Calcium together with phosphorus and vitamin D is involved with the formation, repair and maintenance of bones throughout life. In humans, it is well established that calcium metabolism is regulated by vitamin D, and the extent of calcium absorption is totally dependent on the levels of vitamin D. If vitamin D levels are low in the intestines, only 15% of dietary intake of calcium may be absorbed, and most will pass through the body unused.
If the amount of calcium circulating in blood becomes too low, the body responds by releasing a calcium-regulating hormone. This hormone, known as the parathyroid hormone, is the most powerful regulator of healthy bones and is the guardian of calcium concentrations in blood. Once summoned, the parathyroid hormone will draw calcium from bone to replace any lost circulating calcium. The loss of calcium in bone will continue as long as the parathyroid hormone levels are elevated. The parathyroid hormone plays a dominant role in maintaining blood levels of calcium, within a very narrow range, but it does so at the expense of bone loss.
Therefore, controlling parathyroid hormone levels during one’s lifetime is important to reduce the risk of bone disease later in life. This is done by balancing the dietary intake of calcium with the intake of phosphorus represented by the calcium to phosphorus ratio, (Ca / P).
In both animal and human studies, a high phosphorus intake, relative to calcium intake, elevated the parathyroid hormones levels and interfered with calcium metabolism.
Consider the following summaries of a few studies:
High phosphorus intake in young women caused persistent changes in parathyroid hormones, which could adversely influence bone formation.
The low calcium to phosphorus dietary pattern of American women results in persistent changes in the parathyroid hormones that are neither conducive to peak bone mass nor to slowing the rate of bone loss.
A mechanism that contributes to low bone mass in the United States involves a low calcium to phosphorus dietary ratio.
Healthy women were found to have a positive association between a calcium to phosphorus dietary ratio and bone density.
In healthy women, even when calcium intake is adequate, diets with a low Ca/P ratio increases the loss of calcium from bones.
Each case above involves elevation of the parathyroid hormone because it is the dominating regulator of bone loss at any stage of the human life cycle and is controlled by diet. Any dietary pattern contributing phosphorus above the RDA will potentially contribute to an abnormally low calcium to phosphorus (Ca/P) dietary ratio, which will trigger elevated parathyroid hormone levels. This in turn, will leach calcium from bone and contribute to the loss of bone mass. During the time this is taking place, there will be no symptoms or pain; blood levels of calcium and intake of calcium may appear normal even though the bones are being depleted of calcium.
Because blood levels of calcium do not accurately reflect the body’s total calcium status, a severe deficiency must occur before any clinical symptoms can be recognized. When symptoms do become evident, it will require immediate medical care.
The parathyroid hormone levels must remain within the lower range of what is considered to be a normal range rather than in the upper range. To suppress persistent elevations of the parathyroid hormones, the dietary intake of Ca / P must be balanced to some degree. The preferred method for balancing is to reduce the intake of phosphorus rather than increasing the intake of calcium. In a large population based study of 9,000 subjects, calcium intake was associated with bone density only among women who had low levels of vitamin D. In all other groups there was no relationship between calcium intake and bone density.
This could explain in part why, although Americans have higher calcium intake than people in other countries, we still have high incidences of osteoporosis.
This brings us to the theoretical optimal dietary Ca/P ratio for humans. Dividing the RDA for calcium by the RDA for phosphorus equates to a Ca/P dietary ratio of 1.4 by weight for adults. In infants and children, a Ca/P ratio of 1.5 is considered to be ideal for optimal growth. In fact, federal law requires the ratio of Ca/ P in infant formulas be no less than 1.1 and no more than 2.0. Of course you can eat foods above and below these numbers, but the literature shows that habitual daily intakes of a Ca/P dietary ratio below .40 resulted in persistent elevated parathyroid hormone levels.
In summary, high phosphorus diets are associated with loss of calcium in bones, and elevated levels of the parathyroid hormones are indicators of a vitamin D insufficiency and a sign of impaired calcium metabolism.
This chapter lists the total amounts of calcium in milligrams (mg) and the Ca/ P ratios for the same foods.
Recommended Daily Intakes Levels for Calcium