Calcium interactions with other nutrients
Calcium and phosphorus work together to form healthy bones and teeth. High phosphorus intakes lead to increased calcium excretion. The intake ratio for calcium to phosphorus should be 1:1.
Calcium competes with zinc, manganese, copper and iron for absorption in the intestine, and a high intake of one mineral can reduce absorption of the others. This is of particular concern in the case of iron. Calcium reduces both heme and nonheme iron absorption. The practical implications of the inhibitory effect of calcium mean that addition of milk or cheese to common meals such as pizza or hamburgers can reduce iron absorption by 50-60%. Some experts recommend eating foods that provide most of the daily iron intake at a different time to foods which provide most of the daily calcium intake. Thus it is advisable to reduce the intake of dairy products with the main meals providing most of the dietary iron, especially for children, teenagers and women of childbearing age whose iron requirements are high.
This interaction is also a concern in relation to supplements as calcium and iron are both commonly recommended for women. A study done in 1990 on postmenopausal women showed that calcium supplements decrease iron absorption from supplements and from food sources. Orange juice helped to avoid this reduction in absorption probably because it contains citric and ascorbic acids, both of which are known to enhance iron absorption. However, calcium citrate does not appear to reduce iron absorption.
Lead absorption is blocked by calcium in the intestines. Boron supplementation may reduce the excretion of calcium. Aluminum-containing antacids can inhibit calcium absorption. It is unclear whether magnesium inhibits calcium absorption. A 1994 study found no effect of magnesium supplements on calcium absorption. Calcium supplements have been shown to decrease zinc absorption. High calcium diets are being increasingly recommended to prevent osteoporosis and a 1997 study done in the US showed that high calcium diets decreased zinc absorption by 50 per cent and may raise requirements.
Boron seems to be beneficial to calcium metabolism. Calcium interacts with several vitamins, in particular, vitamin D and vitamin K.
Interactions with drugs
Some diuretics, corticosteroids and antidepressants can lead to calcium deficiency. Calcium supplements may decrease the effectiveness of tetracycline antibiotics; the anticonvulsant, phenytoin; and aspirin; and should not be taken at the same time as any of these drugs.
Calcium supplements should not be used in people who have impaired kidney function, cardiac arrhythmias, a history of kidney or bladder stones, constipation or dehydration. The calcium citrate form of supplement is less likely to cause kidney stones than calcium carbonate.
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