Proteins are formed from amino acids that link together to form chains.
Chains containing 2 to 10 amino are known as peptides (or oligopeptides) while longer, linear chains are known as polypeptides.
Most polypeptides within the body contain between 50 and 2000 amino acids and fold into complex three-dimensional shapes which are commonly referred to as proteins.
Proteins are an important dietary source of energy and act as building blocks to make body proteins.
Dietary proteins, from plant and animal based foods, are digested down into their basic building blocks before absorption.
The digestion of dietary proteins starts in the stomach when an enzyme, pepsin, dissolves some of the protein bonds to produce short, peptide chains. When these reach the small intestines, they are further digested by pancreatic and intestinal enzymes (eg trypsin, chymotrypsin, elastase and exopeptidases) to produce chains of two or three amino acids (dipeptides and tripeptides). These are absorbed by cells in the gut wall and separated into single amino acids which are released into the circulation.
Absorbed amino acids building blocks are then used by your cells to make all the different proteins – over 30,000 – that are found in your body, such as:
- Blood clotting factors
- Collagen and elastin in your skin
- Muscle fibres
- Cell receptors
- Transport proteins
- Metabolic enzymes
Altogether, over half the dry weight of your body consists of proteins, and these are broken down and regenerated, at rate of around 80-100g/day. Most of this activity occurs during sleep under the control of growth hormone. Most muscle protein is renewed every six months and 98% of your total body proteins are renewed within one year.
The essential and non-essential amino acids
Twenty one amino acids are important for human health. Twelve of these are synthesised by your own cells, from other building blocks, when needed. The remaining nine cannot be made in sufficient amounts and must therefore come from your diet. These are known as the nutritionally essential amino acids.
|Essential Amino Acids||Amino Acids you can make|
|Methionine||Cysteine (synthesised from methionine which is essential)|
|Tyrosine (synthesised from phenylalanine, which is essential)|
Dietary sources of protein
Animal sources of protein, such as meat, fish, eggs and dairy products, including whey, contain significant quantities of all the essential amino acids including leucine, isoleucine and valine which are important for muscle repair.
Most plant sources of protein (eg rice, beans, peas, nuts, seeds) each contain some, but not all, of the essential amino acids. If you follow a plant-based diet, you therefore need to eat a variety of foods, including beans, soy products such as tofu, nuts and seeds, plus rice and quinoa to ensure a balanced intake of essential amino acids.
Your body cannot store excess amino acids in their original form. When your diet is rich in protein, excess that is not needed for immediate growth or repair of body tissues is used directly as a fuel for energy, or converted into glucose for use as a fuel or, if energy is plentiful, converted into glycogen or triglycerides (body fat) for storage.
Energy from protein
As each of the 21 amino acids has a different chemical structure, 21 different metabolic pathways are needed to process all the amino acids. This means that proteins are an inefficient source of energy. The metabolism of protein also produces a toxic by-product, ammonia, which is converted into urea in the liver and transported to the kidneys for excretion via the urine.
When following a balanced diet, these protein pathways account for between ten and fifteen per cent of the energy your body uses, and generates significant amounts of heat – especially in the liver.
When you follow a high protein diet, however, the amount of energy your cells derive from dietary protein increases, and your liver produces more of the enzymes needed to process amino acids, which increases the efficiency of using protein as an energy source.
Thermogenic effect of protein
Most text books state that a gram of protein provides 4 kilocalories energy, the same as a gram of carbohydrate, but less than the 9 kcal energy derived from a gram of fat and the 7 kcals energy derived from a gram of alcohol.
This is true in laboratory tests, in which a single calorie of energy obtained from fat has the same overall value as a single calorie obtained from protein or from carbohydrate. In the body, the different metabolic reactions used to generate and use energy from different dietary sources incurs different costs, however.
Between 25% and 30% of the energy derived from eating protein is lost as heat during its metabolism, compared with 6% to 8% of the energy derived from eating carbohydrates, and 2% to 3% of the energy derived from eating fats.
|If you ate a hypothetical meal providing 2000 kcals of which half came from protein and half from carbohydrate, for example, you would derive around 1830 usable kcals of energy.
If you increased the protein content so that 80% came from protein and only 20% from carbohydrate, you would ‘only’ derive 1750 kcals usable energy – an extra 80 kcals is dissipated as heat.
|The dependence of effective calories on % carbohydrate in a 2000 kcal diet. Effective calories were determined by subtracting the losses due to thermogenesis. Derived from Feinman & Fine, Nutrition Journal 2004 3:9||
This thermogenic effect gives higher protein diets a metabolic advantage when it comes to weight loss. A higher protein diet also has less effects on blood glucose levels, and does not trigger a rise in insulin, which is the main fat-storing hormone in the body.
