Nutrient - Protein - taking a closer look
How much is too much? Why is it so important? We have delved a little deeper and uncovered some interesting research.
Every cell in the body contains protein. It is a major part of the skin, muscles, organs and glands. Protein is needed to make neurotransmitters, for growth and development, and for the creation of DNA and RNA. Protein is also found in all body fluids, except urine(1).
Proteins are made up of nitrogen-containing molecules called amino acids. Amino acids combine together in specific variations to make different types of proteins that form our cells and organs, in much the same way that letters make words that form sentences and paragraphs.
Eight amino acids are generally regarded as essential for humans. These are phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, leucine and lysine, and other amino acids can be made from these eight.
Foods such as meat, poultry, eggs, fish and milk are called ‘complete’ or high-quality protein, as they contain all these amino acids.
Vegetables sources of protein; such as beans, peas, nuts and seeds; are called ‘incomplete’ proteins, as they lack one or more of the essential amino acids. However, hemp seed, which has been around for many thousands of years, is a complete vegetable protein with high amounts of all essential amino acids and with the added benefit of containing the healthy, polyunsaturated fats. The two main proteins in hemp seed are edestin and albumin(2). Typically, hemp contains 33% albumin and 65% edestin (a Greek word meaning ‘edible’). Both are globular proteins and can be used by the body to manufacture functional proteins, such as hormones, haemoglobin, enzymes and antibodies(3).
It is particularly suitable for vegetarians and vegans, who need to be careful to get all the essential amino acids by otherwise combining proteins; for example, eating corn and beans, or rice and beans, together. In the past, it was thought that these complementary proteins needed to be eaten at the same meal for your body to use them together. Now, studies show that this is not necessary as your body can combine complementary proteins that are eaten within the same day(4).
Proteins are complicated molecules so it takes a long time, and more than one enzyme, to completely break them down into amino acids.
Protein digestion starts in the stomach. Hydrochloric acid starts to break down food and pepsinogen is released, which is converted into its active form, pepsin. When pepsin starts acting on the proteins in swallowed food, a hydrolysis reaction occurs, which breaks apart the long protein chains, or polypeptides, into smaller pieces called dipeptides or peptides.
There are two more enzymes in the pancreatic juice of the small intestine that help digest proteins. Like pepsin, their inactive forms are secreted and they become activated in the cavity of the small intestine. The inactive forms are trypsinogen and chymotrypsinogen. Trypsin and chymotrypsin are the active forms that do the work of breaking apart peptide fragments. Trypsin activates two more enzymes; elastase and carboxypeptidase. Once the peptides are broken down into small chains, peptidases finish them off, by breaking apart the peptides into individual, absorbable amino acids.
Most proteins are absorbed as amino acids by active transport processes that occur mainly in the duodenum and jejunum.
Daily protein requirements
People with certain diseases, such as diabetes, gout or kidney disease, do have to be careful about their protein intake, as too much can increase the amount of work for the kidneys. However, according to Don Layman PhD, Professor Emeritus of Nutrition at the University of Illinois, it is a myth that the majority of adults are eating more protein than they need(5).
The importance of high-quality protein, particularly at breakfast time, is supported by a growing body of evidence. Research shows that dietary protein intakes over the current recommendations may be beneficial in managing diseases such as obesity, heart disease and type II diabetes. In fact, researchers recommend that adults consume 25-30g protein at each meal(6).
Protein is not stored in the body the way fats and carbohydrates are, so it is important that enough is eaten every day. Protein is particularly important for children, whose bodies are growing and changing daily.
A lack of protein in our diet can slow growth, reduce muscle mass and bone density, lower immunity and weaken the heart and respiratory system. Symptoms of low protein include muscle fatigue, a drop in muscle strength or recent weight loss. However, these symptoms may also be signs of other problems, so should be investigated thoroughly.
Daily requirements(7, 8)
Children (aged 2-3): 5-20% of daily calories (approximately 13-50g for 1,000 daily calories)
Children (aged 4-8): Girls: 10-30% of daily calories (30-90g for 1,200 daily calories)
Children (aged 4-8): Boys: 10-30% of daily calories (35-105g for 1,400 daily calories)
Children (aged 9-13): Girls: 10-30% of daily calories (40-120g for 1,600 daily calories)
Children (aged 9-13): Boys: 10-30% of daily calories (45-135g for 1,800 daily calories)
Children (aged 14-18): Girls: 10-30% of daily calories (45-135g for 1,800 daily calories)
Children (aged 14-18): Boys: 10-30% of daily calories (55-165g for 2,200 daily calories)
Adults: 10-35% of daily calories (50-175g for 2,000 daily calories)
Typical protein content of foods
A cup of milk contains 8g protein
A 3oz piece of meat contains about 21g protein
A cup of dry beans contains about 16g protein
An 8oz container of yoghurt contains about 11g protein
N.B. The availability of amino acids is also affected by Net Protein Utilisation (NPU). This is the ratio of amino acid converted to protein to the ratio of amino acid supplied. In general, the higher the amino acid content, the higher the NPU.
