Milk and dairy products offer the biggest source of calcium. Milk is a particularly good source of calcium. A single cup (8 fluid ounces) of milk usually contains about 300 milligrams (mg) of calcium. That is about 25 percent of the calcium that children and teenagers need every day. People older than age 2 are urged to choose nonfat or low-fat milk over whole milk, which contains more fat.

Other dairy products that are rich in calcium include cheeses and yogurt. Some people who have an inability to digest the milk sugar lactose (lactose intolerant) may not be able to consume standard dairy products. The enzyme lactase helps people digest lactose and lactase-fortified dairy products are available. Approximately 25 percent of Americans are lactose intolerant, according to the National Institutes of Health (NIH). People who do not eat animal products (vegans) also may not wish to eat traditional dairy products. In such cases, non-dairy calcium foods are available.

Other food sources of calcium include:

• Almonds
  
  
• Fish that is canned (mackerel) or canned with bones (salmon, sardines)
  
  
• Fortified breakfast cereal, cereal bars, orange juice and soy milk
  
  
• Green leafy vegetables such as broccoli, collards, Chinese cabbage, kale, spinach, turnips and mustard greens
  
  
• Some dried beans
  
  
• Some bottled waters
  
  
• Tofu, soy beans and other soy products, such as soy-based beverages and soy yogurt

Vitamin D is also vital for the body to process calcium. Good sources of vitamin D include fortified milk, tuna fish, fortified cereal, liver, egg yolks, fish and fish liver oils. In addition, 15 minutes of exposure to the sun provides a healthy dose of vitamin D. However, people are urged not to expose themselves to the sun without first wearing sunscreen.

Experts generally recommend that people get their calcium from dietary sources, which also provide other essential vitamins and minerals. However, in some cases a physician may recommend that a person take calcium supplements. These are usually taken several times over the course of a day. Most are available in over-the-counter formulations, although there is an injectable calcium supplement available only by prescription.

There are various types of calcium supplements, including those that contain calcium bound to compounds such as carbonate, citrate, lactate or gluconate. The various types of supplements contain different amounts of elemental calcium, which is the amount of calcium in the supplement that the body can absorb. Calcium carbonate usually contains the highest amount of elemental calcium. However, all of these supplements provide absorbable calcium.

Typically, the supplement lists the amount of elemental calcium on the label. However, some brands list only the total weight of each tablet. This amount also includes the weight of any substance to which the calcium is bound. People can still determine the amount of calcium available in such tablets by looking at the nutrition label. The percentage of the daily value (DV) listed is based on 1,000 mg of elemental calcium. Therefore, every 10 percent of DV corresponds to 100 mg of elemental calcium.

In addition, people are urged to choose brands of supplements that contain the abbreviation USP (U.S. pharmacopeia). This indicates that the supplements are of a quality that meets the voluntary standards established by the USP. Calcium supplements that contain bone meal, unrefined oyster shell, coral calcium or dolomite should be avoided because they may contain toxic substances such as lead, mercury or arsenic.

To check the absorbability of a calcium supplement, drop it into a small glass of warm water for 30 minutes, stirring it occasionally. If the supplement has not dissolved in that time, it likely will not dissolve well in the stomach. More expensive supplements called chelated calcium appear to have no advantage over other, less expensive types of calcium supplements.

Some medications may interact poorly with calcium supplements. Such drugs include:

• Aluminum or magnesium containing antacids
• Antibiotics such as tetracycline and fluoroquinolone
• Anticonvulsants such as phenytoin
• Digoxin (heart medication)
• Diuretics (blood pressure drugs) such as thiazide
• Glucocorticoids  (anti-inflammatory drug)
• Levothyroxine (drug used to treat thyroid condition)
• Mineral oil or stimulant laxatives

One recent study found that calcium supplementation during childhood has little effect on reducing fractures. This supports the notion that children should instead receive most of their calcium through dietary sources.