A compound which is 3-hydroxybutyl 3-hydroxybutyrate enantiomerically enriched with respect to (3R) hydroxybutyl (3R)-hydroxybutyrate of formula (I): is an effective and palatable precursor to the ketone body (3R)-hydroxybutyrate and may therefore be used to treat a condition which is caused by, exacerbated by or associated with elevated plasma levels of free fatty acids in a human or animal subject, for instance a condition where weight loss or weight gain is implicated, or to promote alertness or improve cognitive function, or to treat, prevent or reduce the effects of, neuro degeneration, free radical toxicity, hypoxic conditions or hyperglycaemia.