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Urine and outDoes cranberry juice cure cystitis? If it does, how does it work – surely urine is just urine by the time it is excreted from the body? If it doesn’t, how did the myth arise?

n Cranberry juice can help cure cystitis thanks to its proanthocyanidins, which have an anti-adhesive effect on bacteria in the bladder.

We have known since the 1980s that bacteria can stick to the bladder wall, and bury themselves in mucus – which protects them from antibiotics. So while the antibiotics taken for an infection clear the bladder of free-floating bacteria, the ones hidden in mucus can then come out of the woodwork a fortnight or so after the course of antibiotics is finished. Then the misery of cystitis is back.

Taking cranberry juice can help to prevent this. Many people with cystitis find that cranberry juice on its own works well enough, though not all and some will still need an antibiotic too. James Wakely (part-time GP)Colchester, Essex, UK

n The cystitis for which cranberry juice is effective is caused by a bacterial infection, most frequently by the gut bacterium Escherischia coli but Staphylococcus saprophyticus is behind about 1 in 10 cases.

Cranberry juice contains proanthocyanidins that block the ability of these bacteria to adhere

to the bladder and urinary tract. The proanthocynadins alter the molecular structure of the fine protein filaments, or fimbriae, by which the pathogens attach.

As with all remedies, several factors can affect the juice’s efficacy, meaning it can vary between individuals and between episodes within one individual. Also, the onset of cranberry-juice intake may simply coincide with the body’s natural recovery, while the placebo effect could be involved too. People disposed

to drinking cranberry juice may have a healthier diet and lifestyle. Quantity and frequency of intake, and level of infection, will be important and the juice may augment or diminish the action of other medications, and vice versa.

If you enjoy drinking cranberry juice as part of a balanced diet, by all means continue. Just don’t rely on it as a cure. And certainly not to the exclusion of any medicines you have been prescribed.Len WinokurLeeds, UK

n Firstly, almost everything about the cranberry “cure” is beset with doubt and controversy. The consensus is that, to the extent that it does work, it does not so much cure cystitis as assist in discouraging recurrence, allegedly by preventing bacteria

from adhering to the urinary mucus membrane.

This is not implausible, but it is unclear how much cranberry product is necessary for a useful effect; a couple of dozen glasses of pure cranberry juice per day are proposed, while juice cocktails are considered worthless. That hardly sounds practical. Commercially available capsules seem to be equivalent to a small number of cranberries, so generous helpings on your breakfast cereal should be more economical and perhaps more effective.

Urine is urine? Undeniably so, but urine does vary drastically with diet and physiology. Eating liver turns it yellow; beetroot turns it red; asparagus, cooked mutton, corned beef, coffee and so on, all have olfactory effects, and practically everything affects urinary pH. Urine is the product of your kidneys’ action on whatever your body absorbs into the blood, and its character varies accordingly, sometimes with dramatic effects on the normal (or abnormal) microflora of your urinary tract. Jon RichfieldSomerset West, South Africa

Meta-analysis suggests that cranberry juice may have value for women who have recurrent urinary tract infections. The evidence is not conclusive and further stringently designed studies are needed. To find out more, see the Cochrane Database Systematic Reviews at 1.usa.gov/d525a5 – Ed

What’s the buzz?We have dimmers on many of the incandescent lamps in our house. When the lamp is dimmed, a faint buzzy whine can be heard. The sound doesn’t come from the dimmer but from the light bulb itself. Why does the dimmer make the bulb buzz?

n A tungsten lamp is dimmed when the controlling electronics switches off the power to the bulb each time the alternating current of the mains voltage crosses the zero point in its sine-wave cycle, restoring it at a later point in the half-cycle. The longer the power is switched off, the lower the average current passing through the bulb. This reduces its brightness.

The loss and restoration of current happens very quickly, generating a spiky waveform through the lamp that is rich in the harmonics of the mains frequency. Bulb filaments often have a natural resonant frequency of vibration of a few hundred hertz, so can be excited into resonance by these harmonics. When it reaches this frequency, the bulb will buzz. Chris CollinsLlandrindod Wells, Powys, UK

This week’s questionLangUage cLassesWill humans ever be able to understand or even speak dolphin?Riccardo PesciRome, Italy

“several factors can affect the efficacy of cranberry juice, making its effects vary between individuals”

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