- Adequate Intake for adults (19-50 years old): 1,000 mg/day
- Upper Intake Level for adults (19-50 years old): 2500 mg/day
- Absorption varies between food sources – calcium in cruciferous vegetables is well absorbed; calcium in fortified products not as well absorbed
- Cooking, soaking, and sprouting can increase absorption of calcium found in veggies, nuts, and seeds
- Absorption dependent on vitamin D; excessive intake of zinc and magnesium can inhibit absorption
- Food sources – Dairy, tofu (fortified), grains (fortified), legumes, green leafy vegetables, cruciferous vegetables
- Calcium citrate best absorbed
- Calcium Citrate – $0.12/serving
Calcium is the most abundant mineral in the body. Calcium occurs in two forms. The first form is called hydroxyapatite and it makes up the structure of bone and teeth. The second form is ionized calcium and it is present in bodily fluids for physiological activity. About ninety-nine percent of calcium is incorporated into bones and teeth while one percent is ionized. The calcium from the bones can be pulled into the bloodstream to sustain essential physiological processes.
Most calcium absorption occurs in the proximal small intestine and is dependent on vitamin D. Transient receptor potential vanilloid 6 (TRPV6) and calcium transporter 1 (CaT1) are transporter proteins which allow calcium to enter the enterocytes (intestinal cells). Vitamin D upregulates the expression of these transport proteins as well as calbindin which is a protein that shuttles calcium across the cytosol. Ca-ATPase then releases calcium into the plasma (blood stream) on the basolateral membrane of the enterocyte; Ca-ATPase is dependent on vitamin D as well. So, if vitamin D is not adequate, calcium absorption can become disregulated.
Calcium can be absorbed via paracellular diffusion (absorption between the cells). This occurs when the concentration of intestinal calcium is high and there is a need for calcium. Claudins, proteins which hold cells together, become relaxed due to the action of vitamin D, allowing for calcium to be absorbed. The claudins relax just enough to allow calcium to get though but not other things such as waste products. If the claudins relaxed too much, “leaky gut syndrome” would ensure and waste products would enter the bloodstream along with calcium. Calcium is a small element so it can be absorbed by itself. Fructooligosaccharides (FOS), inulin, and non-digestible saccharides may help calcium be absorbed through this mechanism.
A third mechanism of calcium absorption occurs when gut bacteria digest fermentable fibers and release calcium that is bound in those fibers. It is estimated that 4-10% of calcium in fiber-rich diets can be absorbed daily via this mechanism.
On average, we absorb about 25-30% of the calcium in the foods we eat and about 27-39% of the calcium found in supplements (depending on the form of calcium being used).
Calcium absorption is dependent upon the food source. So, even though a food is high in calcium, it does not necessarily mean we are great at absorbing it. Alternative milks such as soy and almond milk contain a lot of calcium but only about 30% of this form of calcium (tricalcium phosphate) is absorbed. The calcium found in dairy products and calcium-set tofu are absorbed at about 30% as well. Legumes, nuts, and seeds contain calcium but they also contain phytic acid which makes calcium less bioavailable. The absorption rate of calcium in almonds, sesame seeds, pinto beans, and sweet potatoes is about 20%. Processing such as cooking, fermenting, soaking, and sprouting can break down phytic acid and allow for more absorption of calcium. Only about 5% of the calcium found in spinach, rhubarb, and swiss chard is absorbed. The calcium found in cauliflower, watercress, cabbage, brussels sprouts, rutabaga, kale, mustard greens, bok choy, broccoli, and turnip greens are best absorbed at about 50%.
Calcium absorption is inhibited by oxalic acid in addition to phytic acid. Oxalates are found in spinach, swiss chard, beets, celery, squash, berries, pecans, peanuts, and tea. Phytates are found in grains, legumes, and seeds. The processing of vegetables, such as in sautéing them, will actually increase calcium absorption by reducing phytates. Adding 50 mg of a vitamin C source will reduce phytates too. Eating a high fiber diet will contribute to a healthy gut; this allows for normal stomach pH which reduces phytates as well.
