Best Practices for Dietary Supplement Assessment and Estimation of Total Usual Nutrient Intakes in Population-Level Research and Monitoring

Abstract

The use of dietary supplements (DS) is pervasive and can provide substantial amounts of micronutrients to those who use them. Therefore when characterizing dietary intakes, describing the prevalence of inadequacy or excess, or assessing relations between nutrients and health outcomes, it is critical to incorporate DS intakes to improve exposure estimates. Unfortunately, little is known about the best methods to assess DS, and the structure of measurement error in DS reporting. Several characteristics of nutrients from DS are salient to understand when comparing to those in foods. First, DS can be consumed daily or episodically, in bolus form and can deliver discrete and often very high doses of nutrients that are not limited by energy intakes. These characteristics contribute to bimodal distributions and distributions severely skewed to the right. Labels on DS often provide nutrient forms that differ from those found in conventional foods, and underestimate analytically derived values. Finally, the bioavailability of many nutrient-containing DS is not known and it may not be the same as the nutrients in a food matrix. Current methods to estimate usual intakes are not designed specifically to handle DS. Two temporal procedures are described to refer to the order that nutrient intakes are combined relative to usual intake procedures, referred to as a “shrinking” the distribution to remove random error. The “shrink then add” approach is preferable to the “add then shrink” approach when users and nonusers are combined for most research questions. Stratifying by DS before usual intake methods is another defensible option. This review describes how to incorporate nutrient intakes from DS to usual intakes from foods, and describes the available methods and fit-for-purpose of different analytical strategies to address research questions where total usual intakes are of interest at the group level for use in nutrition research and to inform policy decisions. Clinical Trial Registry: NCT03400436.

Total Usual Micronutrient Intakes Compared to the Dietary Reference Intakes among U.S. Adults by Food Security Status

“…A strength of this analysis is that the models applied to examine total usual intakes adjust for the effects of random measurement error to the extent possible, in addition to using the recommended method of adding mean daily nutrient intakes from supplemental sources to the adjusted usual nutrient intakes from dietary sources [18,22]. USDA's automated multiple-pass method is a state-of-the-art method for capturing dietary data, as is the Food and Nutrient Database for Dietary Studies that supports it.…”

“…Furthermore, the analysis of nutrients in DS relies on label declarations on products, rather than analytically derived values, that are likely to result in an underestimation of micronutrients from DS [35]. Furthermore, we assume that the DS intake reported for the past 30 days during the in-home interview reflects long-term, habitual DS intake, but little is known about the measurement error structure of DS reporting [18]. NHANES is a nationally representative survey of the U.S. noninstitutionalized population.…”

“…Thus, folic acid was the only form of folate used to estimate the proportion of the population exceeding the UL. Sodium was excluded since negligible amounts are found in DS [18].…”

“…The NCI method is used to estimate the distributions of "usual" or "long-term mean daily" intakes by accounting for random measurement error (i.e., within-person variation). It was adapted to estimate the contributions of DS to usual micronutrient intake estimates through the incorporation of reported DS intakes from the dietary supplement questionnaire, using the method described by Bailey et al [18,21]. Covariates incorporated in the usual intake models included day of the week of the dietary recall (weekend/weekday), interview sequence (first or second dietary recall), and DS use overall.…”

“…Categorical variables for sex and food security status were used for subgroup analyses. Mean daily nutrient intakes from DS and their relative contribution to total intakes were estimated by adding nutrients from supplemental sources to the adjusted distributions of usual intake from dietary sources to estimate the distributions of total usual micronutrient intake among the adult total population (DS users and nonusers combined) [18,22]. The relative contribution of DS to total micronutrient intakes was calculated by dividing the total usual micronutrient intake from DS by the total usual micronutrient intake from all sources (inclusive of foods and DS) at the population level (Table 1).…”