Ian Spreadbury of the Gastrointestinal Diseases Research Unit, Queen's University, Kingston, Ontario, Canada published “a novel hypothesis of obesity” in the May 2012 issue of Diabetes, Metabolic Syndrome and Obesity: Targets and Therapy. (Abstract & PDF link)
He compares the cellular carbohydrates of the ancestral diet with the acellular carbohydrates of “flours, sugars, and processed foods.” The key issue is carbohydrate density:
Due to being made up of cells, virtually all "ancestral foods" have markedly lower carbohydrate densities than flour- and sugar-containing foods, a property quite independent of glycemic index.
Replacing refined grains with whole grains doesn’t seem to help much:
Whole grains are mooted to be healthier than refined grains, yet comparisons between grain consumption habits in industrialized societies indicate the effects of replacing refined grains with whole grains yield only modest improvements to health.
The dense acellular carbohydrates of modern foods may trigger obesity (known to be associated with low-level inflammation) by inflamming the GI track:
The present hypothesis suggests that in parallel with the bacterial effects of sugars on dental and periodontal health, acellular flours, sugars, and processed foods produce an inflammatory microbiota via the upper gastrointestinal tract …” (emphasis added)
A diet of grain-free whole foods with carbohydrate from cellular tubers, leaves, and fruits may produce a gastrointestinal microbiota consistent with our evolutionary condition, potentially explaining the exceptional macronutrient-independent metabolic health of non-Westernized populations, and the apparent efficacy of the modern "Paleolithic" diet on satiety and metabolism.” (emphasis added)