Protein

Protein supplies the body's building materials

Protein is a macronutrient: a nutrient needed in relatively large amounts. Proteins are made from long chains of smaller units called amino acids. These chains fold into particular shapes, allowing different proteins to perform different jobs.

The body needs protein for growth, repair and maintenance of tissues. Growing children need materials to build new tissues; adults still need protein to maintain and repair existing ones. Protein is therefore not only important for people trying to build muscle.

Proteins also form enzymes, which help chemical reactions take place, and antibodies, which help defend the body against infection. Some hormones, the chemical messengers that help regulate body processes, are proteins. Protein can also provide energy, although building and maintaining the body are its main nutritional roles.

Choosing foods that supply protein

Animal sources include meat, fish, eggs, milk, cheese and yoghurt. Other sources include soya products, beans, lentils, peas, nuts, seeds and quinoa. Cereals and cereal products, such as bread, rice, oats and pasta, also contribute protein to the diet.

A food can contain several nutrients at once. Cheese supplies protein as well as fat, for example, while beans supply protein alongside carbohydrate and fibre. Choosing a protein source therefore affects more than the protein content of a meal.

Why the amino acid balance matters

Some amino acids can be made by the body and are called non-essential amino acids. This does not mean they are unnecessary: it means they do not all have to come directly from food. Essential amino acids must be supplied by the diet because the body cannot make enough of them.

A high biological value (HBV) protein supplies all the essential amino acids in sufficient proportions for the body's needs. Meat, fish, eggs and dairy foods are familiar examples. Soya and quinoa are also HBV sources, so HBV does not mean ‘animal food’.

A low biological value (LBV) protein supplies too little of one or more essential amino acids to meet the body's needs on its own. Many cereals, pulses and nuts are LBV sources. The GCSE distinction is often expressed as HBV containing all essential amino acids and LBV lacking one or more; the important idea is whether enough of each essential amino acid is supplied.

Think of building a model that needs several different types of piece. Having many pieces is not enough if one necessary type is in short supply. Similarly, the total quantity of protein and its amino acid balance are different considerations.

Protein complementation combines different strengths

Protein complementation means combining protein foods whose amino acid profiles complement each other. An essential amino acid that is relatively scarce in one food can be supplied by another.

A cereal and a pulse provide a useful illustration: beans on toast or lentils with rice combine different plant protein sources. The foods do not chemically turn into a new protein in the meal; together, they supply a better balance of amino acids for the body to use.

This helps explain how a varied vegetarian diet can provide the essential amino acids needed for health. It is not necessary to rely entirely on meat substitutes or on one protein-rich food.

Recognising protein alternatives

Protein alternatives can replace meat in dishes while providing protein and a suitable texture:

• Soya is a plant protein source used in a range of foods and meat alternatives. It is an HBV source.
• Textured vegetable protein (TVP) is a processed vegetable protein product, commonly made from soya flour or concentrated soya protein. It has a meat-like texture and can be sold as dried mince-like granules or chunks. Dried products need soaking to soften and rehydrate them before cooking; follow the packet instructions.
• Tofu is a soya-based protein food, also called bean curd. It can be cut into slices or cubes and absorbs flavours from marinades and sauces.
• Mycoprotein is protein produced by growing a fungus through fermentation. It is used in products such as Quorn, including mince-style meat alternatives.

These names are not interchangeable: tofu and soya TVP are both soya products, but mycoprotein is not soya. Check labels for allergens and dietary suitability. Some meat alternatives contain egg or milk, so a product suitable for vegetarians is not automatically suitable for vegans.

Too little and too much protein

A prolonged shortage of protein can cause poor growth and reduced tissue repair. It may also contribute to muscle loss, brittle hair and nails and a weakened immune system. These effects connect directly to protein's roles: without enough building materials, the body struggles to maintain tissues and produce the proteins it needs.

Severe protein-related malnutrition can involve kwashiorkor, associated with swelling, including a swollen abdomen, and an enlarged liver. This is a serious form of malnutrition, not an inevitable result of occasionally eating a low-protein meal.

More protein is not always better. Protein beyond the body's needs is broken down, and the kidneys remove waste products such as urea. This can increase fluid needs, creating a risk of dehydration if fluid intake is inadequate. A high-protein diet that provides too little fibre may also cause digestive problems, such as constipation. Excess energy intake, including energy from protein, can contribute to weight gain.

People with reduced kidney function are particularly vulnerable: excessive protein can increase the kidneys' workload and worsen existing kidney disease. This does not mean that a moderate increase in protein inevitably damages healthy kidneys. The foods supplying protein also matter: a diet rich in fatty meats may contain substantial saturated fat, which can negatively affect heart health.

Dietary reference values depend on the population group

Dietary reference values (DRVs) provide reference amounts for different population groups. They are useful for planning and assessing diets, but are not exact targets for every individual.

The UK protein reference figures below are expressed in grams per day:

For adults aged 19–49, this is often rounded to 45 g per day for women and 55 g per day for men. Notice that the table gives females first and males second, and that the figures change with age.

Requirements relate to factors such as body size and growth. Pregnancy and breastfeeding increase demand, while active people may need additional protein to support muscle maintenance and repair. A higher requirement does not mean that unlimited protein is beneficial.

