Calcium is the most plentiful essential mineral found in the body. In fact, it makes up approximately 2 percent of the total body weight and 99 percent of the body's calcium can be found in the bones. Perhaps this mineral is best known for its role in helping to build and maintain strong bones and teeth, but it is also critically important for the contraction of muscles, release of neurotransmitters, regulation of the heart beat, and clotting of the blood. 
Calcium and magnesium also have a relationship, namely, if calcium is high, then magnesium absorption will be low and if magnesium is high, then less calcium will be absorbed. Typically calcium and magnesium are supplemented in a ratio of 2:1 or 1:1 (calcium:magnesium). 
The following foods have the highest concentration of calcium. For a more expansive list on food sources of specific nutrients visit Health Canada's Dietary Reference Intakes for Elements or USDA's National Nutrient Database
- Primary Sources: dairy products (cheese, yogurt, goat's milk, whole milk, cottage cheese)
- Plant Sources rich in calcium: tofu, kale, spinach, turnip greens, other green leafy vegetables such as collard and mustard
The calcium in kale is better absorbed than the form found in spinach. In fact, the rate of calcium absorption from kale is better than that found in milk products.
In terms of absorption of dietary sources of calcium from food, only about 25% is absorbed. Calcium is better absorbed in the presence of vitamin D, protein, lactose, and an acidic environment. If levels of phosphorus are increased, the rate of calcium absorption is decreased. Absorption of calcium is also impaired by higher levels of phytic acid, oxalate, fiber, fat, alkalinity, and stress. 
Other food sources include:
- grains: white rice
- protein sources: Sardines in oil, whitebait, salmon
- vegetables: Broccoli, watercress, okra, red kidney beans, chick peas,
- fruit: apricots, figs, currants, oranges
- dairy: milk, yoghurt, blue cheese, edam, feta, cheddar
- Other sources include: almonds, brazilnuts, hazel nuts, walnuts, soy
The following are the primary uses of calcium. 
- Bone formation: involves both mineral and non-mineral components of the bone matrix suggesting that it is not just a calcium issue; many nutrients are important for the health of our bones. Calcium has been shown to reduce bone loss in postmenopausal women and is also important for the prevention of osteoporosis. Calcium lactate, calcium citrate, or calcium gluconate are the preferred forms of calcium for supplementation. This is because these forms of calcium can be absorbed even if a person has low stomach acid. In contrast to this, calcium carbonate cannot be absorbed by a person who has low stomach acid.
- Hormone release
- Muscle contraction
- Nerve and brain function
Parathyroid hormone (PTH) and vitamin D help manage calcium balance in the body.
Deficiency in children can lead to rickets, which results in bone deformations and growth retardation.
Deficiency in adults can lead to osteomalacia which is a softening of the bones. Again, vitamin D, when deficient, can also lead to osteomalacia due to its important role in helping with calcium absorption.
Extremely low serum calcium levels may lead to:
Low calcium intake may eventually result in:
- High Blood Pressure: A link between deficient calcium (as well as magnesium) and high blood pressure has been made. Reductions in blood pressure have been shown in individuals with salt-sensitive hypertension but not in salt-resistant hypertension. The best results have been shown with calcium citrate over calcium carbonate.
- Pregnancy: Observational studies have shown that there is a greater incidence of pre-eclampsia and gestational hypertension in pregnant women who are low in calcium.
- Colorectal Cancer
Excess of calcium is not typically from supplementation, but from an associated underlying condition. Hypercalcemia is the term to describe excess calcium in the blood. Conditions associated with excess calcium include:
- Primary hyperparathyroidsim
- kidney stones
- soft-tissue calcification
- Adrenal conditions
- Kidney Diseases
Best specimens to collect: 
- Urine - positive result is low
- Hair - positive result is high
- Lead levels in calcium supplements is a concern; therefore be sure to use supplements from a reputable company who tests their products for lead content. Avoid natural oyster shell calcium, dolomite, and bone meal products unless manufacturer is sure that lead content is negligible .
- The best calcium supplements to use in terms of lead content are refined calcium carbonate or chelated calcium especially calcium citrate or calcium gluconate. Calcium citrate is better absorbed than calcium carbonate. Calcium carbonate requires solubilization and ionization by stomach acid. People with low stomach acid, therefore, have issues with proper absorption of calcium carbonate. Calcium citrate is already ionized and soluble. Other soluble forms can also be used such as calcium lactate, aspartate, orotate etc. In addition, Kreb cycle intermediates such as fumarate, malate, succinate, and aspartate can be used in combination with citrate and fulfill the requirements of being easily ionized, almost completely degraded, without toxicity, and helpful in increasing the absorption of calcium and other minerals.
- Calcium citrate also bypasses the issue of calcium supplementation increasing one's risk for developing oxalate kidney stones 
Calcium supplementation comes in many forms which are not all created equally in terms of their ability to absorb in the body. Calcium carbonate is the supplement most widely used, however, it is not absorbed as well as calcium citrate, calcium lactate, or calcium gluconate. If an individual has low stomach acid, this further reduces the absorbability of calcium carbonate. 
