Your body generally absorbs about 30 percent of the calcium you consume, depending on the type and amount of food you eat and other factors, including your age and life stage, your health status, your vitamin D intake, and other components, such as phytic acid and oxalic acid, present in your food. Calcium absorption may be as high as 60 percent in infants and young children, but decreases with age, according to the Office of Dietary Supplements. Your specific rate of calcium absorption from a meal or dietary supplement depends on a combination of factors, the bioavailability of calcium in each food or supplement and other metabolic conditions and interactions.
Calcium occurs most abundantly in milk and milk products, but nuts, seeds, tofu and dark green leafy vegetables also can be major sources if you do not drink milk. Multiple factors affect calcium absorption. Depending on your body’s needs and the availability of calcium in your food or supplements, you may absorb as little as 5 percent or more than 50 percent of the calcium you ingest. A higher rate of absorption occurs when more is needed for growth and in times of inadequate intake of calcium-rich foods.
Stomach acid helps to keep calcium soluble and easily absorbed. When your body needs more calcium, it increases production of a calcium-binding protein to improve calcium absorption from your food, according to Eleanor Whitney, Ph.D. and Sharon Rolfes, M.S., R.D., in “Understanding Nutrition.” Vitamin D assists in the absorption of calcium, according to MayoClinic.com. Vitamin D helps make the calcium-binding protein that is required for absorption. Lactose also enhances calcium absorption. These factors make calcium-rich milk a good food to fortify with vitamin D.
A pregnant woman absorbs 50 percent of the calcium from milk. Growth hormones in growing children cause an increase in calcium absorption to a level of 50 percent to 60 percent of the calcium they consume in food and beverages. Later, when bone growth slows down, the rate of absorption falls to a normal adult level of 30 percent.
Conditions that enhance calcium absorption inhibit its absorption by their absence. For instance, a deficiency of vitamin D impairs the absorption of calcium, as does the lack of sufficient stomach acid. A high fiber diet, the phytates found in seeds, nuts and grains and the oxalate binders found in vegetables such as beets, spinach and rhubarb decrease the rate of calcium absorption from other foods eaten at the same time. These foods are nutritious, but are less useful as sources of calcium. Consumption of alcohol reduces calcium absorption by inhibiting the liver enzymes that help convert vitamin D to its active form, according to the Office of Dietary Supplements. If your intake of sodium, potassium, protein, or caffeine is high, some absorbed calcium is eliminated from your body, along with waste products.
Less than 5 percent of the calcium in spinach, rhubarb and Swiss chard is absorbed, due to the presence of oxalates that bind the calcium and inhibit its absorption by your body. The bioavailability of calcium in almonds, sesame seeds, sweet potatoes and pinto beans is about 20 percent. About 30 percent of the calcium found in milk, cheese and yogurt is absorbed. Among foods with the highest bioavailability, more than 50 percent absorbed, are Brussels sprouts, kale, mustard greens, broccoli, bok choy, cauliflower and foods that are fortified with calcium.