The low-carbohydrate, high-protein diets which have become popular in recent years may be good at helping people lose weight rapidly, but they could also be damaging their kidneys and their bones, according to researchers at the University of Texas Southwestern Medical Center.
Researchers discovered that the acidic content of animal meat and the lack of alkaline foods in the diet led to an increased risk of kidney stones. Writing in the August issue of the American Journal of Kidney Diseases, co-author Dr Chia-Ying Wang said that acid excretion - a marker for the acid load in the blood - increased as much as 90 per cent while subjects were on diets that severely restricted carbohydrates. Levels of urinary citrate, which inhibits kidney stones, fell by almost 25 per cent in the group during the six-week study.
"People may lose weight on this diet, but this study shows that this is not a healthy way to lose weight," said Wang. "When you restrict the amount of carbohydrates you can go into a state called ketoacidosis. Our body needs a certain source of energy and a quick source are carbohydrates, which are readily available. When you restrict carbohydrates the body then turns to other sources, one of which is fat. Ketone bodies are formed when the body is forced to burn fat for energy which may result in a state of ketoacidosis."
Ketoacidosis is a condition resulting from an accumulation of ketone bodies in the blood and increased blood acidity.
The study focused on 10 healthy subjects who ate a regular diet for two weeks, followed by two weeks on a highly restrictive diet that included some vegetables but no fruits and less than 20 grams of carbohydrates. Participants then ate a less-restrictive diet for the final four weeks. Each member of the group also took a daily multivitamin tablet. During the last five days of each of these stages, the study subjects were tested by the researchers.
Urinary citrate levels fell from 763 milligrams per day to 449 mg per day during intake of the severely carbohydrate-restricted diet, the researchers reported in the journal. Subject readings improved to 581 mg per day during intake of the moderately carbohydrate-restricted diet.
The researchers also reported that net acid excretion rose from baseline levels of 61 milliequivalents per day to 116 mEq per day during the severely carbohydrate-restricted diet phase. Levels were 112 mEq per day when the group switched to a moderately carbohydrate-restricted diet. Chronic acid load suppresses the function of osteoblasts, bone-forming cells, and stimulates the function of osteoclasts, a cell associated with bone resorption.
"This type of diet increases the propensity to develop kidney stones," Wang said. "On the basis of this study alone, there was an increased risk of developing kidney stones and a possible increase in the risk of bone loss. We already know that osteoporosis is going to be a major issue as the population ages and if people are going to eat this kind of diet on a long-term basis, it's unknown what the implications would be for your bones."
The researchers are now studying the effects of this protein- and fat-heavy diet on the bones and are developing methods to counteract the higher risk of kidney stones.
"We are not questioning the value of this diet in producing weight loss," said Dr Kashayer Sakhaee, co-author of the study. "We are investigating a counter-measure so that subjects can benefit from weight loss without experiencing the side effects of increased risk of stones and bone loss."
The results of the study have coincided with the launch of a campaign by the US-based Physicians Committee for Responsible Medicine which is designed to highlight the risks of high-protein diets.
The campaign can be seen at the PCRM website , where consumers will also be invited to register if they believe they have suffered adverse health effects from following such a diet.
PCRM is also concerned about legal liability - both from the physician's perspective and that of patients. The site, which features both a consumer and a physician advisory, counsels doctors that liability could be an issue and explains to consumers that they may have legal recourse.
"Given the health problems associated with high-protein diets, doctors who prescribe them may be assuming serious legal liability," said PCRM's president Neal Barnard. "There's no need to put yourself at risk when there are safer and healthier alternatives, especially low-fat, plant-based diets. Not only are vegetarians, on average, 10 per cent lighter than omnivores, but they enjoy dramatically lower rates of heart disease, diabetes and several forms of cancer."