Even before we knew what vitamin D was, we’ve understood that it is important for building strong bones. Bone isn’t an inert organ. It has critical functions in the body, which can be divided into three main categories: mechanical, synthetic, and metabolic.
Mechanical: Bone provides the mechanical functions of movement, protection (of major organs), shape, and sound transmission (in the inner ear).
Synthetic: All the blood cells of the body (red blood cells, white blood cells, and platelets) are produced within the cavities of bone, in tissue called marrow.
Metabolic: Bone serves a number of metabolic functions that are essential for normal functioning of the body, including mineral storage, acid-base balance, detoxification, and endocrine organ function.
What is bone?
Bones are very strong but relatively lightweight. To make good bone, you need a protein scaffold; then you need to harden the scaffold with crystals of calcium and phosphorus.
There are two different types of bone:
Cortical, or compact, bone: This is the hard, dense bone that makes up the tube part of the long bones such as the femur that is in your thigh. It also makes up your skull and the outer part of the ribs and vertebrae. The marrow lives in the center of the long bones and ribs and is protected by cortical bone.
Cancellous, or trabecular, bone: This type of bone is more porous than cortical bone. It’s found at the end of long bones and inside the vertebrae. This woven structure works like a shock absorber. The marrow can be mixed within the pores of trabecular bone.
Comparing the two different types of bone: cortical and cancellous.
Good, strong bone is produced in two different ways:
When you’re growing, the ends of your bones grow because of the action of cells called chondrocytes. These cells live in the part of the long bones called the “growth plate,” a structure at the end of the long bones.
The other way new bone is made is by another bone-forming cell called osteoblasts. These cells live on the surface of bone. Both osteoblasts and chondrocytes make proteins that serve as the scaffold for bone.
As bone mineralizes, the osteoblast can get embedded in the newly formed bone. When this happens, they change and become osteocytes. Osteocytes are like sensors within the bone that detect when the mineralized bone has tiny stress fractures. This leads the osteocytes to send signals to a cell type called osteoclasts, whose function is to dissolve bone.
These cells reabsorb the bone either when bone needs repairing or when the body needs calcium or phosphorus. After they do their job, the lost bone is replaced by osteoblasts (the bone forming cells). As this process occurs, the bone retains its shape. The entire process is called bone remodeling.
Bone remodeling occurs for two major reasons:
It provides a source of calcium to keep the blood calcium stable.
It maintains the strength of the bone as a structural material.
Vitamin D brings in the calcium and promotes bone growth
Without enough vitamin D in the body, the bones are in trouble. This is because calcitriol (active vitamin D or 1,25(OH)2 vitamin D) has a number of roles in the control of how your body uses calcium as well as in the production of healthy bone:
In the intestine, calcitriol induces the production of proteins that cause calcium to be absorbed from food into the bloodstream. Much of the calcium absorbed ends up being taken up by the protein scaffold that osteoblasts make.
In the parathyroid gland, calcitriol suppresses the production of more parathyroid hormone. Parathyroid hormone is a signal to break down bone and free up calcium. When your vitamin D status is too low, your body can’t make calcitriol and parathyroid hormone levels go up.
In the kidneys, calcitriol induces the production of proteins that keep calcium from being excreted in the urine. By keeping more calcium (but not too much!) in the body, you protect your bones.
In the bone, vitamin D causes the osteoblasts to produce more of the protein scaffold that is then mineralized to make mature bone.
