University of Groningen
The development and implementation of a high-throughput phenotyping platform for
identification of a new mouse models of cardiovascular disease
Svenson, Karen Louise
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Chapter 5
Body mass index does not accurately describe obesity in mice
Karen L. Svenson
Submitted
67
Abstract
Objective: To evaluate the utility of the Body Mass Index (BMI) for describing obesity in mice by
comparing it to other methods of evaluating body composition and adiposity.
Research Methods and Procedures: Inbred mouse strains were fed a high fat diet for eight weeks
and were weighed, nose to anus body length was measured, and body composition was assessed
using dual-energy x-ray absorptiometry (DEXA). A group of intercross mice were fed standard
rodent chow and in addition to weight, length and DEXA measurements, weights of dissected fat
depots were obtained. Regression analyses were performed on each group of animals and on
combined data for both sexes and after separating sexes.
Results: Regression analysis of percent body fat and BMI revealed that variation in body mass
index did not reliably predict variation in percent body fat. By analyzing sexes separately, some
improvement in predictability of body fat percent from BMI was observed. Analysis of two
alternative methods for evaluating degree of adiposity in mice, DEXA and regional fat dissection,
revealed that these methods more reliably describe murine obesity than BMI.
Discussion: Rodent models of obesity are increasingly important for deciphering the etiology of
human obesity. Owing to inherent differences in body shape between mice and humans, using BMI
to evaluate obesity in mouse models can be misleading. Analysis of regional fat depots or methods
of measuring total percent body fat by noninvasive means provide more accurate assessments of
obesity in mice and are recommended as standard methodology.
Key Words: Mouse models, adiposity, regional fat, percent body fat, body composition
68
Introduction
Body mass index (BMI) is been an important clinical measurement for describing both
childhood and adult obesity. In humans, it is calculated from measurements of weight and height,
namely weight in kilograms divided by height in square meters (kg/m2). Charts of BMI for a broad
range of weights and heights are readily accessible to the general public as part of public health
efforts to raise awareness about the increasing prevalence of obesity worldwide and to offer
guidelines towards better general health and overall wellness. BMI measurements have also been
used to describe obesity in rodent models using body weight and nose-to-anus body length (g/cm2).
As research in complex human diseases continues to benefit from animal models, especially
rodents, many parameters used for humans have been extended to the analysis of disease in these
models. Mice and humans share many risk factors for disease. Measurements such as blood
pressure and lipid profiles, glucose and insulin levels, and hematological parameters have been
useful predictors in animal models of cardiovascular disease, diabetes, and autoimmunity,
respectively. Obesity, a condition of excessive weight due to fat, is also common to mice and
humans. Many robust rodent models of obesity have been developed and are widely used (6-8, 19).
Obesity in these models is typically described by growth curves using longitudinal weight data,
dissection and weighing of regional fat depots to calculate an adiposity index, BMI, and more
recently by computed tomography and dual energy x-ray absorptiometry (DEXA) which provide
measurements of percent body fat (%BF). Most of these methods provide reliable assessments of
obesity in rodents. This report, however, provides evidence that BMI can be misleading when used
to describe obesity in mouse models.
Research Methods and Procedures
Animals and diets: Females and/or males of 34 inbred strains (182 females; 172 males) were
obtained from The Jackson Laboratory, Bar Harbor, ME, and housed in pressurized individuallyventilated cages (Maxi-Miser PIV; Thoren Caging Systems, Hazelton, PA) within specific
pathogen-free rooms with a 12:12h light:dark cycle. Mice had ad libitum access to food and
acidified water. Intercross animals from parental strains C57BL/6J and NZO/LtJ (126 females; 122
males) were generated within this animal research facility and consumed only standard rodent chow
containing 6% fat (LabDiet 5K52; LabDiet, Scott Distributing, Hudson, NH). Inbred strains were
fed rodent chow from weaning until age 6-8 weeks and then were fed a diet containing 15% dairy
fat (30% caloric content), 50% sucrose, 0.5% cholic acid and 1.0% cholesterol (15). The Jackson
Laboratory Animal Care and Use Committee approved all procedures described in this report.
Phenotyping: DEXA. Under tribromoethanol anesthesia (0.02 mL/g body weight), animals were
analyzed for body composition using a Lunar PIXImus DEXA machine (Lunar Corp., Madison
WI). To calculate body mass index, animal weight and nose-anus length were obtained prior to
DEXA scanning. Inbred strains had consumed the high fat diet for eight weeks when body
composition measurements were obtained at 16 weeks of age. Intercross animals were scanned at
15 weeks of age.
