Interaction : Calcium and Food

• Due to its dependence on stomach acid for absorption, calcium carbonate is absorbed most efficiently when taken with food
• calcium citrate is absorbed equally well when taken with or without food.
• Calcium citrate is also useful for people with achlorhydria, inflammatory bowel disease, or absorption disorders
• Other calcium forms in supplements or fortified foods include gluconate, lactate, and phosphate.

Absorption

• Not all calcium consumed is actually absorbed in the gut. Humans absorb about 30% of the calcium in foods, but this varies depending upon the type of food consumed
• Amount consumed: 
• the efficiency of absorption decreases as calcium intake increases
• Age:
• Absorption decreases to 15%–20% in adulthood (though it is increased during pregnancy) and continues to decrease as people age; compared with younger adults, recommended calcium intakes are higher for females older than 50 years and for both males and females older than 70 years
• Other components in food:
• phytic acid and oxalic acid, found naturally in some plants, bind to calcium and can inhibit its absorption. 
• Foods with high levels of oxalic acid include spinach, collard greens, sweet potatoes, rhubarb, and beans. 
• Among the foods high in phytic acid are fiber-containing whole-grain products and wheat bran, beans, seeds, nuts, and soy isolates
• The extent to which these compounds affect calcium absorption varies. Research shows, for example, that eating spinach and milk at the same time reduces absorption of the calcium in milk.
• In contrast, wheat products (with the exception of wheat bran) do not appear to lower calcium absorption
• For people who eat a variety of foods, these interactions probably have little or no nutritional consequence and, furthermore, are accounted for in the overall calcium DRIs, which factor in differences in absorption of calcium in mixed diets

Elimination

• Sodium and protein intakes:
• high sodium intake increases urinary calcium excretion. 
• High protein intake also increases calcium excretion and was therefore thought to negatively affect calcium status 
• However, more recent research suggests that high protein intake also increases intestinal calcium absorption, effectively offsetting its effect on calcium excretion, so whole body calcium retention remains unchanged
• Caffeine intake: 
• this stimulant in coffee and tea can modestly increase calcium excretion and reduce absorption. 
• One cup of regular brewed coffee, for example, causes a loss of only 2–3 mg of calcium
• Alcohol intake: 
• alcohol intake can affect calcium status by reducing its absorption and by inhibiting enzymes in the liver that help convert vitamin D to its active form. 
• However, the amount of alcohol required to affect calcium status and whether moderate alcohol consumption is helpful or harmful to bone is unknown
• Fruit and vegetable intakes: 
• metabolic acids produced by diets high in protein and cereal grains increase calcium excretion. 
• Fruits and vegetables, when metabolized, shift the acid/base balance of the body towards the alkaline by producing bicarbonate, which reduces calcium excretion. 
• However, it is unclear if consuming more fruits and vegetables affects bone mineral density. These foods, in addition to reducing calcium excretion, could possibly reduce calcium absorption from the gut and therefore have no net effect on calcium balance

Reference:

https://ods.od.nih.gov/factsheets/Calcium-HealthProfessional/