Of all minerals in the body, both calcium and phosphorus are intimately related to the structure and metabolism of bones throughout a lifetime of dynamic remodeling. Remodeling is a continuous process of simultaneous formation and resorption of bones that involve the activity of bone cells called osteoclasts (also known as the “destroyers”) and osteoblasts (also known as the “formers”.) These two elements are essential for the bones to be stable enough to render humans mobile and to protect the vital organs in the body.
Any factor that disrupts in this homeostasis predisposes an individual to disorders of faulty mineralization like osteoporosis. Therefore, the importance of balancing phosphorus and calcium in the diet cannot be overemphasized. Aside from maintaining bone health, both calcium and phosphorus play other important physiologic functions in the body. Ever wondered how both calcium and phosphorus are metabolized in the body? Which glands regulate calcium and phosphorus metabolism? Do calcium and phosphorus absorption decline in the elderly? This article attempts to answer these questions.
Calcium, the most abundant mineral found in the body at 1-2 kg, is 99% found in the bones. This primarily maintains the stability of bones and acts as a reserve for when calcium in the blood, due to several reasons, is low. The remaining 1% is found in the blood and is critical for metabolic functions like vascular contraction and dilation, nerve transmission, intracellular signaling, and hormonal secretion. Therefore, the function of calcium goes beyond just maintaining the integrity of the bones.
Parathyroid hormone (PTH, produced by parathyroid glands) and Vitamin D (active form) work together to regulate the movement of calcium ions across the intestinal and renal cells, maintaining normal calcium levels in the body. Low serum calcium increases the activity of PTH and in turn, PTH increases calcium resorption (movement of calcium into the blood).
Calcitonin, a hormone produced by the thyroid gland, does the opposite and is stimulated when there is too much calcium in the blood. It acts to increase calcium absorption (movement of calcium back into the bones and/or tissues).
In addition to this, the balance between calcium resorption and absorption changes with age, where breakdown exceeds formation in the aging population, particularly in postmenopausal women. Low estrogen among postmenopausal women negatively impacts the absorption of calcium into the intestines, rendering calcium intake useless. This explains why postmenopausal women have an increased risk for osteoporosis and will benefit from calcium supplements and/or milk.
The recommended dietary allowance (RDA) for calcium among people aged 19 to 70 ranges from 1000 mg to 1200 mg per day. Among food choices, the highest source of dietary calcium includes yogurt, cheese, sardines, and milk. Calcium is also present in supplement forms (i.e. carbonate and citrate), requiring medical consultation before initiation. Excess calcium is excreted through the urine.
Besides maintaining bone health, several studies have shown that calcium plays a protective role against colorectal cancer. A calcium Polyp Prevention study revealed that supplementation with calcium carbonate led to a reduction in the risk of developing adenoma. An adenoma is a benign tumor but considered a precursor to colorectal cancer. On the other hand, several studies have linked high calcium intake to an increased risk of prostate cancer. Other studies also showed the role of calcium in lowering body weight by increasing fat breakdown and preventing fat accumulation.
Phosphorus is another essential mineral of interest. There is approximately 600 g of phosphorus found in the body. Approximately 85% of this is found in the bones and teeth. The remaining is found in the blood and soft tissues. It is widely available in the diet and is absorbed in the small intestines.
Like calcium, phosphorus is also regulated by PTH and vitamin D. High phosphorus with low calcium increases PTH activity, leading to increased calcium resorption. This is the reason why there should be a balanced ratio of calcium and phosphorus in the body. Studies have also shown that increased phosphate levels among patients with chronic kidney disease increase the risk of mortality. Furthermore, high phosphate level is also linked with cardiovascular diseases and mortality, even among those with no history of any cardiovascular problems.
The recommended daily allowance of phosphorus for people aged 19 and above is 700 mg. Rich sources of phosphorus include scallops, chicken, and lentils. Special groups at risk for low phosphorus include preterm newborns, patients with severe malnutrition, and those with genetic phosphate regulation disorders.
In the body, there should always be an adequate level of both calcium and phosphorus to maintain several physiologic functions. More than maintaining the integrity of the bones, calcium is needed for the functioning of the heart and bones. However, phosphorus is essential in cellular functions like the synthesis of proteins and nucleic acids.
Both calcium and phosphorus are widely available in the diet. They are mainly absorbed in the small intestine. They are regulated by the parathyroid glands and the kidneys through PTH and vitamin D. PTH is a hormone that helps in calcium resorption. It is stimulated by high calcium and low phosphorus and is inhibited by the opposite. Another hormone from the thyroid glands called calcitonin is stimulated when there is too much calcium level in the blood. People with kidney problems are at risk of having excess calcium and phosphorus. Excess levels of both are excreted via the urine.
Calcium absorption declines with age. This is why osteoporosis is common in the elderly. This is especially more complicated among women undergoing menopause. During menopause, there is decreased in estrogen levels in the body. This decreases the absorption of calcium in the intestines, which makes dietary intake of calcium useless. In this special population, calcium supplements are of great help.
- Jameson, L., Fauci, A. et. al. 2018. Harrison’s Principles of Internal Medicine 20th Mc-Graw Hill Education.