The late Medieval shift away from carbs and toward meat
The past may have always been worse than the present, but some periods were better than others. And as Thomas Malthus showed, what made for an enjoyable era was plenty of disease, war, and other disasters beforehand -- to clear out a good chunk of the population, leaving much more stuff to go around per person among the survivors.
The 14th C. was overall a very calamitous century, so that during and just after the myriad disasters that plagued it, signs of the good life abounded. How would this affect the diet? Nutritionists from roughly 1950 onward would predict that very little animal fat -- or perhaps animal products at all -- would have been consumed, and that they would have enjoyed a diet based mostly on grains and cereals, and then on fruits and vegetables. (Recall the FDA's food pyramid and its large base of bread, pasta, and cereal.)
These nutritionists are completely ignorant of human evolution, physical anthropology, as well as recorded history. Not surprisingly, they've got it completely backwards -- their recommended diet will keep your insulin levels high chronically, causing you to store fat rather than burn it for fuel, not to mention all the other side-effects of a carbohydrate-rich, fat-deficient diet. (For a good review, read Gary Taubes' Good Calories, Bad Calories or watch a video if you're lazy.) It's no wonder, then, that the dietary sign of higher standards of living in the late Middle Ages was the exact opposite of the nutritionists' prescriptions -- cutting way back on bread and loading up on dead animals.
In this post I mentioned that Gregory Clark's book A Farewell to Alms has a table showing that English farm workers in the late Middle Ages got a fair portion of meat to supplement their wages. In fact, not only did they eat a lot more meat from 1350 to 1450 compared to 1250 to 1350 -- one pound a day! -- they were also eating a lot less bread. So, unlike present-day bodybuilders who hamstring their ability to pack on muscle and burn lots of energy (by carb-loading), late Medieval farm workers were not eating foods that would keep their insulin levels chronically high.
Rather than yammer on at greater length about this change, I'd rather point you to a free copy of the academic article that Clark's table mostly draws from: go to this list of articles and Ctrl F "Dyer." The article is "Changes in diet in the late middle ages: the case of harvest workers." He cautions that autumn farm workers were better off than other laborers, that even they only enjoyed this diet during their autumn work schedule, etc., but the overall change from roughly 1250 to 1450 is pretty clear.
He also provides data showing that elite people, such as the prioress of a nunnery, ate much less bread and much more meat than the lower orders. Dyer has more data on how diet varied throughout the social ladder, and the pattern holds up there too. See his book chapter "English diet in the later middle ages" in this volume, as well as the discussion of diet in his book Standards of living in the later Middle Ages. Barbara Harvey's book Living and dying in England, 1100 - 1540 is about the monks of Westminster Abbey, an elite group. In the chapter on diet, she provides data showing that during the 15th C., they ate a fair amount of meat and not as much bread as Dyer's farm workers from 1250 -- probably above the farm workers of 1450 but below the higher elites.
As I mentioned before, this is a pretty general pattern -- animal products, especially good muscle and organ meat, are more expensive to produce than grain products. So, the elite have always been less reliant on empty carbs, and enjoyed more animal protein and fat, than the commoners. This is why the notion that elites used to be fat or even obese, while the commoners used to be thin, is nonsense. As a rule, they never have been. By consuming so much of their food in the form of non-fibrous carbohydrates, the commoners of the Middle Ages would not have looked very different from today's Wal-Mart shoppers.
A curious thing in Dyer's article is that he seems to think that when people ate more meat, they must have had a vitamin A deficiency, since their new diet also saw a decrease in the amount of dairy they ate. While you can get a decent amount of vitamin A from dairy (usually 5 - 10% of the RDA per serving of butter, milk, cream, or cheese), the key source has always been animal livers. To see this, here is a tool to list foods by how much of some substance they have. Click the "highest in" bar, and scroll down until you see retinol (under vitamin A), and search. Aside from dry cereals (which are irrelevant since once you add everything else to them, their weight will shoot up, and the concentration of vitamin A will plummet), the high-scorers are all from animal organs, especially liver.
True vitamin A is only found in animal products -- the stuff in spinach, carrots, etc. is just a precursor to vitamin A, and is not converted with 100% efficiency into vitamin A. (See the Wikipedia entry for vitamin A.) Vitamin A is fat soluble, so that the excess can be stored away in our fat, although the bulk of the not-currently-in-use vitamin A is stored in the liver.
So, the simple way to get plenty of this vitamin is to steal it. Find an animal that has spent all day processing the plants that are rich in the precursors -- this animal will have created true vitamin A from all this junk, and it will have stored most of the unused portion in its liver. Kill this animal and eat its liver -- and boom, you've hit the vitamin A jackpot. And all without letting a single leaf of spinach enter your mouth.
Returning to Dyer's article, he mentions that the farm workers also ate the offal of animals, not just the muscle meat. And the elites surely did too. If this included liver -- and that's probably true, since it's been prized forever (including among present-day hunter-gatherers) -- then they would've had plenty of vitamin A.
At any rate, the important thing to take home is that elites have always eaten better than commoners, in particular eating fewer easily digestible carbs and more animal protein and fat, so they were never fatter than the commoners. I don't know where this image of the "well fed, rotund aristocracy" came from, but look at who is well fed today -- middle class French people don't look obese at all, while American proles may soon be required to purchase two airline tickets for their one body.
Since the 14th C was a time of improving standards of living -- starting decades before the Black Death, but particularly so after the plague cleared away a bunch of the survivors' would-be labor competition -- these data also show that a lower standard of living, as during the 13th C., is characterized by eating lots of empty carbs and hardly any animal fat and protein, while in better times, such as the 14th and 15th Cs, people can finally junk all of that tasteless bread and dig in to beef, lamb, mutton, liver, and the rest.