Them bones, them bones…

Bone suffers an unjustified reputation as stiff and unchanging, as though our skeletons were little more than a rattle of sticks, playing a supporting role in the bigger drama of living flesh and pulsing blood. After all, skin glows and wrinkles, bellies tighten or sag and muscles tremble and swell; hearts lose and sometimes skip a beat; lungs draw deep and shallow breaths and emit sudden gasps; and all five senses wax and wane with our growing years.

Compared to all that, bone – at least once we’re out of adolescence – is only a minor character in our corporeal stories, right?

Not at all. Bones are among the busiest of body parts; hollow bones, for example, are filled with marrow – red marrow in ribs, vertebrae, pelvic and skull bones, yellow marrow in the body’s long bones. Red marrow is the source of all the red and white cells and platelets that make up our blood; yellow marrow is a reserve of fat cells against a long lean stretch, but, if we become anemic, yellow marrow is converted into red and begins to pump out red cells to bolster up the blood.

^Bones, of course, play a more obvious supporting role in the long drama of growth, maturation andecline: Without our skeletons, we’d be hard pressed to stand, let alone walk, run, lift, carry or perform any other physical motion or activity; without bone, we’d be as shapeless as jellyfish. Given the lifetime of punishment in loads, strains and stresses we impose on our bony understructures, it’s a wonder our skeletons aren’t built a good deal heavier than they are. In fact, bone is remarkably (and thankfully) light; the entire skeleton accounts for only about 14 per cent of our total weight – less than 20 lbs. if you’re under 140 lbs. – yet, ounce for ounce, bone is stronger than steel or reinforced concrete. The femur, for example, the long bone of the upper leg, stands up under a compressive force of approximately 1,200 pounds per cubic inch just in the normal act of walking.

What gives bone its extraordinary (or perhaps we should say perfectly ordinary) strength is its chemical composition, a potent brew of salt crystals and tough collagen fibres. The salts are mostly complex compounds of calcium and phosphorous with traces of other elements, including sodium, zinc and lead, which form rod shaped crystals; that’s what gives bone its hardness and rigidity.

Collagen is a fibrous insoluble protein that’s a major part of nearly all the body’s connective tissues, including skin, ligaments and cartilage. In bone, it winds itself into long, rope like fibres studded with the rod shaped mineral crystals. The collagen keeps bone from being brittle; the crystals confer strength.

This crystal studded collagen forms two different, intricate kinds of structures in bone, either compact or cancellous. Compact bone occupies the middle of long bones, like the femur, while lighter cancellous bone forms at the ends of bones and occupies their hollow centres. Compact bone is built up in Haversian systems (named after the 17th century scientist who first described them) – concentric, circular layers of crystal collagen called lamellae, with central canals that contain blood and lymph vessels, nerve filaments and delicate connective tissue. Cancellous bone, on the other hand, is packed with a spidery coral like construct of load resistant struts and braces called trabeculae. All of it – compact and canellous bone – is wrapped in a thin membrane called the periosteum, which supplies extra blood as needed and sensation through a dense tracery of fine blood vessels and nerves.

Bones are busy inside and out; the hard tissue of bone is in a constant state of flux known as remodelling, continually ridding itself of old bone and laying down healthy new bone to replace it. The process begins with resorption, in which cells called osteoclasts release enzymes that chew up old bone, until they’ve created a pit in the surface of the bone. The renewal process begins with osteoblasts, which fill in the cavity with a meshwork of protein fibres that’s then hardened with mineral deposits.

The whole cycle of remodelling takes place over two to three month periods in healthy young people, but, as we age, the cycle develops a wobble: We continue to lose bone, but we fail to make it up at the same rate. Sometime between the teen years and the mid 20s in women, a little later in men, we start losing from .3 to .5 per cent more bone than we rebuild each year, depending on the individual.

And it’s basically downhill from there, says Dr. Gillian Hawker, Research Director of the Multi Disciplinary Osteoporosis Program at Women’s College Hospital in Toronto. “Women essentially maintain that bone to their mid to late 30s, then there’s a gradual onset of bone loss, which picks up roughly at the age of 50, concomitant with the onset of menopause. At that point, you have fairly rapid loss of bone, as much as five per cent loss of bone per year for up to 10 to 15 years.

“After that, bone loss slows a bit, but continues. To a degree, that happens with men too. Although they don’t physiologically go through menopause, there’s a slow age related loss of bone that occurs.”

The result of all this loss is osteoporosis, which means that bone (osteo) is becoming more porous, less dense. As this happens, the bones weaken and become increasingly subject to fractures, particularly in the hips, the head of the humerus in the arm, the wrist and vertebrae, which is why it’s important to achieve the highest peak bone mass possible: The higher your peak bone mass at the point your bones stop growing, the slower your inevitable bone loss will be and the less its impact as you age.

A number of things determine how much bone you’re going to lay down during childhood and adolescence – heredity, hormones, exercise and certain environmental factors – but one of the key factors is what you eat, especially the amount of calcium you consume. It’s been shown that populations with good calcium/vitamin D nutrition (vitamin D is essential to calcium absorption) attain better peak bone mass than populations that are poorly nourished.

In any case, the key to building good peak bone mass is to start as early as possible, says Dr. Robert Josse, Chief, Division of Endrocrinology and Metabolism at St. Michael’s Hospital in Toronto: “The late Prof. Charles Dent, who was a doyen of calcium metabolism in the world, said – 30 years ago – that osteoporosis was a pediatric disease. A very prescient and clever statement to make. If you don’t generate peak bone mass with which you are genetically endowed in early adulthood, you’ll never be able to make it again, at least not with treatment or anything that we’ve got available.”

That said, there are measures you can take to minimize your risks of developing osteoporosis; there are diagnostic techniques that can detect the early signs of the disease; and there are things you can do to halt the bone drain once it has started – before fractures rob you of your mobility and independence.