Copper plays a role in our immune system. Among infection-fighting white blood cells, one type, known as macrophages, harnesses copper ions from blood as part of their defence, says Matt Sweet, an immunologist at Australia’s Institute for Molecular Bioscience.

“Bacteria require a certain amount of copper and zinc to survive, but too much can be toxic,” he says. “The immune system can manipulate the levels to beat an infection.”

But many men have copper-deficient diets, says Dr Leslie Klevay, a 
professor of pharmacology at the University of North Dakota. You need about a milligram a day. Good food sources include shellfish, wholegrains, beans, potatoes and nuts.

Running really low? Then you should talk with your doctor about supplementing.