What affects calcium absorption and excretion?

Calcium absorption refers to the amount of calcium that is absorbed from the digestive tract into our body's circulation. Calcium absorption can be affected by the calcium status of the body, vitamin D status, age, pregnancy and plant substances in the diet. The amount of calcium consumed at one time such as in a meal can also affect absorption. For example, the efficiency of calcium absorption decreases as the amount of calcium consumed at a meal increases.

Age:
Net calcium absorption can be as high as 60% in infants and young children, when the body needs calcium to build strong bones. Absorption slowly decreases to 15-20% in adulthood and even more as one ages . Because calcium absorption declines with age, recommendations for dietary intake of calcium are higher for adults ages 51 and over.

Vitamin D:
Vitamin D helps improve calcium absorption. Your body can obtain vitamin D from food and it can also make vitamin D when your skin is exposed to sunlight. Thus, adequate vitamin D intake from food and sun exposure is essential to bone health.

Pregnancy:
Current calcium recommendations for nonpregnant women are also sufficient for pregnant women because intestinal calcium absorption increases during pregnancy. For this reason, the calcium recommendations established for pregnant women are not different than the recommendations for women who are not pregnant.

Plant substances:
Phytic acid and oxalic acid, which are found naturally in some plants, may bind to calcium and prevent it from being absorbed optimally. These substances affect the absorption of calcium from the plant itself not the calcium found in other calcium-containing foods eaten at the same time . Examples of foods high in oxalic acid are spinach, collard greens, sweet potatoes, rhubarb, and beans. Foods high in phytic acid include whole grain bread, beans, seeds, nuts, grains, and soy isolates. Although soybeans are high in phytic acid, the calcium present in soybeans is still partially absorbed. Fiber, particularly from wheat bran, could also prevent calcium absorption because of its content of phytate. However, the effect of fiber on calcium absorption is more of a concern for individuals with low calcium intakes. The average American tends to consume much less fiber per day than the level that would be needed to affect calcium absorption.
Calcium excretion refers to the amount of calcium eliminated from the body in urine, feces and sweat. Calcium excretion can be affected by many factors including dietary sodium, protein, caffeine and potassium.
Sodium and protein:
Typically, dietary sodium and protein increase calcium excretion as the amount of their intake is increased. However, if a high protein, high sodium food also contains calcium, this may help counteract the loss of calcium.
Potassium:
Increasing dietary potassium intake (such as from 7-8 servings of fruits and vegetables per day) in the presence of a high sodium diet (>5100 mg/day, which is more than twice the Tolerable Upper Intake Level of 2300 mg for sodium per day) may help decrease calcium excretion particularly in postmenopausal women.

Caffeine:
Caffeine has a small effect on calcium absorption. It can temporarily increase calcium excretion and may modestly decrease calcium absorption, an effect easily offset by increasing calcium consumption in the diet. One cup of regular brewed coffee causes a loss of only 2-3 mg of calcium easily offset by adding a tablespoon of milk. Moderate caffeine consumption, (1 cup of coffee or 2 cups of tea per day), in young women who have adequate calcium intakes has little to no negative effects on their bones.


Other factors:
Phosphorus: The effect of dietary phosphorus on calcium is minimal. Some researchers speculate that the detrimental effects of consuming foods high in phosphate such as carbonated soft drinks is due to the replacement of milk with soda rather than the phosphate level itself.

Alcohol: Alcohol can affect calcium status by reducing the intestinal absorption of calcium. It can also inhibit enzymes in the liver that help convert vitamin D to its active form which in turn reduces calcium absorption. However, the amount of alcohol required to affect calcium absorption is unknown. Evidence is currently conflicting whether moderate alcohol consumption is helpful or harmful to bone.
In summary, a variety of factors that may cause a decrease in calcium absorption and/or increase in calcium excretion may negatively affect bone health.