Proteins in nutrition
For every physical activity, the body requires energy and the amount depends on the duration and type of activity. Energy is measured in Calories and is obtained from the body stores or the food we eat. Glycogen is the main source of fuel used by the muscles to enable you to undertake both aerobic and anaerobic exercise. If you train with low glycogen stores, you will feel constantly tired, training performance will be lower and you will be more prone to injury and illness.
Carefully planned nutrition must provide an energy balance and a nutrient balance.
The nutrients are:
Proteins– essential to growth and repair of muscle and other body tissues
Fats– one source of energy and important in relation to fat soluble vitamins
Carbohydrates– our main source of energy
Minerals– those inorganic elements occurring in the body and which are critical to its normal functions
Vitamins– water and fat soluble vitamins play important roles in many chemical processes in the body
Water– essential to normal body function – as a vehicle for carrying other nutrients and because 60% of the human body is water
Roughage– the fibrous indigestible portion of our diet essential to health of the digestive system
One gram of protein or carbohydrate contains 4 calories, while one gram of fat has 9 calories.
Protein Quality: Nutritive Value
The quality of protein depends on the level at which it provides the nutritional amounts of essential amino acids needed for overall body health, maintenance, and growth. Animal proteins, such as eggs, cheese, milk, meat, and fish, are consideredhigh-quality,orcomplete, proteinsbecause they provide sufficient amounts of the essential amino acids. Plant proteins, such as grain, corn, nuts, vegetables and fruits, arelower-quality,orincomplete, proteinsbecause many plant proteins lack one or more of the essential amino acids, or because they lack a proper balance of amino acids. Incomplete proteins can, however, be combined to provide all the essential amino acids, though combinations of incomplete proteins must be consumed at the same time, or within a short period of time (within four hours), to obtain the maximum nutritive value from the amino acids. Such combination diets generally yield a high-quality protein meal, providing sufficient amounts and proper balance of the essential amino acids needed by the body to function.
The function of proteins
In virtually every biological process proteins are playing a role. Some of the main functions of proteins in the human body are to:
Build, strengthen and repair/replace things, such as tissue. Examples include keratin (strengthens protective coverings, such as hair), collagen and elastin (both provide support for connective tissue).
Make antibodies for our immune system
Make hormones, which help cells send messages and coordinate bodily activities
Muscle contractions – actin and myosin, two types of proteins, are involved in muscle contraction and movement.
Make enzymes. An enzyme facilitates a biochemical reaction.
Carry things – hemoglobin, a protein, transports oxygen through the blood.
Mediate cell response – rhodopsin is a protein in the eye which is used for vision
Store things – ferritin is a protein which stores iron in the liver
Recent developments on protein function.
High protein diets may help promote weight loss. The International Food Council Foundation found that a high percentage of women who eat more protein do not only avoid weight gain, but also report weight loss.
A high protein diet may be good for bone health. An investigation published in theJournal of Clinical Endocrinology and Metabolism, revealed that a calorie controlled diet lower in carbohydrates and higher in protein along with daily exercise has a significantly positive impact on bone health in overweight individuals as well as obese young women.
Protein is not a significant source of energy for the body when there are sufficient amounts of carbohydrates and fats available, nor is protein a storable energy, as in the case of fats and carbohydrates. However, if insufficient amounts of carbohydrates and fats are ingested, protein is used for energy needs of the body. The use of protein for energy is not necessarily economical for the body, because tissue maintenance, growth, and repair are compromised to meet energy needs. If taken in excess, protein can be converted into body fat. Protein yields as much usable energy as carbohydrates, which is 4 kcal/gm (kilocalories per gram). Although not the main source of usable energy, protein provides the essential amino acids that are needed for adenine, the nitrogenous base of ATP, as well as other nitrogenous substances, such as creatine phosphate (nitrogen is an essential element for important compounds in the body).
Dietary requirements of protein
Nobody seems to agree on how much protein we can eat; experts from industry, government agencies, diet companies and nutritional organizations have a varying list of assertions.
