A 2003 study published in the Journal of the American College of Nutrition found that four weeks of treatment with glucomannan supplements helped guard against elevated glucose levels and improve cholesterol levels in diabetes patients. The study involved 22 diabetes patients with high cholesterol who were not taking cholesterol medication. Results showed that glucomannan was superior to placebo in reducing LDL ("bad") cholesterol and fasting glucose levels (a measure of blood sugar after not eating or drinking for at least eight hours).
There's also some evidence that including glucomannan in your diet may help improve diabetes control.
In a 2000 study from Diabetes Care, for example, 11 diabetes patients ate glucomannan-enriched biscuits or biscuits enriched with wheat bran fiber every day for three weeks. Researchers found that consumption of the glucomannan-enriched biscuits lead to greater improvements in blood sugar control and a greater decrease in cholesterol.
Additionally, taking glucomannan in combination with plant sterols (naturally occurring compounds with cholesterol-lowering effects) may help diabetes patients keep their cholesterol in check, according to a 2006 study published in the European Journal of Clinical Nutrition. For the study, 16 diabetes patients and 18 non-diabetic individuals were assigned to one of four treatments for 21 days: supplements containing plant sterols, supplements containing glucomannan, supplements containing a combination of glucomannan and plant sterols, or a placebo. Results indicated that the combination of glucomannan and plant sterols was most effective at lowering LDL cholesterol.