- The recommended dosages varies based on age and health status. To determine what your specific requirements are talk to your naturopathic doctor or other trained medical professional.
- Infants: 400mg (under 6 months); 600mg (6-12 months)
- Child: 800mg (1-10 years);
- Adolescent - Adult: 1200mg (Males and Females 11-24 years); 800mg (Males and Females 25+ years)
- Pregnancy and Lactation: 1200mg
- Children: Some sources suggest that supplementation should only be done under medical supervision due to risk of bowel perforation. Non-dairy foods rich in calcium are preferred; human breast milk is the best source for infants. Liquid forms are also available when supplementation is appropriate.
- Adults: generally regarded as safe
- Seniors: generally regarded as safe
- Pregnancy and Breastfeeding: Calcium supplementation during pregnancy and lactation is generally advised and can reduce risk of pre-eclampsia.
- Contraindications: in some individuals with hyperparathyroidism, chronic renal impairment or kidney disease, sacroidosis or other granulomatous diseases, cancer patients with a history of hypercalcemia, with a history of calcium stones (except the common calcium oxalate stones, where calcium supplementation with meals may reduce the risk of stone formation)
- Precautions: caution is generally appropriate in conditions associated with hypercalcuria and hypercalcemia; hyperparathyroidism, hyperphosphatemia, magnesium deficiency, or vitamin D overdose may lead to soft tissue calcification; hypertensive patients as blood pressure may be altered; high calcium intake, primarily from dairy products can increase prostate cancer risk (evidence is mixed).
- Drug Interactions include:
- Supportive or Beneficial:
- Bisphosphonates - Synergistic interaction. Vitamin D assists calcium absorption, and both enable drug in maintaining bone mineralization, including with HRT.
- Calcitonin - Calcium intake may enhance bone-sparing effect of drug in prevention or treatment of osteoporosis.
- Hormone Replacement Therapy (HRT): Estrogen-Containing and Synthetic Estrogen and Progesterone Analog Medications - Synergistic interaction, especially with osteoporosis. Calcium can protect bone mineral density (BMD) during attainment of maximal peak bone mass in adolescence and early adulthood (with which oral birth control pills can interfere) and minimize later bone loss. Vitamin D assists with calcium absorption and both enable estrogen to inhibit osteoclastic activity and bone resorption and maintain bone mineralization. Progestins may counter-benefit.
- Addresses Drug-Induced Deficiency:
- Aminoglycoside Antibiotics - Nephrotoxicity effects of aminoglycosides can increase calcium excretion. Calcium co-administration may be protective but may potentiate adverse effects, especially of gentamicin, although such nephrotoxic synergy has only been demonstrated with parenteral, not oral, calcium co-administration.
- Amphotericin B - Toxicity of drug associated with increased intracellular calcium concentration. Drug can deplete and disrupt calcium and other minerals.
- Anticonvulsant Medications - Anticonvulsants, especially phenytoin and phenobarbital decrease several nutrients including calcium, which in turn impairs mineralization, leading to increased risk of bone loss, osteoporosis, and fractures. Co-administer calcium and vitamin D.
- Cholestyramine, Colestipol, and Related Bile Acid Sequestrants - Drug can impair calcium absorption and vitamin D and fat-soluble nutrients. Risk of deficiency and sequelae.
- Corticosteroids, oral - Drug reduces calcium absorption and may increase excretion while decreasing vitamin D availability and lowering serum levels. Increased risk of bone loss, osteoporosis, and fractures with long-term drug use.
- EDTA - EDTA binds calcium thereby increasing excretion and increasing risk of hypocalcemia or negative calcium balance.
- Cimetidine and Related Histamine (H2) Receptor Antagonists & Omeprazole and Related Proton Pump Inhibitors - Both drugs can decrease calcium absorption. Supplement with calcium other than carbonate; take separate from meals.
- Heparin, unfractionated - Significant risk of bone loss with extended drug use and nutrient depletion. Co-administer calcium and vitamin D.
- Isoniazid - Drug can lower levels of both calcium and activated vitamin D. Drug-induced vitamin D deficiency can produce hypocalcemia and elevate parathyroid hormone. Co-administer calcium and vitamin D when drug used > 1 month.
- Metformin and Related Biguanides - Drug interferes with vitamin B12 absorption through calcium-dependent ileal membrane antagonism. Supplement vitamin B12, folic acid, and calcium.
- Sulfamethoxazole and Related Sulfonamide Antibiotics - Drug can impair calcium absorption as well as that of magnesium and vitamin B12. Additional calcium supplementation may be require during extended therapy. Separate intake.
- Tetracycline Antibiotics - Chelation between calcium and drug likely to impair absorption of both to clinically significant degree. Discontinue calcium (including dairy foods), or separate intake during short-term therapy. Calcium supplementation usually appropriate with extended therapy, with intake separated by several hours.
- Antacids Containing Aluminum and Magnesium - Aluminum-based antacids may reduce calcium absorption and complex with phosphates to deplete calcium. Calcium citrate may increase aluminum absorption. Avoid calcium citrate during therapy.
- Calcium Acetate - Possible adverse effect from concomitant administration due to additive effect.