An individual’s daily protein requirement depends on several factors, including:
Age – a growing child’s needs will not be the same as an individual aged 80 years
Sex – males generally require more protein than (non-pregnant or non-breastfeeding) females
Weight – an individual who weighs 200lbs will require more protein compared to somebody who weighs 120lbs. In fact, recent studies indicate that weight matters more than age when determining dietary protein requirements.
Muscular exertion – an individual who earns his living delivering pianos will require more protein than a computer programmer of the same age and height
Muscle mass – a muscle-bound weight trainer will need more dietary protein than a marathon runner
Health – a person who is convalescing after an illness or medical procedure may need more dietary protein than other people
Protein Requirement and Nutrition
The recommended protein intake for an average adult is generally based on body size: 0.8 grams per kilogram of body weight is the generally recommended daily intake. The recommended daily allowances of protein do not vary in times of strenuous activities or exercise, or with progressing age. However, there is a wide range of protein intake which people can consume according to their period of development. For example, the recommended allowance for an infant up to six months of age, who is undergoing a period of rapid tissue growth, is 2.2 grams per kilogram. For children ages seven through ten, the recommended daily allowance is around 36 total grams, depending on body weight. Pregnant women need to consume an additional 30 grams of protein above the average adult intake for the nourishment of the developing fetus.
Percentage of energy that should come from protein:
Infants (7 – 12 mo) – 11 grams per day
Infants (0 – 6 mo) – 9.1 grams per day
Teenage boys (14 – 18 y) – 52 grams per day
Teenage girls (14 – 18 y) – up to 46 grams per day
Adult men – approximately 56 grams per day
Adult women – approximately 46 grams per day
Pregnant or lactating (breastfeeding) women – about 71 grams per day
According to the U.S. Centers for Disease Control and Prevention protein intake should be:
Children ages 1 to 3 – 13 grams per day
Children ages 4 to 8 – 19 grams per day
Children ages 9 to 13 – 34 grams per day
Girls ages 14 to 18 – 46 grams per day
Boys ages 14 to 18 – 52 grams per day
Women ages 19 to 70+ – 46 grams per day
Men ages 19 to 70+ – 56 grams per day
In some developing countries protein deficiency is a major cause of illness and premature death. Protein deficiency can lead to mental retardation and reduced IQ,
In most parts of the world where protein deficiency is common, total food energy consumption is also too low – i.e. people are not getting enough food in general. Protein deficiency can lead to:
Wasting and shrinkage of muscle tissue
Weaker immune system, leading to a higher susceptibility to infections and diseases
In several countries where protein deficiency is a serious problem, the leaves and other parts of the Moringa tree can help provide dietary protein.
In developed countries, especially Western Europe where the dietary requirements of poorer people are very carefully monitored and resolved, protein deficiency is quite rare. In developed nations, protein deficiency is more likely to occur among people on crash diets, or among very elderly individuals who do not eat properly.
Sources of dietary protein
Fish and fish eggs
Seeds and nuts
Grains, vegetables and legumes also have protein (less per kilo of total weight)
Proteins are large molecules made up of long chains of amino acids. Amino acids are the building blocks of proteins. The biochemical activity of proteins is characterised by their individual structure, size and shape. These factors are determined by the sequence and characteristics of the constituent amino acids.
There are about 20 different amino acids commonly found in plant and animal proteins. For adults, 8 of these, have to be provided in the diet and are therefore defined as ‘essential’ or ‘indispensable’ amino acids. These are:
In children, arginine, histidine, cysteine, glycine, tyrosine, glutamine and proline are also considered to be essential (indispensable) amino acids, because children are unable to make enough to meet their needs. These are referred to as ‘conditionally’ essential. There may also be certain disease states during adult life when a particular amino acid becomes conditionally essential.
The other amino acids do not have to be provided by the diet. This is because the amino group of these amino acids can be transferred to another amino acid with a different amino group by a process called transamination. In this way the body is able to make some amino acids itself. These are known as ‘non-essential’ or ‘dispensable’ amino acids.
How much protein should we eat?
The Dietary Reference Values for protein are based on estimates of need. For adults, an average requirement of 0.6g of protein per kilogram bodyweight per day is estimated. The Reference Nutrient Intake (RNI) is set at 0.75g of protein per kilogram bodyweight per day in adults. This equates to approximately 56g/day and 45g/day for men and women aged 19-50 years respectively. There is an extra requirement for growth in infants and children and for pregnant and breast feeding women.
