My statements about carbohydrate and insulin in the previous post seem to have kicked up some dust! Some people are even suggesting I've gone low-fat! I'm going to take this opportunity to be more specific about my positions.
I do not think that post-meal insulin spikes contribute to obesity, and they may even oppose it. Elevated fasting insulin is a separate issue-- that's a marker of insulin resistance. It's important not to confuse the two. Does insulin resistance contribute to obesity? I don't know, but it's hypothetically possible since insulin acts like leptin's kid brother in some ways. As far as I can tell, starch per se and post-meal insulin spikes do not lead to insulin resistance.
I am not suggesting that low-fat diets are the ultimate path to health, but I do think they can be compatible with health in most people, if carefully composed. In addition, they may have benefits in certain situations, for example if you want to reduce food reward. Reducing carbohydrate is another way to do that, and the effectiveness of each method may depend on individual differences.
Humans are adapted to eating starch. Hunter-gatherers show genetic evidence for selection for starch tolerance relative to other primates, and agricultural populations show even more (1). The vast majority of people who are reading this descend from agricultural populations that ate high-starch diets for thousands of years. Although there's still some controversy, it appears that modern Europeans descend mostly from agricultural populations that immigrated from the Middle East and replaced European hunter-gatherers (2, 3, 4, 5, 6), and needless to say European hunter-gatherers didn't contribute much to the genetic makeup of people of more recent African origin, or native Americans.
Related the the last point, agricultural and pastoral humans have undergone significant genetic adaptations since the development of agriculture/pastoralism. That does not mean you can live on white bread and soda and be healthy.
Even if our hunter-gatherer ancestors on average ate less carbohydrate than we do today, since hunter-gatherer diets are highly variable, we have been selected over millions of years to be able to tolerate wildly different macronutrient ratios. Maybe mongongo nuts were the main available food for three months (like the !Kung), but during another season all there was to eat was starchy tubers. If you can't adapt to that, you die out, so our broadly omnivorous dietary pattern has selected us to be very tolerant of differing macronutrient ratios. Our metabolism is highly attuned to coordinating the appropriate metabolic response to differing carbohydrate-to-fat ratios, and insulin is central in orchestrating that. That's reflected in the fact that cultures have thrived on practically nothing but carbohydrate (New Guinea highlanders, etc.) as well as mostly fat (Inuit, etc). Other animals are not necessarily endowed with the same metabolic flexibility due to their more restricted diets. If your body evolved to tolerate eating 70% carbohydrate for a month, it's not that big of a stretch to think a culture could begin eating it year-round without any genetic modifications.
Since the ancestors of most people reading this have probably been eating more starch than fat for a very long time, at a minimum thousands of years, but probably closer to a million (because African game meat tends to be pretty lean, and most peoples' ancestors never passed through far Northern latitudes where fat calories predominate), I think the "null hypothesis" should be that humans are best adapted to diets where starch predominates over fat. In other words, that should be the default hypothesis that requires evidence to disprove. The fact that there are so may healthy high-starch cultures, far more than there are high-fat cultures, adds to the weight of the evidence.
That does not necessarily mean eating more fat is unhealthy, it's just a very rough evolutionary framework. I do believe some people legitimately do better on higher-fat diets, and some people do better without grains specifically, particularly wheat. I believe that a diet based mostly on freshly cooked starchy tubers/roots such as potatoes and sweet potatoes is probably superior to one based mostly on grains for most people. Treating grains by traditional methods such as soaking, grinding and fermentation improves their nutritional value (7). I still think pastured butter, red palm oil, virgin coconut oil and extra virgin olive oil are healthy, as well as the fats contained in quality meats.
Let the discussion begin!