> Key et al. (10) found that the risk of diabetes in vegans was 47% lower than in meat-eaters. This was reduced to 1% after adjusting for BMI, implying that lower BMI explains the lower risk in vegans. In the economic model, we use the BMI unadjusted risk of diabetes to account for the reduced weight and lower BMI of vegans compared to meat-eaters, which are both risk factors associated with type 2 diabetes (33). [1]

Apparently, vegans have lower BMI and this reduces their risk of diabetes significantly. To your point about over-indulging (tending to obesity)..

> There is good evidence that plant-based diets are associated with a reduced risk of obesity. For example, Chiu et al. (31) found that each additional year of a vegan diet lowered the risk of obesity by 7% for Taiwanese adults, and similar results have been reported for the UK (34). However, our base case analysis excludes obesity from the model because we expect obesity to be a key risk factor overlapping with other disease outcomes captured elsewhere. This is especially true for type 2 diabetes, where it is likely that we would be double-counting any benefits associated with a plant-based diet. [1]

That all said, I realize now that the sourced paper is a preprint and hasn't been peer reviewed yet. It's also funded by the Vegan Society so might be considered bias by some.

[1] https://www.medrxiv.org/content/10.1101/2023.12.26.23300536v...