Osteoporosis is a condition in which bones become thinner and weaker, increasing the risk of fractures in your hips, wrists and spine (around the chest and lower back regions). Most people don't know they have it until they break a bone.

When do you get it?

Bone is constantly renewed. But in osteoporosis, the new bone is more brittle and there is less of it. Your bone mass (which peaks in your 30s when your bones are at their strongest) gradually gets reduced. It's a natural part of ageing, but best delayed as long as possible. As well as breaks, it can make you stoop and cause back ache.

What causes it?

No one knows, but it increases with age and is more common in women because they have smaller bones and lose bone faster after the menopause. But it is a problem for men too.

Women who have an early menopause have an increased risk. If anyone in your family had osteoporosis (including your mother, father and sisters), or broke a bone when they were over 50 following a trivial injury, or had a kyphosis (a hump on the upper back), then your risk rises. Being underweight (especially if you have had anorexia nervosa) increases your risk, as does smoking – female smokers have a 2% lower bone mineral density each decade after the menopause compared to non-smokers. Some medical conditions such as Crohn's disease also increase your risk, as do some medicines for epilepsy and breast cancer. Drinking more than three units of alcohol a day is thought to reduce your bone mineral density, but the evidence is not as strong as for other risk factors. It used to be thought that caffeine might be a problem but research doesn't show this.

What can I do to prevent it?

Think ahead. You need to have stopped smoking for 10 years to see an improvement in your bone mineral density. Get your bones (and those of your children) in as good a shape, with as high a mineral density as possible in your (and their) 30s. Do this by eating enough calcium – in dairy products, sardines and spinach. You also need vitamin D to absorb calcium properly, so go out in the sun (without sunscreen) for 10-15 minutes a day but not in peak sunshine hours (10am-4pm). And exercise. Exercise in adolescence improves density for years after the menopause. You should encourage younger children to skip, play football and netball and to drink milk.

But it's never too late. If you exercise and drink milk in your 40s, you still reduce your risk of osteoporosis. Exercise should be weight-bearing to stimulate bone cells – so jogging, walking or gardening. Research says you need 100mg of calcium a day (one pint of milk and 50g (2oz) of hard cheese) to reduce your risk of hip fracture by nearly a quarter.

How do I know if I have it?

If you have a fracture after a minimal injury or have a family history you should suspect it, but you will only know for sure with a dual-energy x-ray absorptiometry scan. Then there are drugs you can take to improve your bone mineral density.

• Always consult your doctor before following the suggestions in these guides