Use%20of%20nutritional%20bio-measures%20in%20national%20dietary%20surveys

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Use of nutritional bio-measures in national
dietary surveys
Gillian Swan Food Composition and Diet
Team Nutrition Branch
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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

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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

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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

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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)

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Why isnt dietary assessment enough?

• Diet can only be assessed over a short period
• doesnt 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

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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

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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)

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Bio-measures in NDNS blood samples

• Results dont 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

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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

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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

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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

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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

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www.sacn.gov.uk
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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

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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

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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.

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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

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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

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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