Phosphate

Stores cellular energy

ATP is adenosine with three phosphate groups attached in a chain. Each phosphate carries negative charge, and packing three of them together is like compressing a spring. Removing the last one releases energy, which the cell uses to contract muscle, pump ions and build molecules. ATP is the cell's main energy currency, and phosphate is what makes it work.¹

Phosphate also carries energy in other forms:

• Phosphocreatine is a fast backup store in muscle and brain. It hands its phosphate to ADP to rebuild ATP within seconds during a sprint (see Creatine).
• 2,3-BPG in red blood cells is a phosphate compound that helps hemoglobin release oxygen to tissue. With low phosphate it falls, and hemoglobin holds onto oxygen more tightly.
• Switching proteins on and off. Adding or removing a phosphate is the main way cells turn proteins on and off. Insulin signalling, muscle contraction and cell division all run on it.

What happens when blood phosphate drops shows how much depends on it. In refeeding syndrome, a starved person is fed carbohydrate again, and insulin drives phosphate into cells to make ATP for the sudden flood of glucose. Blood phosphate can crash within days. The result is muscle weakness, a failing diaphragm and heart, broken-down red cells and confusion, which can be fatal unless phosphate is replaced.^2,3

Builds bone

About 85% of the body's phosphorus is in bone and teeth, as hydroxyapatite, a calcium-phosphate crystal laid down on a collagen frame. Collagen gives bone flexibility. The mineral gives it hardness. Neither works without the other. Phosphate is half the mineral, so calcium can't build bone without it. A long phosphate shortage causes soft bones (rickets in children, osteomalacia in adults), the same result as a lack of vitamin D.¹

Three hormones keep blood phosphate in range, and they are tied into calcium control:⁴

• Vitamin D (as calcitriol) increases phosphate absorption from the gut.
• PTH releases calcium and phosphate from bone, but makes the kidney excrete phosphate.
• FGF23, a hormone from bone cells, rises when phosphate is high. It makes the kidney excrete phosphate and lowers vitamin D activation. Genetic disorders with too much FGF23 cause phosphate-wasting rickets.

Builds DNA and membranes

The backbone of DNA and RNA is an alternating chain of sugar and phosphate. Every rung of the genetic code hangs off a phosphate link, and the negative charges along that backbone are what let DNA wrap around its packaging proteins. Copying DNA uses phosphate-carrying building blocks, so growing tissue has a high phosphate demand.¹

Cell membranes are built mostly from phospholipids. Each one has a phosphate-containing "head" that faces water and two fatty acid "tails" that face inward. That shape is what makes the two-layer sheet of every membrane assemble itself. Phosphatidylcholine, the most common one, is also where the body gets choline for acetylcholine.

Phosphate also buffers acid inside cells and in urine. The kidney excretes much of each day's acid load bound to phosphate.

Intake: the additive problem

The RDA is 700 mg/day for adults, and most people eat about twice that. Shortage from diet alone is almost unheard of, because phosphate is in every cell of every food. The real issue is what kind of phosphate:^5,6

• Natural phosphate in meat, fish, dairy and legumes is bound in organic molecules and only partly absorbed: around 40–60% from animal foods, less from plants, where much is locked in phytate.
• Added phosphate in processed food (phosphoric acid in cola; sodium phosphates in processed meat, cheese, baked goods and fast food) is inorganic and almost completely absorbed. It also often doesn't appear on nutrition labels, so national surveys underestimate it.

High phosphate raises FGF23 and PTH, and in people with kidney disease it's strongly linked to calcification of the arteries.⁶ Whether the extra from additives harms people with healthy kidneys hasn't been tested in trials, but the hormone changes appear in healthy people too. The practical answer is the same either way: get phosphate from whole foods, and treat "phos" in an ingredient list as a sign of ultra-processed food.

Top foods

• Salmon: about 270 mg in 3 oz
• Yogurt, plain: about 245 mg per 6 oz
• Milk: about 250 mg per cup
• Chicken or beef: about 180–210 mg in 3 oz
• Lentils, cooked: about 180 mg per ½ cup
• Pumpkin seeds: about 330 mg in 1 oz

Deficiency effects

Low blood phosphate (hypophosphatemia) almost always comes from shifts or losses, not diet. Causes include refeeding after starvation, treating diabetic ketoacidosis, alcoholism, recovery from severe burns, long-term use of aluminium or magnesium antacids (which bind phosphate in the gut), and some genetic and kidney disorders.³ Mild cases cause no symptoms. Severe cases (under about 1 mg/dL) cause muscle weakness including breathing muscles, broken-down muscle and red cells, heart failure and confusion. Long-term mild loss softens bone.