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Proteins are large polymers that are composed of one or more chains of amino acids Whitney and Rolfes, 2008. There are 30,000 to 50,000 different proteins in the body, each containing between 50 to 1000 AA (Whitney and Rolfes, 2008). Proteins contain differing amounts of various AA therefore proteins are not all nutritionally equivalent (Geissler and Powers, 2005). Indispensable AA cannot be synthesised in the body whereas dispensable AA can be synthesised from metabolic intermediates through protein in the diet (Whitney and Rolfes, 2008). Proteins have several roles in the body including maintaining body protein mass as well as regulation of body composition (bone health, gastrointestinal function, bacterial flora, glucose homeostasis, cell signalling) (Millward, 2008).
Dietary protein/AA requirements are defined in metabolic demands and efficiency of utilization whilst achieving nitrogen (N) equilibrium, however requirements may be greater at times due to factors that influence the efficiency of protein use (digestion, absorption, cellular bioavailability) ((Food and Agriculture Organization, FAO, United Nations University, UNU, World Health Organisation, WHO, 2002). The pathway of AA metabolism and interchange is dependent upon sufficient micronutrient status therefore the amount (quality) of food consumed (FAO, UNU, WHO, 2002). Many factors affect protein/AA requirements (separately and together) including digestibility of dietary proteins (smoking, alcohol, digestibility, pathological conditions, metabolic needs, physiological state, maintenance stage, stages of development, reproductive state, environmental factors, diseases, genotype, catabolic stress, operations, burns, injury, infection, multi organ failure, trauma, renal or liver failure, medications, environmental challenges) (Millward, 2008). Requirements of protein/AA vary during pregnancy or lactation, changes in body composition (age, sex, diet) as well as changes in lifestyle (physical activity) (FAO, UNU, WHO, 2002).
Dietary Reference Values for Protein
Dietary Reference Values (DRV) for protein are based on estimates set for the needs of groups and less so for individuals (reference nutrient intakesÂ for protein of 55.5 g/day for men aged 19-50 years and 53. 3 g/day for those aged 50+ years; and 45.0 g/d for women aged 19-50 years and 46.5 g/d for those aged 50+ years) (Department of Health, DH, 1991). The age range used for adults (18 to 50 years) is a set and rather wide range in comparison to children where somewhat narrower ranges have been adopted (The WHO, 2002). In the UK individual dietary intake of protein is on average around 80g per day (90% is hydrolysed to free AA and small peptides and absorbed whist the remainder is faecal loss equivalent to 10g of protein a day) (Geissler and Powers, 2005). The primary dietary source of high biological value protein is animal protein (vegetable proteins have a low biological value as well as incomplete AA profiles requiring an additional source of other vegetable proteins to maximise the effectiveness and AA profile) (Chernoff, 2004). Activity can increase the demand for protein however individuals with physically active lifestyles have been found to consume more than twice the current safe adult level of protein (WHO, 2002). Very high protein intakes may be toxic (45% of dietary energy as protein) leading to nausea, diarrhoea and possible death (Garlick, 2001).
Methods for determining protein requirements
Protein nutrition/AA metabolism can be measured by the Nitrogen (N) balance method (primary approach) whilst there are several other methods which are also used (plasma AA response, carbon balance method, 24 hour indicator AA, Isotope tracer method, factorial model) however no method is entirely reliable (Rand and Young, 1999). Studies are carried out on healthy adults (assumed to be in energy balance, of an appropriate body composition without appropriate measurements) (FAO, UNU, WHO, 2002). Studies have found that dermal N losses vary with N intake however these losses are often neglected in many published studies (skin loss, nails, urine, faeces, sweat, hair, hot climates - increasing sweating) (Rand and Young, 1999). The lack of accuracy of the N balance method and other methods (lack sensitivity, differences between two large numbers, N-intake, N output) has led to the adoption of the protein digestibility corrected AA score (PDCAAS) approach (FAO, UNU, WHO, 1991). When determining protein/AA requirements it is necessary to adapt over a wide range of intakes (differences in digestibility, other dietary factors, chemical reactions, measurement methods, considering the many diverse roles of indispensable AA, quality and quantity of protein) (Millward, 2008). Estimating protein/AA requirements can be difficult due to individual demands whilst unequivocal indicators of the dietary inadequacy of protein/AA can rarely be identified until gross dysfunctions have developed (FAO, UNU, WHO, 2002).
