I'm one of those skinny people. Have been my whole life, along with all the men in my family. From 18 to 37 I literally have not gained more than 6-7 lbs. For me and many other thin to normal people I think it is we are extremely well adapted to an agriculture-based diet.

Remember humans have continued to evolve over the past 10,000 years. That is some 400+ generations. Any lab biologist, dog breeder, or dairy farmer can attest an almost unimaginable amount of variation can express in a mere 20-40 generations. Among humans, we have developed lactase persistence and efficient alcohol metabolization. I suspect there are similar adaptations to carbohydrates in general, and wheat/gluten in particular. Actually, I know there are, as in Celiac disease.

I'm interested in Paleo for the health aspects, particularly as regards the missing vitamins and vital-lipids in the modern diet, but I also feel if one has always been in fine health eating moderate amounts of wheat and grains then don't sweat it. And for those that have NOT been fine with moderate amounts of grain (or couldn't self-regulate to only moderate amounts) and have cut them out completely I think that makes good sense as well.

In fact I have no problem with vegetarians, paleos, and everyone in between. I do have a problem with the evangelists and zealots that swear theirs is the one and only true way.

Genetic or epigenetic differences in the insulin sensitivity ratio between liver and adipocytes. That's what explains skinny people who eat crap.

The short answer is genetic variation. In the same way that some people have natural immunity to a particular disease, skinny people do not have the same insulin response to the SAD that fat people do. Having said that, there may be other health issues apart from obesity associated with the SAD that naturally thin people may still suffer.

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."

Metabolic Syndrome (or The Diseases of Civilization or whatever it's called now) expresses differently in different people and at different ages. Just because a teenager is skinny now (many of us were) doesn't mean s/he won't be an insulin resistant diabetic by age forty.

Obesity is only one manifestation of ill health that the SAD brings us. A favorite subject of mine lately is skull, jaw, dental development. It is common for crowded, skinny mouths/sinus and poor eyesight. The SAD diet for mother and child has left a handful of generations (skinny and fat alike) malformed. It is shocking how many people require braces, tooth removal, corrective lenses, sinus procedures.... and it is not considered odd.

The SAD has so many consequences as other commenters have listed, my path to a Paleo approach was brought by my need to lose body fat, and has led to my understanding of the Paleo implications to ward off cancer, osteoporosis, arthritis, dimentia, Alzheimer's, MS, diabetes, allergies... myriad illness all non-issues on Paleo.

I was one of those people that could eat and drink anything I wanted and never put on weight. Then I left the Army and turned forty. I put on over 20 pounds in 2 years, then another 20 over the next 8 years before I got pissed off and started to fix it. Even so, I was not obese according to BMI ... but I sure didn't like where my waist was going.

There are a lot of people who are not obese according to height & weight charts, but have unhealthy amounts of belly fat. You may have heard the term "skinny fat", so be aware that there is a difference between obsese and overweight.

Addendum - I've cleaned up my diet, and dropped nearly thirty pounds. My exercise now includes a fair bit of muscle building and I've put back about 10 pounds. Net effect is a significant recomposition.

I notice the mothers at Walmart who are nothing more than grotesque accretions of flesh, shopping baskets full of garbage, with their cute, rail-thin teenage daughters. Do these girls look at their mothers and see their future there? I doubt it.

I am also one of those who does not gain weight... I have eaten everything at different point in my life: SAD, vegan, vegetarian, omnivorous whole-food, and now low carb. I am 5' 7'' and the only time I was (slightly) above 125 was in college when I drank lots of beer! I am now 110 (41, after 3 kids). I have aunts in their 70's who are and always were rail thin - if I last that long I expect to be the same.

I agree with the comment above that folks who get fat on carbs are the lucky ones, as I got cancer at age 38... I am still trying to understand exactly how this happened to me. I am guessing something along the lines of what Dr. Harris mentions, although I don't understand the process.

The beauty of an advantageous hormonal profile and/or the genetic upper-hand of possessing an efficient (or inefficient, if seen from a different angle) insulin response mechanism. Some people can live in the weight room and never get close to being "swole" while others brush by a dumbbell and end up jacked and chiseled beyond belief. It is interesting to note, though, the high percentage of Alzheimer's sufferers who are thin. I can't lay my hands on the study at the moment (Mauro Dipasquale speaks to it some, and I've seen it cited elsewhere...Eades, maybe?) -- I guess my point is that being overweight is not the only manifestation of metabolic syndrome; Alzheimer's can be thought of as "diabetes of the brain".

