2.3 Calcium (Ca)
About 40% of the total mineral mass of bones is calcium, making it the most abundant mineral in the body. In bone, it is combined with phosphorus, as well as oxygen and hydrogen, in a mineral compound called hydroxyapatite. Calcium is also present in the fluids in the body, and there it occurs in the form of dissolved ions. An ion is an atom that carries a very small electrical charge, which can be either positive (+) or negative (−), depending on the ion.
You may recall from our study of Vitamin A that the charges are due to the loss or gain of electrons.
A calcium ion is written as Ca 2+. What does this indicate in terms of the number of its electrons compared with an atom of calcium?
A calcium atom must have lost 2 electrons to become Ca 2+, leaving it with two more protons in its nucleus than it has electrons around the outside, and thus an overall charge of +2.
Calcium ions, along with others, play an important role in the transmission of the electrical signals along the nerves of the body and in the brain, and in muscle contraction. Ions are also important in keeping the chemical composition constant inside cells and in the tissues around them. This process is one aspect of homeostasis, which is the maintenance of a stable internal environment in the body, by correcting any changes which occur to disturb that stable state. Calcium ions also play a role in blood clotting.
In the West, calcium is mainly obtained through milk and dairy products in the diet. Soya milk is usually enriched with calcium for vegetarians who do not consume dairy products. Calcium is present at a lower level in cereals and is added to most flour. It also occurs in green leafy vegetables and in those fish, like sardines, whose bones are eaten. Various compounds in food can bind to calcium and prevent it being released from the food so that it can be absorbed from the digestive system into the blood. For example, oxalates, which are present in spinach and rhubarb, may lock up the calcium in a compound called calcium oxalate. A meal containing these foods therefore provides the body with less calcium than would be expected. In general it appears that only about 30% of the calcium in food is actually absorbed into the blood; the rest is lost in the faeces.
Which vitamin is involved in the uptake of calcium from the digestive system and what are the deficiency diseases associated with this vitamin in adults and children? Which other vitamin plays a part in the formation of bone?
Vitamin D is involved in calcium uptake and the deficiency disease in adults is osteomalacia and in children, rickets (see section on Vitamin D). Vitamin K also has a role in bone formation (see section on Vitamin K).
With the natural ageing process, the amount of calcium present in the bones declines, especially in women for the first two to three years after the menopause. When this process has continued to the extent that the bones become fragile and easily broken, the condition is called osteoporosis. Inactivity and changes in some hormone levels, and certain drugs such as steroids, can increase the risk of osteoporosis. In 2000, there were 90 000 cases in the UK of fractures associated with osteoporosis, so it is a significant cause of illness (morbidity) and mortality in the population. The best method of prevention appears to be to achieve the maximum amount of bone mass (known as the peak bone mass, PBM) by the age of 20–25. Although bone composition is largely genetically controlled, various factors under the control of the individual can play an important role in teenagers and young adults, such as:
Taking exercise. Increased muscle development leads to increased bone mass.
Ensuring an adequate calcium intake, maybe as high as 1.3 g per day, i.e. significantly above the RNI value.
Maintaining a normal BMI. Underweight female teenagers are particularly at risk, since a low BMI leads to lower bone mass. It also leads to amenorrhoea (ceasing of the normal menstrual cycle), when steroid hormones, such as oestrogen, are at lower levels than normal, and this also affects normal bone growth.
Ensuring adequate vitamin D and K intake, as already mentioned.
Vitamin C is important for collagen synthesis, and collagen forms part of the structural framework of bones, so adequate vitamin C intake is important too.
Alcohol intake and cigarette smoking are linked with relatively lower bone mass.
In fact, many of these same factors apply to the maintenance of bone mass throughout life.