Hormonal regulators of calcium & Bone Homeostasis

   Bones are the living growing hard tissues that are composed mainly of proteins, collagen and calcium, the most important component in bone formation that giving the bone tissues their hardening and strength. Bones support your body, giving the body its shape, allowing your movement, protect your internal organs and protect the brain even production of your blood cells.

Why it is important to keep your bones healthy?

Before the age of 30, your bones are in a continuous formation and remodeling, you gain more bone mass than you lose and your bones mass reach their peak, after this age, the curve begin to go down because you begin to lose more bone mass than you gain, this affecting your bone density and this often associated by what is called by osteoporosis, a condition in which your bones become more weak and liable for fracture.

Factors affecting the health of your bones:

1. A diet low in calcium considered the most important affecting factor that badly affects your bone mass, making your bones are weak and liable for fracture.

2.Sex hormones are very important in affecting the bone mass, testosterone hormone in male and estrogen hormone in female, the both are considered a calcium regulating hormones. However, bones generally are more liable for osteoporosis in females than in males because of the high density of bones in men.

3. Age of individuals, the small your age is, the weaker and lesser density your bones are.

4. Physical activity, the individuals with a high physical activity are less liable to bone weakness and osteoporosis than others who have a low physical activity and being lazy.

5. Smoking and alcohols are associated by the risk of bone weakness and osteoporosis as smoking can impair the metabolism of bone turnover via affecting the metabolism of hormones, vitamin D level and calcium absorption while alcohols can decrease the absorption of calcium from intestine and impair vitamin D metabolism, this in turn can decrease bone mass.

6. Family history, if you have one or more individual in your family with a history of osteoporosis, you are in a great risk.

7. Long term administration of certain medication like corticosteroids that badly affects your bones, other drugs used for epilepsy like phenytoin, drugs used as anti- cancer like methotrexate.

8. Hormonal Regulators of calcium and bone homeostasis as parathormone hormone, calcitonin hormone, vitamin D, thyroid hormone, estrogen and glucocorticoids.

How can calcium affects your bones?

Calcium is an essential mineral that has a predominant role in the  whole biological body functions, It also has a specific role in skeleton mineralization as it presents in about more than 99% in bone tissues as calcium- phosphate complexes , provides your bones their strength, hardening and structure.while the remaining calcium is found in the blood, muscles, vessels responsible for muscle contraction, vascular contraction and nerve impulse transmission.

Generally, bones present in 2 different types, cortical bone and trabecular bone.

1. Cortical bone is the most compact thick and dense layer which makes the outer surface of most bones and long bone shafts, about 80-90% of cortical bones volume is calcified.

2. trabecular bone is the sponge parts that are found at the end of long bones, inside the flat bones and vertebrae, about 15-25% of trabecular bones volume is calcified.

 The Recommended Dietary Allowance of calcium is 1000 milligrams per day for adults up to 50 years old in females and 70 in males. The recommendations may increase to 1200 milligrams per day for age of 51 and older in women and 71 and older for men. However, Not only dietary calcium or supplements can keep your calcium and bones healthy, there are also certain hormones that can regulate your calcium and bones homeostasis.  

• primary hormonal regulators of calcium:

1. parathormone 

It is a single chain polypeptide hormone secreted by parathyroid glands that regulates serum calcium concentration by its effect on bones, kidney and intestine. It is secreted in response to low calcium level in the blood. so, it stimulates osteoclasts within the bone matrix to release more calcium in blood till the serum calcium maintains its normal levels and this leads to increase bone  resorption.

Parathormone hormone can also increase calcium absorption from intestine through activation of vitamin D and increase calcium re-absorption from kidney in order to maintain calcium in its normal level when serum calcium is being low.

2. Calcitonin:

It is a single chain polypeptide secreted by the C-cells of the thyroid gland that regulates serum calcium concentration by its effect on bones, kidney and intestine. It is secreted in response to low calcium level in bones (not in the blood). its work oppose the action of parathormone hormone. it inhibits the action of osteoclasts that make break down of bones in order to release calcium in the blood. By inhibiting its action, calcium remains in your bones and so calcitonin plays a main role in treatment of osteoporosis and hypercalcemia. When serum calcium level decreases, your thyroid gland decreases the amount of released calcitonin hormone.

Calcitonin hormone can also acts on  specific receptors  on kidney leading to decrease of serum calcium re-absorbtion, thus decreasing serum calcium levels and bone resorption , it can also decrease calcium absorption from intestine. 

3. Vitamin D :

Itis a fat solable vitamin found in 2 forms D2 and D3 . The most active form is D3. Vitamin D is not only necessary for keeping your bones strong and difficult to be broken. It also have an important role in maintaining normal calcium levels within your bones, thus preventing osteoporosis, rickets in children, osteomalacia in adults. Even calcium is sufficient to be precipitated in your bones , deficiency of vitamin D  may impair calcium absorption in bones because vitamin D acts on specific DNA receptors to synthesize proteins necessary for calcium transport. 

Vitamin D can also increase calcium re-absorbtion from kidney and increase calcium absorption from intestine in order to maintains normal calcium levels in bones.

• Secondary hormonal regulators of calcium:

1. Glucocorticoids:

Long-term glucocorticoid use can cause osteoporosis in adults, delayed growth, and rickets in children by increasing bone resorption via renal calcium excretion and lowering bone production and growth. It inhibits vitamin D's ability to stimulate calcium absorption.   

2. Estrogen:

Estrogen is a sex hormone in females has been used in the treatment of postmenopausal osteoporosis, but prolonged administration is not safe at all because it may cause many side effects like breast and endometrial cancers

Certain medications are used to stimulate estrogen receptors in bones and block them in the breast and uterus like Raloxifene