One speculative cause of kidney failure is eating too much protein. The beneficial effect of protein restriction on the symptoms and the course of renal disease, discussed in Chapters 4 and 7, has logically led to the question as to whether a high intake of dietary protein can cause kidney disease. Some authors have gone so far as to recommend that older people in particular should cut down on dietary protein in order to reduce the incidence of kidney failure.
The evidence supporting the idea that high protein intake damages the kidney is unconvincing. The only experimental evidence supporting this idea comes from studies done on rats, especially after removal of one kidney and part of the other. Rats develop renal failure with age almost universally. (Many lab rats don't live long enough to get it.) The predominant lesion is a process called glomerulosclerosis. Although it was at first reported that protein intake was a determinant of this process, caloric intake was later shown to be just as important. Caloric restriction without protein restriction markedly retarded the development of glomerulosclerosis. Also, very high protein feeding for two years in rats had no effect on the extent of glomerulosclerosis.
Studies of this question in people have also cast doubt on the idea that protein harms normal kidneys. In a study of normal people, kidney function was measured in old individuals twice, separated by 10 to 18 years. A dietary history was taken from each person, and the protein content their diet contained was estimated. When the authors looked at the relationship between change in renal function over the 10- to 18-year interval and protein intake, they found no relationship. In other words, people who ate a lot of protein were just as likely to show a decline in kidney function with age as people who ate only a small amount of protein.
Other species do not exhibit the progressive decline in kidney function after partial removal of the kidneys that is characteristic of rats. Neither dogs nor baboons show this phenomenon, and they don't develop kidney damage from eating protein.
From these observations, it is clear that protein restriction does not prevent the decline in kidney function with age. In fact, reduced protein intake appears to be its major cause. A better way to prevent the decline with age would be to increase protein intake. Indeed, protein malnutrition in elderly people is a far more prevalent problem than is kidney insufficiency as a result of age-related decline in kidney function.
It seems safe to conclude that high dietary protein does not harm the kidneys in normal people.
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