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CARDIOVASCULAR JOURNAL OF AFRICA • Volume 27, No 3, May/June 2016
AFRICA
181
other factors, in addition to body fat distribution, are important
determinants of IR in black women.
Another important finding of this study was that central
and peripheral FM were independently associated with fasting
insulin and HOMA-IR values in both the black and white
women. To our knowledge, this is the first study to demonstrate
independent associations among ethnically diverse women, a
finding that has been demonstrated in mostly white men and
women.
9-11
Differences in the contribution of abdominal and
gluteo-femoral fat to IR may be due to phenotypic differences
in adipose tissue depots. Indeed, studies from our laboratory
and others have shown that in white women, inflammatory gene
expression, especially in the abdominal depot, was significantly
associated with higher IR.
28-30
However, despite black women
having a higher SAT inflammatory gene expression profile
than white women, SAT inflammatory gene expression was not
significantly associated with IR in black women.
28
In contrast to abdominal fat, lower-body fat has been
suggested to act as a metabolic sink, storing excess free fatty acids
(FFA) when there is an energy surplus, due to its lower lipolytic
activity and higher lipoprotein lipase (LPL) activity, compared
to upper-body fat stores.
31-34
Lower-body fat has been suggested
to protect against ectopic fat deposition and therefore protect
against risk for CVD and T2D. However, when the capacity to
store excess fat in the periphery is exceeded, peripheral fat is
no longer protective. A small SA study demonstrated reduced
adipogenesis and lipogenesis in obese black women, and this
was associated with increased IR.
18
Furthermore, a recent
study from our laboratory has shown that with increasing
weight gain, black women accumulated more central relative to
peripheral FM, which was associated with the development of
IR.
35
These findings imply that the prevention of an increase in
centralisation of body fat is vital for the prevention of metabolic
risk in black women, and it is important to determine the point
at which peripheral FM is no longer protective.
In addition to body fat distribution, other lifestyle factors
also influenced IR, but these differed between black and white
women. In black women only, contraceptive use, which was
predominantly in the form of injectable contraceptives, was
associated with increased IR, which is supported by previous
studies.
36
In addition, physical activity was differentially
associated with IR in the black and white women. Despite
white women having lower MVPA than black women, MVPA
was associated with IR in white women only. Previous research
from our laboratory has demonstrated that white women mainly
perform leisure activity, typically undertaken at a higher intensity,
whereas black women mainly perform physical activity for travel,
typically undertaken at a lower intensity.
17,37
This may suggest
that the intensity of exercise is an important determinant of IR.
HDL-C and TG concentrations are often used as markers
for IR.
38,39
Despite similar levels of IR, HDL-C and TG
concentrations were lower in the black compared to the white
women. Notably, similar to the findings for IR, higher HDL-C
and lower TG concentrations were associated with reduced
central and increased peripheral FM, and these associations were
similar in black and white women, a finding supported by studies
in the USA.
40,41
The lower HDL-C concentrations of black
women must therefore be explained by other factors.
HDL-C in black women was associated with alcohol
consumption, independent of body fat distribution. Alcohol
consumption may raise HDL-C concentrations by increasing
the transport rate of the major HDL apolipoproteins Apo-I and
Apo-II.
42
Contraceptive use was another significant determinant
of HDL-C levels in both black and white women and TG levels
in the white women. Notably, contraceptive use was negatively
associated with HDL-C concentrations in the black women
and positively associated in the white women, which could
be explained by the type of contraception used. Studies have
demonstrated that women using injectable contraception had
lower HDL-C and TG concentrations compared to those who
were on oral contraception.
36
In our study, we also demonstrated that independent of
body fat, TC and LDL-C concentrations were lower in the
black compared to the white women, which is in agreement
with similar SA studies.
22,43,44
Additionally, we demonstrated that
increased TC and LDL-C concentrations were associated with
increased central FM in white women only. Hosain
et al
.
45
also
demonstrated that the association between central FM and lipid
levels was stronger in white compared to black women.
It is important to note that increased trunk FM was
significantly associated with increased LDL-C concentrations in
the white women, whereas in the black women, abdominal SAT
areawas negatively associatedwithLDL-CandTCconcentration,
suggesting a protective effect of SAT in the black women. There
have been a number of studies demonstrating that SAT is
protective against IR and increased lipid levels, more specifically
TG in women with higher BMI.
46
Additionally, lipodystrophic
loss of SAT results in increased IR and dyslipidaemia.
47
By contrast, other studies in women have demonstrated that
loss of abdominal SAT did not produce the same beneficial
results as VAT in terms of reduced IR and dyslipidaemia.
48
This
is the first study of which we are aware, that has demonstrated
a protective effect of abdominal SAT on cardiometabolic risk in
black women. A possible mechanism for the protective role of
SAT is that it is an alternative depot for excess FFA, potentially
reducing ectopic fat deposition in VAT and the liver, thereby
preventing lipotoxicity and reducing dyslipidaemia.
49
Notably, we found no associations between arm FM and
any metabolic risk factor in black and white women. A few
previous studies have examined the independent effects of arm
vs leg FM on cardiometabolic risk, but the results have been
contradictory.
11,50
Although arm FM is regarded as ‘peripheral
fat mass’, this is from the upper body and therefore may not
exhibit the same protective effects as leg FM. Further studies are
required to understand these disparate findings.
The strengths of this study include the state-of-the-art
measures of body fat distribution, DXA and CT scans, and the
examination of ethnic-specific associations with cardiometabolic
risk. Possible limitations of the study were the inclusion of a
convenient sample of women, which was not representative of
the total population. The black women were more obese that
the white women but this may be reflective of the population,
according to a recent population survey.
5
The cross-sectional
design of the study limits one to derive conclusions in terms
of causality. The number of women in which CT scans were
conducted was low (76% of total sample) and this may have
created type II error. Furthermore, we did not measure other
lifestyle factors such as diet, and this has been shown to affect
body fat and cardiometabolic risk. More objective measures
of physical activity, using accelerometers, should be used, as