Spinach is loaded with nutrients. Eating 1/2 cup cooked spinach adds 190 percent of your daily recommended dose of vitamin A to your diet. You'll also get 20 percent of your daily value of iron from 1/2 cup cooked spinach. However, spinach can cause some unpleasant side effects if you eat it in large quantities.
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Spinach is high in oxalic acid, a chemical that can bind with iron and calcium and cause your body to absorb less of these important nutrients. To combat this problem, take in some vitamin C when you eat spinach; you can do this by having a glass of orange juice or a tomato whenever you eat spinach. Vitamin C will help your body better absorb calcium and iron.
If you are already prone to kidney stones, your doctor may suggest avoiding spinach, among other foods. Spinach contains purines, which are organic compounds that your body turns into uric acid. Having high levels of uric acid in your body increases your risk of developing kidney stones. The oxalic acid in spinach can also cause the levels of calcium oxalate in your urine to rise, which can also promote kidney stones.
A common, but harmless, side effect of eating spinach is feeling as though your teeth are gritty or slimy. This is caused by the oxalic acid found in spinach. Oxalic acid contains small crystals that do not dissolve in water. The grit is harmless and can be removed easily by brushing your teeth.
Spinach is high in dietary fiber; a cup of cooked spinach contains 6 g of fiber. Although your body needs fiber to promote proper digestion, eating too much fiber at once can contribute to stomach upset. You may experience gas, bloating and cramping after eating spinach. Eat a large amount of spinach all at once, and you may experience some diarrhea. If you have recently increased the fiber in your diet, your body will eventually adjust and you will experience less stomach pain as you continue to eat high-fiber foods. Try drinking a full glass of water every time you eat spinach to help your body better process the fiber.