D-Mannose

D-mannose is an aldohexose sugar with the same chemical formula as glucose, differing only in the arrangement of a single hydroxyl group ^[4]. It is found in a range of fruits and, more importantly for biology, as a key component of the branched sugar chains that decorate many proteins ^[4]. Inside the body mannose can be phosphorylated and fed into the same metabolic routes as glucose, and the two sugars are interconvertible, so mannose is not an essential nutrient ^[4].

Mannose is a dominant sugar in N-linked glycosylation, a process that attaches sugar chains to proteins as they are made ^[4]. This role is medically significant: several rare inherited conditions known as congenital disorders of glycosylation stem from faults in mannose-related enzymes, and a few of these can be improved by giving dietary mannose or other simple sugars ^[4]. The immune system also recognizes exposed mannose patterns on the surfaces of certain microbes ^[4].

The most popular use of supplemental D-mannose is for preventing recurrent urinary tract infections, most of which are caused by Escherichia coli ^[1]. The rationale is that D-mannose interferes with the bacteria's ability to cling to the cells lining the urinary tract, so that they are washed away in the urine rather than establishing an infection ^[1]. Because it works physically rather than as an antibiotic, it has drawn interest as a non-antibiotic option at a time of growing antibiotic resistance ^[1].

The clinical evidence, however, is inconsistent. An earlier systematic review and meta-analysis suggested that D-mannose reduced recurrences compared with placebo and might be roughly as effective as preventive antibiotics, but it drew on a small number of modest studies ^[2]. A 2022 Cochrane review concluded that the existing trials were too few and too low in quality to support or refute the treatment ^[1]. Most tellingly, a large, well-conducted randomized trial published in 2024 found that daily D-mannose did not reduce the proportion of women with recurrent infections who went on to have another one, leading its authors to advise against recommending it for this purpose in primary care ^[3].

D-mannose is generally well tolerated, with loose stools or mild digestive upset being the most common complaints ^[1][2]. It is sold as a dietary supplement, typically as a powder or in capsules, rather than as an approved medicine, and it is not classed as an essential nutrient because the body makes its own ^[4].

Very well tolerated; high doses can cause loose stools or bloating because it's an incompletely absorbed sugar. It has minimal effect on blood glucose at usual doses but diabetics should still be mindful. It's a prevention tool, not a treatment for an established, severe infection, which still needs medical care.