levels of physiological functioning and physical activity (IOM, 2000b). Age-related declines in nutrient absorption and kidney function also may occur.

Unique changes in physiology and nutrition needs occur during pregnancy and lactation. For the DRI framework, consideration is often given to the following factors:

• The needs of the fetus during pregnancy and the production of milk during lactation;

• Adaptations to increased nutrient demand, such as increased absorption and greater conservation of many nutrients; and

• Net loss of nutrients due to physiological mechanisms, regardless of intake.

Owing to the last two factors, for some nutrients there may not be a basis for setting reference values for pregnant or lactating women that differ from the values set for other women of comparable age.

Indicators for DRIs are defined as the health outcomes that serve as the basis for estimating a nutrient requirement. Within the fields of biology and medicine, the term “indicators” has been defined differently and in some cases the definition may not be the same used for DRI purposes. In the case of indicators for DRIs, they can take various forms and many different indicators have been used in the more than 15 years of DRI experience (Taylor, 2008). The term in other settings encompasses what are variously referred to as endpoints, surrogates, biomarkers, or risk factors. Additionally, the term clinical outcome, also referred to as health outcome, is used to refer to the ultimate measurable effect of interest for nutrients, which is, of course, an indicator. Other measures preceding the occurrence of a clinical outcome can be predictive of the clinical outcome itself, although this is not necessarily the case and they must be validated before this can be assumed.

The term biomarker, like the term indicator, is defined differently within different fields of study. In the field of nutrition it is often referred to in the same way in which this report uses the term indicator. In order for them to equate, however, the biomarker must be causally related to the outcome indicator. Important terms in common parlance are biomarker of exposure and biomarkers of effect. The former is a validated measure that can be relied upon to reflect intake or exposure in the case of nutrients. A biomarker of