Let's start with a little biochemistry. Hope you are sipping your antioxidant green tea right now. Ahem. Carbohydrates come in two flavors, officially, simple, and complex. Nutrition junkies and medical folks tend to group them thusly: fruits and veggies, starchy carbs, and added sugars. Fruits and veggies are, for the most part, obvious. Starchy carbs include bread, crackers, cake, pastry, flour, tortillas, oatmeal, cereals, rice, corn, and potatoes (regular and sweet), and some other starchy root vegetables. Added sugars are table sugar, corn syrup, honey, maple syrup, molasses - you get the picture. Other types of foods, such as beans and milk products, are also pretty high in carbohydrates.
Carbohydrates are important in our sordid little tale of obesity due to their captaincy position in directing insulin to do its deeds. All carbohydrates that we eat, whether whole grains or white bread or Pepsi, go through our tummies and digestive organs, where some are released immediately (glucose), and others are kept around for further processing (into glucose, eventually). Once they are released, your blood sugar levels rise, signaling the pancreas to release insulin. Insulin is a growth hormone and leads the way in causing a whole host of reactions in our body - its main role is to help us store glucose, fat, and any extra protein we eat as fat. So if we eat a lot of carbohydrates and we don't burn them immediately by being Tour de France bike racers, we store it. Some glucose is stored in the muscles for short term reserves, but the rest is processed by the body and goes into the fat stores.
Here's where the fallacy of high carbohydrate, low fat diets really hit home, especially for anyone with type II diabetes, prediabetes, or insulin resistance (possibly anyone with abdominal obesity) Any kind of high carbohydrate diet will keep insulin levels high unless you really cut back on the calories, or you exercise like a maniac (1). Insulin is vital, but having too much floating around is implicated in type II diabetes, obesity, atherosclerosis, depression, dementia, and all those other diseases of Western civilization we spend all our free cash on as we get older. And guess what, the body has a nasty habit of cutting your metabolic rate when you cut calories, by lowering your base body temperature, making you lethargic so you move less, and pulling in a whole host of hormonal and neural changes designed to make you EAT. Your body doesn't care if you have several months' worth of extra fat - if you go super low calorie and rely on glucose for your primary fuel, you will feel as if you are starving. Same thing happens, unfortunately, when you exercise a lot. You can't help it. Every evolutionary driven instinct will have you eat more, and then you eventually gain weight.
A high carbohydrate, high fat diet is even worse. Carbs are the captain, after all. Sugar and starch tell your body to store all the carbs AND the fat you eat as fat!
However, carbohydrates aren't the whole problem. Let's look at the Kitavans in Papua New Guinea (2). they are a modern hunter-gatherer people who subsist mostly on a diet of starchy root vegetables and coconuts. High carb and tons of saturated fat. Also, 76% of the men and 80% of the women smoke, as tobacco has been farmed on the island for 100 years. They don't get more exercise than a typical westerner who has a physical job, or a moderately active exerciser who has a less active job. And guess what - no western disease! No angina, heart attacks, strokes (even though they have high cholesterol, due to the coconut diet), no diabetes, no cancer (except some squamous and oral cancers due to the tobacco and habits of chewing betel nuts - but no lung cancer!). The average BMI on the island for women is around 18.5 ("model thin"), and for men around 20. Their cholesterol and blood pressure do not rise with age as westerners do. These observations were made by doctors, not anthropologists. They tend to die of infections, accidents, homicide, or quietly of old age after fatigue for a few days. There is no malnutrition - leftover food abounds and is fed to the numerous dogs (3).
Also, let's look at the Raramuri of the Copper Canyons of Mexico, who eat mostly a corn meal mush, an antioxidant drink made of chia seeds, lime juice, and sugar, and corn beer. The kids also play a game before school that involves running 8 miles, and whole groups in the various villages routinely participate in footraces of 50-200 miles. Guess what - their hearts are in fine shape (according to McDougall, but then, I haven't exactly seen a CT scan of their coronary arteries or anything), and blood pressure is nice and low (4).
The Raramuri health can be explained by all the exercise (and running for them I'm guessing is low intensity - long distance high intensity exercise has been associated with developing atherosclerosis and inflammation (5) ), but what about the Kitavans? It must be something about the kind of starchy carbs they eat - no grains, no potatoes, no rice, no corn.
Most hunter-gatherer populations (in modern times or historically) do not eat as the Raramuri or the Kitavans do - they tend to eat mostly fruits, veggies, meat (lean grassfed meat, but also organ meats and bone marrow loaded with saturated fat!), fish, shellfish, nuts, seeds and some of those starchy root vegetables...