Under certain circumstances, spinach can lead to kidney stones. According to the National Kidney and Urological Diseases Information Clearinghouse, spinach is one of a number of foods that are high in oxalate, a substance in urine that helps kidney stones form.
Other foods that are high in oxalate are rhubarb, nuts and wheat bran. Calcium oxalate stones are only one form of kidney stones, however. Others include calcium phosphate stones, which are caused by an abundance of animal protein from sources like meat, fish and eggs, or by a calcium-deficient diet. Uric acid stones can be caused by animal protein as well.