Minerals and trace elements - Calcium
Calcium (Ca) is the most abundant mineral in the body and is essential for a number of vital functions. The body needs adequate dietary calcium (alongside vitamin D and several other nutrients such as vitamin K) to develop and maintain healthy bones and teeth. Calcium also plays a vital role in many systems including intracellular signalling to enable the integration and regulation of metabolic processes, the transmission of information via the nervous system, the control of muscle contraction (including the heart) and blood clotting. Furthermore, it has been suggested that adequate calcium intake (for example from reduced fat dairy products) may help lower high blood pressure and may help protect against colon cancer, although more evidence is needed to fully substantiate these functions.
The skeleton contains about 99% of the body’s calcium with approximately 1kg present in adult bones. The major constituents of bone are calcium and phosphate, forming hydroxyapatite, which is associated within a meshwork of collagen fibres to form a rigid structure. The body’s requirement for calcium fluctuates with the rate of bone development, so as well as protecting vital organs, the skeleton acts as a ‘bank’ of minerals from which calcium and phosphorus may be continually withdrawn or deposited to support physiological requirement.
Calcium levels in the blood are carefully regulated and blood plasma levels are maintained within narrow limits. Calcium absorption is well controlled to match the needs of the body and so calcium balance can be maintained at a variety of different levels of calcium intake. Even at low levels of intake, there is evidence from the Gambia, for example, that calcium balance can be achieved. Calcium status is maintained by balancing calcium absorption from the gut, excretion via the kidneys and mobilisation and deposition in the bone. These sites are regulated by feedback mechanisms controlled by several hormones including parathyroid hormone and the activated form of vitamin D. Plasma levels of calcium only become abnormal if there is a breakdown of this homeostatic mechanism, and not usually as a result of differences in dietary calcium intake. The body invests this effort because small variations in plasma calcium concentrations may have serious consequences to the functioning of vital organs and to health in general. Low blood calcium is called hypocalcaemia and high blood calcium is called hypercalcaemia..
For some nutrients, nutritional deficiency is identified by the existence of a low blood level of the nutrient but for nutrients such as calcium, for the reasons described above, low blood levels rarely occur. Because of the need to maintain blood levels, the impact of a poor supply of calcium is usually reflected in bone density because bone acts as a reservoir in times of need. For example, insufficient calcium in bones can result from an inadequate supply of vitamin D which is essential for absorption of calcium. In children, vitamin D deficiency results in rickets and, in adults, osteomalacia, in which bones become weak owing to lack of calcium.
In terms of dietary supply, a significant proportion of young women have average calcium intakes below the Lower Reference Nutrient Intakes (8% of women aged 19-24 years and 6% of women aged 25-34 years) indicating these intakes are likely to be inadequate. An adequate calcium intake is vital for health, particularly in times of growth (in childhood, adolescence, pregnancy) to establish peak bone mass and also during lactation (breastfeeding). Supplements are sometimes recommended for those at risk of osteoporosis. See nutrient requirements for information on calcium requirements throughout the life course.
Obtaining calcium as part of a varied diet is unlikely to cause any adverse effects but taking high dose supplements sometimes causes stomach pain and diarrhoea.
Milk, cheese and other dairy products provide about half of the calcium in the UK diet. Bread is also an important source in the UK because most bread flour (though not wholemeal) is fortified with calcium by law. Calcium is also provided by some green leafy vegetables such as broccoli and cabbage (but not spinach), fortified soya products and fish eaten with the bones such as sardines, tinned salmon and whitebait, oranges, almonds, chickpeas.
Calcium absorption is influenced by a number of promoting and inhibitory factors. Promoting factors include vitamin D, lactose, dietary protein, non-digestible oligosaccharides and an acidic environment in the small intestine. Calcium is most readily absorbed from milk and dairy products. Inhibitory factors include phytates (e.g. in wholegrain cereals, pulses), oxalate (e.g. from spinach, rhubarb, beetroot), use of antacids, unabsorbed dietary fats, excessive intakes of dietary fibre and large intakes of phosphoric acid (e.g. from carbonated drinks). Calcium is often less available from plant foods where the calcium may be bound by phytates and oxalates in foods, which makes the calcium unavailable for absorption from the intestine into the blood. However, absorption from some plant foods is good e.g. broccoli, although the amount present is usually lower than in milk.
© British Nutrition Foundation