Excessive intake of zinc and magnesium can also inhibit absorption.
Calcium excretion is increased by sodium and caffeine consumption. Two to three milligrams of calcium is lost per cup of coffee ingested. Each additional 500 mg of sodium consumption increase calcium excretion by about 10 mg.
About 40% of calcium found in the body is bound to albumin or prealbumin, which are proteins made in the liver. About 10% of calcium is bound to sulfate, phosphate, or citrate. And 50% is in its free, ionized form. The intracellular and extracellular concentrations of calcium are tightly regulated (8.5-10.5 mg/dL). If calcium levels fall too low or too high, hormonal shifts occur. Calcium will be released from the bone if concentrations are too low. Vitamin D will also be activated which allows more calcium to be absorbed in the gut. If calcium concentrations get too high, calcium excretion is increased and absorption is shut down.
Calcium is a major structural element of bones and teeth, is important in blood clotting as it activates thrombokinase, and transports ions across cell membranes which is important in nerve transmission and muscle contraction. Calcium is involved in the secretion of hormones including insulin. Calcium signals the release of insulin from beta cells in the pancreas, and magnesium controls calcium influx into cells. Therefore, when there is a magnesium deficiency, calcium is not well-regulated which may disrupt insulin release.
Withdrawal and deposition of calcium from the bones are regulated by parathyroid hormone (PTH) and calcitonin (calcium lowering hormone). PTH and calcitonin are sensitive to blood levels of calcium.
When blood calcium concentration drops, the parathyroid glands increase the excretion of PTH. PTH stimulates vitamin D in the kidneys; vitamin D is converted into its active form (calcitriol). Calcitriol rapidly decreases urinary excretion of calcium while increasing phosphorus secretion in the urine. PTH elevation results in the release of calcium and phosphate from the bone. This restores blood calcium concentration. The increased circulating calcitriol stimulates calcium and phosphorous to be absorbed in the intestines. Calcitriol also stimulates calcium to be released from bone. This is done by activating osteoclasts. PTH is no longer secreted when blood calcium levels normalize.
When blood calcium levels increase, calcitonin, a hormone produced by the thyroid gland, is stimulated. Calcitonin inhibits PTH excretion, decreases bone resorption (mineral being pulled from bone), and increases urinary calcium excretion.
Adequate Intake (AI)
The AI for adults (19-50 years old) is 1,000 mg/day.
Upper Intake Level (UL)/Excess/Toxicity
The UL for adults (19-50 years old) is 2500 mg/day.
Calcium toxicity causes calcium rigor. The muscles contract and do not relax. Milk-alkali syndrome can occur when greater than 10 grams of calcium carbonate (Tums) is taken per day. Alkalization of the blood can occur leading to symptoms incudling nausea, vomiting, and diarrhea.
Acute calcium deficiency can cause tetany. Intermittent muscle contractions occur which fail to relax, mostly in the arms and legs. Muscle cramps, spasms, and pain can also be a sign of calcium deficiency.
Chronic calcium deficiency can lead to stunted growth in children and osteoporosis in adults.
Calcium deficiency may be caused by excess sodium intake. Greater than 2,400mg of sodium increases calcium excretion by 24-40mg. Vitamin D deficiency, magnesium deficiency, excessive protein intake, and excessive caffeine intake may contribute to calcium deficiency too.
- Yogurt, plain, low fat, 8 ounces – 415 mg – 42% DV
- Sardines, canned in oil, with bones, 3 ounces – 325 mg – 33% DV
- Tofu, firm, made with calcium sulfate, ½ cup – 253 mg – 25% DV
- Turnip greens, fresh, boiled, ½ cup – 99 mg – 10% DV
- White beans (cooked), 1/2 cup – 81 mg – 8% DV
Calcium citrate and calcium malate appear to be bioavailable forms of calcium. Calcium citrate appears to contribute to the least amount of GI discomfort, especially in doses of less than 500 mg. Calcium carbonate, which is found in products like Tums, does not appear to be as bioavailable.
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