Source of Calcium Dietary

The majority of dietary calcium in industrialized countries comes from milk products; one serving (i.e., 250 ml milk or yogurt or 40 g cheese) contains approximately 7.5 mmol (300 mg). Nondairy sources (fruits, vegetables, and grain products) supply approximately 25% of total calcium. When substantial amounts of grains are consumed, for example, in breads or as maize products, these can be important sources, although the calcium in cereals tends to be less bioavailable than that in dairy products. Other foods high in calcium include tofu set with a calcium salt, kale, broccoli, and, increasingly, calcium-fortified juices and cereals. No matter what the source, a high percentage of people in both industrialized and less wealthy countries fail to meet recommended guidelines for optimal calcium intake.


Several dietary constituents decrease the bioavailability of calcium in food. Increasing fiber intake by, for example, replacing white flour by whole wheat flour in a typical Western diet has long been associated with negative calcium balance even when calcium intakes meet recommended levels. Likewise, the fiber in fruits and vegetables can cause negative calcium balance. In cereals, hytic acid is the main constituent of fiber that binds calcium, making it unavailable for absorption. The fermentation of bread during leavening reduces phytate content substantially, making calcium more bioavailable. In fruits and vegetables, the uronic acids in hemicellulose are strong calcium binders, as is the oxalic acid present in high concentrations in foods such as spinach. 

Calcium bioavailability from beans is approximately half and that from spinach approximately one-tenth of the bioavailability from milk. In contrast, calcium absorption from low-oxalate vegetables, such as kale, broccoli, and collard greens, is as good as that from milk. The difference in calcium absorption between the various forms of supplements is not large. Dietary fat does not affect calcium absorption except in individuals with diseases that impair fat malabsorption (e.g., short bowel syndrome, celiac disease, and pancreatitis). In these conditions, the calcium forms an insoluble and unabsorbable ‘soap’ with the unabsorbed fat in the alkaline lumen of the small intestine, potentially resulting in impaired bone mineralization. 

In addition, the luminal calcium is not available to precipitate the oxalates, meaning that the free oxalates will be hyperabsorbed leading to increased risk for renal oxalate stones. Neither dietary phosphorus nor a wide range of phosphorus-tocalcium ratios affect intestinal calcium absorption in very low-birth-weight infants and adults. Lactose improves calcium absorption in young infants, in whom absorption of calcium is predominantly by passive transport. In adults, the presence of lactose in the diet has little effect on the efficiency of calcium absorption. 

Effects of High Calcium Intakes 

Calcium can inhibit the absorption of both heme iron (found in meat, fish, and poultry) and non-heme iron. The mechanism by which this occurs remains controversial, but the inhibition probably occurs within the mucosal cells rather than in the intestinal lumen. This interaction is of concern because calcium supplements are taken by many women who may have difficulty maintaining adequate iron stores. Approximately 300–600mg of calcium, as a supplement or in foods, reduces the absorption of both heme and nonheme iron by approximately 30–50% when consumed in the same meal. 

The inhibitory effect on iron absorption is inversely related to iron status so that it is relatively unimportant above a serum ferritin concentration of approximately 50–60 mg/l. Thus, consideration should be given to monitoring the iron status of menstruating women with low iron stores who take calcium supplements. There is no inhibitory effect when calcium and iron supplements are consumed together in the absence of food, and inhibition may be less with calcium citrate. In the past, it was common to restrict dietary calcium in patients with a history of calcium oxalate stones. However, recent data suggest that a severe calcium restriction in patients with oxalate stones is not only ineffective but also can lead to bone demineralization. For the prevention of recurrent stone formation, a diet restricted in oxalate, sodium, and animal protein is probably most effective. Only if absorptive hypercalciuria is present should a moderate calcium restriction be imposed.

Long-term consumption of approximately 1500– 2000 mg calcium per day is safe for most individuals, although there will be some reduction in the efficiency of iron absorption. However, higher intakes from supplements (62.5 mmol or 2.5 g per day) can result in milk–alkali syndrome (MAS), with symptoms of  hypercalcemia, renal insufficiency, metabolic alkalosis, and severe alterations in metabolism. Based on risk of developing MAS, the upper limit for calcium intake is 2500 mg per day for adults and children.


Source: Guide to Nutrition Supplements
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