Calcium 

Calcium in the body is found as a divalent cation (Ca2+) and accounts for 1.5% of body mass.¹ Bones and teeth contain most of the calcium within the body - 99%!¹ Calcium can be complexed to other molecules, including calcium carbonate, calcium acetate, calcium citrate, calcium gluconates, calcium phosphates, etc. These are often found in supplements with varying absorption rates estimated at 24-42%.¹

Absorption: Calcium is absorbed in its ionized form and is liberated from molecule complexes.¹ Ionized calcium absorption can occur by two pathways. One pathway is controlled by vitamin D when bound to receptors, forming a vitamin D receptor complex. It involves vitamin D promotion of calcium channels known as calbindin. Calcium binds to vitamin D-regulated calbindin and is exported to the blood via a vitamin D-regulated ATPase.¹ Paracellular diffusion between intestinal cells is another mechanism.¹

Ionized calcium can form chelates (ligand bonded to metal ions), reducing absorption within the body. Chelates can also form with fiber-like substances and other minerals decreasing absorption. Other than chelates, factors that decrease calcium absorption include decreased vitamin D and parathyroid hormones; phytate, oxalate, and fiber found in certain foods; excess zinc, magnesium, vitamin A, and phosphorus presence; and high-fat diets (due to formation of soaps).¹  Factors that increase calcium absorption include increased vitamin D and parathyroid hormone, lactose consumption with calcium, increased need, varying consumption throughout the day, and an acidic pH in the gut.¹

Circulation and Excretion: Dietary blood calcium does not impact blood calcium levels significantly due to endocrine regulation. Parathyroid hormone is released into the bloodstream when blood calcium levels are low.¹ With the presence of vitamin D, calcium absorption is increased by the formation of calcium-binding proteins. Parathyroid hormone mobilizes calcium from bones, reduces excretion of calcium in kidneys, and promotes reabsorption of calcium.¹ If blood levels of calcium are high, calcitonin is secreted to deposit calcium in bones.¹ 

Functions:  

Bone Structure- Calcium composes the structure of bones and teeth; specifically, calcium phosphate and hydroxyapatite are complexes that strengthen and harden bones and teeth. Bones, cartilage, and dentin are associated with bone Gla protein and matrix Gla protein that contains calcium.¹

Muscle Contractions- Within the heart, an electrical signal produced by calcium allows a calcium influx via voltage-gated channels within the cardiac muscle causing a contraction. Increased intracellular calcium levels (derived from sarcoplasmic reticulum)  within skeletal muscle allow for fiber contraction, and calcium is pumped back into the extracellular fluid.¹ Calcium concentration is higher in the extracellular fluid than in the intracellular fluid.¹ Smooth muscle tissue and neurotransmitters also use calcium (located in extracellular fluid) to generate muscle contractions.¹

Glycoprotein Formation - Vitamin A may play a role in glycoprotein synthesis. Glycoproteins help cells recognize and communicate with each other amongst other roles.¹

Messenger- Calcium binds to calmodulin, a protein containing four calcium binding sites, to regulate enzymes and participate in other intracellular activities.¹

Blood Clotting - Blood clotting factors (specifically factors VII, IX, and X) and prothrombin conversion to thrombin require calcium for clotting to occur.¹

Dietary Reference Intake (DRI) - The recommended dietary allowance (RDA) for adult men and women (19-50 years old) is 1000 milligrams (mg).² An increase is noted for women between 51-70 years old at 1200 mg and for both men and women 70 years old or older at 1200 mg.¹

Deficiency - Calcium deficiency in early years can lead to decreased bone mineralization which causes softening of bones.¹ In adults, osteomalacia (bone demineralization) and osteoporosis (loss of total amount of bone) can arise from calcium deficiency. Calcium deficiency can be related to diet, levels of vitamin D, decreased absorption, or increased excretion among other factors.¹

Toxicity - Calcium toxicity may develop with certain medical conditions and supplement usage. Though uncommon, excess calcium can deposit in soft tissues and may lead to kidney stone development.¹

Dietary Sources of Calcium 

Calcium is most commonly found in dairy products, including milk, cheese, yogurt, and ice cream. Other than dairy products, some vegetables, nuts, and fish contain calcium. Calcium is also fortified in foods, such as orange juice and grain products. Oxalates and phytate compounds found in certain foods can bind to calcium, lowering the absorption rate.¹

Examples of foods containing calcium include

Dairy - yogurt, milk, cheese 

Vegetables - spinach, turnip greens, kale

Fish - sardines, salmon 

Legumes - soybeans, tofu, lima beans 

Source(s):

1. Denise M Medeiros and Robert E.C. Wildman, Advanced Human Nutrition, 4th ed. (Burlington, MA: Jones & Bartlett, 2019).

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