What are Nutrient Reference Values?
In the 1991 Recommended Dietary Intakes (RDI) for use in Australia ( NHMRC 1991, Truswell et al 1990) an RDI value, sometimes presented as a range, was developed for each nutrient. The RDI was defined as: "the levels of intake of essential nutrients considered, in the judgement of the NHMRC, on the basis of available scientific knowledge, to be adequate to meet the known nutritional needs of practically all healthy people…they incorporate generous factors to accommodate variations in absorption and metabolism. They therefore apply to group needs. RDIs exceed the actual nutrient requirements of practically all healthy persons and are not synonymous with requirements."
Despite the emphasis on the population basis of the RDI, the RDIs were often misused in assessing dietary adequacy of individuals, or even foods, not only in Australia and New Zealand but also in many other countries. To overcome this misuse, many countries have moved to a system of reference values that retains the concept of the RDI while attempting to identify the average requirements needed by individuals. In 1991, the UK (Dept Health 1991) became the first country to develop a set of values for each nutrient. More recently, the Food and Nutrition Board: Institute of Medicine (FNB:IOM 1997, 1998a, 2000a, 2001, 2002, 2004) adopted a similar approach on behalf of the US and Canadian Governments.
After due consideration, the Working Party decided to adopt the approach of the US:Canadian Dietary Reference Intakes (DRIs) but vary some of the terminology, notably to retain the term 'Recommended Dietary Intake'.
For each nutrient, an Estimated Average Requirement (EAR) was set from which an RDI could be derived. (Note that the US:Canadian terminology is 'Recommended Dietary Allowance', or 'RDA'). Whilst the various NRVs are expressed on a per day basis, they should apply to intakes assessed over a period of about 3 to 4 days. If the standard deviation (SD) of the EAR is available and the requirement for the nutrient is symmetrically distributed, the RDI is set at 2SD above the EAR. Such that
If data about variability in requirements are insufficient to calculate an SD (which is usually the case), a coefficient of variation (CV) is used. A CV of 10% for the EAR is assumed for nutrients unless available data indicate that greater variation is probable. The 10% is based on extensive data on variation in basal metabolic rate and protein requirements (FAO:WHO:UNA 1985, Garby & Lammert 1984, Elia 1992).
If 10% is assumed to be the CV, then twice that amount added to the EAR is defined as equal to the RDI. Thus for a CV of 10%, the RDI would be 1.2 x EAR; for a CV of 15% it would be 1.3 x EAR and for a CV of 20% it would be 1.4 x EAR.
Where evidence was insufficient or too conflicting to establish an EAR (and thus an RDI) an Adequate Intake (AI) was set, either on experimental evidence or by adopting the most recently available population median intake and assuming that the Australian/New Zealand populations were not deficient for that particular nutrient. Both the RDI and AI can be used as a goal for individual intake, but there is less certainty about the AI value as it depends to a greater degree on judgement. An AI might deviate significantly from and be numerically higher than an RDI if the RDI could be determined. Thus AIs should be interpreted with greater caution.
Where AIs were based on median population intakes, these were derived from a re-analysis of the complete databases of the National Nutrition Surveys of Australia, 1995 (Australian Bureau of Statistics 1998) and New Zealand 1991, 1997, 2002 (LINZ Activity and Health Research Unit 1992, Ministry of Health 1999, 2003) using the appropriate age bands. The two-day adjusted data were used for the estimates.
For infants of 0 to 6 months, all recommendations are in the form of Adequate Intakes based on the composition of breast milk from healthy mothers, using a standard milk volume. The bioavailability of nutrients in formulas may vary from that in breast milk, so formula-fed babies may need higher nutrient intakes. As formulas can vary in the chemical form and source of the nutrients, it is not possible to develop a single reference value for all formula-fed infants.
For energy, an Estimated Energy Requirement (EER) was set for a range of activity levels for individuals of a specified age, gender and body size.
For each nutrient, an Upper Level of Intake (UL) was set, which, unless otherwise stated, includes intake from all sources including foods, nutrients added to foods, pills, capsules or medicines. The UL is the highest average daily nutrient intake level likely to pose no adverse health effects to almost all individuals in the general population. In setting the UL, any adverse health effect were considered, including those on chronic disease status. The UL is not a recommended level of intake. It is based on a risk assessment of nutrients that involves establishment of a No Adverse Effect Level (NOAEL) and/or a Lowest Adverse Effect Level (LOAEL) and application of an Uncertainty Factor (UF) related to the evidence base and severity of potential adverse effects. Members of the general population should be advised not to routinely exceed the UL. Intakes above the UL may be appropriate for some nutrients for investigation in well-controlled clinical trials as long as signed informed consent is given and as long as the trials employ appropriate safety monitoring of trial subjects. Readers are referred to the relevant FNB:IOM documents and the report of the UK Expert Group on Vitamins and Minerals (2003) for more details about the potential toxicological effects of high intakes of nutrients. In Australia, vitamin and mineral supplements are regulated under the Therapeutic Goods Act (1989) that also sets some standards for these products. In New Zealand, dietary supplements are generally regulated under the New Zealand Dietary Supplements Regulations (New Zealand Government 1985), but supplements with nutrients at higher/pharmacological doses than the specified maximum daily doses need to meet the requirements of the Medicines Regulations (1984).
