As we age, we slowly but surely lose bone mass.

Bones are rigid organs which help you to move, support and protect the various organs in your body and produce red and white blood cells.

They are generally made of cortical bone, which is compact bone tissue that appears smooth, white and hard, and trabecular bone, which is an interior network of plate-like and rod-like elements, much like scaffolding in construction sites.

Healthy bones should be thick and well-connected on the inside. Dr Lau Tang Ching, consultant at the department of rheumatology, allergy and immunology at Tan Tock Seng Hospital, said that bone strength is determined by bone mineral density and the architecture of the bone.

The former refers to the amount of calcium contained in a bone while the latter is concerned with how strong or fragile trabecular bone is.

Dr Lau said: 'Bones are living structures. New bones are formed as old bones are 'eaten' away, but the process becomes slower as we get older.'

Bones are broken down by specialised bone cells called osteoclasts. In healthy adults, the rate at which osteoclasts break down bone material is about the same as that of the creation of new material by their counterparts called osteoblasts. These processes continuously reshape and repair bones.

To build and maintain healthy bones, you should exercise regularly, have a diet high in calcium and Vitamin D and avoid alcohol and smoking.

Asked why calcium is so important for bones, Dr Lau said: 'Calcium is the building block. If you take less than 500mg of calcium a day, you'll reduce your bone strength because your bones won't have enough building blocks to work with.'

Dr Lau recommended a daily calcium intake of 800 to 1,000mg.

According to the National Nutrition Survey in 2004, the average daily calcium intake of Singaporeans is 627mg.

Low calcium intake has been linked to lower bone density and high risk of fractures.

Clinical Associate Professor Tan Seang Beng, senior consultant and head of the department of orthopaedic surgery at Singapore General Hospital, added that Vitamin D is essential for the body's absorption of calcium and in drawing it from blood into bone.

The best sources of calcium are dairy products like milk, yogurt and cheese. For those who are lactose intolerant, foods like ikan bilis, tofu and dark green leafy vegetables are viable alternatives. But more of these would have to be eaten as they contain smaller concentrations of calcium.

Dr Tan said excess calcium is passed out in urine and faeces if too much is ingested.

'It doesn't really form stones in kidneys,' he said. 'Stones are often formed due to poor kidney function rather than calcium itself.'

Doctors advise those under 30 to build up their bone mass to achieve a high peak. Indians generally have higher bone mass than Malays and Chinese.

Dr Lau said: 'Then even if you lose bone mass later, you may not get osteoporosis.'

Osteoporosis is a disease of the bone in which bone mineral density is reduced, increasing the risk of fractures.

Comparing the maintenance of strong, healthy bones to plumping up one's bank account, Dr Lau said: 'It's like saving money in a bank. If you save very little when you're working, you can't retire on much.'

This article was first published in Mind Your Body, The Straits Times on May 14, 2008.