Glucomannan

What it is

Glucomannan is a soluble dietary fibre taken from the tubers of the konjac plant (Amorphophallus konjac). It absorbs a lot of water, expands rapidly to many times its original size and forms a thick gel, which is why it is sold as a diet aid [1]. Konjac is also eaten as a food [2].

How it might work

Because it swells in the gut, glucomannan is thought to produce a feeling of fullness [1]. Konjac glucomannan is not absorbed intact and is fermented by gut bacteria [3]. In a small trial it raised stool weight and the numbers of lactobacilli and bifidobacteria in stool [4]. These are proposed explanations, not proven ones.

What the research says

Cholesterol. This is the best-supported use. A meta-analysis (a study that pools many trials) of 14 trials with 531 people found glucomannan lowered LDL ("bad") cholesterol by 15.99 mg/dL and total cholesterol by 19.28 mg/dL compared with control [5]. A separate meta-analysis of 12 trials (370 people) found konjac glucomannan lowered LDL by 0.35 mmol/L and non-HDL cholesterol by 0.32 mmol/L, which the authors put at about 10% and 7% at roughly 3 g a day [6]. Two newer meta-analyses also found lower total and LDL cholesterol, but the trials differed widely (heterogeneity above 90% in one), and one team judged the drops not clinically important [7][8]. Triglycerides fell in the 2008 analysis but not in the two newer ones [5][7][8]. HDL ("good") cholesterol did not change [5][8].

Constipation. A meta-analysis of 3 trials in 122 children with constipation found 1.4 more bowel movements per week with glucomannan, though stool consistency and overall treatment success did not improve significantly [9]. In a small trial of 8 healthy adults, 4.5 g a day for 21 days raised the number of bowel movements by about 27% and stool weight by about 30% [4]. A double-blind trial in elite athletes with functional constipation found better constipation symptom scores, quality of life and bowel function after 8 weeks than placebo [10].

Blood sugar. A meta-analysis of 6 trials (440 people) with type 2 diabetes found glucomannan lowered fasting glucose, glucose two hours after a meal, fasting insulin and fructosamine (a blood sugar marker covering the past few weeks) [11]. In the 2008 analysis, fasting glucose fell 7.44 mg/dL overall, but people with impaired glucose metabolism benefited less than others [5].

Weight loss. Results vary by review. A meta-analysis of 8 trials found 0.22 kg more weight loss than placebo, which was not statistically significant [12]. Others found small, statistically significant losses of 0.79 kg [5], 1.27 kg [13] and 1.36 kg (low-certainty evidence) [14]. The 1.27 kg difference fell below the 2.5 kg the reviewers set as clinically meaningful [13].

Blood pressure. The 2008 meta-analysis found no effect on blood pressure [5].

Doses used in studies

The cholesterol meta-analysis supported about 3 g of konjac glucomannan a day, taken for at least 3 weeks [6]. The healthy-adult constipation trial used 4.5 g a day (1.5 g per meal) for 21 days [4]. European regulators cite a study in which adults took 3,000 mg a day for 12 weeks [3]. Weight-loss and diabetes reviews did not report a single dose in their abstracts. These are the doses studied, not recommendations.

Safety and interactions

Side effects. Reported side effects in trials include abdominal discomfort, diarrhoea and constipation [12]. In a study where adults took 3,000 mg a day for 12 weeks, several people had abdominal discomfort, diarrhoea or constipation [3]. Flatulence and abdominal pain have also been reported [1].

Blockages. Because glucomannan swells quickly, case reports describe a woman with a throat (oesophageal) blockage after taking a glucomannan diet aid, who had an oesophageal web [1], and a man in his 50s whose small bowel was blocked by konjac pieces and needed emergency surgery [2]. The case authors flag extra risk for people with a history of upper gut problems [1].

Upper limits. European food-safety scientists set no numerical daily limit, but accepted konjac as a food additive provided total intake from all sources stays below 3 g a day [3].

Medicines. Glucomannan has been reported to possibly change how well other medicines are absorbed [1]. We found no interaction studies. Effects alongside diabetes or cholesterol medicines were not assessed in the reviews we read [5].

Pregnancy, breastfeeding, kidney and liver disease. We found no data for pregnancy, breastfeeding or people with kidney or liver conditions in the sources we read.

Caveats and open questions

Results are inconsistent for weight and triglycerides, and cholesterol meta-analyses show high heterogeneity [7][12]. Several reviews were based on small trials of modest reporting quality [5][12]. Pooled effects mix adults and children, and konjac glucomannan forms [5][6]. We found no recruiting or active trials that would settle these questions.