Non-industrial cultures have an extremely low prevalence of diabetes, whether they are near-vegan or near-carnivorous. This is supported by blood glucose measurements in a variety of cultures, from the sweet potato farmers of the New Guinea highlands to the arctic Inuit hunters. Here is what Otto Schaefer, director of the Northern Medical Research Unit at Charles Camsell hospital in Edmonton, Canada, had to say about the Inuit in the excellent book Western Diseases (Trowell and Burkitt, 1981):
In Canadian Eskimos no case of diabetes mellitus has yet been reported in the traditional-living central and Eastern arctic regions; but an increasing number of diabetics have been reported from the Western arctic, especially in the more acculturated Eskimos living in the Mackenzie delta. Age-adjusted prevalence rates even in this area are less than one-third of those reported in most Western nations. Comparable upward trends in prevalence have been reported in Alaska and Greenland Eskimos. The prevalence of diabetes is reported to be rising slowly in Alaskan Eskimos (Mouratoff and Scott, 1973). Recently a very high prevalence rate was reported in an Eskimoid population subjected to a long period of acculturation, the Aleuts of the Pribilof Islands (Dippe et al., 1976).The low prevalence of diabetes in traditionally-living cultures is consistent with their leanness, high levels of physical activity, and younger mean age (although they have a very low prevalence of diabetes even when age is taken into account). These three factors, plus genetics, explain the large majority of type 2 diabetes risk.
The Adventist Health Study examined the health of vegans, vegetarians, semi-vegetarians, pescetarians, and omnivores in a Seventh-Day Adventist community in California. All non-omnivore diet groups had a reduced diabetes risk compared with omnivores, but vegans had the lowest risk (1). After adjusting for BMI, physical activity and other measured confounding factors, the relationship was attenuated but vegetarians and vegans still had a lower diabetes risk than omnivores-- about half the risk of omnivores in the same community. This suggests that the difference in risk is related to their relative leanness, as well as other factors related to diet and/or lifestyle. After adjustment, vegans did not have a significant advantage over lacto-ovo vegetarians in the initial study, although they did in a follow-up study (2).
What about specific types of meat? In general, unprocessed poultry and seafood consumption have neutral associations with diabetes risk (3, 4, 5, 6, 7, 8). Unprocessed red and processed meat, on the other hand, tend to be associated with increased risk, although the association with unprocessed red meat is very weak (9, 10, 11).
While we're at it, I may as well cover dairy. Dairy intake typically has neutral or favorable associations with diabetes risk (12, 13, 14). In 2013, my colleagues and I published a review paper on the association between high-fat dairy consumption and obesity and cardiometabolic disease risk (15). We conducted a comprehensive literature review, and found that the results on diabetes were mixed. Some found harmful associations, others found protective associations. This year, we updated the analysis, identifying seven new studies. Of the seven, three reported protective associations, three reported no association, and one reported a harmful association (16, 17, 18, 19, 20, 21, 22). The literature overall provides little support for the idea that dairy, including full-fat diary, contributes to diabetes risk.
As always, it's worth keeping in mind that these are observational studies, and it isn't necessarily easy to tease apart the health effects of meat from everything else that comes along with eating it. This is particularly true for culturally distinct groups such as the Seventh Day Adventists.
Body weight is probably one mechanism that links processed meat to diabetes risk. Processed meat tends to be calorie-dense and highly palatable, so it lends itself to overconsumption.
The association between unprocessed red meat consumption and diabetes risk is so weak it's hard to take seriously, but there is actually a plausible mechanism by which red meat (processed and unprocessed) could promote type 2 diabetes in some people: excess iron. Red meat is a particularly rich source of highly bioavailable dietary iron-- which is helpful in certain situations-- but excess iron can degrade metabolic control.
Several lines of evidence support this conclusion. Serum ferritin is the best blood marker of iron status, and it's strongly correlated with the risk of developing diabetes and metabolic syndrome (23, 24, 25, 26). Iron depletion by repeated blood removal improves glucose control in diabetics and non-diabetics (27, 28). Iron overload can be a problem for men and post-menopausal women, while pre-menopausal women often benefit from higher iron intake. Eating less red meat, avoiding iron supplements, and avoiding iron-fortified cereals are three effective ways of reducing iron intake. The body does not have any natural way to dispose of excess iron once it's absorbed. Blood donation can help reduce the body's iron load, but it must be done regularly for a prolonged period of time to be effective.
Vegan diet interventions
Neal Barnard has been involved in several low-fat vegan diet trials with type 2 diabetics (29, 30, 31). These trials show that a very low-fat vegan diet causes weight loss and improves blood glucose control more effectively than commonly used diets for diabetics.
One might be tempted to conclude that avoiding animal foods improves glucose control, but when we consider the literature as a whole, that explanation seems unlikely. Older studies show that very low-fat diets high in unrefined carbohydrate improve glucose control in diabetics at least as well as a low-fat vegan diet, and although they were low in animal foods, they were not vegan (32, 33, 34). These older studies found that many patients are able to discontinue insulin therapy after 1-2 months on the diet, due in part to increased insulin sensitivity. Furthermore, the Paleo diet has been shown to improve glucose control more effectively than standard diabetes diets as well, and lean meat is a key component of these trials (35, 36, 37).
Although low-fat vegan diets can improve glucose control, there is currently little evidence that the effect is directly related to meat avoidance. The effect may instead be attributable to 1) weight loss, and 2) the insulin-sensitizing effect of very low-fat diets high in unrefined carbohydrate.
Synthesis and conclusions
Poultry, seafood, and dairy consumption don't appear to be linked to type 2 diabetes risk, nor is there any clear mechanism linking them to diabetes risk (to my knowledge). Processed meat may increase diabetes risk, and I speculate that this is mostly due to its ability to promote overconsumption and weight gain. Unprocessed red meat may contribute to diabetes in some people due to its ability to increase body iron stores.
However, the potential effect of red meat appears to be very small, and it's important to keep it in perspective. Obesity, age, physical activity, and genetics are the dominant risk factors for type 2 diabetes. Everything else is probably peanuts.