still suffering on the inside. Especially as they age, which I think she's about to figure out. I suppose that when talking to a skinny person on the SAD it's easiest to go through the diseases of civilization other than obesity and diabetes. Allergies, arthritis, irritable bowel, narrow face, headaches, eye problems, blood pressure, hypoglycemia, crooked teeth, sugar crashes, heart issues, etc, etc? It's your diet, baby, or least least exacerbated by it. The fat ones are lucky, because the cause of the problem is obvious, even if the correct dietary solution isn't. The skinny ones just say "guess it's one of those things.... Pass the corn flakes."