Further details of the approach used in setting ULs are given in the FNB:IOM publication Dietary Reference Intakes. A risk assessment model for establishing upper intake levels for nutrients (1998b) and in the relevant nutrient chapters of the DRI publications.
The uses of the various NRVs are summarised in the table below that was adapted from the FNB:IOM (2000b) publication, Dietary Reference Intakes. Applications in Dietary Assessment. This document also provides further details of potential applications.
|Nutrient Reference Value||For individuals:||For groups:|
|Estimated Average Requirement (EAR)||Use to examine the probability that usual intake is inadequate||Use to estimate the prevalence of inadequate intakes within a group|
|Recommended Dietary Intake (RDI)||Usual intake at or above this level has a low probability of inadequacy||Do not use to assess intakes of groups|
|Adequate Intake (AI)||Usual intake at or above this level has a low probability of inadequacy. When the AI is based on median intakes of healthy populations, this assessment is made with less confidence||Mean usual intake at or above this level implies a low prevalence of inadequate intakes. When the AI is based on median intakes of healthy populations, this assessment is made with less confidence|
|Upper Level of Intake (UL)||Usual intake above this level may place an individual at risk of adverse effects from excessive nutrient intake||Use to estimate the percentage of the population at potential risk of adverse effects from excessive nutrient intake|
In contrast to the US:Canadian approach, the committee agreed to retain the traditional concept of adequate physiological or metabolic function and/or avoidance of deficiency states as the prime reference point for establishing the EAR and RDIs and to deal separately with the issue of chronic disease prevention. It was felt that assessing nutrient needs for chronic disease prevention in a quantitative manner was still problematical. Research findings related to chronic disease prevention often relate to nutrient mixes or food intake patterns, rather than the intake of individual nutrients.
To address the issue of chronic disease prevention, two additional sets of reference values were developed for selected nutrients for which sufficient evidence existed. The set dealing with the macronutrients was adapted from the work of the FNB:IOMDRI review of macronutrients (2002) and is called the Acceptable Macronutrient Distribution Range (AMDR). The second set of reference values was termed Suggested Dietary Targets (SDTs). These related to nutrients for which there was a reasonable body of evidence of a potential chronic disease preventive effect at levels substantially higher than the EAR and RDI or AI. As the evidence base for chronic disease prevention is mainly derived from studies and health outcomes in adults, these AMDRs and SDTs apply only to adults and adolescents of 14 years and over.
The nutrients reviewed
Having considered emerging evidence on the connections between diet and health and the recent recommendations from other countries, the preliminary workshops identified more than 40 nutrients for the Working Party to consider. The document Recommended Dietary Intakes for use in Australia ( NHMRC 1991), which had also been adopted for use in New Zealand, contained recommendations for 19 nutrients and dietary energy. During this review, dietary energy requirements and requirements for the nutrients were considered. Those for which values were set are listed below:
|Macronutrients||Vitamins||Minerals & trace elements|
|Carbohydrate (for infants only)||Niacin||Fluoride|
|Dietary fibre||Vitamin B6||Iodine|
In addition to the nutrients listed above, we also reviewed the literature on total fat (for ages and life stages other than infancy), carbohydrate (for ages and life stages other than infancy), cholesterol, arsenic, boron, nickel, silicon and vanadium. For these nutrients or age bands and life stages, it was agreed that there was little or no evidence for their essentiality in humans. This was generally in line with the findings of the US:Canadian DRI review recommendations. However, the DRI reviews set upper limits for some of these nutrients (FNB:IOM 1998, 2001) and the reader is referred to these for information.
The reviews were based on assessment of the applicability of the recently developed US:Canadian Dietary Reference Intakes (FNB:IOM 1997, 1998a,b, 2000a,b, 2001, 2002, 2004) to Australia and New Zealand, with reference to recommendations from other countries such as the UK (1991, 2003), Germany:Austria:Switzerland (DACH recommendations 2002) and from key organisations such as the FAO:WHO (2001).
Reference body weights
In developing the recommendations it was necessary to standardise body weights for the various age/gender groups. Assessment of the data on measured body weights and heights for relevant age/gender categories from the most recent National Nutrition Survey of Australia, 1995 (ABS 1998) and New Zealand, 1997 and 2002 (MOH 1999, 2003) showed that the body weights were similar to those used in the earlier US:Canadian DRI publications. From the 2002 publication onwards, the US:Canadian DRI review panels changed their standard body weights in response to availability of new data showing markedly lighter body weights than previously used. As the most recent Australian/New Zealand data more closely resembled those in the earlier US:Canadian reports, these were adopted for use throughout these recommendations.
