Use of nutritional biomeasures in national dietary
surveys
Gillian Swan
Food Composition and Diet Team
Nutrition Branch
Overview
• Why use bio-measures in national nutrition
surveys
• What information do they give us
• Challenges
• Examples of how bio-measures data are
used in policy
Healthy Lives, Healthy People:
strategy for public health in
England
Recognises:
Public health context. i.e:
• that life style (smoking,
drinking, poor diet and low
levels of physical activity) is a
major contributor poor health
and premature death and to
equalities in health
• diet and obesity related
disease is common
• major cost saving gained if
diets improved and weight gain
stopped
• the need to improve diet and
lose weight
NDNS Rolling Programme
• Continuous cross-sectional survey of the general
population
• 1000 people per year (adults and children 1½ years
upwards)
• Designed to be representative of the UK population
• Data collected on individuals
• Food consumption, nutrient intakes, nutritional
status and other measurements
NDNS Components
•
Face to face interview
• Dietary assessment (4 day un-weighed diary)
• Physical measurements
• Blood sample (nutritional status analyses)
• 24 hour urine collection (sodium intake)
• Doubly labelled water (energy expenditure)
Why isn’t dietary
assessment enough?
• Diet can only be assessed over a short period
- doesn’t give longer term picture
• Diet assessment subject to self-reporting errors
• Some nutrients difficult to measure in the diet
– for example sodium (salt)
• Vitamin D – diet not the only source
What do bio-measures in
NDNS tell us?
• Objective indicator unaffected by reporting bias
• Measures of dietary intake
– 24-hour urine sample
• Measures of nutritional status
– Blood sample
• Energy expenditure
– Doubly labelled water
Bio-measures in NDNS –
blood samples
• Nutritional status of population
– level of nutrients available to body for metabolic
processes
– Measure level of nutrient in the blood or activity of
vitamin-dependent enzymes
– Reflect recent intake (plasma vitamin C) or longer term
body stores (plasma retinol; serum ferritin)
– Threshold levels set to indicate low status
• risk of deficiency
– Affected by factors other than diet
• Iron (controls on absorption, blood loss)
• Vitamin D (sunlight exposure)
Bio-measures in NDNS
– blood samples
• Results don’t correlate well with diet
– may not reflect short term intake
– Blood samples collected several weeks after diet
• Logistical issues in collecting
– Fasting samples
– Need for rapid processing for some micronutrients
• Poor response rates, especially for children
• Importance of comparability over time – difficult
when methods / laboratories change
Bio-measures in NDNS –
urine samples
• Sodium excretion in urine samples is best way of
measuring of sodium intake
• Sodium level in urine fluctuates during day – need
24-hour collection
• Single collection sufficient to give population
estimate
• Need to assess completeness of sample
– Para-amino benzoic acid (PABA) marker
– Ask participant if collections are complete
Bio-measures in NDNS –
doubly labelled water
• Measures energy expenditure
– Give a known dose of stable isotope as a drink
– Collect urine samples over 10 days
– Measure rate of disappearance of stable isotopes 2H
and 18O from urine
– Calculate CO2 production – energy expenditure
• Compared with reported energy intake – assess
under-reporting
• Isotope expensive / limited supply
• Complex analysis
NDNS 2008/09-2009/10
Findings on nutritional status
• Evidence of iron deficiency anaemia in a
proportion of adult women and older girls
• Evidence of low vitamin D status in adults and
older children
• Low functional riboflavin status in substantial
proportion of adults and older children
• No evidence of low status for other micronutrients
– including vitamin C, A, E, thiamin, B6, B12
How bio-measures data are
used in policy
• Scientific Advisory Committee on Nutrition
(SACN) use bio-measures from NDNS and other
sources in nutrient risk assessments
• Monitor progress towards recommendations (e.g.
salt)
• Secondary analysis to look at diets of individuals
with poor status
www.sacn.gov.uk
Salt (1)
• High salt intake contributes to high blood
pressure – risk factor for cardiovascular disease
• NDNS 2000/01 showed adult salt intake 9.5g/day
• SACN (2003) recommended salt intake should
reduce to a maximum of 6g/day
• Nationwide salt reduction initiative launched by
Government in 2003
– Targets for industry to reduce salt in processed food
– Consumer-facing campaigns
Salt (2)
• Salt reduction programme now part of Public
Health Responsibility Deal
• Series of urinary sodium surveys to monitor
progress towards 6g/day recommendation
• Latest survey published 21 June
– Salt intake for adults in England 8.1g/day
Current vitamin D
recommendations are:
Based on maintaining plasma 25 (OH) D above
25nmol/l
This is above the level associated with risk of rickets and
osteomalacia
Takes into account UVB production of vitamin D in
the skin in the summer.
Vitamin D
• NDNS shows significant proportion of population
below 25nmol/l plasma 25(OH)D.
• SACN reviewing vitamin D requirements
• SACN (2007) concluded there was an urgent
need to standardise laboratory measurement of
plasma 25(OH)D
– International vitamin D standardisation project
underway
Conclusions
• Nutritional bio-measures in national surveys
provide valuable information on nutritional wellbeing of population
• Complement information on diet – not a
substitute for it
• Objective measures – not subject to participant
self-reporting error
Challenges
• Lack of comparability of analytical data over time
or between laboratories
– Folate, vitamin D
– Need international laboratory standardisation
• Establishing threshold levels for low status and
understanding health implications
• No bio-markers for many nutrients
• Response and compliance in national surveys
– Agreement to collect blood samples
– Completeness of urine collections