Calcium: How Does It Work?

Calcium: How Does It Work?

By reducing absorption of oxalate, a substance found in many foods, calcium may be able to indirectly reduce the risk of kidney stones. However, people with a history of kidney stones must talk with a doctor before supplementing calcium because such supplementation might actually increase the risk of forming stones for the small number of people who absorb too much calcium.

Calcium also appears to partially bind some fats and cholesterol in the gastrointestinal tract. Perhaps as a result, some older research suggests that calcium supplementation may help lower cholesterol levels.

Animal studies have established a role of calcium in the activation and maturation of female egg cells (oocytes). Although the precise role of calcium is not clearly defined, normalization of calcium levels in humans has potential applications for conditions of the human ovary, such as polycystic ovary syndrome (PCOS).

People with a high dietary intake of calcium were found to have a lower incidence of polyps in the colon and rectum (colorectal adenomas). In one preliminary study, high dietary calcium intake was associated with a decreased risk, while calcium supplementation was not. However, a double-blind study demonstrated a lower risk of developing colorectal adenomas in people taking 3 grams of calcium carbonate (providing 1,200 mg of elemental calcium) per day compared to those taking a placebo.


Key uses for calcium

  • Gestational hypertension (GH): Calcium supplementation has significantly reduced the incidence of GH in preliminary studies and in many, though not all, double-blind trials.
  • Lactose intolerance: As lactose-containing foods are among the best dietary sources of calcium, alternative sources of calcium (from food or supplements) are important for lactose-intolerant people.
  • Osteoporosis: Calcium is needed to form bones and teeth. The importance of calcium for preventing osteoporosis is probably its most well-known role.
  • Preeclampsia: Calcium deficiency has been associated with preeclampsia. In numerous controlled studies, oral calcium supplementation has been studied as a possible preventive measure.
  • Premenstrual syndrome: Disturbances in calcium regulation may underlie the development of PMS symptoms. Women who consume more calcium from their diets are less likely to suffer severe PMS.
  • Rickets: Vitamin D and calcium supplements should be used to treat rickets only if a medical professional has diagnosed rickets in a child and has also determined the cause is a nutritional deficiency. Amounts needed to treat rickets should be determined by a doctor and will depend on the age, weight, and condition of the child.


Other potential uses for calcium

  • Celiac disease (for deficiency only): The malabsorption that occurs in celiac disease can lead to multiple nutritional deficiencies, including calcium.
  • High blood pressure: 800–1,500 mg per day.
  • High cholesterol: 800–1,000 mg per day.
  • High triglycerides: In a preliminary trial, supplementation with 800 mg of calcium per day for one year resulted in a statistically significant 35% reduction in the average triglyceride level among people with elevated cholesterol and triglycerides.


Where is it found?

  • Most dietary calcium comes from dairy products.
  • Other good sources include sardines, canned salmon, green leafy vegetables, and tofu.


Which form is best?

  • Choosing a form of calcium supplement can be confusing. While fewer pills of the calcium carbonate form are needed, this form doesn’t absorb as well as some other forms of calcium.
  • Most, but not all, studies suggest that calcium citrate is better absorbed than calcium carbonate.
  • Virtually all comparative studies find that calcium citrate/malate (CCM) absorbs somewhat better than calcium carbonate. CCM is increasingly the form of calcium recommended by doctors.
  • Microcrystalline hydroxyapatite (MCHC), a variation on the bone meal form of calcium, has been shown to improve bone mass, but the absorption of MCHC appears to be poor.
  • Only preliminary research exists regarding the amino acid chelates of calcium, so conclusions cannot be drawn at this time.


How much is usually taken?

  • For ages 19 to 50, calcium intake is recommended to be 1,000 mg daily.
  • For adults over age 51, the recommendation is 1,200 mg daily.
  • The most common supplemental amount for adults is 800–1,000 mg per day.
  • General recommendations for higher intakes (1,200–1,500 mg) usually include the several hundred milligrams of calcium most people consume from their diets.


Are there any side effects or precautions?

  • Constipation, bloating, and gas are sometimes reported with the use of calcium supplements. A very high intake of calcium from dairy taken with calcium carbonate used to cause a condition called “milk alkali syndrome.” This toxicity is rarely reported today because most medical doctors no longer tell people with ulcers to take this combination.
  • People with hyperparathyroidism or chronic kidney disease should not supplement with calcium without consulting a physician. People who have had kidney stones should read the section on kidney stones before considering supplementation. For other adults, the highest amount typically suggested by doctors (1,200 mg per day) is considered quite safe.
  • In some cases, calcium supplements in the forms of bonemeal (including MCHC), dolomite, and oyster shell have higher lead levels than permitted by California regulations, though generally less than the levels set by the federal government. “Refined” forms (which would include CCM, calcium citrate, and most calcium carbonate) had low levels. In that report, only bonemeal exceeded federal levels. People who decide to take bonemeal, dolomite, or oyster shell for long periods of time should contact the supplying supplement company to request independent laboratory analysis showing minimal lead levels.
  • Vitamin D is needed for calcium to absorb. Therefore, many doctors recommend that those supplementing with calcium also supplement with 400 IU of vitamin D per day.
  • Animal studies have shown that essential fatty acids (EFAs) increase calcium absorption from the gut, in part by enhancing the effects of vitamin D and reducing loss of calcium in the urine.
  • Calcium competes for absorption with a number of other minerals. Therefore, individuals taking calcium for more than a few weeks should also take a multimineral supplement.
  • Lysine supplementation increases the absorption of calcium and may reduce its excretion. As a result, some researchers believe that lysine may eventually be shown to have a role in the prevention and treatment of osteoporosis.


Potential adverse drug interactions

  • Ciprofloxacin (Ciloxan®, Cipro®): Minerals such as calcium can bind to ciprofloxacin, greatly reducing the absorption of the drug.
  • Doxycycline (Atridox®, Doryx®, Doxy®, Monodox®, Periostat®, Vibramycin®): Many minerals can decrease the absorption and reduce effectiveness of doxycycline, including calcium.
  • Nadolol (Corgard®): Calcium supplements, if taken at the same time as some beta-blocker drugs, may reduce blood levels of the drug. However, whether calcium affects nadolol in this manner is unknown. Until more information is available, people on nadolol should take calcium supplements an hour before or two hours after the drug.
  • Ofloxacin (Floxin®, Ocuflox®): Minerals, including calcium, can bind to ofloxacin, greatly reducing drug absorption. Ofloxacin should be taken four hours before or two hours after consuming.
  • Tetracycline (Achromycin®, Sumycin®): Many minerals, including calcium, can decrease the absorption of tetracycline, thus reducing its effectiveness.
  • Thyroid hormones (Armour® Thyroid, Cytomel®, Synthroid®, and others): Simultaneous ingestion of calcium carbonate and one form of thyroid hormone (levothyroxine) reduces the absorption of the thyroid hormone. Separating calcium and thyroid hormones by at least four hours is recommended.
  • Verapamil (Calan®, Covera-HS®, Isoptin®, Verelan®): Calcium supplementation has been reported to reverse the blood pressure-lowering actions of verapamil when used to treat arrhythmias. It remains unclear whether people taking verapamil for the purpose of lowering blood pressure should avoid calcium supplementation. These people should discuss the matter with the prescribing doctor.