Dairy and bone physiology
Bone is a composite material made up principally of proteins and calcium phosphate. Other bone components include sodium, magnesium, potassium and zinc. Getting just the right mix of nutrients is important for maintaining strong bones. In addition to calcium, vitamin D, phosphorus, magnesium and protein are all important for keeping our bones healthy. All of the aforementioned mineral elements provide bones with their strength and rigidity.
In humans, the bone skeleton is the body’s calcium reserve, and it is living. This means that it’s renewed in a continuous process referred to as bone remodeling. This process typically begins with activation of a microscopic site, followed by bone tissue removal by osteoclasts and the filling of the cavities by osteoblasts. Overall bone size is largely determined by a combination of mechanical loading and dietary calcium availability. A very simplified outline of the most important bone physiology actors is displayed in the graphic below.
The continuous remodeling of bone serves for repair and growth, and it also contributes to the control of blood calcium levels. When the absorption of dietary calcium fails to match daily losses, blood calcium levels begin to fall, producing an increase of parathyroid hormone (PTH) secretion. PTH is essential for many regulatory processes, including bone mineralization, calcium extraction by the kidneys and the synthesis of the biologically active form of vitamin D (calcitriol or 1,25(OH)2 vitamin D3). In turn, vitamin D contributes to proper absorption of calcium from the gut. PTH secretion is relatively low when there are high calcium intakes, and increases when calcium intakes are low. Calcitonin is a second hormone which serves to regulate overly high calcium levels in the blood, by inhibiting the release of calcium from bone and promoting urinary calcium excretion.
Milk and bones
Per calorie, milk products provide more protein, calcium, phosphorus and potassium than any other typical food found in the average adult diet.1-3 As all of these nutrients are involved in the development and maintenance of bone, the beneficial effects of milk on bone health may be due to the added or synergistic effects of multiple milk components. Whichever the case, sufficient intake of dairy can contribute to bone health in all stages of life.
- Heaney RP. J Am Coll Nutr. 2000 Apr;19(2 Suppl):83S-99S. Calcium, dairy products and osteoporosis. http://www.ncbi.nlm.nih.gov/pubmed/10759135
- Heaney RP. J Am Coll Nutr. 2009 Feb;28 Suppl 1:82S-90S. Dairy and bone health. http://www.ncbi.nlm.nih.gov/pubmed/19571166
- Bonjour JP. J Am Coll Nutr. 2011 Oct;30(5 Suppl 1):438S-48S. Calcium and phosphate: a duet of ions playing for bone health. http://www.ncbi.nlm.nih.gov/pubmed/22081690