The standard body weights for all adults were based on that for 19–30 year olds, although body weight in most western populations tends to increase throughout adulthood because of increasing body fat.
Reference body weight
Experimental data are often only available for a limited age/gender group. The setting of recommendations for other groups may require extrapolation of the data. This is sometimes based on energy requirements, but more commonly on a metabolic body weight. In extrapolating data from one group to another, the processes and formulae used were those developed by the US:Canadian DRI panels unless otherwise indicated in the text.
Extrapolations from adult Estimated Average Requirements (EAR) to children’s requirements were mostly done using the formula:
|EARchild||=||EARadult x F|
|where F||=||(Weightchild/Weightadult)0.75 x (1 + growth factor).|
The growth factors used were 0.3 from 7 months to 3 years of age and 0.15 for 4–13 years of age for both genders. For boys aged 14–18 years, the growth factor used was 0.15 but for girls of this age, the growth factor was set at zero.
When extrapolating from the Adequate Intake (AI) for younger infants aged 0-6 months, to older infants aged 7-12 months, the formula used was:
|AI7–12 months||=||AI0–6 months x F|
|where F||=||(Weight7–12 months/Weight0–6 months)0.75|
|ULchild||=||ULadult x (Weightchild/Weightadult)0.75|
This allows both body mass and metabolic differences between adults and children to be incorporated as necessary. For more details can be found in the methodology sections of the United States:Canadian FNB:IOM reports.
The implications for adoption of these revised NRVs include:
The need to address ongoing education of both health and food industry professionals in the end use of the various reference values and related tools for their use.
The need to update a number of documents and educational tools based on the previous RDIs, including:
The Australian Guide to Healthy Eating and the Dietary Guidelines for Australian Adults, the Dietary Guidelines for Children and Adolescents in Australia and the Dietary Guidelines for Older Australians
The New Zealand Food and Nutrition Guidelines for the ages and stages of the lifecycle.
In Australia, the Core Food Group analysis addressed the translation of the nutrient recommendations into amounts of core foods (eg cereals, fruits and vegetables, meats, fish, poultry, dairy, fats and oils) required to meet these nutrient recommendations in Australia. These in turn were used as the basis for the development of the Australian Guide to Healthy Eating and the Australian Dietary Guidelines for Adults, the Dietary Guidelines for Children and Adolescents in Australia and the Dietary Guidelines for Older Australians.
New Zealand has Food and Nutrition Guidelines covering the ages and stages of the lifecycle. There are currently seven in the series including infants and toddlers (0–2 years), children (2–12 years), adolescents, pregnant women, breastfeeding women, adults and older people. These publications include a background paper for health professionals and an accompanying health education pamphlet for the public.
The interrelationships between these various recommendations and the underpinning evidence are shown in Figure 1.
The need for regular monitoring of dietary intake and nutrient status in the population, including the use of fortified foods and supplements, to underpin the ongoing revisions of the NRVs, notably the Adequate Intake values which, by definition, are often based on population median dietary intakes.
The need for research funds to enable more accurate assessment of requirements for both sustenance and prevention of chronic disease, including studies on issues such as biomarkers for nutritional status and nutrient bioavailability, and adverse effects of high intakes.
The need to update and expand existing food databases for the analysis of national nutrition survey data, including information on the levels of fortification in foods.
The need to change computerised dietary analysis programs that use the existing RDI values as reference values.
The need for the redevelopment of relevant standards for the use of NRVs for food legislative purposes, including issues such as food labelling and food fortification.
The need to consider the implications of changes in the NRVs for the food and dietary supplementation industry.
What are the implications of changes in recommendations for certain nutrients?
Consumption of a diet conforming to the NRVs need not, in itself, be more expensive for the individual (Baghurst 2003), however addressing the needs for implementation outlined above will involve ongoing costs that are difficult to quantify. The financial expense associated with inadequate nutrition in the community is likely to far outweigh that of implementing the necessary changes. Crowley et al (1992) have estimated the economic cost of diet-related disease in Australia in terms of both direct health care (hospitals, medical expenses, allied health professional services, pharmaceutical expenses and nursing homes) attributable to diet and indirect costs (due to sick leave and the net present value of forgone earnings due to premature death). The estimate of direct costs, excluding consideration of alcohol, was $1,432 million and that for indirect, $605 million, giving a total of $2,037 million for 1989–1990.
