Let’s face it – most people don’t spend too much time thinking about their bones. Which is surprising given the vital roles they play in our body and overall health.
They support us as we stand, move and sit, and protect our inner organs, including our lungs, heart and brain. They are a storage site for minerals we need to survive (like calcium) and house the bone marrow, where blood cells are made.
It isn’t until something goes wrong that we start to take our bones less for granted, like when a bone breaks or we develop osteoporosis.
So, what exactly are bones?
Bones are living, growing tissue.
Although they may appear solid, they are in fact a mix of hard and soft tissue. They are made up of different types of cells with different functions, have a rich blood supply and are home to specialised immune cells. Some bones, like the spine and hip, also contain active bone marrow – spongy tissue that makes red and white blood cells.
Our bones live in a constant state of change or remodeling – older bone is continuously being broken down while new bone is formed. Our overall health and lifestyle can affect this process.
There are 3 main types of bone cells that are involved in bone remodeling:
- Osteoblasts – these are cells that create new bone. They release proteins like collagen that form the bone matrix (3D scaffolding) and then release minerals (including calcium) that bind to this matrix. When bone surfaces are not undergoing formation or resorption, they become lined with old osteoblasts that have flattened. These are then called bone lining cells. Under certain conditions, the bone lining cells can return to become bone-forming osteoblasts again.
- Osteoclasts – these are cells that break down and resorb old or damaged bone. They do this by releasing enzymes at the bone surface which digests the bone matrix and frees calcium from the bone.
- Osteoctyes – these cells can live for up to 25 years and make up most (90-95%) of the calcified bone. Osteocytes are surrounded by mineralised bone matrix, but have long branches (or extensions) that connect them to other bone cells. These extensions allow bones cells to transport molecules and signal each other, which can start the bone repair or bone remodeling process.
Maintaining healthy bones is a balancing act
A balance between bone formation and bone breakdown is important for maintaining bone mass and density. If this balance is not maintained, problems can occur that might affect the strength of your bones, and how easily they break. For example:
- Too much bone breakdown without a corresponding amount of new bone being formed can lead to bone loss and osteoporosis. This is where bones lose density and strength, and are easily broken.
- Too much bone formation without a corresponding amount of breakdown is seen in a rare condition called osteosclerosis, where bones become harder and more dense.
What hormones keep bone remodeling processes under control?
A number of hormones play key roles in keeping bone remodeling processes in check.
- Parathyroid hormone (PTH): The main role of this hormone is to keep blood calcium levels stable. If calcium levels fall too low, the parathyroid glands release PTH. This causes the bones to breakdown so that calcium can be released from the bones into the bloodstream. If this happens over a long period of time, the bones can become too weak, leading to osteopenia or osteoporosis.
- Calcitonin: This hormone also acts to keep calcium levels stable but works in the opposite direction to parathyroid hormone. If calcium levels in the blood get too high, calcitonin reduces bone breakdown by osteoclasts. This means more calcium stays in the bones to keep them strong.
- Growth Hormone (GH): This hormone plays key roles in the growth of bones during childhood, but also maintains bone health in adulthood. Growth hormone stimulates osteoblast activity, promoting bone formation. GH deficiencies can lead to a gradual loss of bone density.
- Vitamin D (or calcitriol): This hormone helps the body absorb calcium. If Vitamin D levels are low for a long time, calcium levels in the body can fall. This causes PTH release, which in turn causes the bones to breakdown to release their calcium1.
- Glucocorticoids: These hormones act on osteoblasts to reduce bone formation, as well as causing a period of increased bone breakdown. The effect of these hormones is seen when taking high dose, oral or injected glucocorticoid medications2, such as prednisolone, for a prolonged period.
- Estrogen – The major effect of the loss of estrogen is an increase in bone resorption, which leaves a gap between bone formation and resorption3. This is why the rapid fall in estrogen levels that occurs in the 5-10 years following menopause can cause significant bone loss in women.
- Androgens – These male sex hormones are also important for keeping bones healthy. This mainly happens because some testosterone is naturally converted to estrogen, which protects bone health. Low testosterone levels can therefore also lead to increased bone loss.
Ways to keep your bones healthy and strong, now and into the future.
Keeping active and getting enough calcium through your diet and through sun exposure will help keep your bones strong and healthy.
- Keep active. Ideally, this includes a mix of weight-bearing exercises (e.g. walking and jogging), some resistance training (e.g. weights) and exercises that improve balance (e.g. tai chi, gentle yoga).
- Include calcium in your diet: Dairy foods like milk, yoghurt and cheese are a great source of calcium, as are soy milk, tofu, sardines and bony fish. Calcium is also found in figs, apricots, almonds, lentils, beans and green vegetables. Three serves a day is recommended.
- Get some sun: Sunlight is the best way to get Vitamin D naturally, however it is important to balance this with the risk of sun damage. Osteoporosis Australia has produced a map showing how much sunlight you need in different parts of Australia to get enough Vitamin D. You can find it here.
- Seek medical advice to optimise management of any hormone conditions: Some health conditions affect your hormone levels. When this happens, it is important to listen to your doctor’s advice about medications and supplementation and have regular bone health checks.
Know your bones
Osteoporosis Australia has created an online self-assessment tool for you to check your own bone health. You can find this tool by visiting the Know Your bones website here.
The medical content on this page has been reviewed by A/Professor Frances Milat, an endocrinologist at Monash Health and the leader of the Metabolic Bone Research Group at Hudson Institute.
- Fischer V, Haffner-Luntzer M, Amling M & Ignatius A (2018). Calcium and Vitamin D in bone fracture healing and post-traumatic bone turnover. European cells and Materials, 35, 365-385.
- Compston J (2018). Glucocorticoid-induced osteoporosis: an update. Endocrine, 6(1), 7-16.
- Cauley JA (2015). Estrogen and bone health in men and women. Steroids, 99A, 11-15.