Chemical score
This compares the amount of each indispens- able amino acid in the test protein with the amount of this amino acid in the reference pro- tein; the chemical score is the value of this ratio for the limiting amino acid:
Amount of amino acid in Chemical test protein (mg/g)
100 score Amount of amino acid in
reference protein (mg/g)
The reference scoring pattern for the most frequently limiting amino acids was developed by the FAO/WHO/UNU (1985):
The calculation does not take into account the digestibility of the protein and is, therefore, a very theoretical value. Digestibility is particu- larly an issue for many plant-based diets and some cereals, for example, millet and sorghum, and needs to be considered especially for chil- dren in developing countries.
Biological value
The biological value (BV) of a protein is a measure of how effectively a protein can meet the body’s biological need. To make this measurement, the test protein is fed to an experimental animal as the sole source of protein, and the nitrogen retention and loss are measured. The greater the nitrogen retention, the more of the protein has been used. (Remember that, if a protein cannot be used because it contains limiting amino acids, it cannot be stored and, therefore, is broken down and the nitrogen excreted as urea.)
or more precisely:
For egg protein, BV is 100; and for fish and beef the value is 75. It is generally agreed that a BV of 70 or more can support growth, as long as energy intakes are adequate.
For both BV and chemical score, the result for a single food is of relatively little relevance
Amino acid Reference score
(mg/g protein) Leucine 19 Lycine 16 Threonine 9 Valine 13 Methionine cystine 17
The use of complementary foods to make up for limiting amino acids in some plant foods
TABLE 4.4
Plant food Limiting amino acid Useful complementary food Example of meal Grains (or cereals) Lysine, threonine Legumes/pulses Beans on toast
Nuts and seeds Lysine Legumes/pulses Hummus (chickpeas
with sesame seeds) Soya beans and other Methionine Grains; nuts and seeds Lentil curry and rice
legumes/pulses
Maize Tryptophan, lysine Legumes Tortillas and beans
Vegetables Methionine Grains; nuts and seeds Vegetable and nut roast
BVNitrogen retained 100 Nitrogen absorbed
BV
Dietary nitrogen (urinary nitrogen faecal nitrogen) 100 Dietary nitrogen faecal nitrogen
because most people consume a mixture of foods in their daily diet.
Millward (1999) reports that ongoing research into the adequacy of protein intakes in diets around the world has shown that, even with revised amino acid scoring methods, the prob- lem of inadequate protein or severely limiting amino acids is not as widespread as commonly assumed.
Protein requirements
Requirement figures for protein are calculated on the basis of nitrogen balance studies, which estimate the amount of high-quality milk or egg protein needed to achieve equilibrium. The safe level of protein intake was established by FAO/WHO/UNU (1985) as 0.75 g/kg body weight per day. In addition to nitrogen balance results, increments were included for growth in infants and children, calculated from estimates of nitro- gen accretion. In pregnancy, protein retention in the products of conception and maternal tissues was calculated and, for lactation, the protein content of breast milk in healthy mothers was used to obtain the reference value.
Uncertainty is expressed about the accuracy of these balance studies because they give results that are considerably higher than minimum nitrogen losses in adults on protein-free diets. Also the duration of the studies may not be suffi- cient for adaptation to occur. Finally, it is unclear how the amount of energy given to the subjects affects the results.
Figures recommended in the UK (DoH, 1991) for adults are calculated on the basis of 0.75 g protein/kg body weight per day. Values obtained using reference body weights for adults are shown in Table 4.5. Current intakes in the UK are considerably higher than the values recom- mended here. The National Food Survey (DEFRA 2001) shows that the mean daily total protein intake was 67 g, of which 41.1 g was of animal origin. This intake represents 147 per cent of the mean RNI for protein and, even in the largest households, the intake is well above the mean RNI at 117 per cent.
Major food groups providing protein were shown to be:
Milk products and cheese 21.3 per cent of total Meat and meat products 32 per cent Cereals and cereal products 26 per cent
It is assumed that, in the UK, there is a suffi- cient variety of different protein sources to elim- inate concerns about protein quality. However, for those individuals whose diet contains a con- siderable amount of unrefined cereal and vege- table, a correction for digestibility of 85 per cent is to be applied.