At diagnosis, 1/3 of celiacs are underweight, 1/3 are normal weight and 1/3 are obese.

Good point Scott. I think another question could be asked in this panel.

How do we explain the mentally stable and happy people on the SAD diet? My obese grandmother (250lbs+) is 87 and has no diabetes, no heart disease, no high blood pressure, and no arthritis. Did I mention that she used to chain smoke? However, she is not free to move and be active. She is limited to her chair for most of the day while she exercises her mind. She is a happy lovely and very conversational woman who loves and remembers every little detail of history. She also meditates. Meanwhile, there are other elderly who experience dementia and Alzheimer's, despite their healthy body weights. There is no body weight that reflects how our minds age.

Do you guys think we should try to learn from our grandparents and reflect on our family history?

Most skinny people I know of explode when they turn 30... that's insulin resistance for you.

Also, skinny does not imply healthy. They often have too little muscle mass.

I'm skinny and always have been so. I've often wondered about how I remained at a constant weight even when I ate horribly - donuts, cake, cookies, etc every day, tons of pasta, chips, candy etc etc.

The two answers I can think of are:

1)I was never afraid of fat. Eating whatever I want and never gaining weight literally gave me the freedom to eat whatever I wanted - I never did "low fat" or "reduced fat" or cut out butter or anything like that. It's my personal experience that fat when eaten with sugar helps mitigate the worst effects of a sugar "rush".

2)I've never been a pop (soda) or beer drinker. I think liquid sugar calories can be among the worst for weight gain. My very skinny brother only put on weight when he started drinking beer.

Putting aside all that, I think the effects of the SAD has manifested within me as a skinny person in more invisible ways, from some quite directly correlated to diet to others with a more tenuous connection:

depression, anxiety, ADHD, braces/crowded teeth/very yellow teeth at a young age, very poor eyesight, scoliosis, migraines

I've often had friends or acquaintances comment that they wished they could eat whatever they wanted and stay slim, like me. Would I rather struggle with my weight and not struggle with depression? I'm not entirely sure I wouldn't mind being heavier instead.

You mean like my skinny dad ... with diabetes.

Some people will get obese. Some diabetic. Some will get osteoperosis, IBD, ADHD, etc. Some will get multiple of those, all of those, none of those. It's simply a question of frequency. The worse your lifestyle, the more likely that you'll suffer more of the above, but it's always a genetic, environmental, etc crap shoot.

I don't know I'll live to 85 with my new lifestyle. I just know that my odds are somewhat better than they otherwise would have been, and that I'm healthier now than I otherwise would be. For me, that's well worth it.

I was one of those skinny people... 'til I hit 40 or so, then I had to start working at it. So was my dad, and my kids look to be going the same way. Genetics seems like the obvious answer.

Most people would have called me skinny before paleo. Now I'm down 30+ pounds AND I can do pull ups. I think I was one of those "skinny fat" people too.

This gets to be a tricky logical problem. I've had people ask me, "If Taubes knows a lot about obesity, why is he overweight?" Now that's just bad grammatical construction; it's not pointing out a contradiction. There are really two parts to that gibberish, a statement "Taubes knows a lot about obeisty," with which I agree, and a totally unrelated question, "Why is Taubes overweight?" and my answer is I don't know.

If their goal is to point out a contradiction to a Taubes' hypothesis, they'd have to say something like

If Taubes knows so much about obesity, then why has he said example of something contradicting the biological sciences? But they can't provide such an example.

It's almost as absurd as saying Beethoven couldn't know a lot about music, because he didn't listen to it much.

Or this,

If Phil Jackson knows so much about basketball, why was he only a journeyman bench player for the NY Knicks during his playing days.

I have severe digestion issues after going raw food vegan. I would like to do paleo but I'm having a hard time digesting fats. I am also "fat skinny" however I was in better shape when I was completely raw vegan, but once I went back to cooked foods, oh my it was awful. Also how do you get enough calories from just meat and vegetables and fruit? Where do you start? Do you need to use digestive enzymes first?

Something else that no one talks about is how skinny people actually look without clothes. If you saw me walking down the street you'd probably say that I looked super fit, skinny, and DEFINITELY not like I need to lose weight. I'm 5'4" and 115 pounds, I wear a size zero, I can't even shop in regular mall stores because their size zero is too big. BUT, what you don't see is that when I take my shirt off, I have a ring around my torso of fat. I have fat between my thighs. And I have that muffin top, big enough to grab it. So, just because you see someone who "looks skinny", doesn't mean you know what their body is like.