(copied and pasted from another thread, as per request)
There are two basic premises behind Traditional Foods. First is the idea that you want the most nutritional bang-for-buck out of your food - that is, a very high nutrient density. We're not talking caloric density but rather the most vitamins, minerals, antioxidants etc. Much of the TF movement references the work of dentist Weston Price, who in the early part of the 20th century went on an epic (several epic actually) journey around the globe looking at various ethnic groups in order to basically figure out how people managed to survive before modern dentistry, since his brain kept boggling at the sheer numbers of people in his practise who basically had all their teeth falling out - it was the norm back then. Price figured the human race wouldn't have survived very long with no teeth, so he went checking on the folks who were eating what they'd eaten since the dawn of time - what we now refer to as "traditional cultures". Sure enough, they had all their teeth, and were darned healthy besides. So Price, being the little science-dude he was, took samples of their foods back to his lab and analyzed them and found waaaaaay higher levels of all vitamins and minerals than are/were found in the typical foods of his patients. He concluded that the food products of industrialized nations lacked the necessary vitamins and minerals for human health.
(Sorry if you knew all this already, but it's key to the arguments.) The rationale behind eating all the animal products, particularly fatty meat, is that meat is one of the most nutrient-dense foods you can eat. (seafood too) It's got loads of minerals AND (this is key) they are in highly absorbable forms. Many of the key vitamins (A, E and D) are fat-soluble - and animal meat is not only a good source of these, but essential for adequate uptake. Moreover, the cardiopulmonary issues the medical community frequently associates with moderate to heavy meat intake are a relic of Ancel Key's lipid hypothesis, which has been pretty soundly disproven by epidemiological and experimental data (see Gary Taubes
). Also, for children especially, saturated fat is extremely important for brain development and maintenance of neuron health. It's also important for the immune system and digestion. BUT it's critical that people understand that all meat is not created equal, and a lot of the health problems people have today started rising right along with factory farming. Fat can carry a lot of good stuff, but it can also carry a lot of bad stuff too - hormones, antibiotic residue, pesticides, etc. So most TF proponents stress that meat must be organic and pasture-raised, since pasture provides the optimal diet for the animals themselves, and increases the nutrient density of their meat while decreasing the amount of bad stuff. (The same goes for plant foods - organic is best because of the reduced pesticide load AND the fact that they contain more nutrients.)
The second main idea behind Traditional Foods is the logical premise that if people have been eating these foods (as prepared traditionally - a loaf of bread is not necessarily a loaf of bread) for thousands if not millions of years without any problems (and, as Price found, with better results than a modern diet), they're probably not going to do you any harm. Meat and veg for sure are exactly what human bodies evolved to run on, so they are definitely a-ok. Dairy and grains are newer additions, but most people do just fine with them provided that they're prepared in the ways our ancestors figured out were the safest and most nutritious.
The other argument for Traditional Foods is ecological. Traditional Foods are, by their nature, sustainable. Have a look at Polyface Farm
as a model of how food should be produced - it's a highly productive farm, more so than most monocrop operations, yet its impact on the environment is actually beneficial. Rotational grazing, which is how meat SHOULD be produced, actually results in a net sequestration of carbon rather than an increase. True diversified organic agriculture that incorporates animals such as chickens enhances the soil and prevents toxic run-off. Moreover, a traditional diet dovetails beautifully with the notion of local eating, further reducing the ecological footprint of your diet.