A 24-year-old man with type 1 diabetes presents to the emergency department with abdominal pain, nausea and polyuria. Capillary glucose is 26 mmol/L, venous pH 7.15, serum bicarbonate 12 mmol/L and serum ketones are strongly positive, confirming diabetic ketoacidosis. What is the most appropriate initial intravenous fluid for resuscitation?