Did you know that your skeleton is brand new every ten years? It’s true, bone is living tissue and is continually going through a remodeling process. Just like when our home becomes old and we remodel the kitchen, when our bones become old the body replaces it with new bone. This process is called remodeling. In this process there are two big players; osteoblasts and osteoclasts. Osteoclasts remove bone or are more commonly thought of as the vehicle that steals calcium from the bones, to use where it is needed in the body. Osteoblasts deposit new bone or take absorbed calcium and deposit it to form new bone. A bone remains dense when osteoclasts are removing bone from the inside and osteoblasts are depositing bone on the outside at a balanced rate.
One big factor in keeping this system in balance is stress. Remodeling and the formation of additional bone can modify the strength of bone in direct response to the amount of stress that is applied to it. Stress to the bone increases osteoblast activity in the bone tissue. A removal of stress decreases osteoblast activity. Stress is defined as load bearing. Does it feel heavy, if yes then you might be able to call it stress. This is why swimming is not in the top running for sports for your grandma should pick up.
Calcium and Bone Health
In the US calcium intakes are one of the highest in the world. However, the US also has one of the highest rates of bone de-mineralization (osteoporosis or muted osteoblast activity, increased osteoclast activity). Bone mineral content is dependent on not just on calcium intake (where we focus our attention), but on a net calcium balance (intake – excretion).
Excretion is initiated by MANY factors, however they are mostly diet and exercise related. Calcium needs to maintain a base level in the blood. If you go below the base level you die. No joke! This is probably why the human body was designed with one big calcium storage (the skeleton) to keep these levels in balance. You see calcium is not only used to make bone, but it initiates muscle contraction (this includes the heart), blood clotting and the release of neurotransmitters. It’s pretty darn important, imagine ANY of those functions being blocked. No calcium in the blood, muscles stop contracting, heart stops, you die, nuf said.
Dietary calcium excretion from bone is dependent on acid/base balance. All foods upon digestion ultimately must report to the kidney as either acid or base. When the diet yields a net acid load, the acid must be buffered by the alkaline stores of base in the body. Calcium salts in the bones represent the largest store of alkaline base in the body and are depleted and eliminated in the urine when the diet produces a net acid load. The highest acid-producing foods are hard cheeses, cereal grains, salted foods, soda (diet included), meats, and legumes, where as the only alkaline, base-producing foods are fruits and vegetables. Because the average American diet is overloaded with grains, cheeses, salted processed foods, soda and fatty meats at the expense of fruits and vegetables, it produces a net acid load and promotes bone de-mineralization. By replacing hard cheeses, cereal grains, and processed foods with plenty of green vegetables and fruits, the body comes back into acid/base balance which brings us also back into calcium balance. Diets containing high acidity contribute to the development of osteoporosis and renal stones, loss of muscle mass, and age-related renal insufficiency due to the body’s use of calcium to buffer pH.
What promots calcium intake and bone density? Eat your veggies and lift weights.
Reference: Anatomy & Physiology eighth edition, Seeley, Stephens, and Tate, 2008.