Key Takeaways (expand)
- Phosphorus is the second most abundant mineral in the human body, making up about 1% of a person’s weight!
- Phosphorus plays an important structural role as a component of cells (as the phospholipid bilayer), bones (as a calcium phosphate salt called hydroxyapatite), and nucleic acids (as sugar-phosphate backbones).
- A variety of cell-signaling molecules, enzymes, and hormones require activation by phosphorylation—a process where a phosphate group is added to a molecule.
- Phosphorus helps maintain the acid-base balance in the body, serving as a buffer to maintain a normal pH in extracellular fluid.
- A phosphorus-containing molecule called 2,3-DPG binds to hemoglobin in red blood cells, giving phosphorus a role in regulating oxygen delivery to tissues.
- Studies suggest a number of health issues associated with excess phosphorus intake, including a significantly higher risk of all-cause mortality among people eating double the recommended dietary allowance for this mineral.
- High phosphorus intake is also linked to an increased risk of bone fractures and osteoporosis—possibly due to excess phosphorus causing disordered regulation of some bone-building hormones, impairing vitamin D synthesis, and triggering mineral loss from skeletal tissue.
- Excess phosphorus has also been linked to an increased risk of cardiovascular disease, with studies showing that high-phosphorus meals can impair endothelial function, and that high phosphorus levels in the body can alter cell differentiation in a way that promotes vascular calcification.
- The ratio of dietary phosphorus to calcium is particularly important, with studies suggesting that high phosphorus intake is less harmful to bone health when dietary calcium is sufficient.
- Phosphorus deficiency typically only happens in times of starvation, but some inherited disorders can cause phosphorus levels to drop too low and produce symptoms such as muscle weakness, bone pain, increased susceptibility to infection, appetite loss, numbness in the extremities, and respiratory failure.
- Most foods contain phosphorus, but particularly rich sources are dairy products, poultry, fish, shellfish, seeds, whole grains, and legumes.
Phosphorus is an essential nutrient that’s the second most abundant mineral in the body (right after calcium); in fact, it makes up about 1% of a person’s total weight! Phosphorus was first discovered in 1669 by a German alchemist named Henning Brand, who was conducting experiments with urine in attempt to create the “philosopher’s stone”—a legendary substance that alchemists believed could transform ordinary metals (like lead) into precious ones (like gold). Although he wasn’t successful in his quest for the philosopher’s stone, Brand did manage to produce a white material from evaporated urine that burned brightly and glowed in the dark. As a result of these properties, it was named phosphorus: the Ancient Greek word for what we now call the planet Venus, originally deriving from the words phōs (light) and phoros (bringing)—essentially meaning the bringer or carrier of light!
Phosphorus serves as a building block of nucleic acids and cell membranes. It’s also involved in a number of biological processes such as energy production, acid-base regulation, bone mineralization, and cell signaling.
Phosphorous is present in most foods, but it’s particularly abundant in protein-rich items such as dairy products, fish, shellfish, poultry, seeds, and legumes. Whole grains and breads are also common food sources of phosphorus. This mineral is also found in many common food additives (such as the phosphoric acid used in soft drinks), so it’s often abundant in processed foods.
The Biological Roles of Phosphorus
One of the most important roles of phosphorus is as a structural component within the body: it’s a constituent of bone, cells, and nucleic acids! In bone, it’s part of a calcium phosphate salt known as hydroxyapatite, which makes up mineral portion of bone tissue and tooth enamel (the other components being bone matrix, cells, and water). In cells, phosphorus is part of the phospholipid bilayer—a polar membrane surrounding the cell that’s made of a water-loving (hydrophilic) exterior layer and a water-repelling (hydrophobic) interior layer. And, nucleic acids (DNA and RNA) have sugar-phosphate backbones that form their structural framework—giving phosphorus a role in the body’s very ability to store and transmit genetic information.
What’s more, a number of enzymes, cell-signaling molecules, and hormones can only be activated with the help of phosphorylation—a process in which a phosphate group is added to a molecule. For example, energy production and storage depends on compounds that are phosphorylated, including adenosine triphosphate (ATP, the energy currency of cells) and creatine phosphate (the high-energy phosphate storage molecule in muscle).
