• SHORT syndrome features insulin resistance and reduced adiposity without dyslipidaemia and fatty liver. • A mouse model with a pathogenic human PI 3-Kinase mutation recapitulates this uncoupling. • Surprisingly, no adipose injury nor increased liver de novo lipogenesis is seen. • Energy expenditure is increased, causing resistance to diet-induced obesity. • This increases evidence for some beneficial metabolic effects of PI 3-Kinase inhibition.