On The Shelf
Mary Warner
Calcium is the most abundant mineral in the body, with approximately 99% contained in the bones and teeth and the rest in extracellular fluid, intracellular structures, and cell membranes. Calcium must be absorbed from food or dietary supplements to support the continuous remodeling of bone as well as other metabolic processes.
Function
In addition to formation of bone and teeth, calcium is essential for muscle contraction, blood clotting, normal heart rhythm, and activation of several enzymes. The mineral is necessary for acetylcholine synthesis and for the functional integrity of cells, particularly those of the neuromuscular and cardiovascular systems.
Two hormones—parathyroid hormone and calcitonin— primarily regulate blood calcium levels. To keep blood calcium levels steady, the body moves calcium from the bones into the blood. When people do not consume sufficient calcium, the body compensates by drawing calcium from the bones, which weakens them.
As we age, calcium absorption efficiency decreases, resulting in a calcium deficiency that can lead to osteopenia and eventually osteoporosis. Calcium deficiency produces no obvious symptoms in the short term, so it’s important that patients consume enough calcium to prevent longer-term problems.
Daily requirements and dietary sources
The recommended amount of calcium intake per day varies by age and sex, ranging from 200–260 mg for infants up to 12 months and 700–1,000 mg for children up to 9 years to 1,300 mg for teenagers.
Most people need a daily calcium intake of at least 1,000 to 1,500 mg to maintain a normal blood calcium level without weakening the bones. Teenagers experiencing rapid growth and bone maturation need more calcium per day, as do older adults, particularly postmenopausal women, who more often experience lowered bone density and are at increased risk for osteoporosis.
Major food contributors to calcium intake include dairy products (milk, yogurt, and cheese) and green vegetables such as Chinese cabbage, kale, and broccoli. While spinach provides calcium, it has low bioavailability and should not be used as a source of calcium. Many foods are fortified with calcium, including fruit juices and drinks, tofu, and cereals.
According to FDA, most adults can easily meet calcium RDAs by incorporating dairy products into their diets daily. Each 8-oz serving of nonfat milk contains about 300 mg of calcium, and 3 oz of salmon contains almost 200 mg.
Calcium supplements
For those who do not regularly consume sufficient calcium as part of their diet, calcium supplements are available in multiple forms and are usually well tolerated in doses of less than 2 g per day. Many common calcium supplements also contain vitamin D to ensure effective absorption of the calcium.
Recommendations for calcium intake are based on elemental calcium content rather than the calcium salt in supplements. Most labels list elemental calcium as well as the amount of the salt, but pharmacists should be familiar with the salt forms and different percentages of calcium in each, including carbonate (40%), citrate (21%), lactate (13%), gluconate (9%), and phosphate salts (23%–39%).
What to tell your patients
While adequate calcium is critical to health, ensure patients know that calcium in doses greater than 3 g daily can be harmful. Large amounts taken as dietary supplements or antacids can lead to high levels of calcium in the urine and the formation of renal stones.
Development of hypercalcemia, particularly in those also taking high-dose vitamin D supplements, is associated with anorexia, nausea, vomiting, constipation, and polyuria.
Encourage patients taking more than 500 mg of supplemental calcium per day to take their supplements in divided doses, as optimal absorption occurs with individual doses of 500 mg or less.
Those taking calcium carbonate or phosphate supplements should take them with meals to increase absorption. Patients who take histamine-2 antagonists or proton pump inhibitors should take a supplement that contains a water-soluble salt formulation, such as calcium citrate, as these are more readily absorbed in basic environments.
Some patients may experience GI adverse effects, including gas, bloating, constipation, or a combination of these symptoms. Calcium carbonate appears to cause more of these adverse effects than calcium citrate, so a switch in the form of calcium supplement may be helpful if patients report these GI symptoms.