Phosphorus also serves as one of the body’s most important buffers to help maintain the acid-base balance, helping sustain a normal pH in extracellular fluid. And, a phosphorus-containing molecule called 2,3-diphosphoglycerate (2,3-DPG) regulates oxygen delivery to tissues by binding to the hemoglobin in red blood cells.
The body tightly regulates phosphorus levels through a multi-hormone endocrine axis. After being absorbed in the small intestine, the kidneys excrete excess phosphorus through the action of parathyroid hormone, vitamin D, and fibroblast growth factor 23 (a glycoprotein produced in bone). When blood calcium levels drop, the parathyroid glands begin secreting more parathyroid hormone, which reduces calcium excretion but increases phosphorus excretion. And through another mechanism, parathyroid hormone can help increase phosphorus absorption: it assists in the conversion of vitamin D into its active form (1,25-dihydroxyvitamin D), and when more active vitamin D is circulating, more calcium and phosphorus get absorbed in the intestine. Lastly, the hormone fibroblast growth factor 23 helps regulate phosphorus homeostasis by inhibiting the production of active vitamin D whenever phosphorus intake increases, as well as by independently directing the body to excrete more of it!
It’s worth noting that although most phosphorus is very readily absorbed, the phosphorus in botanical seeds (which includes beans, peas, cereals, and nuts) is only about 50% bioavailable due to the presence of phytates, which bind to phosphorus and some other minerals. Similarly, a high calcium intake, especially from supplements, has been shown to inhibit the absorption of phosphorus in the intestine.
Drug Interactions with Phosphorus
Some drugs and supplements have potential interactions with phosphorus. For example, aluminum-containing antacids interact with phosphorus to form aluminum phosphate, which the body can’t absorb, and high doses of these antacids can sometimes lead to abnormally low blood phosphorus levels. Magnesium can inhibit the absorption of phosphorus. Meanwhile, supplementing with high doses of active vitamin D can potentially result in elevated blood phosphorus levels due to the regulatory effects of vitamin D on phosphorus homeostasis. Taking potassium-sparing diuretics or potassium supplements along with phosphorus supplements can cause hyperkalemia (high blood potassium), which itself can produce life-threatening heart arrhythmia. And lastly, hormone replacement therapy (such as for preventing bone demineralization in postmenopausal women) can increase the urinary excretion of phosphorus, leading to lower blood phosphorus levels.
Problems From Too Much Phosphorus
Most research conducted on phosphorus and human health has focused on the adverse effects of high dietary intakes and/or high levels of phosphorus in the blood. For example, even just consuming twice the recommended dietary allowance of phosphorus (upwards of 1,400 mg daily—an intake still considered very safe by most health authorities) has been associated with a significantly increased risk of all-cause mortality. Some studies have also linked high phosphorus intakes with increased risk of bone fractures and osteoporosis, as well as an increased risk of cardiovascular disease. And beyond just observational associations, trials of healthy individuals have shown that high-phosphorus meals significantly decrease flow-mediated dilation, indicating a negative effect of phosphorus on endothelial function.
Elevated blood phosphorus—known as hyperphosphatemia—is also associated with adverse health outcomes. But, this condition more commonly results from impaired kidney function (since the kidneys are in charge of phosphorus excretion) than from high dietary intake (since the body works so hard to maintain phosphorus homeostasis regardless of how much is ingested!). In fact, if the kidneys drop to 20% of their typical function, even normal phosphorus intakes can lead to elevated blood levels due to the compromised ability to remove it from the body; dialysis helps remove this excess. People with low parathyroid hormone levels (called hypoparathyroidism) can also be susceptible to high blood phosphorus, since parathyroid hormone helps stimulate phosphate excretion by the kidneys.
When people with existing kidney disease develop elevated blood phosphorus, it can be associated with more rapid disease progression and mortality—even at blood levels on the upper end of the normal range (2.5 to 4.5 mg/dL). In fact, according to one large meta-analysis of over 90,000 patients with chronic kidney disease, every 1 mg/dL increase in blood phosphorus above 3.5 mg/dL was associated with an 18% increase in all-cause mortality.
