It's taken a few hundred years, but scientists have finally figured out why it is that an apple a day keeps the doctor away. An apple - and, indeed, oats and nuts, too - is rich in soluble fibre, which boosts the immune system. Scientists are recognising that many diseases, such as heart disease and diabetes, are related to inflammation, and apples transform cells into anti-inflammatory ones that can accelerate healing. While health authorities recommend we eat around 25 to 35 grams of fibre a day, they don't point out that you need a mix of both soluble and insoluble. Insoluble fibre is found in whole wheat and green, leafy vegetables such as spinach, and it helps aid digestion. (Source: Brain, Behavior, and Immunity, 2010; doi: 10.1016/j.bbi.2010.01.015).