Dairy cows mobilise body tissues to support milk production and, because glucose 
supplies are limited, lipids are used preferentially for energy production. 
Lipogenic activity is switched off and lipolytic mechanisms in adipose tissue 
increase through changes in the expression of several key enzymes. This results 
in a loss of body condition, together with high circulating concentrations of 
non-esterified fatty acids. Changes in the synthesis, secretion and signalling 
pathways of somatotrophic hormones (insulin, growth hormone, insulin-like growth 
factor 1) and adipokines (e.g. leptin) are central to the regulation of these 
processes. A high reliance on fatty acids as an energy source in the peripartum 
period causes oxidative damage to mitochondria in metabolically active tissues, 
including the liver and reproductive tract. The expression of genes involved in 
insulin resistance (PDK4, AHSG) is increased, together with expression of TIEG1, 
a transcription factor that can induce apoptosis via the mitochondrial pathway. 
Polymorphisms in TFAM and UCP2, two autosomal mitochondrial genes, have been 
associated with longevity in dairy cows. Polymorphisms in many other genes that 
affect lipid metabolism also show some associations with fertility traits. These 
include DGAT1, SCD1, DECR1, CRH, CBFA2T1, GH, LEP and NPY. Excess lipid 
accumulation in oocytes and the regenerating endometrium reduces fertility via 
reductions in embryo survival and increased inflammatory changes, respectively.