Dietary assessment Flashcards
Objectives of dietary assessment To characterise the diet of an individual
# Therapeutic # Research
Objectives of dietary assessment To characterise the diet of a population
# Sociological # Public Health # Research
Components required to measure food and nutrient intakes
[5]
# A report of all food consumed by an individual # Identification of foods such that an appropriate item can be chosen from a standard food table # Quantification of the portion size of each food item # Determination of the frequency of consumption # Calculation of nutrient intake, (portion size (g) x frequency x Nutrient content per gram.)
Food composition tables
[3]
Average macro and micronutrient content of foods
UK - McCance and Widdowson’s The Composition of Foods integrated dataset
USDA reference database
Computer database programmes [4]
Based on Food Composition Tables # COMPEAT # Foodbase # Whisp etc # Dietplan
Limitations of Food Composition databases and computer programmes [4]
Seasonal variation in the nutrient content of foodstuffs
Batch to batch variation in the composition of processed foods (depends on the origin of the food ingredients)
Lack of information on specific foods (especially important for minority ethnic groups)
Incomplete nutrient analysis for all foods.
Tools for assessing diet in the individual [2types & examples]
Prospective : # Food diaries # Weighed food diaries # Estimated food records
Retrospective # Food frequency questionnaires # 24 hr recall methods
assessment tool selection depend on
assessment purpose: # Measuring food consumption # Measuring nutrient intake # Characterising eating habits
Consideration of which method [4]
Importance of diet to study
Focus on one or many foods/nutrients
Variability within the study population
Resources available
Prospective tools
Tools to capture dietary intake data over a defined future period
Variation in degrees of accuracy: # Estimated food intake # Food Diary # Weighed food Diary
7 Day weighed food diaries
Gold Standard
Volunteer records all that they eat over a 7 day period
They couple each recording to a measurement of the weight of the food consumed
7 Day weighed food diaries strength & weakness
Strength: # Highly accurate # Quantitative- Faithful recording of portion size
Weakness:
# High responder burden
- Selectively representative
- Requires literacy
Reactivity
- The more you invade someone’s dietary habits the more they amend them
- Used as a weight loss tool
- Not necessarily representative of usual diet
an example of the use of food diaries
NDNS
NDNS is the best available survey of the dietary intake of the population # Optimum measure of food consumption by the individual # Provide data on diet, nutritional status, and general health of the population
24 hour recall
Respondent prompted to remember all foods consumed over previous 24 hours
24 hour recall Strengths
# Literacy not required # Low respondent burden= Representative cohort # Adaptable to multi-cultural populations # Recall does not influence behaviour
24 hour recall Weaknesses
Recall bias - Selective bias =Under-reporting of energy
Assumes a very regimented diet = Multiple recalls required
Training/ Time /Cost
Portion size estimation
The Food Frequency questionnaire
Composite food list - Lacks detail # 100 000 commonly eaten foods # FFQ: 10-150 questions
Requires literacy and cognitively demanding
Measurement error can be high = Subjective recall
Factors to consider in the design of a dietary questionnaire
Maximally Informative Minimally invasive
Target population
Fit for purpose - Socio-cultural differences in dietary habits
Questionnaire validation
FFQ structure
Composite food list - Generated through surveys and focus groups
Corresponding multiple response grid in which respondents can estimate frequency of foods eaten over past year.
Participant estimates portion size
food frequency questionnaires Strengths
Minimally invasive: completed in 20-25 minutes by the volunteer
Well suited to large studies
Can capture a lot of data
Low Cost
External validity - validated questionnaires can be used to compare data across cohorts
food frequency questionnaires weakness
# Volunteers subjectively estimate portion size # Tendency amongst volunteers to over report “healthy foods” and under-report “un-healthy foods” # Newly developed FFQs require validation against other food intake methods # Reliant upon accuracy of food composition tables and portion size measures # Poorly indicative of changing dietary habits across the life-course in the individual
Where is an FFQ appropriate
# Prospective epidemiological studies # Case control studies # Observational studies # Interventions aimed at changing overall dietary behaviours
Using the data
Data output- Converting categorical into continuous data
Portions per day- Worked example
Enhancing the data- Food composition tables- Micronutrients
Worked example
Composite scores for food (100g)
Estimated portion size of a serving
Calculate mean contribution to daily diet for beef
Handling the weaknesses of an FFQ
Adjusting to percentage energy per day
+ ameliorates the problem of over-reporting
- Values become relative as opposed to absolute
- decreases contribution of low energy foods
- increases contribution of high energy foods
+ adjustments can be made to account for macro/micronutrients
Adjusting to percentage of total portions per day
+ ameliorates the problem of over-reporting
- Values become relative as opposed to absolute.
- portions per day can be a vague measurement unit
- does not lend itself to the assessment of macro/micronutrients
FFQ/ 24 hr recall validation
Usually against some other instrument: # These aren’t usually designed to look at long-term diet # Food diary # Biomarkers