Any excess protein can be used to provide energy. 1g of protein provides 17kJ (4 kcal) but carbohydrate, and to a lesser extent fat, should be the main sources of dietary energy. At present, protein provides around 16% of energy on average in the British diet.
The nature of protein in the diet
Most foods contain either animal or plant cells and will therefore naturally contain protein. But the processing of foods may change the amounts and relative proportions of some amino acids; for example the Maillard reaction and the associated browning that occurs when foods are baked reduces the available lysine. The quality of the protein is also important and depends on the amino acids that are present. Proteins from animal sources have a higher biological value than proteins from plant sources. This is because the pattern of amino acids in animal cells is comparable to the pattern in human cells. Plant foods may have very different patterns of amino acids compared to animal proteins, and, in the past, this difference has lead to a concept of first-class and second-class proteins, for animal and plant foods respectively. However, diets are typically varied in the UK and rarely made up of single foods. A combination of plant proteins tends to have a complementary effect boosting their overall biological value.
Complementary action of proteins (plant protein)
In most diets, different proteins tend to complement each other in their amino acid pattern, so when two foods providing vegetable protein are eaten at a meal, such as a cereal (e.g. bread) andpulses (e.g. baked beans), the amino acids of one protein may compensate for the limitations of the other, resulting in a combination of higher biological value. This is known as the complementary action of proteins. Thus if vegetarians and vegans eat a variety of vegetable proteins in combination, there is no reason why the quality of protein cannot be as good as in a diet comprising meat, milk, fish, eggs or other foods that contain animal protein. Good sources of plant protein include nuts, seeds, pulses, mycoprotein and soya products. There are also small amounts in grains.
Protein from animal sources contains the full range of essential amino acids required from an adult’s diet. Sources include meat, fish, eggs, milk and cheese. For most of us, low fat options of these foods are preferable as some can be high in saturated fat.
Good sources of protein
Table 1: Protein content of some common foods found in the diet
Protein content (g) per 100g
Chicken breast (grilled without skin) Beef steak (lean grilled) Lamb chop (lean grilled) Pork chop (lean grilled)
32.0 31.0 29.2 31.6
Tuna (canned in brine) Mackerel (grilled) Salmon (grilled) Cod (grilled)
23.5 20.8 24.2 20.8
Prawns Mussels Crabsticks
22.6 16.7 10.0
Whole milk Semi-skimmed milk Skimmed milk Cheddar cheese Half-fat cheddar Cottage cheese Whole milk yogurt Low fat yogurt (plain)
3.3 3.4 3.4 25.4 32.7 12.6 5.7 4.8
Red lentils Chickpeas
Kidney beans Baked beans Tofu (soya bean steamed)
6.9 5.2 8.1
Wheat flour (brown) Bread (brown) Bread (white) Rice (easy cook boiled) Oatmeal Pasta (fresh cooked)
12.6 7.9 7.9 2.6 11.2 6.6
Almonds Walnuts Hazelnuts
21.1 14.7 14.1
Adults and children should consume two to three servings of protein every day. If plant sources dominate, it is important to make sure that different types are consumed.
One typical portion size equates to:
• 100g of lean boneless meat (red and poultry) • 140g of fish • 2 medium eggs • 3 tablespoons of seeds or nuts.
It is important to choose lower fat protein-rich foods, such as lean meats or reduced fat dairy productsas some high protein foods can also be high in saturated fat. This will help minimise the risk of developing cardiovascular disease.
Summary and recommendations
For basic protein synthesis, you don’t need to consume more than 1.4 to 2.0 g/kg (around 0.64-0.9 g/lb) of protein per day.
Nevertheless, consuming higher levels of protein (upwards of 1g per pound of body weight) may help you feel satisfied after eating as well as maintain a healthy body composition and good immune function.
You should consume some protein before and after training to ensure adequate recovery.
Do not eat more than four eggs per week. Although they are a good source of protein and are low in saturated fat, eggs are very high in cholesterol.