Mannose is a “six-carbon-sugar,” as are the better known and closely related substances glucose and fructose. Relying on evidence that is both exceedingly preliminary and highly inconsistent, some alternative medicine practitioners have popularized mannose as a treatment for urinary tract infections.
Mannose plays an important role in human physiology. However, there is no nutritional need for this substance, as the body can easily produce it from glucose. Nonetheless, significant quantities of it can be found in many fruits and vegetables, including peaches, apples, blueberries, green beans, cabbage, and tomatoes.
A typical recommended dose of mannose for the treatment or prevention of bladder infections is 1.5 g daily, often divided into three doses of 500 mg each.
The idea that mannose supplements can help prevent or treat bladder infections derives from a property of the
is one of the common causes of bladder infections. Many, though not all, strains of
have the ability to attach to the mannose present in the wall of the bladder by means of thread-like structures called pili. This process of attachment allows them to initiate the process of infection.
Reasoning from this fact of basic science, medical researchers in the 1980s hypothesized that consumption of mannose as a supplement will increase levels of mannose in the urine to such an extent that this free mannose will saturate the
mannose-binding pili and thereby make the bacteria unable to grapple onto the cells of the bladder wall.
It is essentially this reasoning that is restated by proponents of mannose for bladder infections. However, the argument has at least four problems. First, one of the main ways that the body’s white blood cells recognize and kill
is via these mannose-sensitive pili.
When these pili are saturated by mannose, white blood cells (specifically, macrophages) are less able to consume the
Second, many species of
, including some of the most dangerous, do not have mannose-sensitive pili at all.
Third, there are numerous other bacteria that cause bladder infections, and these are not known or suspected to have mannose-sensitive pili.
But perhaps the most important point is that the use of mannose for preventing or treating bladder infections has rarely undergone any meaningful scientific study in human beings. Almost all of the evidence that is available comes from animal studies performed in the 1980s,
5. However, D-mannose powder was associated with a signficant reduction in bladder infection in one human randomized trial. D-mannose powder was given to 308 women with a history of recurrent bladder infections after the completion of antibiotic treatment. Participants were randomized to D-mannose powder (2 grams in 200 milliliters of water per day) vs a daily oral antibiotic (50 grams once daily) vs no prophylaxis. After 6 months, D-mannose was associated with a significant reduction in recurrent bladder infections when compared to both antibiotic and control groups. There were also fewer side effects (nausea, skin rash, headache, vaginal burning) with D-mannose compared to the antibiotic.9
As a sugar widely present in foods, mannose is assumed to be safe. However, the maximum safe dosage has not been established in healthy adults, nor in pregnant or nursing women or young children. Very weak evidence from test tube studies hints that consumption of gigantic amounts of mannose by pregnant women could conceivably increase risk of birth defects in their offspring.
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