Corm can help blood sugar but all is not sweet

The Irish Times
Health Supplement, p. 13
19 May 2009

DOES IT WORK? Glucomannan can help to regulate insulin levels but there are side effects

GLUCOMANNAN IS a form of dietary fibre that has some properties similar to starch. It is obtained from the corms of a plant called konjac. A corm is an underground stem that is sometimes mistaken for a bulb, but instead of having moist layers, it is solid and hard like a potato. Konjac is grown in Asia and used to make konjac flour and a vegan form of gelatine.

Glucomannan is used like starch in some Asian cooking as a thickening and emulsifying agent. It was introduced into the West as a bulk-forming agent in weight-loss products on the assumption that it would leave people feeling full without adding calories to dietary intake.

Controlled trials have failed to show beneficial effects for weight-loss. As a result, several US companies over recent years have been reprimanded by the US Federal Trade Commission for making misleading or exaggerated claims about glucomannan as a weight-loss agent.

However, during these studies, changes in people’s blood glucose and cholesterol levels were observed which have led to glucomannan being marketed in a different way as a dietary supplement, especially for people who have diabetes.

Evidence from studies 

Glucomannan is a polysaccharide, which means it contains long chains of sugar molecules. Some polysaccharides, like starch, can be digested and provide energy and calories to the body. Others, like glucomannan, are not digested but pass through the intestines creating a bulky gel – hence they are called bulk-forming agents. The gel passes along slowly, trapping glucose, cholesterol and other lipids. These materials are then absorbed from food into the body more slowly and in a more controlled way.

The beneficial effects of glucomannan were first noted in people with diabetes. When glucose is absorbed more slowly after a meal, blood levels of glucose and insulin do not spike and plummet. But insulin does not work as well as normal for people with diabetes, resulting in higher levels of glucose and fatty acids in the blood.

Studies found that people taking a few grams of glucomannan with their meals had better control over their blood glucose levels. In addition, several studies have demonstrated that glucomannan lowers people’s levels of total cholesterol, triglyceride and LDL-cholesterol (the so-called “bad” cholesterol). No changes were found in HDL-cholesterol levels – the “good” cholesterol – nor in blood pressure or body weight.

Problematic aspects 

People taking any medicines, but especially those for diabetes or to lower cholesterol levels, should talk to their doctors before using glucomannan. Since the supplement does consistently reduce blood glucose and total cholesterol levels, these should be carefully monitored when starting glucomannan to ensure the levels do not drop too much or to determine if the doses of other medications should be reduced. Bulk-forming agents are known to reduce the absorption of many medications, thus requiring careful medical monitoring.

Glucomannan can have side effects of diarrhoea, flatulence and other gastro-intestinal problems. The intestinal tract contains numerous helpful microbes which are important in normal digestion. Glucomannan is known to affect these, but the overall or long-term impact on these microbes has not been studied.


Glucomannan can have a beneficial effect on blood glucose and cholesterol levels. Given the increased prevalence of diabetes and high cholesterol levels, such changes could benefit many people. At the same time, some people find the gastrointestinal effects of glucomannan too uncomfortable and discontinue its use.

All of the studies to date have involved relatively small numbers of people and been of short duration, so further research is needed to determine how best glucomannan might benefit people. Most studies have used biscuits made from glucomannan so that people take 4-7g of glucomannan per day. It is crucial that people already taking medication talk to their GPs if they are considering trying glucomannan.

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