Well I am a Dr of political sociology not medicine, so I can't advise the person who mailed in about diet and diabetes I am afraid.
Vegetarianism and vegan seem of interest, here is something I wrote in 1988 after finishing my archaeology degree.
The Diet of Early Humans
Vegetarianism and Archaeology
Derek Wall examines the "mighty hunter" myth of human ancestry
from The Vegetarian, September/October 1988, published by The Vegetarian Society UK:
Archaeology and vegetarianism are, at first sight, a rather unlikely combination; most people if asked to consider the diet of our ancestors would tend to conjure up images of cavemen roasting mammoth steaks or early medieval monarchs spitting venison over a roaring fire, not a lentil in sight. Many academics have taken these simplistic visions to their logical and dangerous conclusion; to argue, that in the past we have eaten meat, therefore eating meat is 'natural' and that vegetarianism is an unhealthy regression to the period when we were full fruitarians and swung from tree to tree.
Perhaps even more disturbing is the view that we have only become human through eating meat, that according to people like Raymond Dart writing as long ago as the 1930s, it has only been through hunting, aggression and violence that we have eaten non-carnivorous rivals in the evolutionary battle of the fittest. According to Dart and others, changes in human and early hominid dentition show that our teeth adapted to chewing meat. Happily more recent investigation tells a different story; Dr Clifford Jolly suggests that the real evolutionary transition came when our ancestors left the tropical forests of central Africa and took to the open savannah, shifting from a diet made up mostly of fruit to one based on seeds and grains, our ancestors' teeth adapting to cope with the relatively hard particles that needed a lot of grinding down before they could be digested.
Despite this, nutritionalist John Yudkin claims that for 99% of our existence we have been hunters with an 'ideal' diet where '. . . people tend to have a quite high proportion of meat.' Yudkin goes on to draw the conclusion that we suffer nutritional problems today (especially allergies) only to the extent we have shifted from this all animal diet with the occasional root or tuber thrownin. A major pitfall associated with this line of reasoning is the fact that much illness is caused by over consumption of animal fats; heart disease cancer, weight problems . . And again the archaeological evidence tells another story as does the existence of so-called 'hunters' in the modern world.
Groups such as the Kalahari bushmen and the Australian aborigines are not so much hunters as 'hunter-gathers', gathering much of their diet in the form of roots and tubers, seed grains, fruit, nuts and other nutritious plant products. Gould, who spent some time studying the aborigines of the Western Desert, states quite clearly that, 'The diet is primarily vegetarian'. In a very detailed study of the Kalahari bushman's diet it is revealed that: 'The proportions by weight of vegetable food and animal food in the total diet are, respectively, 81.3 per cent cent and 18.7 per cent. If the plants taken as water sources (such as melons and tubers) are included in the vegetable food count, the ration of animal food to vegetable food is even lower . . . Although the proportion of animal food of the total (18.7 per cent) diet is quite large, the Kade San can survive in the Kalahari without it, whereas they could not survive without vegetable food.'
In fact, out of existing hunter-gathers and those recorded by early anthropologists (before we made them extinct), only the Eskimos/Innuit, living in a climate where they have little choice, eat anything like the proportion of meat we consume today in Western society. This said we can't have it all our own way, there have probably been as few pure vegetarians as 20th century European style carnivores amongst our hunter-gatherer ancestors. Even so, if we go by the evidence of modem hunter-gathers, our ancestors probably ate meat in a more sensible fashion than that of present. Meat tended to be, as we have seen, only a very small part of diet, sauce to make the vegetables and grains more palatable rather than the other way round. Hunter gathers also tend to treat their prey with rather more respect than the way we treat our poor factory farmed, hormone and antibiotic ridden livestock. The Ainu of Japan traditionally pray to the spirits of the animals they kill and ask their forgiveness; similar practice is known amongst both North American Indians and African bushmen.
Any way, back to the strictly archaeological evidence. Can we tell for certain what our ancestors in the very distant past before the existence of written records ate? Dentition gives us at best only a very rough idea and anthropology provides only possible parallels. Food remains found in the course of archaeological 'digs' are a help but tend to be biased to animal products because, in most conditions, bone is far better preserved than highly biodegradable vegetable matter, if we excavated a Kalahari bush camp, abandoned for the sake of argument for 50 years (a tiny span of time in archaeological terms), we would find bones from the occasionally eaten gazelle but would miss almost entirely the staple gongo nuts or the 50 other plants exploited from the desert as food.
Tools used for food preparations may help as well, but flint 'tool' to take one example, have tended to be misinterpreted by meat eating archaeologists. Palaeolithic (old stone age) axes originally thought to be butchering tools would have been just as servicable for digging up root vegetables. In a paper under the title 'Mesolithic Europe - the economic basis', Clark shows how middle stone age people in Britain could have exploited nuts, fungi and a rich variety of plant foods from a landscape which has since become so degraded by human damage, that we have overlooked this vegetarian possibility almost entirely. He goes on to show that flints previously interpreted as tips of hunting arrows, may have components of composite vegetable grating boards!
Since the arrival of farming, the written word and 'civilisation' in general some 7,000 years ago, archaeologists have been able to discuss the diet of our more recent ancestors with more certainty than that of earlier stone age peoples. The Aztecs and Incas combined maize, beans and squash, so that the different amino acids in the maize and beans could be complemented by the carbohydrate content of the squash. Classical India was vegetarian, as was Japan up until a generation or two ago. The staple of Egyptian workers building the Pyramids was boiled onions. Pythagoras was a vegetarian, although he had a weird distaste for beans. Even the Roman army marched on its vegetarian stomach. It is clear that 90% of humanity have subsisted on a 90% vegetarian diet. Modern carnivorous men and women are the exception not the rule.