Perhaps the biggest historic attitude-change when it comes to "ideal diet" is not an accent on carbohydrate intake, but the ebb-and-flow opinion about dietary fat.
As i pointed out yesterday, with voices from the past as corroboration, a moderate-to-high-carbohydrate diet has been the norm for a very long time, and the farther into the neolithic past and the further down the socioeconomic ladder you go, the larger the ratio of coarse carb to first-class protein you observe to be eaten.
Fats have always been treasured, though. A source of light as well as of fuel for the body, common people hoarded as much as they could of it. If you weren't rich or holy you couldn't acquire wax candles -- you used a "tallow dip" or put any kind of rough-and-nasty grease in your "betty lamp" or rushlight that you preferred not to eat. Whale-oil wasn't even widely available till comparatively recently, and kerosene (coal oil) is much newer. Interestingly, in the new American colonies, bayberry bushes were protected by law, they were so valuable a source of lighting material.
Even soap-making, as a craft requiring fat, is a newish thing. For centuries, washing clothes (if done at all) was done with a weak lye bath or a mechanical water-only process -- the linens that people often wore for an entire week before laundering contained a significant amount of body-oils already! Certain plants provided an alternative to the lye-and-grease-containing true soaps, and were valued because they spared fats for dietary and lighting purposes. In America, only when animal fats became more abundant with the spread of farming, ranching and buffalo-hunting did people make or buy a goodly amount of soap.
Before the protestant movements in Europe, the church dictated a lot of what kinds of foods people were allowed to eat, and when. Sumptuary laws went further. These often restricted animal foods ... which includes their fats. Ya wanna talk about short and brutish lives? The middle ages beat hell out of the paleolithic.
Of course, "vegetable oils" were in scant supply until very recently. Well, very few plants actually ARE oily, so are only expressible with modern technology. Olive oil was not something that northern Europe or early America saw a lot of, nor coconut or palm oils. Carcass and dairy fats were pretty much IT.
And people knew it was valuable! Tubers and grains may fill the belly, but they don't stick to the ribs very efficiently. Fat fixes that. Native peoples in the New World prized fatty parts of their prey animals for themselves, and threw the lean stuff to the dogs.
It was only when nutritionism invented calorie-counting that fat started to get a bad name amongst people interested in curing obesity. William Banting's version of low-carb doesn't restrict it, but by the 1920s the view seems to be "why consume something you're trying to remove from your body" (paraphrased from Lindlahr's "Eat and Reduce"). Besides, every gram of fat contains NINE calories, as opposed to only four in carbohydrates and proteins.... :-P Decades ago when i started to gain weight a bit, i tried the Lindlahr version of low-carb, but being fat-poor it was very unsatisfying.
Well, with technology and some decent RCTs, we now know an awful lot more about what dietary fat actually does in the body. Banting and Stefansson KNEW it was good stuff, but they didn't know precisely WHY. We do.
Twentieth-century nutritionism was an ignorant, deluded, arrogant asshole, especially as personified by Keys, McGovern, and a few more jerks i could list. They thought their version of science was more enlightened than millennia of observation, trial and error. Boy were they wrong.
Fat is more than just fuel, and especially more than just the sum of its calories. Telling people that eliminating most of it from their diets, and making sure most of what remained was PUFA, was the beginning of catastrophe for public health, in my humble opinion.