Fat depot dissection. Cervical and inguinal fat depots (left sides only) were dissected from
intercross mice and weighed. Previous investigation has shown that weights of left and right
inguinal fat depots do not differ significantly (19). Likewise, left and right cervical depots are not
significantly different in weight (Svenson et al., unpublished observation). Hence, only the left
depots were harvested and their weights multiplied by two to obtain a value for total fat depot. To
derive the adiposity index (AI), total weight of fat depots was divided by body weight.
69
Data Analysis. Regression analyses were performed using Excel (Microsoft Corp., Redmond, WA).
Results
As shown in Figure 1, a simple
observation provided the rationale for further
investigation of the relationship of BMI to
%BF in mice using previously generated
phenotype data. When 34 inbred strains
were ranked by %BF and compared to their
ranking by BMI, 13 strains fell into different
categories (below, at or above one standard
deviation unit from the average for all
strains). Six strains moved towards a
ranking suggestive of obesity, and seven
strains moved towards a leaner category on
re-ranking from %BF to BMI.
Comparison of BMI to % Body Fat. Percent
body fat (%BF) ranged from 9.1-47.0%
among 354 animals of 34 inbred strains, and
BMI ranged from 20.7-49.6. Regression
analysis of BMI and %BF (Figure 2A) shows
R2 = 0.35 for both sexes combined (n= 182
females + 172 males). When sexes were
separated, R2 values improved to 0.42 among
females and 0.55 among males (not shown).
Among 248 intercross animals (126 females
Fig. 1. Comparison of 34 inbred strains ranked by
+ 126 males), %BF ranged from 13.9-48.6%,
percent body fat (%BF) to their ranking by body
and BMI ranged from 23.1-59.2. Regression
mass index (BMI). For each ranking, gray areas
2
analysis shows R = 0.22 for BMI and %BF
denote strains with average values within one
(Figure 2B). As was observed from the
standard deviation unit from overall strain mean for
2
inbred strain data, improved R values
the parameter (left column %BF; right column
resulted when sexes were analyzed separately
BMI). Dashed lines indicate that strains moved
(R2=0.64 females, 0.53 males; not shown).
towards an obese classification on ranking by BMI;
When inbred strain and intercross data are
solid lines indicate that strains moved towards a
leaner classification on ranking by BMI.
combined (n=602), regression analysis of
BMI and %BF for all animals shows R2 =
0.38 (Figure 2C), and after separating the sexes R2 = 0.52 among all females and 0.55 among all
males (not shown).
Comparison of Adiposity Index to % Body Fat. Comparison of AI to %BF among all intercross
animals (Figure 3A; R2 = 0.53) showed that more of the variation in %BF is explained by the
variation in fat pad depots than by the variation in BMI (compare Figure 3A to Figure 2B). When
sexes were analyzed separately, R2 improved in females (R2=0.63; not shown) and slightly
decreased in males (R2=0.49; not shown). Regression analysis of AI to BMI shows R2 = 0.28
(Figure 3B), providing further evidence against the utility of BMI in predicting adiposity.
Coefficients of determination improved in both females and males when sexes were separated (R2
70
female=0.42; R2 male =0.43; not shown). Fat depots were not obtained from the inbred strains;
therefore AI could not be compared to BMI or %BF in these mice.
BMI v % Fat
Inbred Strains
A
Fig. 2. Regression plots of BMI and %BF for
mice analyzed in this report. A) Females and
males of 34 inbred strains (n=354); B)
Intercross females and males (n=248); C) All
females and all males from each group
(n=602). Coefficients of determinarion (R2)
are as shown per plot.
50
45
% Fat (DEXA)
40
35
30
R2 = 0.3537
25
20
15
10
5
20
25
30
35
40
45
50
BMI
B
Discussion
BMI v % Fat
Intercross Mice
50
Percent body fat and BMI were
measured in two groups of mice included in
this report. In one group, inbred strains, mice
consumed a high fat diet. The second group
is comprised of intercross progeny derived
from one non-obese and one obese parental
strain and were fed standard rodent chow.
Regression analysis of BMI to %BF was
BMI v % Fat
Inbred Strains + Intercross
performed to investigate the efficacy of BMI,
C
a standard used to describe human obesity, to
define obesity in mice. Fat depot dissection
performed in the intercross group allowed an
additional comparison of adiposity index to
%BF and BMI. Both groups represent a
broad range of %BF from lean to obese
phenotypes. The intercross group had higher
BMI
BMI values than the inbred strains. Data was
evaluated as separate groups and as one
combined population. The combined data more closely represents the broad range of phenotypes
and dietary conditions observed in humans. The outcomes of regression analyses for separate and
combined data reveal that BMI is not a robust predictor of obesity in mice. It is notable that when
sexes are evaluated separately there is a marked improvement in the utility of BMI. However,
obesity is more reliably predicted in mice using methods that measure body fat more directly such
as dissection of fat depots to derive the adiposity index or by DEXA scanning. When included as a
phenotype in genetic analyses, BMI is less informative than percent body fat in both mouse (12, 21)
and human studies (4, 14).
