Diets heavy on foods that are high in protein and low in carbohydrates can increase the risk of kidney stones and reduce the body's 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 Robert 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 body's 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.