Someone on a forum was going on and on about grains being A.O.K. because traditional societies like the Japanese or some of Weston A. Price's healthy cultures ate them and were not obese. I think he misses the point, but also underscores a very annoying misconception. Many of my friends have told me that they have no need for anything like the paleo diet because they are skinny and always have been.
But last time I checked skinny does not equal healthy. There are all kinds of health problems a skinny person can have and new studies show that within a single person insulin sensitivity may vary. Skinny programmers chugging Mountain Dew might be lucky enough to have belly fat tissue that is not insulin sensitive, but they might still be damaging other organs.
A paleo diet is about avoiding diseases of civilization. Obesity is just one of those diseases. I think well-planned veganism can do wonders for improving weight, cholesterol, and other basic measures of metabolic syndrome, but I do not believe it is the diet that brings out the best in the human body. I did raw grain-free veganism for time and like many people I initially felt good, probably because of all the wheat and dairy I wasn't eating, but eventually I just felt diminished. I alternated between fruit-induced sugar highs and extreme fatigue. I mainly just felt hungry. I realized that vegetables just don't have many calories and you have a choice of eating massive amounts of sugar from fruit or massive amounts of omega-6 fatty acids from nuts. I feel the paleo diet has the optimal amount of nutrients, in the best ratios, and in forms that are easily utilized by the human body (bioavailable). That's important, because many of us have damaged our bodies with junk and we need to do more than avoid obesity, we need the nutrients to repair.
Many other raw vegans, including The Raw Model, a popular raw blogger, have found that their health has improved dramatically since they added animal products.
Societies like the Japanese avoided many problems by eating a diet low in total calories, but they did not reach their potential for height and bone development until fairly recently (incidentally as consumption of meat and fish has increased). It's the same with many agrarian societies: they aren't obese, but they aren't completely healthy either. There is plenty we can learn from peasant diets, but we can do better than peasants who worked a backbreaking day on very little in the way of calories.
I think a sugar-free vegan or agrarian diet is certainly a step in the right direction and an agrarian diet can be made optimal with the careful addition of small amounts of meat and fish and by the fermentation of grains, dairy, and legumes. But the animal component of the diet has to be foods like liver and sardines, not skinless boneless chicken breast.
In the end people can go on and on about fruits and vegetables, but that's not where the calories are. The big question is where you are going to get the calories and whether you want to burn sugars or fats. Plants might enhance your health, but your fuel is going to make a bigger difference. I encourage anyone who hasn't read Good Calories, Bad Calories to get a copy or at least check out the detailed notes.