Calcium

Functions

Calcium is the most abundant mineral in our bodies. It is an essential mineral that is stored in our teeth and skeletons and contributes to their hardness. Calcium is also required for proper functioning of our neuromusclar and cardiac body systems; as it is intricately involved in the contraction of muscles, conduction of nerve impulses and blood clotting.

Between birth and puberty bone mass increases seven-fold, during adolescence there is a tripling in bone mass which then remains relatively stable until approximately age 50 years in men and menopause in women when it starts to decrease. Women lose bone mass more rapidly than men around the time of menopause, for approximately 5-10 years, thereafter they lose bone mass at the same rate as men: around 0.5 - 1.0% per annum. At menopause calcium balance deteriorates as intestinal absorption of calcium declines and/or urinary calcium excretion increases. Increasing dietary calcium intake post-menopause can help to slow this accelerated loss of bone mass and reduce the risk of fractures.

Role in Disease Prevention

Osteoporosis

Diets low in calcium have been associated with lower bone density, in particular the condition known as osteoporosis. The lower bone density associated with osteoporosis results in the bone being more porous and fragile, thereby increasing the risk of fractures. Osteoporosis is common in western populations and particularly amongst postmenopausal women. In fact, it is one of the leading causes of morbidity amongst post-menopausal women in New Zealand and Australia. Calcium intake is not the only factor affecting osteoporosis incidence, low vitamin D status and limited exercise have also been shown to increase the risk of osteoporosis.

Food Sources

Rich natural sources of calcium include milk and dairy products such as cheese, yoghurt and icecream. It is also found in plant-based foods such as legumes, nuts, soy beverages and breakfast cereals - however this calcium is not as bioavailable (see below). Calcium is also contained in the bones of small or tinned fish, such as tinned salmon.

Calcium Absorption and Excretion

When considering food sources of calcium consideration must be given to not only calcium content but also calcium bioavailability. Calcium absorption from different food groups varies considerably. For instance, calcium in spinach, rhubarb and beans is poorly absorbed due to the oxalic acid found in these plant foods. Compared to cow's milk, calcium absorption from spinach is about 10%. From dried beans it is slightly higher at around 50%. Absorption of calcium from seeds, nuts, grains and some soy isolates is also inhibited due to the phytic acid found in these foods. Absorption of calcium from soy milk, however, is typically as high as that from cow's milk.

Other factors which can affect calcium status are sodium and protein intake. Sodium excretion is linked to calcium excretion in the kidneys, hence if excessive sodium is consumed and therefore excreted by the kidneys it will take with it proportionally more calcium. For every 2300mg of sodium that is excreted around 40mg of calcium is also excreted. Protein intake can also affect calcium status as high protein intakes increase urinary calcium excretion.

Typical Contributions from Food

Food	Calcium (mg)
Milk, standard (1 cup)	299mg
Milk, low fat (1 cup)	590mg
Cheese, colby (20g)	130mg
Cheese, camembert (20g)	110mg
Yoghurt, low-fat, unflavoured (1 cup)	306mg
Yoghurt, low-fat, fruit (1 cup)	290mg
Ice-cream, vanilla (1/2 cup)	150mg
Spinach, boiled (1/2 cup)	40mg
Salmon, tinned (1/2 cup)	110mg
Almonds, flesh (10 nuts)	30mg

Source: The Concise New Zealand Food Composition Tables 5th Edition.

Recommended Dietary Intake (RDI)

Following are the recommended dietary intake (RDI) levels for New Zealand and Australia. 

Calcium RDIs – New Zealand and Australia

Life Stage	Age	Males (mg/day)	Females (mg/day)
Children	1-3 yr	500	500
	4-8 yr	700	700
	9-11 yr	1000	1000
	12-18 yr	1300	1300
Adults	19 - 50 yr	1000	1000
	51 - 70 yr	1000	1300
	70+ yr	1300	1300
Pregnant	14-18 yr	-	1300
	19+ yr	-	1000
Breastfeeding	14-18 yr	-	1300
	19+ yr	-	1000

Suggested Dietary Target for Reducing Chronic Disease Risk

The New Zealand and Australian governments have not set a suggested dietary target (SDT) for calcium intake.

Deficiency

A diet that is chronically low in calcium during childhood growth may limit the attainment of peak bone mass. Once peak bone mass is attained (in early adulthood) a chronically low calcium intake may contribute to bone loss and ultimately osteoporosis (see above discussion). 

Toxicity

The highest level of average daily intake of calcium for children and adults that is believed to not cause any adverse health effects is 2500mg/day. It should be noted, though, that the Second Expert Report Food, Nutrition, Physical Activity, and the Prevention of Cancer: A Global Perspective, produced by the World Cancer Research Fund and American Institute of Cancer Research, states that diets high in calcium are a "probable cause of prostate cancer" in men. Their definition of a diet high in calcium is one that contains in excess of 1500mg/day of calcium.

References

Garrow JS, James WPT & Ralph A. (2000). Human Nutrition and Dietetics (10th Ed). London: Churchill Livingstone.

Ministry of Health. (2006). Nutrient reference values for Australia and New Zealand. Canberra: Commonwealth of Australia.

Last Updated: 9 October 2008

The material provided by Thinking Nutrition Ltd on this website is for information purposes only. It is not a substitute for appropriate health advice from a qualified medical practitioner.