What is Protein?
Before I get into the nitty-gritty aspects that I want to touch on in this post about protein, let's go back to basics. As already mentioned in my 'What's In My Food' introduction post, proteins are complex organic molecules that contain nitrogen in addition to carbon, hydrogen, and oxygen. The 'building blocks' of proteins are called amino acids. As you can see in the sketch below, each amino acid contains a central carbon atom, a carboxyl (-COOH) group, an amine (-NH2) group, a lonely hydrogen atom, and a unique side chain . There are 20 standard amino acids, each with a different side chain, that are important to humans and of these, nine are essential amino acids, which cannot be made in the body . The essential amino acids include:
Amino acids join together via peptide bonds (the -O-N- section that you can see in the chain above) to form long chains called polypeptides, which bend and twist to form three-dimensional molecules. Proteins are important for proper growth and repair of body tissues, and in addition to these functions proteins also act as:
- Enzymes, which catalyse biochemical reactions in the body
- Hormones, which regulate various body functions
- Carrier proteins, such as haemoglobin which transports oxygen
- Contractile proteins in muscles, which are responsible for movement
- Structural proteins, such as collagen
- Immune system components, which protect our bodies by fighting infections
We need to consume about 0.8 g/kg body mass of protein each day to prevent protein deficiency but need a bit more for optimal protein synthesis, immune function, satiety, and weight management. Proteins can be found in both plant and animal food sources. Animal sources contain high-quality protein, supplying all of the amino acids required by the body, whilst plant-based proteins need to be combined throughout a day in order to supply all of the essential amino acids.
Proteins from different sources can be classified according to their quality, which basically describes their ability to be absorbed into the body and achieve the desired metabolic actions. There are a few commonly used methods of assessing protein quality:
- Protein Efficiency Ratio (PER) - This method is based on the assessment of animal growth (weight gain) divided by the intake of a specific food. It is able to assess protein needed for growth, but not for maintenance .
- Net Protein Utilisation (NPU) - This is a ratio of the amino acids that are converted into usable body proteins to the ratio of amino acids supplied . This method of assessing protein quality is influenced by the amounts of essential amino acids found in the body and limiting amino acids in a food .
- Biological Value (BV) - This is a measure of the absorbed amino acids that are incorporated into body proteins and tissues. BV does not take into consideration how a food is digested and absorbed and can be influenced by dietary intake and the way in which a food is prepared .
- Protein Digestibility Corrected Amino Acid Score (PDCAAS) - In light of the practical difficulties associated with using each of the other methods described, the PDCAAS method has been adopted as the current internationally approved method for assessing protein quality . In the simplest of terms, this method of assessing protein quality takes into consideration the amino acid requirements of humans and their ability to digest protein in a food.
How does the body use it?
When we eat a meal our body digests the food before it can be absorbed into our bloodstream. I have written a very comprehensive post on the overall digestion process here but will touch a bit more on digestion in this post. Protein digestion begins in the stomach, once food has been chewed and swallowed, through the action of an enzyme called pepsin . This enzyme breaks the peptide bonds mentioned earlier, breaking up big proteins into smaller chains of amino acids . These chains then move from your stomach to your small intestine where they are broken apart to release individual amino acids and di-/tripeptides, which contain two or three amino acids, by a number of pancreatic enzymes and brush-border enzymes (trypsin, chymotrypsin, and carboxypeptidase) . Amino acids are transported across the wall of the small intestine into the bloodstream, which transports them throughout the body to cells that need them . We can only use a set amount of protein at any one time, and whatever amino acids aren't used in the body end up being processed and eliminated from the body.
Do I need to eat/drink protein powder to consume enough protein each day?
Let's start by answering this question - what exactly are protein powders? They essentially supplement the diet with extra protein and can be useful when you are unable to get enough of it from whole food sources . Many powders purchased in the shops contain extra ingredients, including carbohydrates, fats, vitamins, minerals, stabilisers, anti-caking agents, and more. Common sources or protein used in these supplements include:
- Milk (whey)
It often doesn't make sense to me when people (wellness bloggers, Instagram health gurus etc.) claim to never eat processed foods, but believe it or not, protein powders don't grow on trees. Most protein powders are actually highly processed food supplements that are refined in such a way that they taste good and have a very long shelf-life . There's a pretty good summary of how different types of protein powders are processed in this article by Precision Nutrition, so give it a read if you are interested to learn a bit more. At the end of the day if you decide to use a protein powder make sure that you first consider the reason why you want to use it and do your research before purchasing it. Try and stick to a simple product without all the extra added ingredients, and if you realise that you don't want to spend your money on protein powder don't worry, a healthy individual really can get more than enough good quality protein from whole foods every day.
