Lecture 18 Flashcards
Dietary assessment challenges and how to overcome them
it is quite common for people to report very low energy intakes when reporting what they eat. if they are under reporting their energy can we believe their nutrient intakes
no not really
what is under reporting
deliberate or unintentional omission of underestimation of foods
what is under-eating
process of diet reporting can make people change their eating habits
what is the mean energy intake of a 19-30y old male from the adult nutrition survey (2008)
11.9MJ
what is the mean energy intake of a 19-30y old female from the adult nutrition survey (2008)
8.4MJ
what is a method of identifying low energy reporters
“doubly labeled water”
doubly labeled water technique can determine
degree of diet underestimation
how accurate is doubly labelled water
very accurate estimation of free living energy expenditure (only 1-2% error)
what is meant by energy balance in terms of dietary assessment
if they are in energy balance
the energy they report to be eating should be = to their energy expenditure
what are 3 forms of indirect calorimetry
- douglas bag
- portable gas analysis
- on line gas analysis
what does doubly labelled water contain
stable isotopes : deuterium and oxygen 18
what does doubly labelled water estimate
CO2 production without needing breathing apparatus
how does doubly labelled water work
drink a small amount of doubly labelled water and collect urine samples to determine how quickly isotopes disappear from body
what are the two ways we lose oxygen from the body
through urine and breathing out CO2
how do we know how much of the doubly labelled water is lost through CO2 without a breathing apparatus
measure the amount of oxygen lost in the urine, calculate difference by the doubly labelled water oxygen that wasnt lost in urine
what happens to the doubly labelled water when it enters your body
labels the water pool, evenly disperse in your body
once you know how much CO2 is exhaled from the doubly labelled water, can use that to
calculate energy expenditure and then you can compare this to the energy they have reported and this should be the same if they are in energy balance
what are the advantages to doubly labelled water
- once you know CO2 losses for time period can calculate total energy expenditure
- if in energy balance then reported energy intake should = energy expenditure
- very accurate
- relatively simple for participant
what are the disadvantages to doubly labelled water
- very expensive
- only get average total energy expenditure (not for individual days)
- assumes energy balance so amount of energy expended = amount eaten
what are the steps to using doubly labelled water to identifying low energy reporters
- collect dietary data
- measure energy expenditure by doubly labelled water at the same time
- calculate difference
the doubly labelled water method can be expensive and inconvenient so what can be used instead
equations based on estimated basic energy requirements
basal metabolic rate + allowance for basic activities of daily life (usually assumes a sedentary lifestyle)
the goldburg equation allows you to determine whether
reported intake is high enough to represent : plausible intake for the period measured
in the goldburg equation intakes vary day to day so the cut off ….
the cut off to determine plausible intake is lower than it would need to be to determine whether intake represents habitual intake
what are the steps to using the goldburg equation
- collect data on : energy intake, sex, age, weight for each individual
- calculate estimated basal metabolic rate for each individual using Schofield et al (1985)
- use goldburg equation et al (1991) to determine cut off value for 1 person with data collected over x days
- low energy reporting is indicated if EI:BMRest < cutoff
what are the advantages to the goldburg equation
- quick, easy, cheap
- can be done retrospectively if have age, sex and weight
- non invasive
what are the disadvantages to the goldburg equation
- not as accurate as DLW - misses some LERs
- assumes participants are in energy balance
- assumes sedentary level physical activity
what happens if you delete participants with low energy reporting
- misses people who have high intakes who are LER
- LERs have other characteristics e.g overweight, higher social desirability so could > bias
- we dont measure HERs
what happens if we keep participants of low energy reporting but report the prevalence
keeps data that are inaccurate
what should we do with low energy reporters
collect data to quantify LER
minimise risk of low energy reporters
what are biomarkers
substances in biological specimens such as blood, urine of hair which reflect intake sufficiently closely to act as objective indices of true intake
examples of biomarkers and their use
doubly labelled water = energy
24hr urine nitrogen = protein
adipose fatty acids = some fatty acids
biomarkers are independent of
many errors associated with dietary methods
biomarkers can be used to
- assess measurement error (how well the dietary assessment is working)
- calibrate measurement error in dietary data
why not use biomarkers instead of dietary assessment ?
- only available for some nutrients
- often expensive
- invasive
- transport and storage issues
- assumptions
- dont necessarily capture total intake
- generally reflect recent intake only
- may be affected by disease status
why are biomarkers invasive
- biological specimens
- complete collection required
- multiple collections may be required
what are some assumptions of biomarkers
- doubly labelled water assumes energy balance
- 24hr urine nitrogen assumes nitrogen balance
what is meant by biomarkers don’t necessarily capture total intake
- low energy reporters are often under collectors
- urine nitrogen ~80% intake
