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