Report 41 (DoH, 1991) suggests that it is pru- dent to avoid protein intakes that are in excess of an ‘upper safe limit’ of 1.5 g/kg per day, suggest- ing that such high intakes may contribute to bone demineralization and a decline in kidney function with age. It has been found that there is a linear relationship between increases in animal protein intake and calcium loss in the urine, although the relationship with bone demineral- ization is still unclear. More recent evidence has largely failed to support concerns about effects of high protein intakes on kidney function, unless there is pre-existing renal disease. A number of cross-sectional studies in the USA and Britain have shown an inverse relationship between protein intakes and blood pressure and stroke. However, the possible mechanisms involved are
Dietary reference values for protein for adults
TABLE 4.5
Gender/age Estimated Reference
average nutrient requirement intake (g/day) (g/day) Males 19–50 years 44.4 55.5 50 years 42.6 53.3 Females 19–50 years 36.0 45.0 50 years 37.2 46.5
From DoH, 1991. Crown copyright is reproduced with the permission of Her Majesty’s Stationery Office.
PROTEIN DEFICIENCY ❚
yet to be discovered, and intervention studies have been unable to replicate these effects.
Finally, it should be noted that those individ- uals with high energy needs, such as athletes, con- suming a typical Western diet, are likely to ingest protein in excess of this ‘upper safe limit’, unless they make adjustments to the balance of macro- nutrients in their diet. This should preferably be achieved through consuming more carbohydrate, rather than more fat, but may be impractical in terms of the volume of food required.
Many questions remain to be resolved and new issues are continually being raised in this field. Some of these are briefly discussed below.
■ Newer research on individual amino acids suggests that requirement figures for indis- pensable amino acids may need to be increased and, therefore, the safe level of pro- tein intake may need to be revised upwards. Given the prevalence of undernutrition in the world, such a revision would have enor- mous implications in terms of global food policies.
■ The distinction between essential and non- essential amino acids may become less clear- cut, as studies have shown that there is some potential for amino acid synthesis from urea residues, by the colonic bacteria. Factors that might influence this synthesis and availabil- ity to the host may need to be considered in the future.
■ It is already clear that needs for specific amino acids vary between individuals and at
different times of life and conditions. The ability to cope with these life situations may also depend on optimal amino acid avail- ability and the presence of other nutrients.
Protein deficiency
Insufficient protein intake is a problem for many people of the world, especially the poor in many countries. It is rarely their only nutritional problem; the diet is likely to be low in energy and fat, and may contain marginal amounts of many nutrients. In addition, there are likely to be social, economic and environmental problems, which increase the likelihood of infection and reduce the availability of health care. Low levels of educational achievement are also likely to be found in these societies owing to a lack of opportunity.
Children are most likely to suffer from pro- tein deficiency in a complex picture of protein– energy malnutrition, which can take a number of different forms. Classically, the two main forms are marasmus and kwashiorkor; there is consid- erable debate as to whether these are separate conditions or two ends of the spectrum of the same condition. They have been seen to occur in the same village and even in the same child at different times, suggesting a common cause. The child exhibits growth failure, in particular, a slowing of linear growth, resulting in stunting (Figure 4.6). Usually the child is miserable and
irritable. There is likely to be liver enlargement, possibly oedema, changes to the hair and skin; the eyes may be sunken and also show signs of vitamin A deficiency. Susceptibility to infection is increased, and the coexistence of infection and malnutrition may precipitate death.
The exact causes of this clinical picture are unclear. Low protein intake can result in many of the signs, with low albumin levels resulting in oedema. It is possible that an imbalance of amino acids may be responsible, as the syndrome does not occur in wheat-growing areas, but is com- mon where cassava, yams, maize and plantain are the staple. Most recently, it has been sug- gested that food contaminated with moulds may be responsible, or that a lack of antioxidants pre- vents the body coping appropriately with the free radicals produced by toxins or infections.
Treatment involves restoring the nutritional status of the body, while treating infections, elec- trolyte imbalances and hypothermia, all of which may be present in a sick child with protein– energy malnutrition. In addition, the whole fam- ily may need to be educated about nutrition and health to provide long-term improvement and to prevent the condition recurring.