Studies have found no difference in protein/AA requirements between men and women (between younger and older adults) however results are taken from a small sample (52 subjects) using the N balance method (Campbell et al, 2008). Further studies have ascertained that the metabolic demand for protein/AA is variable between individuals, in the same individual in different times during the day as well as at different stages in life (FAO, UNU, WHO, 2002) High protein diets (in excess of the safe level) have been found to lead to weight loss, (increasing satiety and carbohydrate (CHB) displacement) in the short-term, however long-term data was lacking (He, 1999). Weight loss is possible due to the thermic effect of protein (Westernterp et al, 1999). Contrasting studies found there to be no significant weight loss amongst individuals consuming high protein diets, however these were carried out on small groups or have been inconclusive (Barkeling et al, 1999, Millward, 2008). Large long term studies have found that high dairy consumption has been found to have a positive effect on body weight, (hypertension, insulin resistance syndrome) (Zemel and Miller, 2004)
Consuming high-protein intakes (in excess of the safe level) was found to decrease triglycerides, low-density lipoprotein (LDL) cholesterol, reduce risk of ischaemic heart disease) however studies were carried out over a short period (3 months) and on a small sample group (21 subjects) whilst further studies found no significant differences in blood lipid measures (amongst small samples) (Hu, 2005, Kelemen, 2005). Peptides derived from certain animal proteins have been found to be beneficial (blood pressure regulators, anti-ulcer agents, appetite regulation) therefore proteins should be distinguished for reasons other than their AA contents (FAO, UNU, WHO, 2002). High intakes of animal protein foods (in excess of the safe level) increases the risk of Coronary Heart Disease (CHD) (Long term study on large group) as well as stroke (containing high levels of sodium), (Hu et al 1999, Schluze et al, 2003 Kelemen et al, 2005).Fish (omega 3 fatty acid) reduces risk of CHD (He et al, 2004).
High-protein diets (high omega-6 fatty acids with low omega-3 fatty acids) may have a negative effect on bone quality resulting in ketoacidosis, leading to an imbalance increasing bone turnover (Albertazzi and Coupland, 2002). Further studies have argued that a high protein diet had no effect on bone turnover however these were carried out on a small group of (30) subjects over a short period (3 months) (Carter et al, 2005). During pregnancy increased requirements for protein are based on birth weight, however there is limited information related to the outcome of the health of the baby (short or long term) (FAO, UNU, WHO, 2002). Body-composition measurements do not show any maternal storage in early pregnancy (increasing amounts are recommended for each trimester) (FAO, UNU, WHO, 2002). In pregnancy consuming protein supplements have been found to reduce birth weight however small samples were used (Kramer et al, 1993). Studies found that high protein intakes increase the risk of kidney stones (uric acid, stones, calcium stones) whilst decreased protein consumption slows the progression of renal disease as found in a small sample (12 patients) (Hu, 1999). High intakes of protein have been found to promote anabolic influences on muscle (leucine helps regulation of muscle protein synthesis) however there is a lack of evidence supporting any benefit in terms of athletic performance or physique as found in small sample groups (Millward, 2004)
Older adults may have higher dietary protein requirements due to various changes as found amongst a small sample (19 older adults) (metabolic, physiologic changes, loss of muscle mass, decline in physical activity, physical functional capacity, total food intake) (Campbell et al, 2008). Diseases (trauma, infection) tend to lower the efficiency of nitrogen utilization and retention is more common in the older adults than in the young (Chernoff, 2004) Further studies have proved inconclusive regarding the efficiency of protein utilization in older adults, however all studies were carried out on small samples (Millward et al, 1999, Fereday et al, 1997). Older adults inadequate protein intakes contribute to a decrease in reserve capacity (increased skin fragility, decreased immune function, risk for pressure ulcers, chronic infection, bone fracture, longer recuperation from illness, susceptibility to infectious disease, poor wound healing) (Chernoff, 2004). Adults adapt to lower dietary intakes however older adults adapt by compromising their functional capacity (affecting their immune status whilst loosing muscle mass) (Castaneda et al, 1995). Studies are inconsistent with some reports concluding that safe intakes of protein should not be lower than 0.75 g/kg per day whilst other studies showed that the demand was lower in older adults (Millward et al, 1999).
Recommendations regarding optimal intakes of protein for long-term health as well as defining a safe upper limit is difficult due to a lack of knowledge of the relationship between protein intake and health (FAO, UNU, WHO, 2002). It is important to identify the extent and form of dietary N needed to enable a flow of AA sufficient to maintain health (body weight, nitrogen balance, physiological, metabolic, psychological) (FAO, UNU, WHO, 2002). Dietary protein should provide for maintenance, special needs of growth, reproduction as well as lactation (FAO, UNU, WHO, 2002). Recommendations on protein/AA requirements are essential to set national food and nutrition guidelines worldwide whilst supporting adequacy of population protein/AA intakes therefore further research (larger samples, longer periods) on the relationships between excessive or deficient protein intakes and long-term health outcomes or the occurrence of diseases is required (FAO, UNU, WHO, 2002).