EGGS

Eggs are one of the few foods that are a natural source of vitamin D, with one egg providing nearly one-third of your daily requirement. Vitamin D deficiency (which affects one in five people in the UK) may increase your chances of upper respiratory infections, according to research published in the British Medical Journal. Scientists have found that your immune cells have vitamin D receptors, which are important for regulating your body’s natural defence mechanisms.

Eat it: Knock up a simple frittata by mixing beaten eggs with a handful of baby spinach, mushrooms or red peppers; pour into a hot frying pan and cook until almost set, then pop under a hot grill until puffed up and golden.