chapter 7: eat to improve your immunity – the micros and phytonutrients
The wide range of nutrients involved in supporting immunity indicates that a nutritious, balanced and diverse diet is key. There are 13 vitamins and 20 minerals, and we need to meet all these micronutrient requirements to ensure our immune system is able to perform at its best. Various micronutrient deficiencies can have a significant and negative impact on our immunity, thereby increasing our risk of infection and disease. Let’s have a look at the key vitamins and minerals involved in strengthening our immunity.
The vitamins
Vitamin A
This micronutrient plays an important role in our immunity, as it helps maintain an active thymus, which is key for strong immunity. Also, vitamin A is a potent antiviral vitamin as it helps to make cell walls stronger and more resistant to viral infection. Furthermore, the immune-system cells need vitamin A for lysozyme production, which is an antibacterial enzyme. Vitamin A is known as an anti-inflammatory vitamin because of its critical role in enhancing immune function62. It plays a vital role in helping to keep the skin, the lining of the respiratory system and the gut healthy, thereby protecting us from infections. Along with vitamin D and zinc, vitamin A is crucial for the health of the gut barrier. It also helps to maintain vision, particularly in dim light, and support growth. When you are fighting an infection, you need more vitamin A, and you may also require more if you are a smoker, experiencing intense stress, or are exposed to pollution.
Vitamin A derived from animal sources is called retinol, and that from plant-based sources is carotenoids. Retinol is more bioavailable, since the body has to convert carotenoids to retinol.
How much do we need a day?
In the UK, government guidelines for adults aged 19–64 is 0.7mg per day for men and 0.6mg per day for women. You should be able to get all the vitamin A you need from your diet. Also, any vitamin A your body doesn’t need immediately is stored for future use, which means you don’t need it every day.
In the US, recommendations are slightly different. The National Institutes of Health advise that the recommended daily amount (RDA) is actually given as retinol activity equivalents (RAE) to allow for the different bioactivities of retinol and provitamin A carotenoids – all are converted into retinol by the body63. For adults aged 19 and over, the guidelines are 900mcg (RAE) and 700mcg (RAE) per day for men and women, respectively.
Which foods can vitamin A be found in?
Rich food sources of vitamin A include beef liver, carrots, sweet potatoes, squash, mango, watercress, cantaloupe melon and pumpkin.
Should you supplement?
Studies have shown that consuming in excess of approximately 1.5mg a day of vitamin A over a long time may adversely impact bone health, putting you at an increased risk of fracture as you get older. Therefore, if you do take a multivitamin containing vitamin A, just check that your intake from supplements and food together doesn’t exceed 1.5mg per day. If you eat liver or liver pâté more than once a week, be cautious, as you may be getting too much vitamin A.
Vitamin C
Vitamin C, also known as ascorbic acid, has an essential and extensive role in the immune system. Numerous studies have reported that vitamin C has powerful antimicrobial properties, which decrease infection risk, as well as exerting immunomodulatory functions64. Research has shown that a deficiency of the vitamin appears to make you more susceptible to infection. Some research suggests that, although regular vitamin-C intake likely won’t prevent you from catching a cold, it may decrease the duration or severity of cold symptoms. A review of 31 studies found that consuming 1–2g of vitamin C daily reduced cold duration by 18 per cent in children and 8 per cent in adults. The review also confirmed that vitamin C may be beneficial for individuals who undergo brief periods of intense physical exercise65.
As well as supporting our immune system, vitamin C has an important role in helping to protect the cells in our body. This potent antioxidant helps to heal wounds and maintain skin health, as well as helping to keep our blood vessels, bones and cartilage in tip-top condition. In addition, it’s known that vitamin C increases iron absorption; thus, individuals with iron deficiency might benefit from increasing their vitamin C intake.
How much do we need a day?
In the UK, government guidelines for adults aged 19–64 is 40mg a day. You should be able to get all the vitamin C you need from your daily diet. Unlike vitamin A, it can’t be stored in the body, so you need it in your diet every day. As we get older, our absorption of vitamin C decreases significantly, so older people need to increase their vitamin C intake, and spread it out over the course of the day.
In the US, the RDA for men aged 19 and over is 90mg a day, and for adult women it is 75mg a day. Pregnant women are advised to have 85mg of vitamin C a day, while 120mg a day is recommended during breastfeeding. Furthermore, US guidelines suggest that smokers need 35mg of vitamin C more a day, compared with those who don’t smoke.
What foods can vitamin C be found in?
Foods rich in vitamin C include kiwi fruit, guavas, yellow and red (bell) peppers, blackcurrants, red chillies, broccoli, tomatoes, cabbage, cauliflower, strawberries and oranges.
Should you supplement?
Whether to supplement completely depends on your individual circumstances. Supplementing with 1–3g a day has been reported to have benefits if you are battling a cold, such as decreasing the severity of symptoms and reducing recovery times.
Vitamin D
This micronutrient is super-important for maintaining a strong immune system, with research indicating that vitamin D is involved in the activation of our important T cells, by ‘priming’ them ready to fight infection66. Furthermore, vitamin D has been shown to regulate gene expression and have an immunomodulatory influence on our immune cells. This powerful micronutrient can also promote the function of our NKT cells, increase Treg activity and inhibit Th167.
