So you last trip to McDonalds made you feel guilty hey? Well, while you (hopefully) realise it was bad for your waistline, you might not have realised it could also be having a negative impact on your immune system function. Dietary deficiencies have long been associated with immune dysfunction in the medical world. What does this mean to you? It means that you need to ensure your nutrition is well-balanced, containing adequate amounts of protein, and specific micronutrients. This information is particularly relevant if you're an athlete or regular gym-goer. While you're probably thinking ‘WHAT!? I thought going to the gym was improving my health, not detracting from it', it's important that we establish strenuous exercise and heavy training are associated with depressed immune cell function. This doesn't mean you should stop working out to ensure you don't get ill. It also doesn't give you another excuse to skip that gym session. All it means for the healthy individual is that its essential you tailor your nutrition to the demands you're placing on your body.

Our immune systems protect us against all manner of nasty invaders in the body. When we train hard, whether this is lifting weights in the gym, or participating in a marathon, or just regular hockey practice, it can lead to immune impairment. Immune impairment is connected with increased susceptibility to infection. Studies have theorised that our immune impairment as a result of our strenuous exercise is likely caused by the increase in stress hormones such as cortisol and adrenaline. With the possibility that despite our best intentions, training hard could lower our bodies defences, it's of the utmost importance we avoid nutrient deficiencies. Malnutrition decreases our immune defences, not good, especially when we see the ‘uninformed' training like crazy and eating a few rice cakes a day. This is why you should never follow ‘celebrity fad' diets. Never, never, ever.

To reiterate the importance of this point to you, it's the athletic population, on a crash-diet, restricting food groups severely to lose weight, who are most at risk. It's of particular importance that we don't restrict essential micronutrients, and protein in our diets. This applies even when trying to lose weight. Studies have illustrated that immunity is affected negatively by protein-energy restriction. We must take into account that athletes are very unlikely to reach severe malnutrition, however defence mechanisms can still be negatively influenced by protein deficiencies at a moderate level.

So how to do we go about ensuring we can train hard, diet, and keep our immune system defence working well. Adequate intakes of those foods, vitamins, and minerals shown to aid the immune function are an significant part of the puzzle. This means that intake of iron, vitamins A, E, B6 and B12, should be at present. Protein is another key ally in the battle to keep our immune systems firing. This means sufficient protein, even if you're dieting and trying to lose weight, don't lower your protein significantly, if at all. Micronutrients must be ingested at ample levels too. No single food we eat contains all the micronutrients we need to ensure optimum nutrition. Make sure you're getting enough vegetables and fruits throughout the day. Some coaches I've worked with have recommended ‘eat the rainbow', this translates to eating fruits and vegetables of different colours, containing differing nutrients. So for example, you may have some green broccoli and yellow pepper with lunch, and have red tomatoes and orange carrots with your evening meal.

Don't go mad with supplements either, you might think this is an easy route to boosting your immune system, but you'd be wrong. There is a distinct lack of evidence that any of the supposed ‘immunity boosting' supplements out there (google it, there's tonnes), actually work. It's an expensive leap of faith at best. Some scientific studies also have concluded that ‘megadoses' of vitamins and minerals could actually impair immune function rather than increase it. It's worth noting some studies do show vitamin supplementation for athletes training hard on weight-loss diets, may be beneficial. For the majority of us, it's probably best to concentrate on  getting plenty of mixed vegetables into your daily nutrition. This should see you on the right path. Remember, nutrition isn't all about making you look good, it can help to keep you healthy too. Avoid the crash diets, take a long-term and educated approach to your nutrition, and remember it is possible to diet too hard.