In the general population, too, high blood phosphorus levels have been linked to adverse health outcomes—again, even when those levels are just at the high end of normal! For example, multiple studies drawing from both the general population and from patients with previous cardiovascular disease have shown that high-normal blood phosphorus levels (3.5 mg/dL and above) are associated with greater risk of cardiovascular disease, atrial fibrillation, heart failure, and left ventricular hypertrophy (where the wall of the heart’s main pumping chamber thickens). Some observational studies have also shown that when blood phosphorus levels are at least 4 mg/dL, the risk of developing chronic kidney disease and end-stage renal disease more than doubles among people who initially had no kidney disease at all.
The problems caused by excess phosphorus likely stem from the ways phosphorus is regulated—particularly the secretion or action of parathyroid hormone, vitamin D, and fibroblast growth factor 23. Both high phosphorus intake and elevated blood levels can cause disordered regulation of these hormones, such as impairing the synthesis of active vitamin D (in turn increasing the risk of kidney disease, heart disease, and calcium phosphate buildup in the organs and other soft tissue). High phosphorus concentrations can also trigger a process called osteochondrogenic differentiation, in which the expression of bone-specific markers in blood vessel-forming cells get stimulated, causing vascular smooth muscle cells to transform into bone-like cells; this leads to vascular calcification, which is associated with a greatly increased risk of cardiovascular disease and mortality. Likewise, by stimulating parathyroid hormone secretion, excess phosphorus can induce mineral loss from bone (especially in the context of low-calcium diets), potentially increasing the risk of osteoporosis over time. So, even when the body manages to keep blood phosphorus levels in the normal range amidst high dietary intake, the hormonal process needed to maintain homeostasis could be causing damage—potentially explaining the links between phosphorus and kidney failure, cardiovascular disease, and osteoporosis.
As a result of the issues associated with excess phosphorus, the tolerable upper intake level for phosphorus is set at 4,000 mg per day in adults up to the age of 70. For the elderly, the tolerable upper intake level is 3,000 mg daily, to account for the increased likelihood of impaired kidney function in older age.
In some cases—especially where skeletal health is concerned—the dietary ratio of calcium to phosphorus may be more important than just phosphorus intake alone. For example, some small trials have found that high phosphorus intakes don’t negatively impact bone health (as measured by biochemical markers of bone resorption and bone-related hormones) when dietary calcium is sufficient (around 2,000 mg daily). Likewise, when higher phosphorus consumption has shown up associated with increased fracture risk in cross-sectional studies, other minerals (including calcium) have generally been below the recommended dietary intake. So, it’s not fully clear how much the observed effects of phosphorus on bone health are mediated by other nutrients!
Health Effects of Phosphorus Deficiency
Due to its abundance in foods (as well as its high bioavailability), phosphorus deficiency is rare, and usually only happens during times of starvation. Even when dietary intake is low, the body is generally able to compensate by increasing renal absorption so that blood levels of phosphorus can remain stable. However, some inherited disorders that cause renal phosphorus wasting can cause phosphorus deficiency (either by increasing the amount of phosphorus excreted in the urine, or by decreasing phosphorus reabsorption by the kidneys); likewise, alcoholics, people with respiratory alkalosis, anorexic patients on high-calorie refeeding regimens, and diabetics recovering from diabetic ketoacidosis are all at higher risk of low blood phosphorus.
When low blood phosphorus does occur (a condition called hypophosphatemia), it can cause such as muscle weakness, numbness in the extremities, bone pain and fragility, increased susceptibility to infection, loss of appetite, respiratory failure, and bone softening. When severe and left untreated, low blood phosphate can be life threatening.
How Much Phosphorus Do We Need?
The recommended dietary allowance (RDA) for phosphorus is 700 mg per day for adults (1,250 mg daily while breastfeeding).
Best Food Sources of Phosphorus
The following foods have high concentrations of phosphorus, containing at least 50% of the recommended dietary allowance per serving, making them our best food sources of this valuable mineral!
Good Food Sources of Phosphorus
The following foods are also excellent or good sources of phosphorus, containing at least 10% (and up to 50%) of the daily value per serving.
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