Protein can help weight loss in a number of ways. Firstly, protein helps to slow down the release of sugars in the body, so has a positive effect on blood sugar balance. One study, published in the 2010 issue of Nutrition Research, found that the blood sugar-balancing effect of a protein-rich breakfast reduced hunger and calorie consumption by 18% throughout the day(9). Efficient energy expenditure is also increased by protein, with complete proteins showing larger increases in energy expenditure than lower quality proteins. Protein-induced energy expenditure may be due to gluconeogenesis, a process where glucose is generated from non-carbohydrate sources, such as lactate and pyruvate, which also help to stabilise blood sugar levels(10).
Numerous studies show that consuming protein, as opposed to fat and carbohydrate, has a greater effect on our satiety levels. Protein stimulates the production of cholecystokinin (CCK), an intestinal hormone that is released after a meal and signals satiety. It also decreases levels of ghrelin, a hormone produced in the stomach and pancreas that stimulates hunger.
A study published in Cell Metabolism found that eating a high-protein diet can increase our hunger-fighting hormone, known as peptide YY (PYY). The study, conducted on both obese and normal-weight people, found that a protein-rich meal stimulated a greater release of PYY and, therefore, a greater reduction of hunger than a high-carbohydrate or a high-fat meal. The participants of the study reported feeling fuller quicker. Therefore, food and calorie intake was reduced by a third, improving weight loss and weight loss maintenance(11).
The ageing population is one group that is at risk of not eating enough protein. According to a recent study of men and women in their 70s, carried out by the National Institute of Health, those who ate the least protein lost significantly more muscle than those who ate the most protein. Maintaining muscle is particularly important as you age, as muscle loss in the legs and hips can cause falls and broken bones. Those who don’t maintain strong muscles as they age might also have trouble doing basic things, such as getting up from a chair or walking up stairs(12).
Additional supporting research for the older population’s need for protein comes from a study published in the American Journal of Clinical Nutrition. Researchers in this study found that older men and women who ate the most protein-rich foods lost approximately 40% less muscle mass over three years compared to those who ate the least amount of protein.
According to Clinical Nutrition & Metabolic Care, older patients should be advised to consume 25-30g high-quality protein per meal, in order to prevent or slow down sarcopenic muscle loss(13).
Protein is a major constituent of bone and has long been identified as being both beneficial and detrimental to bone health. Low protein, in particular, is associated with lower bone mass density but it has generally been thought that too much protein can cause calcium to leak from the bones and increase bone loss. However, in a recent study published in the American Journal of Clinical Nutrition, researchers found that this was not the case, as protein did not have a negative effect on bone mineral density. The study looked at 560 women aged from 14 to 40 years old and assessed their protein intake. Yearly changes in hip, spine and whole-body bone mass density were measured using a DEXA machine. The team found that high protein does not adversely affect bone mineral levels(14). This research was confirmed by a study in the same year, published in Clinical Calcium that found high protein intake does not promote calcium loss and so does not negatively affect BMD. In fact, they found that a high protein intake has a beneficial effect on bone mineral density and bone mineral content(15).
Calcium and protein work together, constructively, to affect bone health and intakes of both must be adequate for the best results. The concerns about protein’s impact on acid production appear to be very minor, compared to the excellent alkalinising effect of fruits and vegetables. Therefore, it appears that it would be more beneficial for practitioners to recommend that their clients increase their fruit and vegetable intake rather than reducing their protein intake(16).
Approximately a quarter of the protein in our body is collagen, a major structural protein found in skin, bones, cartilage and other connective tissues. Collagen is also the key protein that cushions the ends of bones so that our joints move smoothly and comfortably.
Collagen is composed of three chains wound together in a tight triple helix. A special amino acid sequence makes the tight collagen triple helix particularly stable: every third amino acid is a glycine and many of the remaining amino acids are proline or hydroxyproline.
Studies show that collagen, which is found in high quantities in joint tissues, is essential for healthy joint metabolism. Maintaining normal collagen integrity in bone, cartilage, tendons, ligaments and joint tissues is vital for joint homeostasis. Research has shown that collagen supplements that are generally well tolerated and digested help the formation of new cartilage, repair existing cartilage and can help to reduce pain and increase mobility in patients with osteoarthritis. Collagen has also been shown to have a positive effect on chondrocyte metabolism and the stability of cartilage matrix(17).
Debbie Paddington has been a practising Nutritional Therapist for the last nine years. As well as seeing clients for one-to-one consultations Debbie also writes for various health publications and is a member of Higher Nature’s busy Nutrition department.
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17. Collagen Hydrolysate and its Relationship to Joint Health www.aminoinfusion.com/pdf/Collagen%20report.pdf