Body mass index is derived from measurements of height and weight and does not consider
body tissue composition. Obesity is a clinical condition of overweight due to excess body fat.
While BMI has guided clinicians in assessing human obesity, it can be misleading if subjects have a
relatively high degree of lean tissue content. Furthermore, because women generally have higher
%BF than men (10), different thresholds for evaluating women and men based on BMI should be
used, but usually are not. Measurements of waist circumference (WC) or waist to hip ratios (WHR)
are often more descriptive of overweight due to increased fat (3, 9, 13). Clinical studies are finding
R2 = 0.2204
% Fat (DEXA)
40
30
20
10
20
25
30
35
40
45
50
55
60
BMI
50
R2 = 0.383
% Fat (DEXA)
40
30
20
10
0
20
25
30
35
40
45
50
55
60
71
A
Fig. 3. Regression plots of adiposity
index (AI) as determined from fat
depot weights, and A) %BF; and B)
BMI for intercross mice. Females and
males are included in each plot
(n=248). Coefficients of determination
(R2) are as shown in each plot.
AI v % Fat
45
40
% Fat (DEXA)
35
30
R2 = 0.5295
25
20
15
that more precise assessments of body
fat improve the evaluation of obesity in
both children and adults, although
thresholds for %BF have yet to be
B
AI v BMI
60
established (11, 20). Validation of
DEXA analysis in mice (5, 17) and
50
humans (2, 16) has established DEXA
as an accurate estimation of body fat
40
content.
For mice there are no clear
30
thresholds for BMI that define healthy,
20
overweight and obese states as are used
0
1
2
3
4
5
6
in humans. If human obesity thresholds
for BMI were used in the present
analysis of 602 mice, 4% would be considered healthy, 14% overweight, and 82% obese. Table 1
summarizes BMI and %BF for mice in the current analysis and compares these values to normal
human values (1, 18). Interestingly, for both mice and humans average BMI is higher in males
while %BF is higher in females. Compared to humans, average mouse BMI values are all in the
obese range. The mouse %BF values are a better comparison to human values.
Animal models with features of human disease are important tools for biomedical research and are
becoming increasingly important in developing highly specific targets for pharmaceutical
intervention. As we rely on these mammalian systems to help find genes underlying complex
human traits, accurate phenotypic definitions must be used to compare animal and human disease
states.
10
0
1
2
3
4
5
6
Fat Pad Weight/Total Weight (%)
BMI
R2 = 0.2774
Fat Pad Weight/Total Weight (%)
Table 1. Percent body fat and BMI values for female and male mice and humans.
Trait
BMI
Female
Male
F+M
40.1 ± 5.1
47.8 ± 4.6
43.9 ± 6.2
31.9 ± 4.5
36.2 ± 6.1
34.0 ± 5.7
35.3 ± 6.2
41.0 ± 7.9
38.1 ± 7.7
24.5 ± 2.6
26.6 ± 2.6
25.6 ± 2.8
26.0 ± 5.0
27.5 ± 4.4
26.3 ± 4.9
%BF
29.6 ± 5.4
27.6 ± 4.5
28.6 ± 5.2
(DEXA)
24.0 ± 7.9
22.3 ± 7.5
23.2 ± 7.8
26.3 ± 7.5
24.5 ± 7.0
25.4 ± 7.3
30.8 ± 6.3
21.8 ± 6.0
26.1 ± 7.6
36.7 ± 7.6
25.0 ± 7.2
34.7 ± 8.7
Healthy Humans†
*From Ball et al. 2005; n=24 females, 26 males. †From Sun et al. 2005; n=491 females, 100 males.
72
Population
Intercross Mice (F2)
Inbred Mouse Strains
F2 + Strains
Healthy Humans*
Healthy Humans†
Intercross Mice (F2)
Inbred Mouse Strains
F2 + Strains
Healthy Humans*
Acknowledgements
Willson Roper and Stacey Dannenberg provided expert technical assistance in DEXA
scanning, and Holly Savage meticulously performed fat depot dissections with a high degree of
accuracy and efficiency. This work was funded by NHLBI Program for Genomic Applications
grant HL66611.
73
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