If you are diabetic or pre-diabetic, you need help sorting your nutritional needs. So individualized counseling is important. Individualized counseling has reduced the Hba1c level (blood sugar measured over 3 months) by 2%!
Mediterranean diets for adults with Type 2 diabetes works well, and compared to those on other diets studied, most diabetics did the best with this diet.
Low Carbohydrate diets. This is defined by getting less than 45% of your calories from carbs. First of all, these can be very hard on the kidneys, and diabetics may have kidney problems. While some diabetics can do well on these, and some can even lose weight quickly on these it is suggested you be monitored. Both the Hba1c and the blood fat triglyceride decreased on this. Those who also cut calories will lose weight.
Paleolithic diets do better than old fashioned diabetic diets, and nearly all measures of diabetes lab tests got better.
Glycemic index diets involve eating foods that are known to have a low glycemic index. (skip potatoes, wheat flour, most breakfast cereals and rice). Most diabetics can control their sugar levels while eating this way verses their traditional diet, however, most diabetics do not lose weight on this type of a diet.
Vegan diets, like the glycemic index diets help diabetics control their sugars and blood lipids like cholesterol, but diabetics do not lose weight just by switching to vegan.
The studies that looked into these diets, generally are comparing the effects of the diet alone, and did not try to look at strategies such as lifestyle changes or CoolSculpting.