- Thiazide Diuretics - Drug increases calcium retention by decreasing urinary calcium excretion. Concomitant use theoretically increases risk of hypercalcemia (low probably of occurrence).
- Atenolol and Related Beta-1-Adrenoceptor Antagonists (Beta-1-Adrenergic Blocking Agents) - Simultaneous intake may inhibit absorption and bioavailability fo both agents. Separate intake by 2 hours.
- Fluoroquinolone/Quinolone Antibiotics - Chelation between this class of antibiotics and calcium likely to impair absorption of both, as well as other minerals. Calcium depletion and effects of bone loss plausible with extended use but not established. Discontinue calcium or separate intake during short-term therapy. Mineral supplementation may be necessary with extended therapy.
- Levothyroxine and Related Thyroid Hormones - Chelation between drug and calcium impairs absorption of both, particularly with calcium carbonate. Thyroid hormones increase urinary calcium excretion. Calcium depletion and effects of bone loss plausible with extended use. Risk is greater in women with history of hyperthyroidism or thyrotoxicosis. Comorbid conditions may require both agents. Separate intake.
- Verapamil and Related Calcium Channel Blockers - Drug may decrease endogenous vitamin D synthesis and induce target-organ PTH resistance. Calcium administration can be used to reduce adverse effects of calcium channel blockers. Concurrent sue of drug and IV calcium salts, in particular, can be therapeutically efficacious (i.e., control cardiac tachyarrhythmias) yet avoid hypotensive effect of calcium channel blocker. Conversely, increased calcium availability may oppose drug's activity as calcium antagonist, particularly when antihypertensive effect is desired. Theoretically, excess calcium (or vitamin D) might contribute to hypercalcemia, which in turn might precipitate cardiac arrhythmia in patients on verapamil. Rare occurrence but potentially severe. Concurrent supplementation with calcium (and vitamin D may be appropriate, but only under close supervision.
- Nutrient Interactions include 
- Alcohol - intake may reduce calcium absorption.
- Essential Fatty Acids - Essential fatty acids may enhance calcium absorption, at least in part, by enhancing the effects of vitamin D, to reduce urinary calcium excretion, to increase calcium deposition in bone and improve bone strength, and enhance bone collagen synthesis.
- Iron - Concomitant use of calcium and iron can decrease GI absorption of iron, particularly from non-heme sources.
- L-Lysine - Lysine can enhance intestinal calcium absorption and improve the renal conservation of absorbed calcium thus reducing excretion.
- Magnesium - Concomitant administration of calcium and magnesium, particularly with high calcium, decreases GI absorption of magnesium.
- Milk and Dairy Products - There is controversy as to whether calcium in dairy products 1) has limited bioavailability for individuals who are not able to digest dairy properly, 2) reacts to the characteristic proteins.
- Phosphorus - Oral administration of calcium, as a divalent cation, may bind with oral phosphate and interfere with phosphorus absorption in the GI tract. High phosphorus intake also has a negative impact on calcium function. Separating intake of supplemental phosphorus and calcium by at least 2 hours will minimize impairment of absorption of either nutrient.
- Protein - The effect of dietary protein on calcium balance is an unresolved issue. As intake of dietary protein increases, urinary calcium excretion increases which can lead to an increase risk of bone loss over time. It is probably reasonable to suggest increased calcium supplementation when someone is consuming a high protein diet. However, individuals consuming low protein diets are probably at even greater risk of developing osteoporosis.
- Sodium - Evidence suggests that high sodium intake is associated with adverse effects on calcium and increased risk of bone loss.
- Soy - Phytic acid found in soy products may interfere with calcium absorption although some research shows that soy products have a fairly high calcium bioavailability despite it's phytic acid content. Some recommend separating intake of calcium supplementation and soy products by 2 hours.
- Vitamin D - A normal physiological function of vitamin D is to facilitate intestinal calcium absorption and supplementation is a part of an overall approach for the prevention of bone loss.
- Zinc - Most studies show that simultaneous administration of calcium and zinc decreases GI absorption of zinc.
- ↑ 1.0 1.1 1.2 1.3 Murray Michael T (2005) Encyclopedia of Nutritional Supplements, The Essential Guide for Improving Your Health Naturally, Prima Publishing
- ↑ 2.0 2.1 2.2 Hoffer Abram, Prousky Jonathan (2006) Naturopathic Nutrition, A Guide to Nutrient-Rich Food & Nutritional Supplements for Optimum Health, CCNM Press
- ↑ Medlineplus 
- ↑ Pizzorno Joseph, Murray Miachel (1999) A Textbook of Natural Medicine, 2nd Edition CCNM Pres.
- ↑ Bralley J Alexander and Lord Richard S (2005) Laboratory Evaluations in Molecular Medicine, Nutrients, Toxicants, and Cell Regulators Institute for Advances in Molecular Medince, GA
- ↑ 6.0 6.1 Stargrove Mitchell Bebell, Treasure Jonathan, McKee Dwight L (2008) Herb, Nutrient, and Drug Interactions, Clinical Implications and Therapeutic Strategies. Mosby