High protein diet brings risk of kidney stones

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BMJ 2002;325:408 ( 24 August )

News extra

High protein diet brings risk of kidney stones

Gottlieb New York

Diets heavy on foods that are high in protein and low in carbohydrates can

increase the risk of kidney stones and reduce the body¹s ability to absorb

calcium after just six weeks.

These findings from a new study come at a time when an increasing number of

Americans, seduced by anecdotal accounts of fast weight loss, are turning to

low carbohydrate, high protein diets. Popularised by Dr Atkins, these

diets are having a revival after a recent cover story in the New York Times

Magazine said that relying on diets heavy in carbohydrates and low fat

products has caused people to hold on to fat, explaining a rising incidence

of obesity and perhaps diabetes.

Although low carbohydrate, high protein diets result in weight loss in the

short term, they are less successful in the long term and may even be

hazardous to health, researchers have warned. Protein rich foods can be high

in fat, which increases the risk of heart disease and type 2 diabetes. Diets

heavy in foods with low levels of carbohydrate, such as fruit and

vegetables, can also leave the body hungry for essential vitamins and

minerals, and insufficient glucose from carbohydrates, the body¹s preferred

fuel source, can lead to fatigue and dizziness (American Journal of Kidney

Diseases 2002;40:265-74).

The researchers, led by Dr Shalini Reddy from the University of Chicago,

found that six weeks on a low carbohydrate, high protein diet increased the

acid load to the kidneys, raising the risk of kidney stones. Animal protein

has been shown to boost urinary excretion of oxalate, a compound that

combines with calcium and other compounds to form kidney stones.

The new study included 10 healthy adults aged 21 to 52 who ate their usual

diet for two weeks, then a low carbohydrate, high protein diet for two

weeks, and finally a diet that restricted carbohydrates only moderately for

four weeks. The diets included three litres of fluid a day.

Study volunteers lost an average of 9 lb (4 kg), but most developed

ketones‹compounds formed when the body uses its own fat as fuel and can

raise acid levels in the blood. Acid excretion, a marker of acid levels in

the blood, rose by 90% in some volunteers, but none of the volunteers

developed metabolic acidosis. Urinary calcium levels also rose sharply.

Although it is not clear from the study whether bone mass was affected, the

findings show that such diets may increase the risk of bone loss over the

long term.

http://bmj.com/cgi/content/full/325/7361/408/d

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