Short answer: Both are soluble fibers that thicken into a gel. In separate meta-analyses, psyllium (about 10 g a day) and glucomannan (about 3 g a day) each lowered LDL cholesterol by about 0.33 to 0.35 mmol/L.12 Neither clearly caused weight loss.34 NIH warns that glucomannan tablets have caused choking blockages.5 No trial compared them alone.
| Psyllium | Glucomannan | |
|---|---|---|
| What it is | Fiber from the seed husk of Plantago ovata1 | Fiber from konjac root that can absorb up to 50 times its weight in water5 |
| LDL cholesterol | 28 trials (1,924 people), median about 10.2 g a day: 0.33 mmol/L lower1 | 12 trials (370 people), about 3 g a day: 0.35 mmol/L lower, about 10%2 |
| Blood sugar | 19 trials (962 people): fasting glucose 6.89 mg/dL and HbA1c 0.75 points lower than placebo6 | 14 trials (531 people): fasting glucose 7.44 mg/dL lower7 |
| Weight | 22 trials: no significant change3 | 8 trials: no significant change (0.22 kg); an older review found 0.79 kg47 |
| Safety | Not covered by an NIH fact sheet in the sources checked | Belching, bloating, loose stools and constipation; tablets linked to 7 cases of esophageal blockage5 |
| Head-to-head trial | None found | None found |
The two reviews came from the same research group and used the same methods. Psyllium lowered LDL by 0.33 mmol/L, non-HDL cholesterol by 0.39 and apolipoprotein B by 0.05 g/L, with the evidence rated moderate to high quality.1 Glucomannan lowered LDL by 0.35 mmol/L and non-HDL by 0.32, with no change in apolipoprotein B, in 12 smaller trials that included 4 in children.2 The numbers look similar, but they come from different trials and amounts, so they are not a direct comparison.
Not clearly. A review of 22 psyllium trials found no significant change in weight, BMI or waist size.3 For glucomannan, a 2014 review of 8 trials found no significant weight loss and listed stomach discomfort, diarrhea and constipation as side effects.4 NIH calls glucomannan's weight effects inconsistent and notes that powders and capsules have not been linked to the tablet blockages.5
For cholesterol, both have similar average results in their own trials; psyllium has more trials and people behind it. Neither is a weight-loss tool on current evidence. Fiber supplements can affect how some medicines are absorbed, so ask a pharmacist about timing. This is general information, not personal advice. More detail: the Psyllium Husk Fiber guide and Psyllium vs inulin: which fiber does what?.