Dr Brunton describes meat, fish, eggs, and fat (as well as low carb veggies) to treat diabetes
Lectures on the Pathology and Treatment of Diabetes Mellitus
T. Lauder Brunton
"The patient must be supplied with a diet consisting of nitrogenous food, such as butcher-meat, fish, eggs, and soups. Fat (which does not contribute in the least to the formation of sugar) may be given in all its forms, such as cream, butter, cheese, and oil.
Spinach, lettuce, and cresses may be freely used, but celery and radishes only sparingly; while potatoes, carrots, parsnips, turnips, peas, French beans, cabbage, Brussels sprouts, cauliflower, brocoli, asparagus, seakale, and fruit of all kinds, both fresh and preserved, should be avoided, with the exception of nuts and almonds."