Use of nutritional bio- measures in national dietary surveys Gillian Swan Food Composition and Diet...
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Transcript of Use of nutritional bio- measures in national dietary surveys Gillian Swan Food Composition and Diet...
Use of nutritional bio-measures 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 2008/09-2009/10Findings 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/lThis 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 well-being 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