Protein is one of the more popular and positive nutrition buzzwords today, with a myriad packaged processed foods declaring that they are high in protein. But what exactly is protein, what are the best sources, how much are we consuming, and do we really need those protein balls and shakes?
Proteins are chemical compounds made up chains of amino acids which themselves are composed of the elements carbon, hydrogen, oxygen and nitrogen. There are 20 amino acids that are important to humans, and nine (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) of these are considered essential in that they must be obtained from foods and drinks in our diets, whereas the other non-essential amino acids can be synthesised within our bodies. Amazingly, all of the proteins in our body are made from these 20 amino acids.
Proteins are essential parts of the structure and function or every cell in our body and the body of an average 80kg adults will contain about 13g of protein, with 43% found in muscle, 16% in blood and 15% in skin. The protein in our cells are constantly turning over, over a period of minutes to months, through the process of protein synthesis and degradation, which is one reason why we ideally need to obtain a range of proteins from foods and drinks each day. In addition to being used to make proteins, amino acids can be used for specific purposes within cells, like the formation of nerve transmitters and hormones. Amino acids also can be used as a source of energy – particularly when other sources of energy like carbohydrate and fat are restricted.
In the great scheme of things, our body’s top priority is to meet its energy requirements, and when energy from other sources is limited, it will break down protein to meet its needs. It does this by stripping off the nitrogen from the rest of the amino acid molecule, leaving carbon, oxygen and hydrogen skeletons to be used as fuels, just like carbohydrate, and provides 17 kJ per gram of protein. On the other hand, if you consume adequate carbohydrate and fat, your body spares protein from being used for energy. When amino acids are used for energy, the nitrogen molecules are converted to urea and excreted in your urine.
Most people eat a variety of foods containing many different proteins. Some foods contain all 9 essential amino acids in amounts suitable for humans and these are called complete protein foods. Commonly eaten foods that are complete proteins include animal foods like meat, poultry, seafood, eggs, and dairy foods and in addition, soy protein is considered to be complete. Proteins in plant foods (vegetables, grains, legumes, nuts and seeds) are rarely complete, but when they are eaten in combination, either at the same meal (e.g., rice and beans, or bread and peanut butter), or another time of the day, you nearly always end up with a complete source of protein, which is one of the reasons why vegan diets (where you do not eat any animal foods) can meet all of our nutritional requirements if well designed.
Unfortunately, most people don’t realise that both low and high protein intakes are detrimental to health. Low intakes can impair the functioning of our immune systems, making us more prone to infections, and of course will stunt the growth of children and adolescents, and slow the healing of wounds and rates of recovery after surgery, for example. It is also possible to eat too much protein as our kidneys don’t have an unlimited capacity to remove urea from our blood. The upper limit of protein consumption in humans is estimated to be 35% of energy from protein, which for a typical person consuming 8,700 kJ (2080 Calories) a day would be 179 grams a day. Needless to say, this rarely occurs under normal circumstances, but has been observed in populations where food is scarce, and people are forced to rely heavily on eating wild animals like rabbits for food, and that’s why it is known as rabbit starvation or mal de caribou. Symptoms include diarrhoea, headache, fatigue, low blood pressure and slow heart rate, and a vague discomfort and hunger that can only be satisfied by consuming foods that are high in fats or carbohydrates.
The Estimated Average Requirement (the daily nutrient level estimated to meet the requirements of half the healthy population) for protein for men is 0.68g per kg body weight per day and for women it is 0.60g per kg body weight per day. Australia’s most recent nutrition survey (part of the Australian Health Survey, or AHS) in 2011/12 determined that men consumed an average of 104.6 g of protein per day or 1.2 g per kg body weight and women consumed 77.9g of protein per day or 1.1g per kg body weight. In other words, the average Australian adult consumes nearly twice as much protein as the estimated average requirement each day.
Perhaps unsurprisingly, Australians have increased the amount of protein they consumed each day since the last national nutrition survey in 1995 – as low-carbohydrate diets have become more fashionable, people have replaced some of the carbohydrate with protein. The primary sources of high quality protein in the Australian diet are lean meats, poultry, seafood, eggs and alternatives (e.g., soy-based products, other legumes, nuts and seeds, etc…) and milk, yoghurt, cheese and alternatives. The AHS found that only 17% (less than 1 in 5) of Australian adults consumed the recommended number of serves of meats and alternatives and less than 14% (around 1 in 7) consumed the recommended number of serves of milk, yoghurt, cheese and alternatives. Legumes are also a good source of protein, but again, less than 10% of Australians consume the recommended number of serves. Overall, these results suggest that most of the extra protein Australian adults are consuming is coming from discretionary foods. This may be highly refined protein devoid of other nutrients – or in other words “junk” protein.
In Australia at least, we should beware the high protein health halo – we don’t need to eat more processed foods high in refined proteins. We should instead be eating more of those high-quality protein-containing foods like lean meats, poultry, seafood, dairy and alternatives that generally don’t make protein claims on their packaging.
Alan Barclay, PhD is a consultant dietitian and chef (Cert III). He worked for Diabetes Australia (NSW) from 1998–2014 . He is author/co-author of more than 30 scientific publications, and author/co-author of The good Carbs Cookbook (Murdoch Books), Reversing Diabetes (Murdoch Books), The Low GI Diet: Managing Type 2 Diabetes (Hachette Australia) and The Ultimate Guide to Sugars and Sweeteners (The Experiment, New York).