Why do we need protein in our diet?

What is protein?  What does it do for us?  And where do we find it in our foods?

Proteins come in many different forms and have many different functions, for example:

  • Part of your DNA – your genetic inheritance!  Proteins combine with nucleic acids to form nucleoproteins, in the nucleus of every cell in your body;
  • Enzymes – These are the proteins which make everything happen, e.g. to break down food for absorption; to regulate the entry of nutrients through cell walls, and the removal of waste-products; to grow, develop, move, reproduce.  (Many enzymes also need specific vitamins and minerals to function);
  • Haemoglobin – the protein which, with iron, carries oxygen around your body;
  • Myoglobin and elastin – These are the two main proteins in muscle fibres;
  • Bones are mainly proteins, with calcium, magnesium and phosphate;
  • Hormones which send chemical messages between nerve cells and to regulate metabolism;
  • Antibodies which circulate in your blood to protect you against viruses; and
  • Keratin which forms your hair and nails

Literally every function of your cells, organs and whole body is controlled by proteins.  They are all made within the body from smaller molecules which ultimately have come from foods.  None of our body proteins arrives ready made.

Did you know?  Every single molecule of protein in the food you eat is broken down (digested) completely into small units.

So don’t be fooled!  You may see proteins on sale as ‘nutritional supplements’, like chondroitin (a component of joint cartilage).  Not one molecule is actually absorbed!!

What exactly is protein?

Think of protein as strings of sausages. Long strings – some many thousands.  Each sausage represents one of 15 similar small molecules called amino acids. The order of amino acids in the chain is programmed by DNA.  “Amino” means that they contain nitrogen, but they also contain carbon, hydrogen and oxygen atoms.  In order to do their jobs, proteins curl up into characteristic shapes, and many of them need to incorporate minerals or vitamins in order to function.

Some proteins are solid, some are flexible in cell membranes, others are mobile in solutions

So, what happens when we eat protein in our food? 

The cells in your intestine can’t absorb whole proteins, only single amino acids or very small chains of two or three amino acids – called peptides.  So digestive enzymes (specialised proteins themselves!) break down the proteins into their component amino acids – essentially, cutting the string between the sausages.

The amino acids and peptides are absorbed into the bloodstream and safely delivered at different parts around your body where they are required, either to make new proteins or to be used to release energy.  Most arrive first at the liver, where most new protein are made, and where excess protein is broken down ready for burning as a fuel for metabolism.

Did you know? There are around 30,000 to 50,000 different proteins in the human body!  Each has its own unique gene in our DNA which arranges the 22 amino acids in a unique sequences.  Proteins in the body cells are continuously being formed and broken down by enzymes, so amino acids are continuously being recycled from broken down proteins as well as the diet.

The essential amino acids

Nine “essential amino acids” must be originally be supplied by your diet,  because your body does not have the ability to make (synthesize) them.

Those amino acids are (alphabetical order):

  • histidine
  • isoleucine
  • leucine
  • lysine
  • methionine
  • phenylalanine
  • threonine
  • tryptophan
  • valine

Our requirements for essential amino acids vary, depending on factors such as stage in life for growth and development, injury or illness.

Protein-rich foods are often rated in terms of how “complete” their amino acid profile is, in relation to needs for essential amino acids.  Because we are, biologically, much closer to a cow than a cauliflower(!), the cow’s protein content is much more similar to us.  Therefore, food that we get from animals and animal products (meat, fish, eggs, dairy) usually score highly on their amino acid profile and are subsequently regarded as “high-quality proteins”. Proteins from these food sources supply all of the essential amino acids.

However, vegetables are also perfectly good sources of protein and good amino acid profiles can be obtained from appropriate combinations, such as cereals (eg bread, pasta, rice) combined with legumes’ (beans, peas, lentils).  This is the basis of many traditional diets, which have evolved to provide the right balance.   Where neither animal foods or this traditional balance isavailable, traditional diets have adopted less usual foodstuffs, like seaweed as rich sources of amino acids.

All healthful nutrition is about achieving the right balance of nutrients from an attractive variety of foods and meals.  The example of protein shows us just how complicated it can be, but also how evolution can teach us some lessons.

These examples are easy food combinations which all achieve the required essential amino acid profiles:

  1. Pulses (beans, peas, lentils) with dairy products (e.g. milk, cheese) = Baked beans with grated cheese, lentil dhal with natural yoghurt
  2. Whole grains (brown rice, noodles, couscous, whole-wheat bread) with pulses (beans, peas, lentils) = baked beans on toast, risotto with peas, mexican tortilla with refried beans
  3. Pulses (beans, peas, lentils) with seeds and nuts = hummus (chick peas with sesame seed oil), mixed bean salad with flax seed oil dressing.
  4. Dairy (e.g. milk, cheese) with whole grains (brown rice, noodles, couscous, whole-wheat bread) = Cheese sandwich with wholemeal bread, porridge with milk.

So, now that you understand why dietary protein is so crucial to our health, how we use it and good sources of it, keep munching!  But don’t eat too much.  Excessive intakes can damage your kidneys.  On average we eat about 13% of our energy (calories) from protein.  That is already well above what we actually need for good health (8-9%) but it allows for having a relatively poor amino acid profile.

Do not be fooled!  Eating extra protein does not encourage muscle growth!   You will see ‘high-protein’ food supplements on sale, eg for athletes, or sometimes to build people up after illness.  They do neither of these things.  Eating extra protein does not improve muscles, and tends to reduce appetite so can even lead to weight loss.