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by CHUNG, Yik-Sham INTENDED READERS: FORM 4 OR ABOVE STUDENTS STUDENTS Nowadays, more and more people are aware of the importance of their health. They would try to have a healthy diet, by eating more vegetables and less meat. However, eating too much vegetable would inhibit the absorption of calcium ions, as well as increasing the chance of formation of kidney stones. These surprising facts can be explained by a simple chemical reaction: PRECIPITATION. Precipitation Reaction Precipitation is the formation of insoluble solids in a solution1. An example is the reaction, which should have been taught in F.3 chemistry, between silver nitrate solution (AgNO3 (aq)) and halide solution, such as sodium chloride solution (NaCl (aq)). White precipitates are formed because the product, silver chloride (AgCl), has very low solubility in water. White Precipitates NaCl (aq) + AgNO3 (aq) NaNO3 (aq) + AgCl (s) Precipitation reactions do not occur only in test tubes. They can be carried out in our bodies, and the one between calcium and oxalate ions, in fact, plays an important role in the formation of kidney stones and the inhibition of the absorption of calcium ions2. 1 Mismatches between Food Oxalate and Calcium Many vegetables (Fig 1) are rich in oxalic acid. Examples include spinach, parsley and carambola1. Oxalic acid in these vegetables can be absorbed into our bodies in the form of oxalate, with two protons being dissociated out1, 2. Fig 1: Some common vegetables with high oxalic acid content. spinach (top left), parsley (top right), and carambola (bottom)@ 2003 green recipe.com and @ 2001-2007 PCC Natural Markets Calcium ions in our bodies come from a variety of food, including cheese, milk and butter1, 3, 4. They can be absorbed simply by passing through the intestinal wall into the bloodstream. Some of the absorbed ions would then deposit in bones with the help of magnesium ions5. Therefore, in addition to the dietary source, calcium ions in blood can be from dissolved bone calcium ions1. The following table shows the calcium content of some common food: Food Cow’s milk Swiss cheese Yogurt Ice-cream/frozen yogurt Canned salmon Egg Soybean curd (tofu) Almonds Cooked beans Serving Size 1 cup (250ml) 1 oz. (slice) 1 cup (175ml) 1/2 cup (125ml) 3 oz. 1 medium 4 oz. 1/2 cup (125ml) 1 cup (250ml) Calcium Content (mg.) 300 250-270 240-300 90-100 170-210 55 145-155 200 90 Table 1: Calcium content in some common food3, 4 2 Mismatches between Food Formation of Kidney Stone One of the possible causes of the formation of kidney stone is the intake of vegetable with high oxalic acid content. The acid in vegetable can be absorbed in form of oxalate and can react with calcium ions in blood to form calcium oxalate, which is insoluble and would precipitate out1, 2, 6. Fig 2: Structure of human urinary system. Note that ureter is the narrow tubule connecting bladder and kidney © 2001- 2007 wikipedia Ca2+(aq) + C2O42-(aq) CaC2O4(s) precipitates Most of the small calcium oxalate solids can pass through kidney and bladder in urine, thus can be excreted out of the bodies. However, if a large quantity of vegetables with high oxalic acid content is taken, calcium oxalate will be formed in a significant amount and the solids will grow into a large size before excretion and block a narrow tubule called ureter (see Fig 2), which connects the outlet of kidney with the bladder1,6. The solids obstructed ureter are called kidney stones1,2,6 (Fig 3). Patients suffering from kidney stones will result in acute flank pain, vomiting, restlessness and even fever if an infection happens1,6. Fig 3: Photo of kidney stone. Note that the kidney stone is large in size, with 8mm diameter. © 2001 – 2007 wikipedia 3 Mismatches between Food As a result, over-intake of vegetables which are rich in oxalic acid can lead to the formation of kidney stones and bring a severe painful sensation to the patients. Wrong Matching in Diet Oxalic acid in vegetables can inhibit absorption of calcium ions, too1,2. Some Chinese dishes, such as ‘bean curd and spinach soup’ (Fig 4) and ‘bean curd with chopped green onion’, are rich in both calcium and oxalic acid. Taking these dishes will not lead to formation of kidney stones, but our bodies are impossible to absorb the nutritive calcium ions. Fig. 4: Bean curd and spinach soap © 1999 – 2000 recipe. goolu.com When both calcium and oxalic acid reach our stomach, they will combine to form calcium oxalate, which precipitate out as solids and cannot pass through intestinal wall. These solids would pass through the gut and then be excreted out of the bodies in stool. Therefore, the the bodies in stool. Therefore, the calcium ions cannot be absorbed into the bodies and wasted1,2. Simple Science Principles: Simple yet IMPORTANT! In conclusion, taking too much vegetable with high oxalic acid content alone can result in the formation of kidney stones by the reaction of absorbed dietary oxalate ions with calcium ions in bloodstream, either from previously absorbed calcium ions or dissolved bone calcium ions. On the other hand, having dishes with both calcium and oxalic acid would result in inhibition of absorption of calcium ions. This is common to Chinese because a number of Chinese dishes contain food with high content of the above two. Both of them are governed by a simple chemical reaction called precipitation. Therefore, if we look into science, even with simple principles, it is possible to have some important discoveries! 4 Mismatches between Food References: 1. Wikipedia. Home page. [online]. Available: http://en.wikipedia.org/wiki/Main_Page 2007. Accessed: 2007 March 6 2. Oxalic acid and Foods. Home page. [online]. 2007. Available: http://growingtaste.com/oxalicacid.shtml 2007. Accessed: 2007 March 2 3. iVillage. Calcium content of various calcium-rich foods [online]. Available: http://health.ivillage.com/eating/ewdairy/0,,bh5x,00.html 2007. Accessed: 2007 March 2 4. Manbir online. Calcium content of some common food items [online]. Available: http://manbir-online.com/htm2/osteo.4.htm Accessed: 2007 March 2 5. Magnesium and Calcium absorption. Home page [online]. Available: http://www.mgwater.com/calmagab.shtml 2002. Accessed: 2007 March 2 6. MedlinePlus. MedlinePlus Medical Encyclopedia: Kidney stones [online]. Available: http://www.nlm.nih.gov/medlineplus/ency/article/000458.htm 2007. Accessed: 2007 March 6 In-Text Image Sources: Green Recipe.com. Home page [online]. Available: http://www.greek-recipe.com/static/ingredients/spinach.html 2003. Accessed: 2007 April 12 PCC Natural Markets. Carambola [online]. Available: http://www.pccnaturalmarkets.com/ 2007.Accessed: 2007 April 12 Recipe.goolu.com. Home page [online]. Available: http://recipe.goolu.com/01_catalog/index.php?php=prototype/h020_070_a. htm 2000. Accessed: 2007 April 12 5