Dietary Assessment Primer

Learn More about Dietary Supplements and Estimating Total Nutrient Intakes

Dietary supplements are defined as products intended to supplement the diet that contain one or more dietary ingredients. Supplements are taken orally as a pill, capsule, tablet, or liquid. The most commonly used dietary supplements are multivitamin-mineral products. Because these products usually contain 100% of the daily value of some nutrients and because supplements may provide doses of some nutrients greater than what can be found in foods (e.g., vitamins D and E), incorporating nutrient intakes from dietary supplements is critical to estimating [glossary term:] total nutrient intakes.

Traditionally, dietary self-report instruments queried intakes of foods and beverages only. However, because the use of dietary supplements has increased over time, it is generally recognized that the contribution of dietary supplements must be accounted for if total nutrient intakes are to be assessed. Supplements can be measured like foods and beverages, using [glossary term:] short-term instruments and [glossary term:] long-term instruments. For 24-hour dietary recalls (24HRs) and food records, participants are often asked to have containers for all supplements consumed available during the assessment (see 24-Hour Dietary Recall Profile or Food Records Profile). This aids in the accurate reporting of types, formulations, and amounts consumed. Assessment using [glossary term:] long-term instruments is usually accomplished using a food frequency questionnaire (FFQ)-like instrument with questions about type, frequency of intake, dose, and duration of use (see Food Frequency Questionnaire Profile). Compared to [glossary term:] short-term instruments, FFQs are likely to better capture episodically consumed supplements.

For all types of dietary supplement assessment methods, if participants cannot remember a specific product consumed, a default can be applied if the participant has general knowledge about the product type (e.g., multivitamin). This likely will not reflect the actual nutrient composition of what was consumed but may be close.

Determining the best methods for estimating usual nutrient intakes (Learn More about Usual Dietary Intakes) that include contributions from both foods and supplements is complex. This is because data may be collected over different periods of time and obtained using different dietary assessment instruments that provide data that are not comparable and subject to different types of [glossary term:] measurement error. Like foods, some supplements are consumed daily whereas others are consumed episodically. If statistical modeling of short-term instruments (e.g., 24HR) for at least one of the components of total intake (foods and beverages or dietary supplements) is required, it is recommended to estimate [glossary term:] usual dietary intakes from foods and beverages first and then add estimated usual dietary intakes from dietary supplements (Learn More about Statistical Modeling). It also is recommended to investigate the possibility that dietary supplement users differ in their food and beverage habits from supplement nonusers. This investigation can be done by performing analyses on users and nonusers separately, or by including supplement use variables as [glossary term:] covariates in a statistical model.

For More Information

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