There are a number well known low-carb diets. The most widely cited is that of the Inuit, but the Masai are often mentioned as well. I came across another example in Jack Weatherford’s Genghis Khan and the Making of the Modern World (see here for earlier discussion).
Mongols lived off of meat, blood, and milk paste. This diet, as the Chinese observed, allowed the Mongol warriors to ride and fight for days on end without needing to stop for meals. Part of this is because they could eat while riding, but there is a more key factor. This diet is so low-carb as to be ketogenic. And long-term ketosis leads to fat-adaptation which allows for high energy and stamina, even without meals as long as one has enough fat reserves (i.e., body fat). The feast and fast style of eating is common among non-agriculturalists.
There are other historical examples I haven’t previously researched. Ori Hofmekler in The Warrior Diet, claims that Spartans and Romans ate in a brief period each day, about a four hour window — because of the practice of having a communal meal once a day. This basically meant fasting for lengthy periods, although today it is often described as time-restricted eating. As I recall, Sikh monks have a similar practice of only eating one meal a day during which they are free to eat as much as they want. The trick to this diet is that it decreases overall food intake and keeps the body in ketosis more often — if starchy foods are restricted enough and the body is fat-adapted, this lessens hunger and cravings.
The Mongols may have been doing something similar. The thing about ketosis is your desire to snack all the time simply goes away. You don’t have to force yourself into food deprivation and it isn’t starvation, even if going without food for several days. As long as there is plenty of body fat and you are fat-adapted, the body maintains health, energy and mood just fine until the next big meal. Even non-warrior societies do this. The meat-loving and blubber-gluttonous Inuit don’t tolerate aggression in the slightest, and they certainly aren’t known for amassing large armies and going on military campaigns. Or consider the Piraha who are largely pacifists, banishing their own members if they kill another person, even someone from another tribe. The Piraha get about 70% of their diet from fish and other meat, that is to say a ketogenic diet. Plus, even though surrounded by lush forests filled with a wide variety of food, plants and animals, the Piraha regularly choose not to eat — sometimes for no particular reason but also sometimes when doing communal dances over multiple days.
So, I wouldn’t be surprised if Spartan and Roman warriors had similar practices, especially the Spartans who didn’t farm much (the grains that were grown by the Spartans’ slaves likely were most often fed to the slaves, not as much to the ruling Spartans). As for Romans, their diet probably became more carb-centric as Rome grew into an agricultural empire. But early on in the days of the Roman Republic, Romans probably were like Spartans in the heavy focus they would have put on raising cattle and hunting game. Still, a diet doesn’t have to be heavy in fatty meat to be ketogenic, as long as it involves some combination of calorie restriction, portion control, narrow periods of meals, intermittent fasting, etc — all being other ways of lessening the total intake of starchy foods.
One of the most common meals for Spartans was a blood and bone broth using boiled pork mixed with salt and vinegar, the consistency being thick and the color black. That would have included a lot of fat, fat-soluble vitamins, minerals, collagen, electrolytes, and much else. It was a nutrient-dense elixir of health, however horrible it may seem to the modern palate. And it probably was low-carb, depending on what else might’ve been added to it. Even the wine Spartans drink was watered down, as drunkenness was frowned upon. The purpose was probably more to kill unhealthy microbes in the water, as was watered down beer millennia later for early Americans, and so it would have added little sugar to the diet. Like the Mongols, they also enjoyed dairy. And they did have some grains such as bread, but apparently it was never a staple of their diet.
One thing they probably ate little of was olive oil, assuming it was used at all, as it was rarely mentioned in ancient texts and only became popular among Greeks in recent history, specifically the past century (discussed by Nina Teicholz in The Big Fat Surprise). Instead, Spartans as with most other early Greeks would have preferred animal fat, mostly lard in the case of the Spartans, whereas many other less landlocked Greeks preferred fish. Other foods the ancient Greeks, Spartans and otherwise, lacked was tomatoes later introduced from the New World and noodles later introduced from China, both during the colonial era of recent centuries. So, a traditional Greek diet would have looked far different than what we think of as the modern ‘Mediterranean diet’.
On top of that, Spartans were proud of eating very little and proud of their ability to fast. Plutarch (2nd century AD) writes in Parallel Lives “For the meals allowed them are scanty, in order that they may take into their own hands the fight against hunger, and so be forced into boldness and cunning”. Also, Xenophon who was alive whilst Sparta existed, writes in Spartan Society 2, “furnish for the common meal just the right amount for [the boys in their charge] never to become sluggish through being too full, while also giving them a taste of what it is not to have enough.” (from The Ancient Warrior Diet: Spartans) It’s hard to see how this wouldn’t have been ketogenic. Spartans were known for being great warriors achieving feats of military prowess that would’ve been impossible from lesser men. On their fatty meat diet of pork and game, they were taller and leaner than other Greeks. They didn’t have large meals and fasted for most of the day, but when they did eat it was food dense in fat, calories, and nutrition.
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Ancient Spartan Food and Diet
from Legend & Chronicles
The Secrets of Spartan Cuisine
by Helena P. Schrader
This is what Mongol MREs looked like
by Cam Rea
Marmalade 
The Original Mediterranean Cuisine: Medieval Recipes for Today – Barbara Santich
https://books.google.com/books?id=JhKmCa_w9c0C
Here’s a book with original Mediterranean recipes, before all those newfangled ingredients like tomatoes started being introduced.
Do you have that book? If so, I’d be curious about what were the most common ingredients.
Of course, the food eaten might have been far different from the earlier time of the Spartans. By the Medieval period, agriculture had spread much further in creating grain surpluses and making bread more available. Even during the Axial Age of the Spartans, bread was more common than during the Bronze Age. I read about the first accounts of white bread being in the Axial Age, although white bread didn’t become affordable to most Westerners until the past few centuries.
It’s those changes across history that interest me the most. There is a strong correlation between changes in diet and changes in culture and psychology. It’s hard to prove that correlation is causation, but such an argument is plausible.
When you look at numerous societies across the millennia, food laws and regulations are among the most important. Maybe that is because diet always has been and remains one of the central ways of maintaining a particular social order and way of being in the world. What if diet combined with other factors can fundamentally alter our sense of reality? Consider the Australian Aboriginal songlines that often involved knowledge and customs about food, and that was a radically and profoundly different way of experiencing the world. Maybe the mnemonic systems were inseparable from the food system.
I know that is hard for most people to imagine. But what if we can’t imagine a different kind of society is for the simple reason that we are trapped in the reality tunnel of our shared diet and food system?
Here is the central correlation I’ve observed. Notice how many of these traditional societies were communal compared to later agricultural societies. Maybe their traditionalism had something to do with diet, not only that the Spartans ate communally but that maybe what they ate supported that communal identity and behavior. Modernity came with a simultaneous increase of grains and increase of hyper-individuality. But as far as I know, no one has yet explored this correlation, as it doesn’t fit into any specific field of study.