Berberine is the most studied natural substance showing positive effects on sugar control. Here is an abstract summary from a recent study showing a beneficial effect of berberine in diabetes both as far as the glucose and the blood fats are concerned:

Berberine, a natural  plant  alkaloid,  is usually used as an antibiotic drug. The potential glucose-lowering effect of berberine was noted when it was used for diarrhea in diabetic patients. In vitro and in vivo studies have then showed its effects on hyperglycemia  and dyslipidemia. Objective: The objective of the study was to evaluate the efficacy and safety of berberine  in the treatment of type 2 diabetic patients with  dyslipidemia. Design: One hundred  sixteen patients with  type 2 diabetes and dyslipidemia  were randomly  al- located to receive berberine (1.0 g daily) and the placebo for 3 months. The primary outcomes were changes in plasma glucose and serum lipid concentrations. Glucose disposal rate (GDR) was mea- sured using a hyperinsulinemic euglycemic clamp to assess insulin sensitivity. Results: In the berberine group, fasting and postload plasma glucose decreased from 7.0 ± 0.8 to 5.6 ± 0.9 and from 12.0 ± 2.7 to 8.9 ± 2.8 mM/liter, HbA1c from 7.5 ± 1.0% to 6.6 ± 0.7%, triglyceride from 2.51 ± 2.04 to 1.61 ± 1.10 mM/liter, total cholesterol from 5.31 ± 0.98 to 4.35 ± 0.96 mM/liter, and low-density lipoprotein-cholesterol from 3.23 ± 0.81 to 2.55 ± 0.77 mM/liter, with  all param- eters differing from placebo significantly (P < 0.0001, P < 0.0001, P < 0.0001, P = 0.001, P < 0.0001, and P <0.0001, respectively). The glucose disposal rate was increased after berberine  treatment (P = 0.037), although no significant change was found between berberine and placebo groups (P = 0.063). Mild to moderate  constipation was observed in five participants in the berberine  group. Conclusions: Berberine is effective and safe in the treatment of type 2 diabetes and dyslipidemia.

(Yifei et al, Treatment of Type 2 Diabetes and Dyslipidemia with the Natural Plant Alkaloid Berberine,  J Clin Endocrinol Metab 93: 2559 –2565, 2008)