The dietary requirement for a micronutrient is deﬁned as an intake level which meets a speciﬁed criteria for adequacy, thereby minimizing risk of nutrient deﬁcit or excess. These criteria cover a gradient of biological effects related to a range of nutrient intakes which, at the extremes, include the intake required to prevent death associated with nutrient deﬁcit or excess. However, for nutrients where insufﬁcient data on mortality are available, which is the case for most micronutrients discussed in this report, other biological responses must Be deﬁned. These include clinical disease as determined by signs and symptoms of nutrient deﬁciency, and subclinical conditions identiﬁed by speciﬁc biochemical and functional measures. Measures of nutrient stores or critical tissue pools may also be used to determine nutrient adequacy.
Functional assays are presently the most relevant indices of subclinical conditions related to vitamin and mineral intakes. Ideally, these biomarkers should be sensitive to changes in nutritional state while at the same time be speciﬁc to the nutrient responsible for the subclinical deﬁciency. Often, the most sensitive indicators are not the most speciﬁc; for example, plasma ferritin, a sensitive indicator of iron status, may change not only in response to iron supply, but also as a result of acute infection or chronic inﬂammatory processes. Similarly anaemia, the deﬁning marker of dietary iron deﬁciency, May also result from, among other things, deﬁciencies in folate, vitamin B or copper. The choice of criteria used to deﬁne requirements is of critical importance,
Since the recommended nutrient intake to meet the deﬁned requirement will clearly vary, depending, among other factors, on the criterion used to deﬁne nutrient adequacy . Unfortunately, the information base to scientiﬁcally support the deﬁnition of nutritional needs across age ranges, sex and physiologic states is limited for many nutrients. Where relevant and possible, requirement estimates presented here include an allowance for variations in micronutrient bioavailability and utilization. The use of nutrient balance to deﬁne requirements has been avoided whenever possible, since it is now generally recognized that balance can be reached over a wide range of nutrient intakes. However, requirement levels deﬁned using nutrient balance have been used if no other suitable data are available. The traditional criteria to deﬁne essentiality of nutrients for human health require that a) a disease state, or functional or structural abnormality is present if the nutrient is absent or deﬁcient in the diet and, b) that the abnormalities are related to, or a consequence of, speciﬁc biochemical or functional changes that can be reversed by the presence of the essential dietary component.
Endpoints considered in recent investigations of essentiality of nutrients in experimental animals and humans include: reductions in ponderal or linear growth rates, altered body composition, compromised host defense systems, impairment of gastrointestinal or immune function, abnormal cognitive performance, increased susceptibility to disease, increased morbidity and changes in biochemical measures of nutrient status. To establish such criteria for particular vitamins and minerals requires a solid understanding of the biological effects of speciﬁc nutrients, as well as sensitive instrumentation to measure the effects, and a full and precise knowledge of the amount and chemical form of nutrients supplied by various foods and their interactions.
Nutrient balance calculations typically involve assessing input and output and establishing requirement at the point of equilibrium (except in the case of childhood, pregnancy and lactation where the additional needs for growth, tissue deposition and milk secretion are considered). However, in most cases, balance based on input–output measurements is greatly inﬂuenced by prior level of intake, that is, subjects adjust to high intakes by increasing output and, conversely, they lower output when intake is low. Thus, if sufﬁcient time is provided to accommodate to a given level of intake, balance can be achieved, and for this reason, the exclusive use of nutrient balance to deﬁne requirements should be avoided whenever possible . In the absence of alternative sources of data, a starting point in deﬁning nutritional requirements using the balance methodology is the use of factorial estimates of nutritional need.
The “factorial model” is based on measuring the components that must be replaced when the intake of a speciﬁc Nutrient is minimal or nil. This is the minimum possible requirement value And encompasses a) replacement of losses from excretion and utilization at Low or no intake, b) the need to maintain body stores and, c) an intake that Is usually sufﬁcient to prevent clinical deﬁciency. Factorial methods should be used only as a ﬁrst approximation for the assessment of individual requirements, or when functional clinical or biochemical criteria of adequacy have not been established. Furthermore, although nutrient balance studies may be of help in deﬁning mineral needs, they are of little use for deﬁning Vitamin requirements . This is because the carbon dioxide formed on the oxidation of vitamins is lost in expired air or hard to quantify, since it becomes part of the body pool and cannot be traced to its origin unless the vitamin is provided in an isotopically labelled form .