Reduce excessive sugar intake
Sugar has shown to negatively affect immunity. One study in the American Journal of Clinical Nutrition found that oral 100-g portions of carbohydrate from glucose, fructose, sucrose, honey, or orange juice all significantly decreased the capacity of neutrophils to engulf bacteria meaning your ability to fight infection could be reduced. Starch ingestion did not have this effect. The decrease in phagocytic index was rapid following the ingestion of simple carbohydrates (1). Therefore, it is important to be mindful of your sugar intake in order to maximise your immunity.
The nutritional importance of Zinc has been known for a long time however recently it has been discovered that Zinc could play a central role in the immune system, and zinc-deficient persons experience increased susceptibility to a variety of pathogens (2). One study in particular found that zinc administered within 24 hours of onset of symptoms reduces the duration of common cold symptoms in healthy people (3).
The old famous immune boosting vitamin! Vitamin C functions as an antioxidant to neutralise free radicals as well as supporting a healthy immune system. Studies have suggested that a Vitamin C deficiency can result in reduced resistance against certain pathogens whilst a higher supply enhances several immune system parameters (4). It should be noted however that these results have been found when Vitamin C levels were consistently adequate- not just dosing up when you feel a cold coming on (5). So, with in the case of Vitamin C- consistency is key!
Get enough micronutrients
Many people nowadays are just not getting enough micronutrients. This is usually from a diet high in processed food and not enough vegetables. There is some evidence that suggests that deficiencies in certain micronutrients (such as zinc, selenium and iron) can alter immune response in animals. Although the effect on human immune response is yet to be assessed the research at this stage is promising and there is no harm in ensuring that you get enough micronutrients! This is where ensuring you are getting at least 5 handfuls of vegetables per day and even supplementing with a good quality greens powder can be great.