So what whole foods are good sources of protein?
So as I have already mentioned, you don't need to spend money on expensive protein supplements if you are consuming enough good quality, whole foods that contain protein. Some of the more obvious sources of protein include animal products such as [6,7]:
- Beef steak (31 g per 100 g)
- Chicken breast (32 g / 100 g)
- Pork chop (31.6 g / 100 g)
- Lamb chop (29.2 g / 100 g)
- Tuna, canned (23.5 g / 100 g)
- Salmon (24.2 g / 100 g)
- Eggs (6 g / 1 large egg)
- Whole milk (3.3 g / 100 g)
- Cheddar cheese (25.4 g / 100 g)
- Cottage cheese (12.6 g / 100 g)
- Full-fat plain yoghurt (5.7 g / 100 g)
Despite the common notion that those that follow vegetarian and vegan diets will struggle to consume enough protein, it really is possible to consume enough good quality plant-based protein when you eat a wide variety of different grains, seeds, legumes, nuts, vegetables, and fruits. In a position statement by the American Dietetic Association  it is mentioned that:
"Plant protein can meet protein requirements when a variety of plant foods is consumed and energy needs are met."
Research has shown that eating a variety of plant foods over the course of a day can provide all of the essential amino acids that our bodies need for sufficient nitrogen retention and use . Some plant-based protein sources are not digested as well as others, for example, cereals and legumes, and different plant foods have varying amounts of each essential amino acid . Consuming an array of plant-based foods from different groups is key to absorbing enough protein and enough of each essential amino acid on a vegetarian or vegan diet . I found the sweetest poster by Simple Happy Kitchen that you can download from here. It shows a variety of different plant-based protein sources in a visually pleasing and straightforward way. Just a few examples of plant-based protein sources include [7,9]:
- Red lentils (7.6 g / 100 g)
- Chickpeas (8.4 g /100 g)
- Kidney beans (6.9 g / 100 g)
- Tofu (8.1 g / 100 g)
- Oatmeal (11.2 g / 100 g)
- Almonds (21 g / 100 g)
- Tahini (22 g / 100 g)
- Walnuts (14.7 g / 100 g)
- Hazelnuts (15 g / 100 g)
 Annigan J. How Many Amino Acids Does the Body Require? [Internet]. SFGate. [cited 2017 Aug 10]. Available from: http://healthyeating.sfgate.com/many-amino-acids-body-require-6412.html.
 Andrews, R. All About Protein Powders [Internet]. Precision Nutrition. [cited 2017 Aug 4]. Available from: http://www.precisionnutrition.com/all-about-protein-powders.
 Millward DJ, Layman DK, Tomé D, Schaafsma G. Protein quality assessment: impact of expanding understanding of protein and amino acid needs for optimal health. Am J Clin Nutr. 2008 May;8(5):15765-815.
 Gillapsy R. Protein Digestion and Absorption Process [Internet]. Study.com. [cited 2017 Aug 10]. Available from: http://study.com/academy/lesson/protein-digestion-and-absorption-process.html.
 Open Learn. Nutrition: Proteins: 1.7 Protein Digestion and Absorption [Internet]. 2017 [cited 2017 Aug 10]. Available from: http://www.open.edu/openlearn/science-maths-technology/science/biology/nutrition-proteins/content-section-1.7.
 Today's Dietitian. Protein Content of Foods [Internet]. Today's Dietitian. 2013 [cited 2017 Aug 10]. Available from: http://www.todaysdietitian.com/pdf/webinars/ProteinContentofFoods.pdf.
 British Nutrition Foundation. Protein [Internet]. British Nutrition Foundation. 2016 [cited 2017 Aug 10]. Available from: https://www.nutrition.org.uk/nutritionscience/nutrients-food-and-ingredients/protein.html?limit=1&start=4.
 Craig WJ, Mangels AR. Position of the American Dietetic Association: Vegetarian Diets. J Am Diet Assoc. 2009 Jul;109(7):1266-82.
 Simple Happy Kitchen. Our Daily Nutrition: Protein, Calcium & Iron [Internet]. Simple Happy Kitchen. 2017 [cited 2017 Aug 10]. Available from: https://www.simplehappykitchen.com/protein-calcium-iron/.