Many doctors believe in cutting off entirely or almost entirely that class of food which contains starch and sugar - namely, the carbohydrates. Others believe it is necessary and even best to only omit the quantity taken. Some of our commonest articles of diet contain starch : for example, bread, potatoes, peas, beans and turnips. The best guides for the regulation and restriction of the diet are (1) body - weight (2) condition of the urine, both as regards the quantity passed and the amount of sugar contained therein.
It never fails to amaze me how the knowledge of our elders took a backseat when that (true) miracle, injected insulin, came along. The diet therapy was learned through trial-and-error, and science, over the course of more than a century in what amounted to experiments on real people. And it worked, on some people. This was well known and had been quite well refined by the early 1920s.
Diabetes back then was usually fatal, as your mum's nursing handbook makes clear, with many not surviving beyond about four years. But they also knew that a few of the people developing diabetes in middle age (who would almost all have been Type 2s) could enjoy a "more or less complete cure" with dietary changes. We now know that the word "cure" is misleading in that context, but the gist of that is correct.
Funny thing is, this knowledge never went away. I remember people talking about it when I was a child. This may have been because my British grandfather was a classic hypochondriac (and a former civil servant in various health-related roles) and was always talking knowledgeably about various diseases.
Anyway, when my diabetes diagnosis came (in a snail-mailed letter) and before I saw my doctor, my wife asked, "do you need to change your diet?" So I said, "nah, don't think so, that was the way diabetes was treated in the old days" and assumed (incorrectly, as it turned out) that the doctor would be prescribing pills! Of course it might have been appopriate to do so if control could not be achieved otherwise, but for the time being the "old ways" are working fine for me and, judging from this forum, a fair number of other people.
And of course the discovery of insulin was an absolute miracle, no doubt about it. Someone has probably, by now, figured out how many lives it has saved in the past century -- it must be in the tens, possibly hundreds, of millions by now. (Not to mention the other diabetes meds developed over the past few decades.)