Calcium is an essential metallic element that only occurs naturally in the form of its compounds, never uncombined. It is the main elemental constituent of bones, shells and teeth.
The majority of the calcium in the body (in terms of weight) is used in the form of an insoluble calcium phosphate, known as hydroxyapatite. This is the structural component of bones and teeth, where the calcium phosphate is deposited on a protein matrix. However, the 100 grams or so not deposited in the bones and teeth takes the form of a soluble compound, ionic calcium, which plays essential roles in many metabolic functions throughout the body.
Calcium is derived from the diet only, although drinking water can be a significant source particularly in hard water areas, providing up to 250 mg per day in some instances. Some mineral waters also contain useful amounts of this essential metallic element.
Calcium is present in virtually all foodstuffs but in extremely varied concentrations. In a mixed diet, about half the daily dietary intake, (between 350 and 1200 mg) comes from milk and milk-derived foods, which are the richest sources of calcium.
Hard cheeses, soft cheeses, milk (particularly the dried skimmed variety) and yoghurt hold pride of place as the most important dietary sources. However, other foods such as nuts, pulses, fish (particularly the canned variety where the bones are soft and edible), eggs, wholegrain cereals and root vegetables are good sources of calcium. These can supply adequate intakes of the mineral even in the absence of milk and milk-derived foods.
Meats, maize, rice, potatoes and green leafy vegetables are poor sources of calcium. However, even these foods can provide useful quantities of the mineral, since they are regularly consumed in large quantities by some people.
Calcium is easily destroyed by food processing and refining techniques, so generally, the more highly processed the food, the lower its calcium content.
The absorption of calcium from the intestine and its uptake by the bones is completely dependent upon the presence of vitamin D, whether the mineral is taken in dietary or supplementary form.
Vitamin D stimulates the production of a certain protein called calcium-binding protein or CaBP, which transports calcium from the intestine to the blood and from the blood to the skeleton and teeth. It is possibly also involved in the excretion of calcium via the kidneys.
Hence, lack of vitamin D means that CaBP cannot be made and so calcium cannot be transported via its various routes within the body. Vitamin D deficiency and inadequate calcium have the same result: rickets in the growing child or osteomalacia in the adult, both of which are characterised by weakened bone structure due to lack of calcium deposition.
Although vitamin D is vital to the absorption of calcium from the diet, the efficiency and extent of the mineral’s absorption is also affected by the form in which it is presented.
Calcium must be in a soluble state to cross the intestinal barrier, the main solubilising factors being stomach acid and the presence of food. It has been calculated that nearly 40% of post-menopausal women are deficient in gastric acid and so their calcium absorption is impaired.
Calcium, like all minerals, cannot be destroyed but it can be lost during food refining and processing methods. It can also be immobilised by other food factors and made unavailable for absorption. The food factors most likely to impair calcium absorption are:
- Excessive intakes of raw dietary fibre in which the phytic acid combines with the calcium to form insoluble calcium phytate. Cooking the high fibre source, e.g. bran, will reduce the immobilising effect of the phytic acid. A daily intake of two tablespoons of raw bran as part of a mixed, sensible diet is unlikely to affect calcium uptake, although this should be regarded as the limit.
- A high intake of foods containing oxalic acid such as spinach, rhubarb, Swiss chard, beet tops and cocoa may immobilise calcium by forming the insoluble compound calcium oxalate. Average intake of these foods as part of a balanced diet is unlikely to impair calcium absorption.
- Excessive dietary intakes of saturated fats may also impair calcium absorption because the mineral and the saturated fatty acids present in the fats combine to form insoluble calcium soaps.
- Many medicinal drugs prevent calcium absorption either by a direct action on the mineral or via an indirect adverse reaction on vitamin D. Typical drugs are prednisone, diphosphonates, glutethimide, phenytoin, primidone, long term antacid therapy, certain diuretics (frusemide and ethacrynic acid), some laxatives (phenolphthalein), neomycin and colchicine. In all cases, extra calcium should be taken.
- Many phosphates naturally present in foodstuffs are unlikely to have an adverse effect on calcium absorption, but when high levels are added to foods during processing, as in the case of soft drinks or frozen foods, insoluble calcium phosphates may be formed.
In children, calcium deficiency causes rickets, which can also be caused by lack of vitamin D.
Rickets is characterised by excessive sweating of the head, difficulty in sleeping, constant head movements, slowness in sitting, crawling and walking, bowlegs, knock-knees and pigeon breast.
In adults, calcium deficiency, like inadequate vitamin D, gives rise to osteomalacia which causes bone pain, muscle weakness, delayed healing of fractures, twitches and spasms of the muscles of the face, hands and feet.
Osteoporosis is the condition where calcium is lost from the skeleton and is not replaced. It is not to be confused with osteomalacia where the prime problem is lack of absorption of calcium from the diet.
Osteoporosis is associated mainly with the post-menopausal period of life but it can occur in young females who regularly undergo intensive training, e.g. ballet dancers and top-class athletes. The symptoms are bone pain and increased tendency to fracture.
The value of calcium therapy in osteoporosis is controversial because the condition is caused essentially by low levels of the female sex hormones, notably oestrogen. Calcium supplementation in isolation appears to be more of a prophylactic than a therapeutic approach, but even in severe cases of osteoporosis that warrant oestrogen therapy, additional calcium is beneficial. It is more important to build up strong bones in the years leading to the menopause by ensuring adequate dietary and supplemental calcium, than treating an existing osteoporotic state with calcium alone. However, additional calcium at a level of 1 to 2 grams daily can slow down demineralisation of the bones in an existing condition.
Similar advice can be given to physically active young females but in their case, once their excessive exercising is finished, sex hormone production returns to normal levels and the condition of osteoporosis is reversed.
High intake of calcium should always be accompanied by small amounts of vitamin D to ensure that the mineral is absorbed from the intestines.Calcium and magnesium appear to have an inverse relationship; i.e. low calcium levels produce high magnesium levels. The same is true of calcium and potassium. Excessive intake of phosphorus that has been added to foods can adversely affect calcium levels.