Calcium

The two key nutrients to consider for bone health are the mineral calcium, and vitamin D. Calcium is a major structural component of bone tissue. It is deposited in bone in the form of a mineral complex called hydroxyapatite, which confers strength to the skeleton. Ninety-nine percent of the calcium in the body is stored in the skeleton, which therefore also acts as a ‘reservoir’ of calcium for maintaining calcium levels in the blood. Blood calcium levels have to be tightly maintained at a constant level, for calcium’s other roles such as in nerve and muscle functioning. Recommended daily calcium intakes for populations vary between countries, often as a result of differing use and interpretation of the available scientific data. The FAO/WHO (2) recommendations, based on data from several countries, are shown below.

			Calcium (mg/day)
	The 'recommended allowance' refers to the amount of calcium that each age group is advised to consume (with daily intake corresponding to an average intake over a period of time), to ensure that calcium consumed compensates for calcium excreted from the body each day. The calcium allowance figures for children and adolescents also take account of skeletal growth (net calcium gain), and those for postmenopausal women and the elderly also take account of a lower intestinal calcium absorption efficiency. *Particularly during the growth spurt. Figures based on Western European, American and Canadian data. Source: FAO/WHO: Human Vitamin and Mineral Requirements, 2002.
	Infants and Children:
	0-6 months		300-400
	7-12 months		400
	1-3 years		500
	4-6 years		600
	7-9 years		700
	Adolescents:
	10-18 years		1300*
	Women:
	19 years to menopause		1000
	Postmenopause		1300
	During pregnancy (last trimester)		1200
	Lactation		1000
	Men:
	19-65 years		1000
	65+ years		1300

Calcium Physiology

Calcium is absorbed in the small intestine both by passive diffusion, and by an active mechanism which requires vitamin D. An inadequate intake of calcium results in a reduced amount of calcium being absorbed, which in turn leads to a lower blood level of calcium. As a result our bodies produce a hormone, called parathyroid hormone (PTH) that causes calcium to be released from the bone ‘reservoir’ into the blood stream, because as stated above it is vital that blood calcium levels are kept constant at all times. It is thought that if calcium intake is habitually low on an ongoing basis (over years), then the small but sustained increase in the PTH level over time could cause a gradual loss of calcium from the bones, making them thinner and weaker.

Calcium requirements are high in the teenage years, during the time of rapid growth of the skeleton, and during this time, calcium absorption efficiency increases. The efficiency of calcium absorption, however, declines with age. This means that over time we need to consume higher amounts of calcium, which is why higher intakes are usually recommended for people over 65 years.

Sources of Calcium

Milk and other dairy foods are the most readily available sources of calcium in the diet. Dairy foods have the additional advantage of being good sources of protein and other micronutrients (besides calcium) that are important for bone and general health. Other good food sources of calcium include certain green vegetables (e.g. broccoli, curly kale, bok choy); whole canned fish with soft, edible bones such as sardines or pilchards; nuts (almonds and Brazil nuts in particular); and tofu set with calcium.

Although some other plant foods also contain appreciable amounts of calcium, some contain substances that bind to the calcium and prevent it being absorbed, e.g. compounds called oxalates in spinach and rhubarb, and phytates in dried beans, cereal husks and seeds1. However, oxalates and phytates only bind the calcium in the foods they are in – they don’t interfere with calcium absorption from other foods or drinks. Soy beverages are sometimes called ‘soy milk’, but it is important to look for ones that are fortified with calcium, because soy drinks don’t naturally contain calcium. Some examples of the approximate calcium levels in foods are shown in Table 2.