Eating protein does, however, cause changes in the secretion of intestinal hormones which are associated with satiety, so you feel full more quickly and tend to eat less.
How much protein do you need?
According to the UK Reference Nutrient Intake, an adult needs to eat 0.75g protein for each kilogram of body weight per day. In the US, an intake of 0.8g protein per kilogram of body weight per day is believed to meet the requirements of nearly all (97.5%) of the population.
Someone who weighs 60kg therefore needs to eat around 45g to 48g protein per day, while someone weighing 75kg needs 56g to 60g protein per day. Most people consume around 15% of the daily calories in the form of protein.
How much protein do athletes need?
Official guidelines suggest that no additional dietary protein is needed for healthy adults involved in resistance or endurance exercise. This is controversial, however, and seems to go against common metabolic sense.
If you are highly physically active, and following an intense training or body-building program, you almost certainly need more than the average amount of protein. When muscle cells run out of glucose or fatty acids to burn as fuel during prolonged exercise, they will resort to breaking down and burning body proteins instead, to keep going. This can result in loss of muscle bulk which may be the exact opposite of what you want to achieve.
Athletes seeking to gain muscle mass and strength tend to consume higher amounts of dietary protein than endurance athletes.
An athlete who is training for at least two hours per day may be advised to obtain between 1.2 and 1.8g protein per kilogram body weight per day, depending on the intensity of exercise, to maximise muscle protein synthesis. A typical recommendation is for athletes to obtain 1.5g protein per kilogram of body weight per day – fifty per cent more than for the average active person.
A weight lifter or body builder in training may need as much as 1.8g to 2g protein per kilogram of body weight per day to prevent lean mass loss during periods of energy restriction to promote fat loss before a competition.
One study estimated that when resistance-trained athletes are restricting calories to reduce body fat or make weight, their protein needs may be as high as 2.3g to 3.1g per kilogram body weight per day.
Diet should always come first, so aim to include a 100g to 150g portion (about the size of your fist) of lean, high-protein foods in each meal, such as meat, fish, chicken, eggs, cheese, beans or lentils.
This will allow most normally active people to get sufficient protein in their diet.
Athletes and those taking part in heavy work-outs may benefit from protein supplements as it is impractical to obtain higher amounts of protein from food alone, without the additional fat that accompanies many protein-rich meals. Protein supplements are also most effective when used immediately after training, when muscles are recovering, and you are unlikely to want to eat a full meal soon after heavy exercise. A protein supplement in the form of a shake is then useful option.
Supplements are available in the form of intact proteins, partially digested (hydrolysed) peptides or free amino acids. Most protein shakes contain whey protein, a valuable source of the branched-chain amino acids, leucine, isoleucine and valine, which appear to stimulate muscle protein synthesis and repair – especially leucine.
Some research suggests that the hydrolysates (dipeptides and tripeptides) may be the best options for bulking up muscles as they are most readily absorbed and utilised in the body.
Amino acids are absorbed most effectively if taken between meals – two hours before or after eating. However, it’s usually advised that individual amino acids supplements should not be taken for more than two weeks – switch to a mixed amino acid complex to avoid amino acid imbalances.
This is one area where it’s important to seek individual advice from a sports’ nutritionist to obtain the best protein balance to suit your particular exercise needs.
NB If you have liver or kidney problems do not take protein supplements.
|Optimum Nutrition Gold Standard 100% Whey provides 24g protein per scoop, including 5.5g branched chain amino acids per serving. It available in ten different flavours, including Chocolate Peanut Butter, Double Rich Chocolate (our favourite) and Vanilla Ice Cream. Sizes range from 450g to 4.54kg.
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| SYNTHA-6 Edge contains a blend of protein powders (whey, hydrolysed whey, milk protein concentrate and soy). It provides 24g protein per scoop. Available in 6 flavours, including Chocolate, Salted Caramel and Vanilla Ice Cream. Sizes are 1.32kg and 2.27kg.
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| PHASE 8 is a sustained-release protein formula that provides fast, medium and slow release, milk-derived protein over an 8 hour period. Provides 36g protein per scoop plus digestive enzymes (papain, amylase) for digestibility. Available in six flavours, including Milk Chocolate and White chocolate. Sizes range from 907g to 2Kg.
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| Platinum 100% Casein provides a slow digesting, casein amino acid formula with 24g protein per scoop to prolong amino acid delivery and help prevent muscle breakdown. In three flavours, including Gourmet Milk Chocolate, Strawberry Shortcake and Vanilla Ice Cream. Sizes range from 830g to 1.7kg.
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| MyProtein Impact Whey Protein provides 20g protein per serving, including 2g leucine. Available in over 30 different flavours, including Banoffee, Latte, Raspberry, toffee, and Sticky Toffee Pudding. It comes in 1kg, 2.5kg and 5kg sizes.
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