Cranberry, D-mannose and what they can and cannot do

Bacteria cause a urinary infection by sticking to the bladder wall. If they cannot stick, they get flushed out at the next wee. That is the whole mechanism behind both cranberry and D-mannose, and understanding it tells you exactly where they help and where they do not.

Cranberry provides proanthocyanidins, which interfere with the way E. coli attaches. D-mannose is a simple sugar that is not used as fuel: it binds the same receptors the bacteria use, so they leave attached to it rather than to the bladder. Neither of them kills anything.

The consequence is that they are prevention, not treatment. A dog with an active infection needs a vet and almost certainly needs antibiotics, and reaching for a supplement at that point delays the thing that works. Where they earn their place is the pattern: the dog that has had three or four flare ups in a year, in the quiet stretch between them.

Two practical notes. The evening meal suits both, because the bladder then sits full overnight and contact time is longest. And the number worth comparing between products is the D-mannose weight, which most tubs on the shelf will not tell you.