Introduction

Bones are truly amazing. A unique combination of flexible collagen and brittle calcium phosphate makes them incredibly strong and durable.

Part of the durability has to do with the fact that our bones are under constant maintenance, being partially broken down and rebuilt on a daily basis. Special bone cells called osteoblasts take calcium and phosphorus from the blood and deposit it as calcium phosphate crystals in the bone. In order for this to happen effectively a second set of cells, termed osteoclasts, must make some space for the new bone.

Osteoclasts remove the crystals and return them to the blood as calcium and phosphorus. It is the outer layer of the bones that are susceptible to these ongoing processes. We build up an optimum bone mineral (calcium phosphate) density (BMD) for the first 25-30 years of our life and spend the remainder of our lives gradually losing it, more quickly in women than in men.

(Figure 1). That is one reason why it is important to maximize your bone density when you are young improving your chances of avoiding or at least delaying osteoporosis.

Why all the fuss about calcium?

Calcium is responsible for nerve cell communication, contraction of muscle cells, function of important enzymes, protein synthesis, blood clotting efficiency and bone formation. Calcium levels in the blood are so important that the body will do almost anything to maintain an optimum concentration. If adequate calcium is not available from the diet then calcium will be removed from the bones and delivered to the blood. Unfortunately for your bones, blood calcium levels take priority.

You can imagine, perhaps, a blood calcium budget in your body being run by an accountant who insists on a balanced budget at all costs. The only external source of calcium is the diet (income). Since the digestive tract isn't 100% efficient we must consume more calcium than our bodies really need. In addition some calcium gets excreted in the urine. So if we use 1000 mg as an example (Figure 2). of a typical daily calcium intake then it is estimated that only about 300 mg of that will enter the blood, and of that 300 mg about 150 will be excreted in the urine and another 150 returned to the digestive tract and eliminated in the feces.

In this budget there is a constant daily input and output of calcium to and from the gastrointesinal tract, the blood, the bones and the body's other cells. With this in mind think of your bones as your nest-egg of calcium savings (sort of a calcium RRSP to be used when you're older or on a rainy day). If calcium income at a given time is not adequate to maintain the blood calcium balance then the accountant will borrow from the bones to ensure a steady blood calcium level. Under ideal circumstances this calcium debt will be repaid to the bones.