Metamucil and Miralax are as different as paper and plastic, yet, people confuse the two all the time. I found this out when I ask patients what they have tried for constipation, diarrhea or irritable bowel syndrome. “Isn’t Miralax fiber, too?” (Spoiler: No, not by any stretch). My colleagues confirm, the two are often taken to belong to the same class of over-the-counter drugs, and, indeed, they are on the same shelf in the drugstore. What are they?
Miralax (PEG 3350) – An osmotic laxative – an artificial polymer that draws water into the colon and loosens up stool
Miralax is the brand name for polyethylene glycol 3350. Polyethylene glycol (PEG) is a clear water-soluble synthetic polymer (‘plastic’) that has ethylene oxide as the base unit. The number 3350 indicates the average molecular weight or size [i]. Polyethylene glycol polymers can have different lengths. PEG of this molecular weight is neither digested by the human body, nor by the gut bacteria, and it is not absorbed by the intestine into the blood stream. However, PEG is osmotically active, that is, it draws water into the colon and thus exerts a laxative effect [ii].
Metamucil – A naturally occurring fiber (psyllium) that has many beneficial effects but is not primarily a laxative
Metamucil (brand name for psyllium) is a naturally occurring plant polymer from Plantago species that is soluble, highly viscous and gel forming. It appears that the health benefits (regularity, glucose and cholesterol lowering) are viscosity dependent [iii].
Gastroenterologists use psyllium for its stool normalizing function, not primarily as a laxative. Being a viscous and gel-forming fiber it absorbs water. It softens hard stool in constipation, and firms up loose stool in diarrhea. When constipation is the main complaint, it is often a good strategy to use an osmotic laxative (such as PEG 3350 , magnesium citrate, many others) first to relieve the acute constipation, and then gradually introduce psyllium for its many health benefits.
Only 5 % of adults consume the recommended level of fiber and while it is best to obtain your fiber from natural sources, i.e., fruits and vegetables, psyllium has been shown again and again, in addition to normalizing stool, to improve glycemic control (blood sugar levels) and lower cholesterol [iv].
[i] The use of 3350 seems to be a historical accident. Early research in GI physiology was carried out with Carbowax 4000, as the name indicates, a polyethylene glycol product that was used as industrial lubricant. Because of supply issues, a switch occurred. See: Fordtran JS, Hofmann AF. Seventy years of polyethylene glycols in gastroenterology: the journey of PEG 4000 and 3350 from nonabsorbable marker to colonoscopy preparation to osmotic laxative. Gastroenterology. 2017 Mar 1;152(4):675-80.
[ii] This is an oversimplification. The osmotic behavior of PEG in the gut is complex. PEG 3350 sequester a portion of the water in it which it is dissolved. The remaining water has a smaller volume and the remaining PEG that dissolves in this compartment achieves very high concentration and osmotic activity on the mucosa of the bowel wall. See reference above.
[iii] McRorie JW, McKeown NM. Understanding the physics of functional fibers in the gastrointestinal tract: an evidence-based approach to resolving enduring misconceptions about insoluble and soluble fiber. Journal of the Academy of Nutrition and Dietetics. 2017 Feb 1;117(2):251-64.
[iv] For a great recent review: Lambeau KV, McRorie JW. Fiber supplements and clinically proven health benefits: How to recognize and recommend an effective fiber therapy. Journal of the American Association of Nurse Practitioners. 2017 Apr 1;29(4):216-23.