Protein deficiency as part of more general- ized malnutrition also occurs in the community and particularly in hospitalized patients in Britain. When pre-existing illness, poor appetite, surgical or medical treatment and prolonged hospitalization coincide, there is the likelihood that insufficient nutrients will be consumed. Thus, although the process may not necessarily start as protein deficiency, if food intakes are minimal, protein catabolism will quickly follow.
In addition, the catabolic response to trauma also increases protein breakdown, contributing to the negative nitrogen balance. Particularly vulnerable are overweight patients, in whom adiposity masks muscle wasting. Negative nitrogen balance may persist for some time before action is taken. Reports published during the last 20 years have indicated a prevalence of malnutrition in hospital patients ranging from 20 to 50 per cent in different studies. Several policy documents have been produced during this time and nutrition teams established in many hospitals.
It is recognized that undernutrition has major implications on the clinical outcome for the patient. These include:
■ increased post-operative complications and poor wound healing;
■ increased risk of pressure sores;
■ poor immune response and increased sus- ceptibility to hospital infections;
■ reduced muscle strength, weakness, immo- bility, inability to cough;
■ apathy and depression;
■ reduced quality of life;
■ prolonged hospital stay;
■ increased mortality.
In addition to these, there are increased eco- nomic implications for the hospital.
The causes of undernutrition may be disease- related or arise for social or psychological rea- sons. In addition, they may have their origins in hospital services, routines and procedures. The latter may include the food provided, the timing of meals in relation to other procedures, and the facilities and help available for eating. The dietetic and catering services can address some of these, others need to be addressed at the ward level. Protocols should be in place to identify and man- age patients at risk of undernutrition. The Better Hospital Food project (DoH, 2000), launched in 2001 in the UK aims to improve meal provision, through greater choice and flexibility so that food can be provided to suit patient need and prefer- ences, and ensure adequate nutrition.
It is important, therefore, that patients are weighed regularly and that assessments of nutri- tional status are made, such as grip strength, mid-arm muscle circumference or plasma albu- min levels. Suitable provision and help with oral consumption of foods or supplements is needed and more vigorous nutritional support via other routes when necessary. Careful monitoring of at-risk hospital patients is necessary, with all of the medical team needing an awareness of the potential problem. Increasing knowledge and awareness of the importance of nutrition as treatment among doctors is seen as a corner- stone for any improvements. This has been addressed by the Royal College of Physicians (2002), but will take time to come into effect at ward level.
REFERENCES AND FURTHER READING ❚
References and further reading
DEFRA (UK Department for Environment, Food and Rural Affairs) 2001: National food survey 2000.
Annual report on food, expenditure, consumption and nutrient intakes. London: The Stationery
Office.
DoH (UK Department of Health) 1991: Dietary reference values for food energy and nutrients in the
United Kingdom. Report on Health and Social Subjects No. 41. Report of the Panel on Dietary
Reference Values of the Committee on Medical Aspects of Food Policy. London: HMSO.
DoH (UK Department of Health) 2000: The NHS plan: a plan for improvement, a plan for reform. London: Department of Health.
FAO/WHO/UNU 1985: Energy and protein requirements. Report of a joint FAO/WHO/UNU Expert Consultation. WHO Technical Report No. 724. Geneva: WHO.
Food Standards Agency 2002a: Food portion sizes, 3rd edn. London: The Stationery Office.
Food Standards Agency 2002b: McCance and Widdowson’s The composition of foods, 6th summary edn. Cambridge: Royal Society of Chemistry.
Jackson, A.A. 2003: Human protein requirement: policy issues. Proceedings of the Nutrition Society
60, 7–11.
Lennard Jones, J.E. 1992: A positive approach to nutrition as treatment. London: Kings Fund Centre. 1 Draw a flow diagram to show the movement of
amino acids within the amino acid pool when the body has adequate supplies of protein. Show how this changes when protein is in short supply.
2 Explain why a protein deficiency might result in:
a oedema
b an increased susceptibility to infection.