The RDI for some nutrients has substantially increased from that in the previous edition due to the availability of new data or changes in the way needs are assessed. In the past, needs at the individual level were often assessed in the practical situation by reference to 70% RDI in the absence of a specific EAR value. The NHMRC Core Food Group assessment, which is the basis for the Australian Guide to Healthy Eating, was also modelled on 70% RDI. In the background papers to the previous RDIs (Truswell et al 1990), figures called Lower Diagnostic Levels were given for some nutrients, but these were not officially adopted. They were used to derive the previous RDIs with 'generous factors' to accommodate variation in absorption and metabolism. They were therefore not used in practice. The existence of a specific EAR in the current NRVs overcomes the need to extrapolate from the RDI when attempting to assess adequacy of individual diets.
The new RDI for iron in young women of 18 mg/day appears to have increased from the previous RDI (12–16 mg/day), however the EAR for this group (of 8 mg/day is actually less than 70% of the old RDI of 8.4–11.2 mg/day. This reflects the very high variability in iron requirements in this group because of variability in menstrual loss. Thus if 70% RDI had been used in the past as a benchmark for assessing the needs of individuals, the apparent requirement would likely have decreased somewhat. For pregnant women, 70% of the old RDI was 15.4–29.0 mg/day whilst the new EAR is 22.0 mg/day. For lactation, 70% of the old RDI was 8.4–11.2 mg/day but the new EAR is 6.5 mg/day.
In the case of zinc, another nutrient known to be borderline for adequacy in the community, the estimate of average needs for men has risen from 8.4 mg/day (70% old RDI) to 12 mg/day (EAR) but that for women has fallen from 8.4 mg/day (70% old RDI) to 6.5 mg/day, partly due to recognition that absorptive capacity for zinc varies across the genders and that men have significant losses in semen.
The EAR is well above 70% of the previous RDI for other nutrients, including the B vitamins thiamin, niacin, riboflavin, vitamin B6 and B12, calcium and magnesium, which are all about 50% higher, and folate, which is about 100% higher, than 70% of the respective old RDIs. The increase in the B vitamin reference values reflects the ways they were set in the earlier version. In the 1981–1989 RDIs, the values for B vitamins were generally set in relation to energy needs for thiamine, riboflavin and niacin or protein needs for vitamin B6. Energy and protein needs were, in turn, set on figures recommended at that time by the FAO:WHO. The EARs for B vitamins in the current reference values were set using the results of metabolic studies with specific biochemical endpoints in blood, tissues or urine related to potential deficiency states, or depletion-repletion studies.
For folate, the higher RDI marks a return to the RDI that was in place in Australia before the 1981–1989 revision, when it was lowered from 400 µg to 200 µg/day on the basis that the amount of absorbed folate required to treat or fully prevent deficiency disease was 100 µg/day, that the average absorption from food was 50% and that average total folate consumption in Britain and North America at that time was about 200 µg/day. Other countries such as the US and Germany had an RDI of 400 µg at that time (although they later reduced it) as they felt that the availability of folate was between 25% and 50% and that 100–200 µg/absorbed folate day were needed.
The new Australian/NZ RDI for folate is based on the current recommendations from the US and Canada and new data on dietary intake in relation to maintenance of plasma folate, erythrocyte folate and homocysteine levels that suggest a need for about 300 µg/day. The folate RDI is expressed in terms of dietary folate equivalents in recognition of the difference in bioavailability between food folate and folic acid. The latter, which is the form used for supplements and fortification of foods, is twice as well absorbed as food folate.
In relation to calcium, the difference between the old and new RDIs relates almost entirely to the recognition that losses through sweat of some 60 mg/day were not accounted for in previous estimates. The additional intake required to account for the decrease in absorption of calcium with increased intake is 320 mg.
In the case of magnesium, the new EARs and RDIs were based on maintenance of whole body magnesium over time from balance studies mostly published since the last Australian/New Zealand RDIs were set. Recent studies of people on total parenteral nutrition that indicated lower needs than earlier balance studies were also considered. In the background paper for the earlier magnesium RDI for Australia, Dreosti stated "more, conventional magnesium balance studies are necessary at this stage in order to resolve the question of requirements" (Truswell et al 1990).
Thus, the increased requirements for some nutrients since the previous revision are based on data not available at the time or on a different approach to assessing needs. This outcome may appear to imply that people need to consume more food at a time when obesity is a major public health problem in the community. However, achievement of the new RDIs requires the consumption of different types of foods, not the consumption of more food. If energy-dense, nutrient-poor foods and drinks are replaced with plenty of vegetables, fruits and wholegrain cereals, moderate amounts of lean meats, fish, poultry and reduced fat dairy foods and small amounts of polyunsaturated or monounsaturated fats and oils as well as plain water, then all the nutrients required can be obtained within energy requirements. It should be remembered also that increased levels of activity make dietary choices more flexible and have the benefits of assisting in the maintenance of acceptable body weight and reducing a range of chronic diseases.
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