From our 40s onwards, our bones gradually lose their density as a natural part of ageing.
One in two women and one in five men over the age of 50 in the UK will break a bone, mainly because of osteoporosis. Exactly why this happens is still not fully understood.
Osteoporosis has no symptoms. The first clue may be seen if you have an X-ray for another reason, or if you fracture a bone.
Whether you have osteoporosis or just want to build strong bones for the future, there are several things you can do to maintain your bones.
Eat calcium-rich foods
Adults should eat at least 700mg of calcium a day, and 1,200mg if diagnosed with osteoporosis. You could get your daily intake by eating a yoghurt, a cheese sandwich, a handful of almonds and a spinach salad.
Get your sunshine quota
Your body needs vitamin D to help it absorb calcium. Vitamin D is found in oily fish, liver, fortified spreads and cereals, and egg yolks. Your body also makes its own vitamin D when you're exposed to sunshine.
Most people in the UK get enough vitamin D by spending 15 minutes in the sun two to three times a week. Generally, normal levels built up in the summer will be enough to last through the winter.
Go easy on the protein
Excessive amounts of meat, cheese and protein make body acid, which drains the body of calcium and weakens bones. Keep your diet balanced.
Your meals should contain protein (meat, fish, eggs, nuts, seeds), fresh fruit and vegetables, and carbohydrates (bread, pasta, potatoes and rice).
The more you smoke, the more likely you'll get osteoporosis. Get help quitting smoking for good.
Cut out the salt
Salt is thought to speed up the body's loss of calcium. Most of us consume 9g of salt a day, but the recommended limit is 6g, which is just a teaspoonful. Don't add salt to your food and look at food labels to help you cut down.
Avoid foods that contain 1.5g of salt per 100g (or 0.6g of sodium) or more. Crisps, ham, cheese, cooking sauces and processed foods such as pies, pizza and soups are all high in salt.
Bones get stronger when you use them. The best way to strengthen them is to do at least five hours of weight-bearing exercise a week. This includes walking, running, dancing, golf, tennis or netball. It doesn't include cycling or swimming, although swimming is good for staying flexible.
Bones also benefit if you lift and carry things. Weight training is ideal, but carrying shopping, gardening and housework all count.
Alcohol, tea, coffee, cola and other fizzy drinks reduce the amount of calcium you absorb and weaken bones. Stick to the recommended amounts of alcohol, and swap your caffeine-fuelled drinks for water and diluted juice.
Maintain a healthy weight
Losing too much weight too fast under a crash diet can increase your risk of osteoporosis. The same is true if you're anorexic, or for women, if you're so thin that your periods have stopped.
Weight loss can cut the amount of oestrogen (a hormone that helps to protect your bones) in your body. If you need to lose weight, do it sensibly.