3 It has been suggested that Western diets contain excessive amounts of protein.
a Keep a record for 1 week of how many times you eat protein-rich food. Use the information in Table 4.2 to help you.
b Are your sources of protein mostly from animal or plant foods, or a combination of both of these?
c Can you identify combinations of different protein sources in your meals, such as those discussed in the chapter?
d Compare your results with those of others in your group. What do you find?
STUDY QUESTIONS
1 Proteins are composed of combinations of amino acids, creating an enormous diversity of proteins.
2 Twenty different amino acids occur in proteins. The body uses these very efficiently, and is able to convert some of the amino acids into others. However, eight of them cannot be made in the body, and must be provided in the diet; they are the indispensable amino acids. At certain times, for example, in young children, or in stress and trauma, other amino acids may become indispensable.
3 Proteins fulfil a great many functions in the body, acting as hormones, enzymes, carriers and maintaining homeostasis.
4 Dietary sources of protein may be of animal or plant origin. The ability of the body to make full use of the amino acids supplied depends on the energy intake and the pattern of the amino acids in the protein. An inadequate amount of one amino acid may limit the usefulness of the whole protein, unless it is combined with a complementary source, which provides the limiting amino acid.
5 Protein requirements are based currently on nitrogen balance studies.
6 Intakes of protein in the UK are above the reference nutrient intake (RNI) in healthy adults. The hospital patient may, however, be at risk of protein deficiency, which may compromise recovery.
McWhirter, J.P., Pennington, C.R. 1994: Incidence and recognition of malnutrition in hospital.
British Medical Journal 308, 945–8.
Millward, D.J. 1999: The nutritional value of plant-based diets in relation to human amino acid and protein requirements. Proceedings of the Nutrition Society 58, 249–60.
Royal College of Physicians 2002: Nutrition and patients, A doctor’s responsibility. London: Royal College of Physicians.
Scrimshaw, N.S., Waterlow, J.C., Schurch, B. (eds) 1996: Protein and energy requirements sympo- sium. European Journal of Clinical Nutrition 50 (suppl. 1).
Waterlow, J.C. 1995: Whole body protein turnover in humans – past, present and future. Annual
fats
The aims of this chapter are to:
❏ describe the nature and characteristics of fats important in human nutrition;
❏ explain the importance of the essential fatty acids;
❏ discuss the role of fat in the diet and trends in fat consumption;
❏ study the transport of fats in lipoproteins;
❏ discuss the role of fat in the body;
❏ discuss the part played by adipose tissue in metabolism.
On completing the study of this chapter you should be able to:
❏ discuss the nature of various fats in the diet and the nutritional importance of the different types;
❏ discuss the advantages and disadvantages of fat in the diet;
❏ describe the current levels of fat intake in the UK;
❏ discuss the importance of omega-3 fatty acids in the body;
❏ explain the role of adipose tissue;
❏ understand the general importance of fat in the body and its role in health.
All living cells contain some fat in their structure, since fatty acids are essential components of cell walls and intracellular membranes. In addition, mammals and birds store fat throughout the body, especially between the muscles, around internal organs and under the skin. Many fish have fat stored exclusively in the liver, but in the oily fish (like herring and mackerel) it is present through- out the flesh. In the plant kingdom, fats are found in the fruits of various plants such as olives, maize, nuts and avocados. Plants manufacture fats by photosynthesis, the same process that they use to make carbohydrates. Animals use or store the fat they ingest, or can synthesize fat from sur- plus energy taken in as carbohydrates or proteins. This does not generally apply in humans under normal circumstances. Advice on healthy eating encourages us to reduce our intake of certain
types of fats and increase others. Names such as cholesterol, polyunsaturates, omega-3s are used by food manufacturers and can be very confusing for many people. To be able to understand the rationale and the details of this advice, it is neces- sary first to understand the nature of fats, and how this is related to their behaviour in the body. Only then can we interpret the advice both for ourselves and others.
What are fats?
Fats are substances that are insoluble in water, but soluble in organic solvents like acetone. In addition, fats are greasy in texture and are non-volatile. When we think about fats in the diet, most people make a distinction between
C H A P T E R
fats, which are solid at room temperature, and