AQA specification - Sources, functions, effects of deficiency, effects of excess, complementation, biological values.

Info from BBC/Wikipedia mainly.

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  • Created by: Katherine
  • Created on: 03-04-10 21:50


  • Meat, poultry, fish, shellfish and eggs
  • Pulses, nuts and seeds
  • Soya products and vegetable protein foods
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Protein is a major functional and structural component of all our cells.

Protein provides the body with roughly 10 to 15 per cent of its dietary energy.

Is needed for growth and repair.

Proteins are large molecules made up of long chains of amino acid subunits.

Some of these amino acids are nutritionally essential as they cannot be made or stored within the body and so must come from foods in our daily diet.

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How much protein should we eat?

Health professionals suggest men should eat 55.5g protein a day and women 45g.

In practical terms, eating a moderate amount of protein - in one or two meals every day – should give you all the protein you need.

Most people in the UK eat far more protein than they actually need.

You should eat two to three servings of protein every day from both plant and animal sources. Here are some examples of one serving (about the size of a standard pack of playing cards):

  • 100g boneless meat (eg lean beef, lamb or pork)
  • 100g boneless poultry (eg chicken or turkey breast)
  • 100g fish (eg salmon, sardines or tuna)
  • 2 medium eggs
  • 3 tablespoons of seeds (eg sunflower or pumpkin seeds)
  • 3 tablespoons of nuts (eg almonds or walnuts)
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Effects of deficiency

Protein deficiency is a serious cause of ill health and death in developing countries.

Protein deficiency plays a part in the disease kwashiorkor.

Symptoms of kwashiorkor include apathy, diarrhoea, inactivity, failure to grow, flaky skin, fatty liver, and edema of the belly and legs.

War, famine, overpopulation and other factors can increase rates of malnutrition and protein deficiency. Protein deficiency can lead to reduced intelligence or mental retardation

Protein deficiency is relatively rare in developed countries but some people have difficulty getting sufficient protein due to poverty. Protein deficiency can also occur in developed countries in people who are dieting or crash dieting to lose weight, or in older adults, who may have a poor diet.

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Effects of excess

The body is unable to store excess protein.

Protein is digested into amino acids which enter the bloodstream.

Excess amino acids are converted to other usable molecules by the liver in a process called deamination.

Deamination converts nitrogen from the amino acid into ammonia which is converted by the liver into urea in the urea cycle.

Excretion of urea is performed by the kidneys.

These organs can normally cope with any extra workload but if kidney disease occurs, a decrease in protein will often be prescribed.

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Protein complementation

Protein complementation is combining plant protein sources to achieve a better amino acid balance than either would have alone.

Because of differences in amino acid make-up, when plant sources are combined, the strengths of one make up for the deficiencies in another.

For example, many grains are notoriously low in lysine, but beans are high in lysine. On the other hand, beans are low in the sulfur-containing amino acids, while grains like wheat contain much of these.

Thus, by eating beans and grains “together,” the strengths of one make up for the deficiencies of the other, making a source of complete protein.

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Biological Values

How much protein needed in a person's daily diet is determined in large part by overall energy intake, as well as by the body's need for nitrogen and essential amino acids.

Physical activity and exertion as well as enhanced muscular mass increase the need for protein.

Requirements are also greater during childhood for growth and development, during pregnancy or when breast-feeding in order to nourish a baby, or when the body needs to recover from malnutrition or trauma or after an operation.

According to US/Canadian Dietary Reference Intake guidelines, women aged 19–70 need to consume 46 grams of protein per day, while men aged 19–70 need to consume 56 grams of protein per day to avoid a deficiency. U.S recommended daily protein dietary allowance, measured as intake per body weight, is 0.8 g/kg. However, this recommendation is based on structural requirements, but disregards use of protein for energy metabolism.

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