Dietary Assessment
"What do people actually eat?" is harder to answer than it looks. Every large claim about diet and health depends on answering it for thousands of people over months or years, and every instrument that attempts to do so has known biases. This skill catalogs the common instruments, documents the biases each one introduces, and gives heuristics for choosing the right tool for a question and reading the resulting numbers with appropriate skepticism.
Agent affinity: lind (controlled-trial design), ancel-keys (population-scale dietary assessment)
Concept IDs: nutrition-assessment-methods, nutrition-study-design, nutrition-measurement-error
The instrument menu
24-hour dietary recall
What it is. A trained interviewer walks the participant through everything eaten and drunk in the previous 24 hours, prompting with portion-size aids and a multi-pass protocol (quick list, meal structure, forgotten-foods probe, detail pass, final review). The USDA Automated Multiple-Pass Method is the reference implementation.
Strengths. Open-ended; captures foods not on any predefined list. Short-term memory is more accurate than long-term. With multiple non-consecutive recalls, can estimate usual intake with statistical modeling (the National Cancer Institute method).
Weaknesses. A single recall does not represent habitual intake; day-to-day variability is large. Portion-size estimation is imprecise without aids. Respondents still underreport energy — roughly 10–15% on average in the AMPM, better than FFQ but not negligible.
When to use. Short-term energy-balance studies, nutrient-intake estimates for large surveys (NHANES uses two recalls per participant), situations where the food list is unknown in advance.
Food frequency questionnaire (FFQ)
What it is. A structured questionnaire asking how often the respondent has eaten each of 100–150 foods over a reference period (typically the past year), with optional portion-size questions.
Strengths. Cheap, rapid, low respondent burden. Well-suited to ranking people for relative-risk epidemiology. Historically the instrument of choice for cohort studies like Nurses' Health, EPIC, and the Harvard cohorts.
Weaknesses. Underreporting of total energy intake is routinely 20–30% and is not random — foods perceived as unhealthy are differentially underreported. Long reference period strains memory. The food list constrains what can be reported. Portion-size estimation over a year is nearly impossible. FFQ-based studies with small effect sizes should be read with substantial skepticism.
When to use. Large cohort studies focused on ranking, not absolute intake. Do not use for clinical or individual-level assessment.
Weighed food diary
What it is. The participant weighs and records every food and drink consumed for a period (typically 3–7 days), ideally with a digital scale.
Strengths. More accurate than recall for the days actually recorded. Portion sizes are measured, not estimated.
Weaknesses. High respondent burden; compliance drops over time. Recording changes behavior — participants simplify their diets to make recording easier. Still does not represent habitual intake unless multiple diaries are collected.
When to use. Shorter-duration metabolic studies where burden can be tolerated; validation studies for other instruments.
Duplicate-plate study
What it is. The participant serves and eats a normal meal, then plates an identical second serving that is collected and chemically analyzed. This removes the estimation step from recall.
Strengths. Nutrient content is measured directly from the actual food eaten. Excellent for trace-element studies where food database uncertainty is large.
Weaknesses. Extremely burdensome; participants know they are being measured. Expensive. Used in specialized contexts only.
When to use. Trace-element studies, contamination studies, validation of nutrient databases.
Biomarkers of intake
What it is. Biological samples (urine, blood, hair, adipose) are measured for compounds that reflect recent or longer-term intake of specific nutrients or foods.
Examples.
- Doubly labeled water — gold-standard measure of total energy expenditure and, in weight-stable subjects, energy intake. Participant drinks water labeled with deuterium and oxygen-18; their differential excretion rates reveal CO2 production, which reflects energy expenditure.
- 24-hour urinary nitrogen — recovers ~80% of protein intake; used to validate protein recall.
- 24-hour urinary potassium — recovers potassium intake reasonably well; biomarker for fruit and vegetable consumption.
- Adipose tissue fatty acids — reflect longer-term fat intake, especially for fatty acids the body cannot synthesize.
- Plasma carotenoids — biomarker for fruit and vegetable intake.
Strengths. Objective — no reliance on respondent memory. Resistant to motivated underreporting.
Weaknesses. Expensive. Not available for every nutrient. Some biomarkers are short-term (plasma) and some long-term (adipose); interpretation depends on which.
When to use. Validation of dietary instruments. Specific-nutrient studies where the biomarker is well-characterized. Situations where motivated reporting bias is expected.
Controlled feeding study
What it is. The study provides all food to participants for a defined period. Intake is known exactly. Typically conducted in metabolic wards for full control, or as provided-meal studies for less control but more generalizability.
Strengths. Intake is known, not estimated. Ideal for mechanism studies, metabolic endpoints, short-term clinical outcomes.
Weaknesses. Expensive. Short duration (rarely more than 4–8 weeks, sometimes longer in ward settings). Artificial — people do not eat prescribed diets in real life, and observed responses may not persist when participants return to self-selected diets.
When to use. When the research question is about metabolic mechanism or short-term clinical endpoint. When a controlled comparison of two dietary patterns is essential and observational confounding would be fatal.
Choosing an instrument — decision protocol
Step 1 — What is the question?
- Is the question about population-level intake patterns? → 24-hour recall (NHANES-style), ideally with multiple recalls and NCI usual-intake modeling.
- Is the question about individual-level adequacy for a clinical assessment? → Multiple 24-hour recalls or a weighed diary, supplemented with biomarkers if available.
- Is the question about long-term intake and chronic disease in a cohort? → FFQ, understanding that ranking is the realistic goal and absolute intake is not.
- Is the question about a specific metabolic response to a dietary change? → Controlled feeding, ideally inpatient.
- Is the question about validating a self-report instrument? → Biomarker study.
Step 2 — What is the target measurement accuracy?
If the hypothesized effect size is small (e.g., a 10% difference in relative risk), the instrument's measurement error must be substantially smaller than the effect. FFQ-based studies with small effect sizes cannot be distinguished from noise. If the effect is large (e.g., scurvy cured vs. not cured), even crude instruments suffice.
Step 3 — Will the population comply?
High-burden instruments (weighed diary, controlled feeding, duplicate plate) drop motivated participants disproportionately, biasing the sample. Lower-burden instruments (FFQ) retain more participants but produce noisier data.
Worked example — reading an FFQ-based relative-risk claim
A 2016 cohort study reports that participants in the highest quintile of "red meat intake" (measured by FFQ) have a 16% higher risk of colorectal cancer than those in the lowest quintile.
How should the department read this claim?
- Instrument fit. FFQ is