The best time to lose weight is when you know you have diabetes. By taking off the pounds, you have a better chance of keeping your blood sugar and blood pressure levels under reasonably good control. If your BMI is over 25, anytime is a good time to start. Mine was 39.6 when I started to manage my weight 10 years after my type 2 diabetes diagnosis. According to the ADA, a weight loss of at least 5 percent benefits overweight or obese adults with type 2 diabetes or prediabetes.