Calcium
CALCIUM
Calcium is the most abundant mineral in the body. It makes up approximately 2 percent of the body weight with 99 percent of it incorporated into the hard tissues, bones, and teeth. The other one percent is present in the blood and extra cellular fluids and within cells of soft tissues where it regulates many important metabolic functions. In addition to building and maintaining bones and teeth calcium is necessary for muscle contraction, blood clotting (stimulates the release of thromboplastin from platelets, facilitates conversion of prothrombin to thrombin), cell membrane transport functions, release of neurotransmitters, synthesis and secretion of protein, hormones and intracellular enzymes, nerve transmission and regulation of heart beat. The proper balance of calcium, sodium, potassium and magnesium ions maintains muscle tone and controls irritability and the muscle membrane's electrical potential.
Calcium is present in bones in the form of hydroxyapitite crystals, composed of calcium phosphate, calcium carbonate, magnesium, zinc, sodium and fluoride. These salt crystals are arranged around a framework of softer protein material (organic matrix). The hydroxyapitite crystal provides strength and rigidity to the softer protein matrix of bone. The same crystals are present in the enamel and dentin of teeth; however, the calcium from teeth is generally not reabsorbed back into the bloodstream in times of need or in conjunction with low circulation levels of estrogen, progesterone, or testosterone. Bone calcium can be reabsorbed back into the blood stream, weakening the skeleton and increasing susceptibility to osteoporotic fractures (often seen in the spine and neck of the femur).
Blood levels of calcium are maintained within a fixed range by various feedback mechanisms. A significant increase in serum calcium can cause cardiac or respiratory failure and a hypocalcemic state leads to tetany (involuntary muscle spasm that can cause asphyxia and death from spasm of airway musculature).
Absorption and Metabolism
Calcium is absorbed primarily via active transport in the duodenum (some via passive diffusion). Active transport requires the assistance of Vitamin D. The body normally absorbs 30-40 percent of ingested calcium, but it can be as low as 10 percent from inorganic sources such as vegetables or grains with a high content of phytic or oxalic acid. Parathyroid hormone (PTH) increases calcium absorption by increasing the conversion of Vitamin D to its active form. In general, factors that increase calcium absorption include: serum levels of Vitamin D, and PTH, lactose, intestinal acidity, possibly fat intake. Factors that hinder calcium absorption include: oxalic acid (chocolate, spinach, beet tops, collard greens, etc.) but this is not of great concern as dietary calcium is usually far greater than dietary oxalate. The same is true for phytic acid found in whole grains (i.e. wheat brand and whole wheat). Low serum levels of Vitamin D and/or PTH decrease calcium absorption.
Following absorption, calcium enters the bloodstream and is transported to body tissues. The major site of deposition is bone. Unabsorbed calcium (approximately 60-70 percent of intake levels) is excreted in fecal matter, but may provide a protective role in regards to colon cancer prevention by binding to bile acids and other sterols and blocking their conversion to cancer-causing secondary sterols (lithocholic acid, deoxycholic acid).
Daily Calcium Requirement (NIH Recommendations)
Age Group and Gender Calcium (mg)
Under 6 months 400
6-12 months 600
1-10 years 800
11 -24 years Male and Female 1200-1500
25-50 years Male and Female 1000
51-64 years (Postmenopausal Women 1500
not taking estrogen replacement (ERT))
51-64 years (Postmenopausal Women 1000
taking ERT)
65+ years (Postmenopausal Women 1500
taking or not taking ERT)
51-64 years Men 1000
65+ years Men 1500
Calcium Preparations and Bioavailability
The bioavailability of various forms of calcium supplements has been evaluated using radio-isotope and other studies. The following is a summary of the key findings to date:
Type Absorptive Fraction of Calcium in Normal Subjects
Milk Approximately 33% on empty stomach
Calcium Carbonate Approximately 3 1 % on empty stomach
Calcium Citrate Approximately 40% on empty stomach
Calcium Gluconate Approximately 26.6% on empty stomach
Calcium Lactate Approximately 34.5 % on empty stomach
Tricalcium Phosphate Approximately 25.2% on empty stomach
Calcium Citrate-malate Approximately 34.9% on empty stomach
Calcium Chloride Approximately 36.4% on empty stomach
Average Diet Approximately 32% on empty stomach
It is best to take calcium supplements with food to capitalize upon the other potential benefits regarding bone/health and blood pressure regulation, as well as the improved bioavailability of calcium that occurs with meals (i.e. calcium carbonate absorption is enhanced by approximately 10 percent when ingested with meals)
Supplementation Studies and Clinical Applications
Osteoporosis
Currently one in four women and one in eight men over 50 have osteoporosis. Nearly one-third of all women and one-sixth of all men will fracture their hips in their lifetimes. Women's mortality rates from osteoporatic fractures are greater then the combined mortality rates from cancer of the breast and ovaries. Up to 20 percent of women and 34 percent of men who fracture a hip die in less then a year.
