The kidneys are two bean-shaped organs located in the lower back. Kidneys filter the blood to remove unwanted waste products broken down from our food and drink. They also remove excess liquid to help maintain correct fluid balance in the body.
There are many diseases and conditions that can affect the kidney function: kidney inflammation (glomerulonephritis); kidney infection (such as pyelonephritis); genetic disorders (such as polycystic kidney disease); hardening of the kidney due to a disease of the arteries (nephrosclerosis); kidney failure due to atherosclerosis (plaques forming in the arteries supplying the kidneys); autoimmune diseases (such as systemic lupus erythematosus); malaria; yellow fever; certain medicines; mechanical blockages (kidney stones) and physical injury.
Surveys have revealed that mild forms of kidney disease are surprisingly common among the general population. The global epidemic of type 2 diabetes has led to an alarming increase in the number of people with chronic kidney disease. Global estimates of people suffering with chronic kidney disease lie at over 50 million, of which one million experience kidney failure every year (Dirks et al., 2005). There may be no apparent symptoms, although small amounts of blood or protein may pass through the damaged filters in the kidneys into the urine. Such small amounts of blood and protein in the urine are not visible but can be detected by certain medical tests.
Normally protein is filtered out by the kidneys and no protein is excreted into the urine. However, when the kidneys are damaged, protein may pass into the urine. Other symptoms include retention of water in the body, called nephrotic syndrome. In some cases the damage to the kidney can be so severe that it leads to a build up of waste in the body and ultimately kidney failure. The symptoms of kidney failure include tiredness, sickness and vomiting.
Certain kidney disorders can lead to the formation of a kidney stone (renal calculi), a small hard mass in the kidney that forms from mineral deposits in the urine. Stones may form when there is a high level of calcium, oxalate or uric acid in the urine; a lack of citrate in the urine; or insufficient water in the kidneys to dissolve waste products.
Traditionally, a low-calcium diet has been recommended to reduce the strain on the kidneys in kidney stone patients. However, over time a low-calcium diet can cause problems in terms of bone health. In the last decade, attention has switched to the effects of animal protein on kidney stone formation. Several studies now suggest that a diet characterised by normal-calcium, low-animal protein and low-salt levels is more effective than the traditional low-calcium diet for the prevention of kidney stones in some people.
The relationship between an animal protein-rich diet and kidney stone formation was investigated by researchers at the Centre in Mineral Metabolism and Clinical Research at the Department of Internal Medicine in Dallas, Texas (Breslau, 1988). In this study, 15 young healthy participants were studied for three 12-day dietary periods during which their diet contained vegetable protein, vegetable and egg protein, or animal protein. While all three diets were constant with respect to sodium, potassium, calcium, phosphorus, magnesium and the total quantity of protein, they had progressively higher sulphur contents (due to the increased sulphur content of animal proteins compared to that of plant proteins). As the sulphur content of the diet increased, urinary calcium excretion increased from 103mg per day on the vegetarian diet to 150mg per day on the animal protein diet. The animal protein-rich diet was associated with the highest excretion of uric acid and therefore conferred an increased risk for uric acid stones (but not for calcium oxalate stones). The link between animal protein and kidney stone formation has since been demonstrated in both men (Curhan et al., 1993; Taylor et al., 2004) and women (Curhan et al., 1997).
Dr Neil Barnard, president of the PCRM, states that animal protein is the worst kind of enemy of people with a tendency towards kidney stones or any kidney disease (Barnard, 1998). The animal protein in red meat, poultry, fish, eggs and milk tend to overwork the kidneys causing their filtering abilities to decline. This may make matters worse in a person who already has kidney disease. Additionally, animal protein causes calcium to be leached from the bones and excreted in the urine, adding further to the burden on the overworked kidney.
A report published in the Lancet in 1992 suggested that soya products may be beneficial in kidney disease. Kidney disease patients with protein in the urine and high cholesterol levels were placed on a cholesterol-free, low-protein, low-fat, high-fibre vegetarian (vegan) diet containing soya products. The amount of protein excreted in the urine dropped considerably as did their blood cholesterol levels (D’Amico et al., 1992). It was uncertain whether these results reflected the reduction in dietary protein and fat or if the favourable results arose from a change in the nature of the food consumed. Either way, switching from a diet containing meat and dairy products to a plant-based diet containing less fat and protein and more fibre was beneficial to patients with kidney disease.
In addition to avoiding animal protein in the diet, increasing the potassium intake has been shown to yield benefits as potassium reduces calcium excretion, which can decrease the risk of stone formation. Additionally, the beneficial effect of increasing the fluid intake and the subsequent dilution of urine is well known (Curhan et al., 1993).