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School ofSocial and Community Medicine
University ofBRISTOL
Longitudinal analysis of diet in ALSPAC
Laura D HoweEUCCONET, Bristol, October 2011
School ofSocial and Community Medicine
University ofBRISTOL
Outline
Trajectories of energy intake and macro-nutrients
Planned analysis
Very preliminary results
School ofSocial and Community Medicine
University ofBRISTOL
120
01
400
160
01
800
200
0m
ean
kca
l
3 5 7 9 11 13Age (average age at measurement)
Diet diary FFQ
School ofSocial and Community Medicine
University ofBRISTOL
50
60
70
80
me
an fa
t int
ake
3 5 7 9 11 13Age (average age at measurement)
Diet diary FFQ
45
50
55
60
65
70
me
an p
rote
in in
take
3 5 7 9 11 13Age (average age at measurement)
Diet diary FFQ
160
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me
an c
arb
ohy
dra
te in
take
3 5 7 9 11 13Age (average age at measurement)
Diet diary FFQ
40
50
60
70
80
90
me
an 'u
nhea
lthy'
sug
ar in
take
3 5 7 9 11 13Age (average age at measurement)
Diet diary FFQ
School ofSocial and Community Medicine
University ofBRISTOL
Data issues
Different # measures per individual Exact ages of measurement vary FFQ and diary data
Want a full trajectory that is comparable for all individuals
Want to reduce the dimensionality of the data
School ofSocial and Community Medicine
University ofBRISTOL
Multi-level models:Random-slopes model
Effect of time varies between individuals (u1i)
The model estimates:The regression coefficients a and b Individuals intercepts (a + u0i)
Individual slopes (b+u1i)The covariance between the intercept and slope
yij = a + u0i + (b+u1i)tij + eij
yij=weight for individual i at occasion j, time tij
School ofSocial and Community Medicine
University ofBRISTOL
Multi-level models in pictures!
kCal
Age
Average regression line
School ofSocial and Community Medicine
University ofBRISTOL
Multi-level models in pictures!
kCal
Age
School ofSocial and Community Medicine
University ofBRISTOL
But the real world isn’t always linear...
Model the data as a curve?
Model the data as piecewise linear?
School ofSocial and Community Medicine
University ofBRISTOL
What shape?
120
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400
160
01
800
200
0m
ean
kca
l
3 5 7 9 11 13Age (average age at measurement)
Diet diary FFQ
School ofSocial and Community Medicine
University ofBRISTOL
What shape?
120
01
400
160
01
800
200
0m
ean
kca
l
3 5 7 9 11 13Age (average age at measurement)
Diet diary FFQ
School ofSocial and Community Medicine
University ofBRISTOL
Raw data1
000
200
03
000
400
0kc
al
3 5 7 9 11 13occ
School ofSocial and Community Medicine
University ofBRISTOL
120
01
400
160
01
800
200
02
200
kCal
3 5 7 9 11 13Age, years
female male
School ofSocial and Community Medicine
University ofBRISTOL
Next steps
Include adjustment for over-reporting
Repeat for fat, protein, carbs, unhealthy sugars
Repeat for energy-adjusted fat, protein, carbs, unhealthy sugars
School ofSocial and Community Medicine
University ofBRISTOL
Using the models: diet as the exposure
Individual-level residuals = how an individual deviates from the normal
Use in standard regression techniques
Obesity NAFLD Cardiovascular risk factors etc
School ofSocial and Community Medicine
University ofBRISTOL
Using the models: diet as the outcome
Include the exposure in the multilevel models
e.g. SEP For each category of SEP, allow:
1. Different intercept
2. Different slope in each period
School ofSocial and Community Medicine
University ofBRISTOL
120
01
400
160
01
800
200
02
200
Kca
l, m
ale
s
3 5 7 9 11 13Age, years
< O-Level O-LevelA-Level Degree
School ofSocial and Community Medicine
University ofBRISTOL
120
01
400
160
01
800
200
0K
cal,
fem
ale
s
3 5 7 9 11 13Age, years
< O-Level O-LevelA-Level Degree
School ofSocial and Community Medicine
University ofBRISTOL
Acknowledgements
Emma Anderson Kate Tilling Debbie Lawlor ALSPAC nutrition team
