It is known that the placenta actively transports vitamin C, vitamin E, and selenium from the mother to the fetus (Hytten and Leitch, 1971). However, for these three nutrients, experimental data that could be used to set an EAR and RDA for pregnancy are lacking. In these cases the potential of increased need for these nutrients during pregnancy is based on theoretical considerations, including obligatory fetal transfer, if data are available, and increased maternal needs related to increases in energy or protein metabolism, as applicable.
For vitamin C, vitamin E, and selenium, it is assumed that the total requirement of lactating women equals the requirement for the nonpregnant, nonlactating woman of similar age plus an increment to cover the amount of the nutrient needed for milk production. To allow for inefficiencies in use of these nutrients, the increment may be somewhat greater than the amount of the nutrient contained in the milk produced. Details are provided in each nutrient chapter.
Substantial changes in analytical methods have occurred during the more than 40 years of studies considered in this report. Although the requirement for vitamin C is based on recent data, the studies that were utilized to determine the vitamin E requirement are 40 years old. Methodological problems have been documented for vitamin E intake assessment from food (see Chapter 6).
Reliable and valid methods of food composition analysis are crucial in determining the intake of a nutrient needed to meet a requirement. For vitamin E and selenium, analytic methods to determine the content of the nutrient in food have serious limitations, the specifics of which are discussed in Chapter 5, Chapter 6, Chapter 7 through Chapter 8.
The quality of nutrient intake data varies widely across studies. The most valid intake data are those collected from metabolic study protocols in which all food is provided by the researchers, amounts