How Your Body Uses Dietary Protein

A chicken sandwich, a bowl of lentils or a glass of milk all contain protein. But your body doesn’t use that protein as it is. It takes it apart into its building blocks, then uses those blocks to build its own proteins — enzymes, muscle fibres, antibodies, hormones — or burns them for energy. Following protein from plate to muscle shows how digestion, genetics and metabolism fit together.

What protein is made of

Proteins are long chains of amino acids joined by peptide bonds (see proteins explained). There are 20 standard amino acids, each with the same backbone — an amine group, a carboxylic acid group and a hydrogen attached to a central carbon — plus a different side chain (see amino acids).

The key element that sets protein apart from carbohydrates and fats is nitrogen. By mass, proteins are roughly 16 % nitrogen. This fact has a long history: in the 1880s, Johan Kjeldahl developed a method for measuring nitrogen in food, and multiplying the nitrogen content by 6.25 (that is, 100 ÷ 16) is still the standard way to estimate protein on food labels. (The same method’s weakness was exploited in the 2008 milk scandal in China, when melamine — a nitrogen-rich chemical — was added to watered-down milk to fake a high protein reading.)

Step 1: digestion — cutting the chains

Proteins are too large to be absorbed. They must be broken into free amino acids and very short peptides by hydrolysis of their peptide bonds (see peptide bonds).

1. Stomach: hydrochloric acid (pH about 1.5–3.5) denatures proteins, unfolding them so enzymes can reach the peptide bonds (see denaturation). The acid also activates pepsin, which cuts proteins into shorter fragments. Pepsin is made in an inactive form, pepsinogen, so it doesn’t digest the cells that produce it.
2. Small intestine: the pancreas releases trypsin, chymotrypsin and other proteases (also as inactive precursors). Each cuts next to particular amino acids — trypsin, for example, cuts after lysine or arginine. Enzymes on the surface of the gut lining (peptidases) trim the fragments down further.
3. The final products are free amino acids, dipeptides and tripeptides (see the chemistry of digestion).

Step 2: absorption

Amino acids are pumped into gut cells by transporters, many of which carry sodium at the same time, using the sodium gradient as an energy source (see how molecules cross cell membranes). Di- and tripeptides enter through a separate transporter and are broken down inside the cells. The amino acids then pass into the blood and travel first to the liver.

Protein digestion is very efficient: typically over 90 % of the protein in a mixed diet is absorbed.

Step 3: the amino acid pool

Once absorbed, amino acids join the body’s amino acid pool — the free amino acids in blood and cells. Importantly, this pool also receives amino acids from the body’s own proteins being broken down. The body doesn’t store amino acids in a dedicated reserve, the way it stores glucose as glycogen or fat in fat tissue.

From the pool, amino acids have several possible fates.

Fate 1: building new proteins

Cells use amino acids to make proteins, following instructions in DNA (see protein synthesis). The body constantly breaks down and rebuilds its proteins — this is protein turnover. An adult turns over roughly 250–300 g of protein every day, far more than the amount eaten, because most amino acids are recycled.

Some proteins last minutes (certain enzymes and signalling proteins); others last months (muscle proteins) or even decades (proteins in the eye lens). Turnover lets the body adapt — building more muscle protein after resistance exercise, making more digestive enzymes after a protein-rich meal, producing antibodies during infection.

Fate 2: making other nitrogen compounds

Amino acids are raw materials for many other molecules:

• Neurotransmitters: serotonin from tryptophan; dopamine and adrenaline from tyrosine (see serotonin, dopamine).
• Haem in haemoglobin (from glycine).
• Nucleotide bases in DNA and RNA.
• Creatine for the phosphocreatine energy buffer in muscle.
• Glutathione, the cell’s own antioxidant (see antioxidants).

Fate 3: energy, glucose or fat

When there are more amino acids than needed — or when energy is short — they’re broken down. The liver removes the amine group (deamination), producing ammonia, and the remaining carbon skeleton is used for:

• energy, by entering the Krebs cycle (see the Krebs cycle)
• glucose, via gluconeogenesis (for many amino acids)
• fat storage, when energy is in surplus

Protein provides about 17 kJ per gram, similar to carbohydrate.

Getting rid of nitrogen: urea

Ammonia is toxic, so the liver converts it to urea, CO(NH₂)₂, in the urea cycle. Urea travels in the blood to the kidneys and leaves in urine (see the kidneys as chemical filters). A high-protein diet produces more urea — one reason people with advanced kidney disease are often advised to moderate protein intake.

Essential and non-essential amino acids

The body can make some amino acids from others, but not all. Nine are essential for adults — they must come from food: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine (see essential amino acids).

Food proteins differ in how well they supply these:

• Animal proteins (meat, fish, eggs, dairy) contain all nine in proportions close to human needs.
• Plant proteins often run low in one: cereals tend to be low in lysine; pulses (beans, lentils) are lower in methionine. Soy and quinoa have fairly balanced profiles.

Eating a variety of plant proteins over the day — grains with beans, for example — provides all the essential amino acids. They don’t need to be eaten at the same meal, because the amino acid pool evens things out.

How much protein do you need?

The body is in nitrogen balance when nitrogen in (from food protein) equals nitrogen out (mainly urea). Growing children, pregnant women and people recovering from illness or building muscle are in positive balance (more in than out); people who are starving or seriously ill may be in negative balance.

Many health authorities recommend about 0.75–0.8 g of protein per kg of body mass per day for healthy adults — about 56 g for a 70 kg adult. Needs are higher in pregnancy, during breastfeeding, for older adults (to help preserve muscle), and for people doing intense training. Most people in high-income countries eat more than enough.

Common misconceptions

• “Eating protein builds muscle directly.” Protein supplies amino acids; muscle is built in response to training signals, using amino acids from the pool.
• “Extra protein is stored as muscle.” Excess amino acids are deaminated and used for energy or stored as fat; the nitrogen is excreted as urea.
• “Plant proteins are incomplete, so vegetarians can’t get enough.” A varied plant diet supplies all essential amino acids.
• “Collagen supplements go straight to your skin.” They’re digested into amino acids like any other protein (see collagen).

Key takeaways

• Dietary protein is hydrolysed by pepsin, trypsin and other enzymes into amino acids and short peptides.
• Amino acids enter a pool shared with the body’s own recycled proteins; there’s no dedicated protein store.
• They’re used to build proteins, make other nitrogen compounds, or are deaminated for energy.
• Nitrogen leaves as urea made in the liver and excreted by the kidneys.
• Nine essential amino acids must come from food; adults need roughly 0.8 g per kg per day.