Adapting a bolognese for a vegetarian diet

A bolognese-style sauce is a useful example because the protein ingredient can change while the vegetables and tomato base remain similar. Replace minced beef with soya mince, suitable mycoprotein mince, lentils or beans. This preserves a protein source rather than simply removing the meat.

For a meat version serving three, use 1 onion, 1 garlic clove, 1 carrot, 1 celery stick, 15 ml oil, 250 g lean minced beef, 400 g canned chopped tomatoes, 15 ml tomato purée, 100 ml water, 5 ml mixed herbs and black pepper. Serve with 240 g spaghetti.

Useful equipment includes a chopping board, sharp knife, peeler, garlic press, scales, measuring jug and spoons, two saucepans, a heat-resistant spoon and a colander.

1. Peel and chop the onion, peel and crush the garlic, peel and slice the carrot, and finely slice the celery.
2. Fry the prepared vegetables in the oil.
3. Add the beef and cook until lightly browned, breaking up the mince. Browning develops the sauce's flavour; it is not by itself proof that the meat is safely cooked.
4. Add the tomatoes, tomato purée, herbs and water, then season with black pepper.
5. Bring to the boil, then simmer gently for 20 minutes, stirring as needed.
6. Meanwhile, boil the spaghetti for about 10–12 minutes, or according to its packet instructions. Drain carefully and serve with the sauce.

For the vegetarian version, replace the beef with a suitable alternative and adjust its preparation and cooking to the packet instructions. Dried TVP needs rehydrating, while lentils need enough cooking to become tender. Check any added stock and cheese for vegetarian suitability.

Knife skills and safe preparation

A steady chopping board and controlled cutting make preparation safer and help produce pieces that cook evenly. Use a suitable sharp knife, keep fingers away from the cutting edge and work slowly enough to stay in control.

With tofu, cut the block into slices, then strips, then cubes. This gives evenly sized pieces for a sauce or marinade. With prepared skinless, boneless fish, use controlled cuts to make even cubes; the ceviche example uses 2 cm pieces. These are examples of using knife skills on a protein alternative or fish rather than only on vegetables.

Keep raw meat and fish separate from ready-to-eat foods. Clean hands, knives, boards and work surfaces after handling them. Vegetarian ingredients still require hygienic preparation.

Acids change protein during food preparation

Denaturation is a change in a protein's folded structure. Acid can disrupt the interactions maintaining that shape. The amino acid chain unfolds rather than being chopped into separate amino acids. Coagulation is the subsequent joining together of denatured proteins into a firmer or set structure. They are related changes, but they are not the same process.

An acidic marinade, such as one containing lemon juice, can denature proteins in meat and help tenderise it. Natural yoghurt can also be used in marinades, as in chicken tikka. A marinade also develops flavour; tofu and mycoprotein readily absorb flavours from ingredients such as soy sauce, garlic, herbs and spices.

Fish provides a clear example of acid changing protein. In ceviche, citrus juice causes the fish to change from translucent to opaque and become firmer as its proteins denature and coagulate. The unfolded proteins join together, changing the fish's texture without applying heat.

These visible changes resemble some effects of cooking with heat, but acid marinating is not a reliable substitute for heat treatment to make raw fish safe. Harmful bacteria and parasites may survive. Appearance alone cannot establish food safety.

Protein's roles and sources

• Functions: growth, tissue repair and maintenance; enzymes, antibodies and some hormones; can supply energy.
• Sources: meat, fish, eggs, dairy, soya, beans, lentils, nuts, seeds and cereals.

Amino acid balance

• Essential amino acids: must come from the diet.
• HBV: supplies all essential amino acids in sufficient proportions; examples include eggs, meat, fish, dairy and soya.
• LBV: short of one or more essential amino acids; examples include many cereals and pulses.
• Complementation: combine different protein sources, especially cereals and pulses, to improve the amino acid balance.
• Alternatives: TVP, soya, tofu and mycoprotein. Mycoprotein is fungal, not soya; dried TVP needs rehydrating.

Intake and health

• Deficiency: poor growth and repair, muscle loss and weakened immunity; severe malnutrition can involve kwashiorkor and swelling.
• Excess: increased fluid needs and possible dehydration; digestive problems if fibre intake is low; excess energy can contribute to weight gain.
• High protein intake can worsen existing kidney disease. Heart-health risks depend partly on the foods supplying the protein.
• Adult reference values, ages 19–49: females 45.0 g/day; males 55.5 g/day.
• Ages 15–18: females 45.0 g/day; males 55.2 g/day.
• Needs vary with age, growth, body size, pregnancy, breastfeeding and activity.

Preparation connections

• Vegetarian bolognese: replace beef with soya mince, suitable mycoprotein mince, lentils or beans; check other ingredients and adjust cooking instructions.
• Knife skills: controlled cuts, even-sized pieces and separation of raw meat/fish from ready-to-eat food.
• Denaturation: protein structure changes. Coagulation: denatured proteins join and set.
• Citrus acid makes ceviche fish opaque and firm; this change does not prove that it is safe to eat.