An important paper by Canadian researcher Ian Spreadbury proposes that high levels of inflammation produced by microbes in the gut may underlie many cases of obesity. Spreadbury's theory is that this inflammation is fueled by foods that are acellular - that is, so overprocessed that they are almost wholly lacking in intact cells.
(For more, see "Carbohydrate Density: A Better Guide to Weight Loss".)
This suggests that the best way to end the inflammation/obesity vicious cycle is to reduce consumption of foods made with flour or added sugar as much as possible. (I would add here that whole-wheat flour is little better than "white" flour - they are similarly acellular, and to be avoided.) Spreadbury recommends focusing the diet on what he terms "ancestral" foods; that is, traditional, relatively unprocessed vegetables, fruits, nuts, fish and meats that promote healthy, non-inflammatory microorganisms in the digestive tract. In other words, pass on the "Modern" foods on this chart, and emphasize the "Ancestral" ones:
Reprinted from Diabetes, Metabolic Syndrome and Obesity: Targets and Therapy, 2012:5, Spreadbury I, Comparison with ancestral diets suggests dense acellular carbohydrates promote an inflammatory microbiota, and may be the primary dietary cause of leptin resistance and obesity, pp. 175-189, Copyright 2012, with permission from Dove Medical Press Ltd.
Spreadbury also suggests that dietary fat, including saturated fat - long demonized in American nutrition science - is "unlikely to be the most pertinent factor in human obesity." In his view, focus on avoiding acellular carbohydrates, rather than natural fats, will yield better weight loss results.
The lesson here is to make sure to eat foods with intact cell structures. This lines up with the observation that native populations eating unprocessed foods can thrive on a wide variety of macronutrient ratios, ranging from equatorial tribes like the Kitava of Papua, New Guinea, who consume 65 percent of their calories from carbohydrates, all the way to the !Kung in Africa's Kalahari Desert who eat large quantities of nuts that are 60 percent fat. Surprisingly, some of these ancestral foods are fairly high on the glycemic index (GI), suggesting that GI is not the optimal measure of how likely a given food is to contribute to obesity.
What neither of these tribes consumes is carbohydrate-dense, acellular foods - as such, they provide excellent examples of healthy eating for all of us.