Raw spinach is high in a type of salt called oxalate, which has a tendency to bind to the mineral calcium. When oxalate joins with excess calcium in our kidneys, it can form hard, yellowish compounds called kidney stones. Although most kidney stones will pass out of your body without causing damage, in some cases they can become as large as golf balls, cause severe back or abdominal pain, induce vomiting, lead to a fever or result in bloody urine. Raw spinach contains relatively high amounts of potassium, providing 167 milligrams per cup. Reduced function of your kidneys causes potassium to build up in your bloodstream, potentially affecting your heart rhythm. Even though spinach is unlikely to damage your kidneys enough to cause kidney failure, you may need to limit your intake of Spinach and other potassium-rich foods if you have advanced kidney disease.