Glucomannan is a soluble polysaccharide, made from the root of the konjac plant (Amorphophallus konjac) and is considered a dietary fiber. Glucomannan is used as a food additive (thickener or emulsifier) and as medicine. It is usually used for weight management, management of high cholesterol, blood sugar control, constipation, and type 2 diabetes.
Role of dietary fiber in weight management
The primary cause of obesity is excess energy intake. However, many other dietary factors need to be taken into account. Studies evaluating beneficial effects of dietary fiber for prevention and treatment of obesity are inconsistent . We found two extensive reviews of observational studies that used eating pattern methods which suggest that a plant-based diet high in fiber-rich foods (vegetables, fruits, whole grains) is inversely related to overweight and obesity [2,3]. Furthermore, excess weight and obesity were substantially lower in vegetarian populations (even if vegetarians consumed some animal products), fiber intake could play an important role in prevention and progression of some metabolic conditions . Studies are proposing several mechanism by which dietary fiber might affect bodyweight, including promoting satiation, decreasing absorption of macronutrients, and altering secretion of gut hormones controlling fat oxidation and storage .
It has been shown that soluble fibers reduce fat and protein absorption , probably because it limits the contact with intestinal villi (hairlike projections in the small intestine).
Can glucomannan aid in weight loss?
Glucomannan is thought to prolong gastric emptying time, which increases satiety, reduces body weight, decreases the ingestion of foods that increase cholesterol and glucose concentrations .
An 8 week double-blind clinical trial from 1984  conducted on 20 obese patients noted a significant mean weight loss (2,5 kg) using 1 gram of glucomannan prior to each 3 meals per day. Researchers from this study speculated that weight loss properties appear to arise from glucomannan bulk-forming properties . 3 grams of glucomannan absorbs approx 300 ml of water, this added bulk just before meal may reduce appetite and cause the subjects to eat less . However, the number of subjects in this study was small.
A more recent large-scale (200 overweight or obese patients) study  analysing the long-term effects of dietary fiber supplementation failed to find evidence that mixed fiber (3 g Plantago ovata husk and 1 g glucomannan) have additional effects on weight reduction.
A systematic review by Sood et al.  included a total of 14 randomized controlled trials that evaluated 531 subjects receiving glucomannan. Results of meta analysis showed that there was a statistically significant but small reduction in weight of 0.79 kg (1%) in studies lasting around 5 weeks. While studied over a slightly longer period of time noted a decrease in body weight by 5% in the first 16 weeks of treatment. Therefore, they ranked glucommanan’s effect on weight as mild.
Another critical review  of glucomannan on obesity concluded that controlled trials ranging from 3 weeks to 4 months with doses of 2-4 grams per day resulted in significant weight loss (up to 2,5 kg) in mostly over-weight and obese.
Glucomanan for cholesterol lowering
Sood et al.  also concluded that glucomannan appears to beneficially affect total cholesterol, LDL cholesterol, and triglycerides, but not HDL cholesterol or blood pressure. The same was noted in the critical review by Keithley and Swanson .
Glucomannan side effects
Based on Sood et al.  meta analysis there is only a limited number of studies evaluating safety and tolerability of glucomannan. Based on available data it appears to be well tolerated. Most commonly reported side effects are flatulence, diarrhea, and abdominal discomfort . There is some concern about glucomannan in tablet form as it is reported to cause blockages of the throat or intestines . However, it is no longer available in tablets because added water may cause them to swell before they reach the stomach . According to WebMD.com:  “Glucomannan powder and capsules are possibly safe for most adults and children. But solid tablets containing glucomannan are possibly unsafe. These can sometimes cause blockages of the throat or intestines”
- Salas-Salvadó, Jordi, et al. “Effect of two doses of a mixture of soluble fibres on body weight and metabolic variables in overweight or obese patients: a randomised trial.” British Journal of Nutrition 99.6 (2008): 1380-1387.
Newby, P. K., and Katherine L. Tucker. “Empirically derived eating patterns using factor or cluster analysis: a review.” Nutrition reviews 62.5 (2004): 177-203.
Togo, P., et al. “Food intake patterns and body mass index in observational studies.” International journal of obesity 25.12 (2001): 1741-1751.
Newby, P. K., Katherine L. Tucker, and Alicja Wolk. “Risk of overweight and obesity among semivegetarian, lactovegetarian, and vegan women.” The American journal of clinical nutrition 81.6 (2005): 1267-1274.
- Slavin, Joanne L. “Dietary fiber and body weight.” Nutrition 21.3 (2005): 411-418.
- Doi K. Effect of konjac fibre (glucomannan) on glucose and lipids. Eur J Clin Nutr 1995;49(suppl 3): S190 –7.
Walsh, David E., Vazgen Yaghoubian, and Ali Behforooz. “Effect of glucomannan on obese patients: a clinical study.” Int J Obes 8.4 (1984): 289-93.
Sood, Nitesh, William L. Baker, and Craig I. Coleman. “Effect of glucomannan on plasma lipid and glucose concentrations, body weight, and blood pressure: systematic review and meta-analysis.” The American journal of clinical nutrition 88.4 (2008): 1167-1175.
Keithley, J. K., and Barbara Swanson. “Glucomannan and obesity: a critical review.” Alternative therapies in health and medicine 11.6 (2005): 30.
Baer, David J., et al. “Dietary fiber decreases the metabolizable energy content and nutrient digestibility of mixed diets fed to humans.” The Journal of nutrition 127.4 (1997): 579-586.
- WebMD.com Retrieved 25. June 2013