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.
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.