The short version
Ask whether plant-based eating lowers diabetes risk and the cleanest number comes from Adventist Health Study-2, a US cohort of more than 60,000 people. The rate of type 2 diabetes climbed steadily as diets included more animal food, and the gap between the ends was wide.
Primary source 01 / 05
Prevalence of type 2 diabetes increased from 2.9% in vegans to 7.6% in nonvegetarians; the prevalence was intermediate in participants consuming lacto-ovo (3.2%), pesco (4.8%), or semi-vegetarian (6.1%) diets.
Vegans carry less body weight on average, and weight drives diabetes risk, so the obvious question is whether the diet did anything on its own. The authors adjusted for BMI and lifestyle and the protection held.
Primary source 02 / 05
Increased conformity to vegetarian diets protected against risk of type 2 diabetes after lifestyle characteristics and BMI were taken into account.
This is observational, so it shows association rather than proof of cause. What it shows is consistent and graded: more plants, less diabetes, across four dietary patterns in the same cohort.
The trials: help, not a cure
Prevention is one question; treating people who already have diabetes is another, and that needs a randomized trial. Barnard and colleagues ran one for 74 weeks, comparing a low-fat vegan diet against the American Diabetes Association's own 2003 guideline diet. The headline blood-sugar change was modest and, in the simplest analysis, not statistically distinguishable between the two diets.
Primary source 03 / 05
Hb A1c changes from baseline to 74 wk or last available values were -0.34 and -0.14 for vegan and conventional diets, respectively (P = 0.43).
Both groups improved. When the researchers accounted for the medication changes that happen in any real diabetes trial, the vegan diet came out ahead on glycemic control, and they were careful about how far to push that.
Primary source 04 / 05
In an analysis controlling for medication changes, a low-fat vegan diet appeared to improve glycemia and plasma lipids more than did conventional diabetes diet recommendations. Whether the observed differences provide clinical benefit for the macro- or microvascular complications of diabetes remains to be established.
Where the position papers land
The Academy of Nutrition and Dietetics, reviewing the whole literature, states the direction plainly.
Primary source 05 / 05
Compared with meat eaters, lacto-ovo-vegetarians and vegans have lower risk of type 2 diabetes.
Read together, the evidence is strong on prevention and encouraging on management. A plant-based pattern tracks with markedly less diabetes in large cohorts, and as a treatment it holds its own against the standard diabetes diet, with a real but modest edge on blood sugar. Calling it a reversal overstates it; calling it one of the better dietary bets does not.