We have three tiers of protection against parasites, bacteria, moulds, yeast, fungi and viruses. First we have natural physical defences. These include the skin, and the mucus membranes in the lungs and digestive tract. Tears, hydrochloric acid in the stomach and resident friendly bacteria in the gut also play a role in protecting our systems from foreign substances that could harm the body.
Then we have what is called the innate immune system. When you cut through skin, a group of white cells called macrophages rush to the site to engulf and destroy any harmful bacteria that could enter the cut, causing inflammation and swelling in the process.
The third is the adaptive immune system. If the innate system is overwhelmed by or ineffective at repelling an invader, the third line of defence comes into play. This is a highly sophisticated system, which protects us against more complex assaults by, for example, viruses.
What we eat can support the immune system and help to ensure it’s in good working order, particularly when we are stressed or have a severe work or training schedule.
Making the right food choices, getting enough exercise and managing our stress can positively help to strengthen our natural defences.
The immune system needs well over 20 different micronutrients to function properly. Normally, we get enough nutrients from foods rather than from high-dose supplements, which sometimes do more harm than good.
Immune cells including monocytes and macrophages contain vitamin D receptors. These cells are activated to fight infection when necessary. Vitamin D has a role in regulating the immune response that helps to suppress pro-inflammatory cells. It protects the lungs and respiratory system from infections by increasing the secretion of anti-microbial immune cells such as neutrophils and natural killer cells.
From April to September we can make some vitamin D by exposing our face, arms and legs to UV rays. However, we can ensure we have enough of this vitamin by regularly including vitamin D rich foods, such as oily fish (salmon, tuna, sardines and mackerel), eggs and fortified foods (milk, yoghurts and cereals) in our meals.
The trace element selenium is crucial for the immune system to function normally. A deficiency results in a weakened immune response and a diminished ability to fight viral infections. Brazil nuts are the richest natural sources of selenium, although this is soil dependent, followed by fish, shellfish, offal, meat, chicken and game.
The intestine is composed of three kinds of defence that work together: good bacteria, the intestinal wall and the immune system. To express their benefits, probiotics in foods must reach the intestine in sufficient quantities and so resist the effects of stomach acids. Certain strains of probiotics have been shown to reduce the duration of respiratory infections. Other strains balance the intestinal microbiome, which helps support normal digestive and immune health. You can boost friendly gut bacteria by consuming low-fat probiotic milk, yogurts and other dairy items.
The mineral zinc helps develop white blood cells, immune cells that fight off foreign bacteria and viruses. A zinc deficiency can greatly increase our risk of infection. Zinc is found in abundance in oysters and shellfish. Lean red meat is another good source but vegetarians have to rely on other foods to ensure adequate zinc, such as pumpkin and other seeds, fortified cereals and low-fat yoghurt and milk.
Vitamins A,C and E
Protective vitamins, minerals and other components found in fruit, vegetables, herbs and spices are called phytonutrients. A steady and consistent intake of these phytonutrients helps to keep our immune systems working properly.
Aim to eat at least two pieces of fruit and three vegetables a day to get a complete complement of vitamins and minerals for immune health. In particular, mushrooms help in the production of white blood cells.
Research suggests that the phytonutrients found in mushrooms such as shiitake and maitake can help white blood cells to act more aggressively against foreign bacteria.
Broccoli and other cruciferous vegetables such as cabbage, kale, Brussels sprouts and cauliflower are good sources of vitamins in general. Broccoli also contains glucosinolates, which help to stimulate the body’s immune system. Vitamin B2, also found in broccoli, is vital for growth and vision and a healthy immune system.
Omega 3 fats are also essential. They work by increasing the activity of phagocytes, the white blood cells that destroy bacteria. These fats also help strengthen cell membranes, thereby speeding up healing and strengthening resistance to infection in the body.
Eating a minimum of one portion of omega-3 rich oily fish (salmon, mackerel, tuna) each week is good for heart, joint and immune function. Maintaining a healthy weight can help your immune health too.
Carrying excessive weight around the middle has been associated with compromised immune function, chronic inflammation and an increased risk of infection.
Paula Mee is a dietitian and member of the Irish Nutrition and Dietetic Institute. Email:firstname.lastname@example.org; Twitter:@paula_mee
Paula Mee’s meals to support your immune system
Apricot and walnut porridge: soak oats overnight with chopped dried apricots. Add three chopped walnuts when cooked. Two thin slices of wholemeal toast topped with manuka honey. Cup of green tea or red bush tea.
Berry good smoothie: blend together one teacupful of frozen raspberries and one teacupful of fresh blueberries with one teaspoon honey and five or six ice cubes until smooth. Serve immediately. Cup of green tea or red bush tea.
Mango and avocado salad with chicken (ripe avocado, 1 teaspoon lemon juice, small mango, 1 tablespoon olive oil, 1 teaspoon wholegrain mustard, 1 teaspoon clear honey, teaspoon cider vinegar, freshly ground black pepper, handful of watercress, 1oz cooked beetroot, finely sliced 2 oz cold chicken, thinly sliced. Mix the olive oil with the wholegrain mustard, honey, vinegar and ground black pepper, and mix well. Remove the avocado from the lemon juice and mix the juice into the dressing. Arrange the watercress and beetroot on a plate or in a salad bowl and add the avocado and mango flesh. Drizzle the vinaigrette over the salad and top with the slices of smoked chicken.
A probiotic yoghurt, three Brazil nuts, three almonds.
A roasted salmon parcel seasoned with lemon juice, ginger or chilli, served with your favourite roasted vegetables and sweet potato.
Two tablespoons homemade fruit and cinnamon compote with a swirl of low-fat Greek yogurt. A cup of green tea or herbal tea.