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Calcium and bones

Calcium is the most abundant mineral in the body. It is well known for its important role in maintaining strong teeth and bones. Most calcium, 99%, is found in the teeth and bones. The remaining 1% is found in the body's fluids and cells. Calcium requires vitamin D for absorption. It also works closely with magnesium, zinc, fluoride and phosphorous. Calcium is also important for proper heart function, nerve transmission, and blood clotting. Complex processes control the amount of calcium in the blood. When there is too little of it in the blood, hormones pull it from the bones to meet the body's demands.

What food source is the nutrient found in? 
Dairy products are the most concentrated source of calcium. Dark green leafy vegetables and fish with edible bones are also good sources. Some vegetables, such as spinach, contain oxalates and some grains contain phytates. Both of these partially block the absorption of calcium. Caffeine and high protein intake can interfere with absorption also.

Sources of calcium include:
  • milk, skim = 320 mg (milligrams) per cup
  • yoghurt = 300 - 450 mg per 200 gram tub
  • mozzarella cheese = 350 mg per 40g
  • tofu = 200 - 400 mg per half cup
  • milk powder = 500 - 650 mg per 1/2 cup
  • fortified soy beverage = 280 - 300 mg per cup
  • broccoli = 30 mg per half cup
  • 50g salmon (with bones) = 150 - 190 mg
  • 50g sardines (with bones) = 190 mg
  • almonds = 155 mg per 1/2 cup
How does the nutrient affect the body? 
The most well known function of calcium is to preserve bone density. This process is aided by other important vitamins and minerals including magnesium and vitamin D. It is estimated that 1 in 2 women and 1 in 3 men over the age of 60 will sustain a fracture (broken bone) due to osteoporosis. An alarming 17% of people with hip fractures die within four months. Osteoporosis is an ongoing condition of brittle bones becoming weaker. This disease puts individuals at greater risk for hip and other bone fractures. Bones act as a reservoir for calcium. If there is not enough calcium circulating in the blood to meet the body's needs for the mineral, it will be pulled from the bones.

Typically, people keep building up calcium in the bones through their late 20's and early 30's. After age 30 to 35 bones begin to naturally lose the minerals that give them strength, like calcium. After age 50, calcium loss from bone becomes even more common, especially among women. The hormone oestrogen is involved in keeping calcium in bones. After menopause, when a woman's body stops producing oestrogen, calcium loss from bone increases. Hormone replacement therapy and weight-bearing exercise such as walking and weight lifting, helps maintain bone density in post-menopausal women.

It's important that people get enough calcium in their youth to build a good reservoir to draw from in later years. Later in life, it's important to consume enough calcium so that it is not robbed from the bones. Not only is calcium needed but also vitamin D to help it absorb into the bones.

Bone health is not the only function of calcium in the body. Calcium is necessary for vital nerve and muscle transmission including proper functioning of the heart. Several recent studies have shown how calcium can help lessen both the physical and emotional symptoms of pre-menstrual syndrome or PMS. The mechanism behind this effect is not yet known. It may be because of calcium's close association with certain hormones. The dose used in these studies was 1200 mg a day.

The Recommended Dietary Intake (RDI) for adults is:
  • adults age 19 through 54 need 800 mg of calcium each day
  • women aged 54 and older should get 1000mg a day
  • during pregnancy and breastfeeding, an extra 300 - 400 mg is needed daily
The goal for all individuals should be to get the recommended amount of calcium each day. However, calcium intake should not exceed 2,500 mg. Large intakes of calcium could cause deposits in the kidneys or heart. Large intake can also reduce the absorption of zinc and iron; impair vitamin K metabolism and encourage the loss of calcium from the bones.

There are many supplements on the market for people who have a hard time getting enough calcium through diet alone. To be stable and absorbable, calcium in supplements is always paired with another compound. Calcium citrate and calcium citrate-malate are usually the best absorbed. Calcium carbonate, the type of calcium common in antacids, is more concentrated and is generally less expensive than other compounds. Individuals should avoid supplements with dolomite or bone meal because they may contain small amounts of lead. Calcium supplements should be taken with meals to help absorption. They should not be taken at the same time as iron supplements, because calcium blocks the absorption of iron. If a person takes more than one tablet, supplements should be taken throughout the day and not all at one time. Individuals should drink plenty of fluids with calcium supplements to avoid constipation. Taking a calcium supplement with milk can help enhance absorption because of the lactose and vitamin D in milk.

Calcium supplements should not be used as a substitute for healthy food choices. They should only be used to supplement the diet. To increase calcium from foods in the diet, individuals can:
  • include yoghurt with breakfast
  • drink a glass of milk with each meal
  • use milk in coffee or tea instead of coffee whitener
  • add low-fat cheese to sandwiches or salads
  • check food labels for calcium content
  • use a soy drink which is fortified with calcium
  • use calcium enriched milk
  • www.osteoporosis.org.au
  • www.healthybones.com.au
Author: Clare Armstrong, MS, RD
Reviewer: eknowhow Medical Review Panel
Editor: Dr John Hearne
Last Updated: 17/10/2004
Potential conflict of interest information for reviewers available on request

This website and article is not a substitute for independent professional advice. Nothing contained in this website is intended to be used as medical advice and it is not intended to be used to diagnose, treat, cure or prevent any disease, nor should it be used for therapeutic purposes or as a substitute for your own health professional's advice.  All Health and any associated parties do not accept any liability for any injury, loss or damage incurred by use of or reliance on the information.


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