Increasing peak bone mass early in life and reducing bone loss later in life are 2 approaches for reducing risk of osteoporosis. This disease of low bone mass and fragility fracture affects a large portion of the elderly population. Milk consumption has long been held as an important strategy for building optimal peak bone mass. Therefore, it is assumed that milk consumption, especially during development of peak bone mass, reduces risk for osteoporosis.
There is substantive mechanistic evidence for this inference from a variety of physiological perspectives, including structure-function relationships, homeostatic regulation of serum cations, and bone balance. Bone mineral composition in many respects reflects the mineral composition of milk. The most abundant mineral in bone is calcium, which composes approximately 32% of bone mineral content. Milk and other dairy products provide the majority of dietary calcium in the diets of most populations. Bone mineral acquisition during growth and retention in later life directly relate to calcium intake up to a threshold intake.1 Other minerals, notably phosphorus and magnesium, are also important constituents in bone. Milk is a major dietary contributor of these minerals, although to a lesser extent than it is for calcium. Maintaining appropriate electrolyte concentrations in the blood and extracellular fluid compartments is essential to support myriad biological functions. If the serum calcium level decreases owing to inadequate calcium intake, parathyroid hormone is released to stimulate bone resorption to return blood calcium levels to the normal range. Long-term low calcium intakes lead to negative bone balance. Bone balance is a net result of bone formation and bone resorption. Increased dietary calcium suppresses bone resorption without altering bone formation rates, thereby improving bone balance.2 It is not only the calcium in milk that influences bone health. Growing rats fed equal and adequate calcium intakes from milk or calcium carbonate with all of the recommended levels of other nutrients achieved larger, denser, stronger bones on the milk diet, and most of these advantages were retained through adulthood after switching both groups to an inadequate calcium intake provided by calcium carbonate.3