In 1917 Rebecca Oppenheimer published “Diabetic cookery; recipes and menus” which described what was already known about the optimal diet for diabetics — more butter, olive oil, meats, cheeses, eggs, and healthy fats (low in carbohydrates), and less sugar, starch, bread, pasta, and other foods that raise blood sugar (due to high carbohydrate content).

This is essentially a low carb / high fat (LCHF) diet, which not only helps diabetics to reduce their blood glucose, but is also effective for weight loss and other metabolic conditions.

The book can be found online at Archive.org and you can download it in PDF format here.

Here are pages 12 and 13 from the book.