An epistemological question: why do people believe what they believe about low carb/paleo/ancestral ways of living? What are your justifying principles? I have written before about how tough it is to justify dietary dogmas. But I think I would like to revisit that question in more detail.
It seems to me that in the low carb world, the overarching belief is that "carbs are bad." Gary Taubes, at least as interpreted by his adherents, seems to be a big proponent of this philosophy. According to this view, carbs are fattening, and therefore, cutting carbs causes people to lose weight. But I have actually read Taubes' two main books, and while he is certainly down on carbs, he does seem to be especially down on fructose and refined carbs, not all carbs. Also, this theory is under attack, with many paleo and primal types rejecting it. As Chris Kresser pointed out, just because cutting carbs is a cure to obesity, it doesn't logically follow that carbs cause obesity. We are confusing a cure with a cause.
Yet, as I peruse the Internet, it seems to me the belief that carbs are fattening is widely held. I don't believe this and I personally think that we lose weight on a low carb diet because when we cut out sugar and refined carb products, we spontaneously consume fewer calories, as fat and protein (the foods with which we replace all those carbs we cut) are much more satiating. And limiting your intake of carbs cuts out an awful lot of food choices. That said, if you eat too much low carb food, you will gain weight, especially if you consume too much fat. Calories do matter.
What is the justifying principle for Paleo? Paleo types try to eat what they guess our Paleolithic ancestors ate. For example, J. Stanton of Gnolls.org suggests, "Eat like a predator, not like prey." Unlike low carb dieters, many Paleo adherents don't do dairy, because have you ever tried to milk a wild buffalo? But do we really know what our ancestors ate? It seems to me that a lot of Paleo adherents have a romanticized view of what our ancestors ate. Some Paleo adherents (e.g., Jaminet, Harris) advocate eating "safe" carbs, such as potatoes, rice, tapioca, etc., in addition to adding dairy. Apparently, Harris is currently eating 40% of his calories as carbs, including a lot of Rice Krispies and half and half (you'll have to listen to a rather long [but interesting!] podcast to hear him admit this).
My main problem with Paleo: its justifying principles are rather shaky. I can find all varieties of people who follow the "Paleo" lifestyle: those who do low carb Paleo; those who eat fruit, since not all ancient fruits were small, bitter, and low in sugar; those who will add some dairy to the previous list; those who will add dark chocolate; those who will add rice or potatoes; those who do vegetarian variants of the Paleo lifestyle; those who do low fat Paleo; and those who try to do some version of Paleo, but who cheat, a little or a lot. There have also been some high profile people quit the Paleo lifestyle, such as Don Matesz. Since there is so much argument about what our Paleolithic ancestors ate, it seems to me that members of the "Paleo" community are more united by what they avoid than by what they eat. They avoid things that make modern man sick, such as refined carbs, dairy, seed oils, processed foods, etc.
I actually appreciate all of the discussions within the Paleo community. As General George Patton once said, "If everyone's thinking the same thing, nobody is thinking." At least some thinking and progressing is going on in the Paleo community. Sometimes, it doesn't seem to me like as much thinking is going on in the low carb community. There does seem to be a lot of agreement in most Paleo camps that modern wheat, seed oils, refined carbs, and processed foods are not healthy to consume. The debate about dairy (fermented, cheese, cream), fruit, potatoes, rice, dark chocolate, etc., is actually very healthy. I know it has helped me change my mind on some topics. And at least they avoid nonsensical arguments like, "All carbs are fattening."