From the cell membrane
to tooth enamel

Basics

Phosphate balance in figures

In the body, phosphorus never occurs as a pure element, but always bound as phosphate. Anyone wanting to read figures on labels or in laboratory results therefore first needs a feel for the orders of magnitude.

Guide values for orientation. Laboratory ranges differ by method and laboratory.

Where the stock sits and how quickly it is exchanged

By far the greatest part of phosphorus is mechanically built into bone and becomes available only slowly. The skeleton exchanges about five to ten percent of its substance per year — osteoclasts dissolve old matrix, osteoblasts deposit new. Phosphate and calcium move along with it.

Less than one percent of the total, by contrast, circulates in the blood. It is precisely this small amount that is measured when a phosphate value is determined in the laboratory. Accordingly, it says little about the reserve in bone; it describes the momentary balance of intake, distribution and excretion.

Membrane

The bilayer that orders itself

A cell membrane is not a rigid skin but a mobile fluid in two layers. Its building blocks are phospholipids: a glycerol backbone with two fatty acids attached and a phosphate group, which in turn is esterified with a small molecule such as choline or ethanolamine.

Phosphatidylcholine and its head group

Phosphatidylcholine makes up about half of all phospholipids in animal membranes. Its phosphate head is charged and binds water; the two fatty acid chains avoid it. If such molecules are placed in an aqueous environment, they arrange themselves into a bilayer without any further intervention: heads outwards, chains inwards. This principle carries every cell boundary and every organelle envelope.

Phosphorus accounts for only a few percent of the mass of a phospholipid molecule — but without this group the molecule loses its water-soluble side and with it the ability to form an ordered interface.

Energy turnover

Phosphate as the currency of metabolism

The body does not store energy as heat or voltage, but in chemical bonds between phosphate groups. Adenosine triphosphate, or ATP for short, is the most commonly used molecule for this.

From ADP to ATP and back again

In the mitochondria, adenosine diphosphate is loaded with a third phosphate group. The energy needed for this comes from the oxidation of carbohydrates, fatty acids and amino acids. When a cell needs energy — for muscle contraction, for ion pumps, for building protein — the outer phosphate group is split off again and the stored energy is released.

Besides ATP, metabolism knows other phosphate-containing carrier molecules: creatine phosphate buffers short-term load peaks in muscle, guanosine triphosphate drives signalling cascades, and nicotinamide adenine dinucleotide phosphate supplies reducing power for biosynthesis. What they all have in common is the phosphate group as carrier.

Hard tissue

Bone and tooth enamel as a mineral depot

Bone is a composite material. Collagen fibrils give it tensile strength, embedded crystals of calcium phosphate compressive strength. The crystals are tiny — typically 50 nanometres long and a few nanometres thick — and lie at regular intervals between the protein strands.

Remodelling of the skeleton over the year

Contrary to the image of a rigid scaffold, bone is constantly broken down and rebuilt. In this remodelling, calcium and phosphate move back and forth between the depot and the blood. In adults, breakdown and build-up are roughly in balance; during growth, build-up predominates, and in later life often breakdown.

Tooth enamel knows no remodelling

Tooth enamel follows a different logic. It consists of about 96 percent mineral, contains no cells and is no longer formed anew once the tooth has erupted. Mineral can only be exchanged at the surface, fed from saliva, which carries calcium and phosphate in dissolved form. The dentine underneath is softer, contains collagen and is slowly regenerated throughout life.

Diet

Phosphorus on the plate

Phosphate accompanies protein. Where there is a lot of protein, there is as a rule a lot of phosphate too — in dairy products, meat, fish, pulses, nuts and cereal germs. The following overview gives guide values per 100 grams of edible portion.

Guide values from common nutrition tables. Variety, origin and preparation shift the figures.

Phytate, additives and the amount actually absorbed

The content on paper and the amount absorbed are two different things. In seeds, kernels and wholegrain, phosphate is present largely as phytic acid; the human intestine lacks the enzyme to break it down completely, so only part of it is used. Soaking, sprouting and sourdough fermentation lower the phytate content and increase the available amount.

The opposite is true of phosphate additives. They appear on ingredient lists as E 338 to E 341 or E 450 to E 452 and serve as acidulants, emulsifying salts or stabilisers. Because they are already in inorganic form, a considerably higher proportion is absorbed than from plant sources. In the balance of a heavily processed diet this carries weight.

1. Read the ingredient list: E numbers in the 300 and 450 series indicate added phosphate.
2. Prefer unprocessed protein sources if intake is to be lower.
3. Where kidney disease is known, intake belongs under medical supervision.

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