A large number of clinical trials have shown that calcium supplementation slows the rate of bone loss after menopause and in conjunction with resistance training, can also increase bone mineral density; even in women not taking hormone replacement therapy. Very strict protocols have been established regarding strength training and the accretion of bone density for this age group.
In general, a variety of calcium supplements (carbonate, citrate, citrate-malate, chloride, gluconate, lactate, microcrystalline hydroxyapitite (MCHC) have demonstrated an ability to retard age-related bone loss. The key factors appear to be to meet the NIH calcium intake recommendations from food and/or supplementation, ingest supplements with meals, perform weight bearing or weight resistance exercise 4-6 times per week, and ensure adequate serum Vitamin D levels. All of these factors enhance calcium absorption and/or calcium retention in bone.
High Blood Pressure
Various clinical studies indicate that calcium supplementation (i.e. calcium carbonate - 1500 mg per day) can reduce blood pressure to a significant degree in sodium-sensitive hypertensive patients. Most of these trials were 8-12 weeks in duration and used 1000-1500 mg of calcium carbonate or citrate. This subject is currently under intensive study to clarify the potential of calcium supplementation as a natural intervention for specific cases of hypertension.
Calcium supplementation (1000-2000 mg per day, calcium carbonate) may also help to prevent pregnancy-induced hypertension or function to reverse existing hypertension during pregnancy. This function is also presently under review.
Dosage Ranges
Most young adult and adult North Americans lack 500-800 mg per day of calcium to match the NIH recommended intake levels. Calcium supplementation represents a viable way to meet the recommendation in many cases.
1. Osteoporosis Prevention and Management - meet the NIH recommended intake levels for calcium, based upon age and gender.
2. Hypertension - Sodium-sensitive hypertensive patients may try 800-1,500 mg of calcium supplementation (8-12 week trial period) to test response.
Side Effects and Toxicity
It is generally acknowledged that calcium intake up to a total of 2000 mg per day appears to be safe in most individuals. The efficiency of calcium absorption decreases as intake increases, thereby providing a protective mechanism to lessen the chances of calcium intoxification. This adaptive mechanism can, however be overcome by a calcium intake of greater than 4000 mg per day. High intake of calcium may increase soft-tissue calcification (4000+ mg or in combination with hyperparathyroidism). In 1981, the FDA cautioned the public to limit its intake of calcium supplements derived from dolomite or bone meal because of the potentially high lead levels in these calcium supplements.
Drug-Nutrient and Other Interactions
Dietary factors such as alcohol, caffeine, sodium and a high protein diet can increase calcium loss from the body. However, studies show that these factors can be compensated for by ingestion of 250-500 mg of additional calcium in most instances.
Drug-Nutrient Interactions
The following drugs have been shown to deplete calcium or reduce its absorption into the body:
EDTA
Tetracycline
Aminoglycosides
Amphotericin B
Anticonvulsants
Salicylates (ASA etc)
Bile Sequestrants (cholestyramine)
Colchicine
Corticosterold drugs
Cimetidine
Isoniazid
Loop diuretics
Magnesium and Aluminum Antacids
Potassium-Sparing Diuretics
Digoxin-(animal studies only)
Drugs that are interfered with if taken at the same time as calcium:
Fluoroquinolone Antibiotics - Calcium can decrease absorption of these drugs and, therefore, calcium supplements and dairy products should not be taken within two hours of ingesting these drugs.
Levothyroxine - calcium carbonate can decrease drug absorption if taken at the same time.
Nutrient - Nutrient Interactions
1. Iron high doses of calcium can reduce iron absorption.
2. Zinc high doses of calcium can reduce zinc absorption.
Pregnancy and Lactation
During pregnancy and lactation the only supplements that are considered safe include standard prenatal vitamin and mineral supplements. All other supplements or dose alterations may pose a threat to the developing fetus and there is generally insufficient evidence at this time to determine an absolute level of safety for most dietary supplements other than a prenatal supplement Any supplementation practices beyond a prenatal supplement should involve the cooperation of the attending physician (i.e., magnesium and the treatment of preeclampsia.)
