5.8: Sources of discrepancies in Nutrient Reference Values (8a.8)

In light of the preceding discussion, it is not surprising that estimates for AR for nutrients and the RI₉₈ derived from them vary among countries. Table 8a.11

compares the ARs for selected micronutrients set by the IOM in the U.S., by EFSA in Europe, and by COMA in the UK. Some of the discrepancies seen in Table 8a.11 arise because of differences in judgment by the expert groups setting the requirement estimates even when the same evidence-base has been consulted. Other potential sources of discrepancies in the AR and thus RI₉₈ include:

• Disparities in philosophy about the most appropriate method­ological approach to use
• Interpretation of data on which ARs are based
• Differences in selection of the criteria used to define nutrient ade­quacy
• Uncertainty in extent of metabolic adaption during pregnancy and lactation due to limited data
• Differences in number of life-stage groupings among countries
• Limited data on require­ments for certain nutrients and life-stage groups
• Differences in scaling (both interpolation and extrapolation) for sub­groups based on known data for other pop­ula­tions (e.g., adults)
• Varying bio­avail­ability factors depending on the composition of habitual national diets
• Unknown factors influencing nutrient require­ments.

Table 8a.11 A comparison of the ARs for male adults for selected micronutrients set by the US IOM in 1997–2011 (Otten et al, 2006; IOM, 2011), the European Food Safety Authority(EFSA, 2017), and the UK (COMA, 1991; SACN 2016).
*1 8–24y; ** > 25y
^A AI as no AR set. ^B Based on an EAR of 2550kcal.
NE niacin equivalents, DFE dietary folate equivalents.
For zinc, the EFSA sets three alternative ARs for levels of phytate intake above 300mg/d, of 600, 900, and 1200mg/d

Nutrient	IOM AR	EFSA AR	UK EAR
Vitamins
Vitamin A (µg/d)	625	570	500
Vitamin D (µg/d)	10	15^A	-
Vitamin C (mg/d)	75	90	25
Thiamin (mg/d)	1.0	0.072/MJ	0.80^B
Riboflavin (mg/d)	1.1	1.3	1.0
Niacin (mg NE/d)	12	1.3/MJ	14.0^B
Vitamin B6 (mg/d)	1.4	1.5	1.2^B
Vitamin B12 (µg/d)	2	4^A	1.25
Folate (µg DFE/d)	320	250	150
Minerals
Calcium (mg/d)	800	860*; 750**	525
Phosphorus (mg/d)	580	550^A	400
Magnesium (mg/d)	350	350^A	250
Trace elements
Iodine (µg/d)	95	150^A	-
Iron (mg/d)	6	6	6.7
Selenium (µg/d)	45	70^A	-
Zinc (mg/d)	9.4	7.5 9.3;11.0;12.7	7.3

Such discrepancies may be especially large for those nutrients and specific age groups for which the available data on require­ments are very limited (e.g., children, adolescents, and the elderly). In such cases, require­ments are often interpolated or extrapolated from data for other age groups, or they are not compiled at all. In addition, during pregnancy and lactation, maternal metabolic adaptation for certain nutrients may occur, but because the adaptation has not been firmly characterized, the additional nutrient needs are still equivocal.

A further source of discrepancy arises from the adjustments required that take into account the bio­avail­ability of nutrients in the habitual national diet. The adjustments required depend on the nature of the diet ingested, the chemical form of the nutrient in the diet, and a variety of systemic factors known to affect the absorption and utilization of the nutrient. For many nutrients, factors affecting their bio­avail­ability have yet to be established, so appropriate adjustments to yield dietary require­ment estimates cannot be made. For others, fixed bio­avail­ability factors are applied, even though the efficiency of absorption may vary with the dietary level of the nutrient or the life-stage group. For example, in U.K. diets, iron is assumed to have a fixed bio­avail­ability of 15%, irrespective of the age and life-stage group (COMA,1991), whereas in the U.S. a factor of 18% is used except for women during the second and third trimester of pregnancy when a bio­avail­ability factor of 25% is assumed (IOM, 2001). Other expert groups such as WHO/FAO (2004) employ differing factors to adjust for bio­avail­ability, depending on the composition of the diet.

Several other factors besides sex, life-stage, and the habitual diet, are known to influence the require­ments for many nutrients. Examples include body size, lean body mass, and activity level. For this reason, the require­ment estimates are often set using a “standard” height and weight, and/or energy intake for a particular age and life-stage group; standards that may vary across countries. Therefore, those nutrients with require­ments expressed per kg body weight or per MJ, may also differ. For example, reference heights and body weights used by EFSA (2017) are from the WHO Child Growth Standard (WHO, 2006) for children aged 0–2y, although for children 2–17y,data from European children are used (van Buuren et al., 2012). In the U.S., reference weights and heights for children and adults were based on anthropometric data collected from 1988–1994 as part of the Third National Health and Nutrition Examination Survey (NHANES III). However, the report on macro-nutrients (IOM, 2003) used data on median BMI and height-for-age from the CDC/NCHS growth charts (Kuczmarski et al., 2002).

In the future, factors such as race or ethnicity, lifestyle (e.g., vegetarians, smokers, oral contraceptive users), the existence of chronic disease (eg asthma, diabetes), environment, family history, and genetic predisposition to disease may also be taken into account when setting require­ment estimates (Gibson, 2012). Examples of potential factors affecting require­ments are summarized in (Figure 8a.7).

Figure 8a.7 Sources of biological variability of indi­vidual nutrient require­ments. Modified from Ashwell et al., 2008.

In the United States and Canada, vegetarianism and some other lifestyle factors are already considered. As an example, the EAR for iron for vegetarians is higher than that for persons consuming a mixed Western diet to take into account the lower bio­avail­ability of iron from a vegetarian diet (i.e., 10% vs 18%) (IOM, 2001), whereas the EAR for vitamin C for smokers is higher than for non-smokers (IOM, 2000).

The life-stage groupings are not defined in the same way among countries. North America has 22 such groups. Fewer groupings are defined in the U.K and Europe: Germany and the Netherlands have 14 each, but the number is presently not standardized, even within the European Economic Community(Trichopoulou and Vassilakou, 1990).

Finally, knowledge of the SD associated with the AR is required to set the RI₉₈. In many cases, however, the SD is calculated from the AR and an assumed CV, because the SD is unknown. Although a CV of 10% or 15% is often assumed, this is not always the case. WHO/FAO (2004), for example, has assumed a CV of 25% for the dietary zinc requirement estimate, resulting in a further source of discrepancy for the RI₉₈ for zinc.