There are many dietary and other factors that distinguish populations without hypertension from those in which hypertension exists. The isolated groups in whom blood pressure does not rise with age also tend to have a low energy intake. The relationship between energy consumption and blood pressure is obscure, but a reducing diet that leads to weight loss is associated with a substantial fall in blood pressure, measured as intra-arterial pressure. A reduction in total energy consumption can therefore reduce blood pressure and this may partially explain the fall in cardiovascular mortality in populations with impaired food supplies, such as in the Netherlands in the Second World War.