Improving diet quality has been a major target of public health policy. Governments have encouraged consumers to make healthier food choices and firms to reformulate food products. Evaluation of such policies has focused on the impact on consumer behaviour; firm behaviour has been less well studied. We study the recent decline in dietary salt intake in the UK, and show that it was entirely attributable to product reformulation by firms; a contemporaneous information campaign had little impact, consumer switching between products in fact worked in the opposite direction and led to a slight increase in the salt intensity of groceries purchased. These findings point to the important role that firms can play in achieving public policy goals.