Calcium, the most abundant mineral in the body, makes up approximately 1.5% to 2% of the body weight and 39% of total body minerals. Approximately 99% of the calcium exists in the bones and teeth.
Numerous factors influence the bioavailability and absorption of calcium within the gut lumen. The greater the need and the smaller the dietary supply, the more efficient is absorption. Increased needs during growth, pregnancy, lactation, calciumdeficient states, or exercise resulting in high bone density enhance calcium absorption. Low vitamin D intake or inadequate exposure to sunlight reduces calcium absorption, especially among older adults. In addition, the efficiency of skin production of vitamin D by older adults is lower than that of younger people. Aging is also characterized by achlorhydria, which results in less gastric acidity and reduced calcium absorption.
In addition to its function in building and maintaining bones and teeth, calcium also has numerous critical metabolic roles in cells in all other tissues. Calcium is required for nerve transmission and regulation of heart muscle function.
Food Sources and Intakes
Cow’s milk and dairy products are the most concentrated sources of calcium. Dark green leafy vegetables such a kale, collards, turnip greens, mustard greens, and broccoli; almonds; blackstrap molasses; the small bones of sardines and canned salmon; and clams and oysters are good sources of calcium. Soybeans also contain ample amounts. Oxalic acid limits the availability of calcium in rhubarb, spinach, chard, and beet greens. Fortified foods (orange juice, soy, nut, grain or rice milks) contain as much calcium as cow’s milk. Many bottled waters and energy bars have calcium and sometimes vitamin D added.
Calcium supplements are now commonly used to increase intake. The most common form is calcium carbonate, which is relatively insoluble, particularly at a neutral pH. Although it has less calcium than calcium carbonate by weight, calcium citrate is much more soluble and would be suitable for patients with a lack of hydrochloric acid in the stomach (achlorhydria). In patients with achlorhydria, the efficiency of calcium absorption is greatly decreased because of the higher pH of the stomach contents; however, calcium absorption is increased by the consumption of a meal, which improves the solubility of calcium ions because of the increased gastric acidity.
An inadequate intake of calcium, in addition to an inadequate intake of vitamin D, may contribute to osteomalacia, colon cancer, and hypertension.
Infants : 200-260 mg/day, depending on age
Children aged 1-8 years: 700-1000 mg/day, depending on age
Children over age 9 and adolescents : 1300 mg/day
Adults (ages 19-50): 1000 mg/day
Adults 51 to 70: 1000 mg/day males; 1200 mg/day females
Adults over age 70: 1200 mg/day
Pregnant : 1000 mg/day; 1300 mg/day aged 14-18 years
Lactating: Same as for pregnancy