Subcutaneous versus visceral adipose tissue
The distinction between SAT and VAT is important because of the
independent association of VAT with increased metabolic risk. People who
carry their weight around their femorogluteal region (SAT) which is the
predominant pattern seen in women, are relatively protected from
metabolic risk in obesity (Booth et al., 2014). In contrast, those who
predominantly lay down visceral adipose are at increased risk (Kwon et
al., 2017). The reason for this different impact on metabolic health is
not clear, but several features of SAT and VAT have been implicated.
- SAT has a greater capacity for expansion by hyperplasia, which results
in more abundant but smaller and “healthier” adipocytes. VAT however
favours hypertrophy for expansion which results in large and less
healthy adipocytes, which can more readily induce lipolysis and
release fatty acids into the circulation or directly to the liver via
the portal circulation (Bergman et al., 2006). A similar pattern of
adipocyte size is found in horses, with peri-renal and retroperitoneal
adipocytes having a significantly larger cross-sectional area than
nuchal adipose (Bruynsteen et al., 2013).
- In humans, VAT has a more inflammatory phenotype, compared with SAT.
This is the case both in ‘normal’ individuals and those in the face of
persistent calorie excess (Ibrahim, 2010). In horses there was
increased mRNA levels of inflammatory cytokines in visceral depots
compared with the nuchal depot (Bruynsteen et al., 2013).
- Leptin secretion is greater from SAT relative to VAT in humans and
horses (Bruynsteen et al., 2013). In humans, adiponectin expression is
higher in SAT than VAT (Fain et al., 2004) but the opposite is found
in horses (Bruynsteen et al., 2013). There is also some evidence in
humans and horses (Warnken et al., 2017) that SAT is more insulin
sensitive than VAT, potentially contributing to “safer” storage of
lipids.
This distinction between SAT and VAT is relevant in horses because
clinical measures of adipose tissues (body condition scoring, weigh
tapes) almost exclusively measure SAT. It is important to recognise this
as a limitation and remember that visceral adipose may be more important
in predicting disease risk. Body condition score is only strongly
correlated with total body fat as determined by deuterium oxide dilution
(eTBF%) (Dugdale et al., 2011) in lean or non-obese horses. As BCS
increases, the predictive ability of BCS reduces significantly (Dugdale
et al., 2012). In addition, reliance on BCS may lead us to miss horses
with body fat carried almost exclusively around the viscera, a state
referred to as TOFIs in human medicine: Thin on the Outside Fat on the
Inside (Thomas et al., 2012). Whilst we cannot use the MRI techniques
employed in human medicine, it is worth considering abdominal ultrasound
in horses if a TOFI phenotype is suspected.