The body creates vitamin D from sunlight on the skin, and from springtime to mid autumn the majority of us should be able to get the vitamin D we need from sunlight. However, mid autumn and spring, we may not be able to get enough vitamin D from sunlight alone, even if we regularly spend time outside, which could result in deficiency.
Vitamin D deficiency can lead to rickets in children, which is a bone deformity, and a condition called osteomalacia in adults. Taking a vitamin D supplement in winter may be recommended for some individuals, especially if a blood check highlights you may be low in this essential micronutrient.
Groups at risk of low vitamin D include:
In addition, vitamin D deficiency has long been associated with systemic autoimmune disease and is prevalent in multiple autoimmune diseases such as multiple sclerosis, Type-1 diabetes and lupus. Vitamin D status is associated with the risk of autoimmunity, therefore this potent micronutrient could be useful in potentially preventing and protecting you from autoimmune diseases.
As well as supporting our immunity, vitamin D has several other important functions, such as regulating the amount of calcium and phosphate in the body. These nutrients are required to help bones, teeth and muscles stay healthy. We will explore the role of vitamin D in more detail in Chapter 12.
How much do we need a day?
In its report ‘Vitamin D and Health’, the Scientific Advisory Committee on Nutrition (SACN) recommends that Recommended Nutrient Intake (RNI) for all people ages 4 and over is 10mcg per day. For younger children and infants, the SACN advised that (as data are insufficient to set an RNI) a ‘safe intake’ of 8.5–10mcg per day has been set.
In the US, the RDA for adults aged 19–70 years is 600 IU (15mcg) for both men and women.
25-hydroxyvitamin D (25(OH) D) is commonly considered as the best indicator of vitamin D status in the body.
Storage of vitamin D and 25(OH)D in adipose and other tissues has been well documented68. This could, in part, explain why serum 25(OH)D levels do not fall to critically low levels during the winter. So, in theory, you could stock up on it during the summer and early autumn ready for the start of the cold and flu season.
Which foods can vitamin D be found in?
Foods that contain good amounts of vitamin D include oily fish, e.g. wild salmon, herring, mackerel, sardines. Egg yolk and meat contain small amounts; some breakfast cereals contain added vitamin D. Increase vitamin D levels further by exposing mushrooms to ultraviolet light.
Should you supplement?
In the UK and parts of the US, it can be tricky to get our daily requirements of vitamin D all year round, since the sun isn’t strong enough in the winter to allow us to make it. If you do opt to supplement, then 10mcg each day should be enough. If you are pregnant, be aware that while cod liver oil can be a good source of vitamin D, it also contains a lot of vitamin A, so do avoid cod liver oil if you are pregnant.
Vitamin E
Vitamin E is a fat-soluble vitamin that protects our cell membranes from damage, and the a-tocopherol form of this micronutrient particularly protects against cellular damage. We want to limit cellular damage as much as possible as it may lead to ‘improper’ immune response. Vitamin E is a powerful antioxidant that protects us from oxidative stress, and also has the ability to modulate immune functions.
How much do we need a day?
The UK government guidelines for adults aged 19–64 is 4mg per day for men and 3mg per day for women. You should be able to get all the vitamin E you need from your diet. Also, any vitamin E your body doesn’t need immediately is stored for future use, which means you don’t need it every day.
In the US, the RDA for individuals aged 14 and over is 15mg a day.
Which foods can vitamin E be found in?
Good sources of vitamin E include nuts and seeds, olive oil, wheat germ oil and avocado.
Should you supplement?
Most people who live in the UK and other developed nations, meet their vitamin E requirements through their diet. However, some digestive conditions, such as Crohn’s disease and coeliac disease can hinder fat absorption, and individuals who suffer from these diseases may experience vitamin E deficiency. The US National Institutes of Health (NIH) state that supplementing with high doses of vitamin E can lead to adverse side effects, including one very serious side effect – haemorrhagic stroke69.
Therefore, if you do suffer from a digestive condition, it would be advisable to speak with a dietician or nutritionist who can advise whether you may need to supplement.
Zinc
Zinc is a mineral and powerful antioxidant that is vital for strong immunity. It is a key player in cellular metabolism, and is needed for enzyme activity, protein and DNA synthesis as well as cell division. Zinc also supports wound healing, and growth and development during childhood. It is also very important in pregnancy, as it helps to build baby’s cells and DNA. Zinc is critical for immune function, and for the development and function of cells that mediate our immunity70. Zinc deficiency has been shown to compromise the number and function of lymphocytes, with T cells being particularly vulnerable to zinc deficiency71. A deficiency in zinc causes the thymus to shrink, leading to low numbers of T cells, and can alter cytokine production, leading to oxidative stress and inflammation.
Even mild zinc deficiency, which is more common than severe zinc deficiency, can suppress aspects of immunity72. The elderly may be particularly at risk of deficiency given that there is a high prevalence of inadequate dietary zinc intake among those 60 years of age and older, and that plasma zinc concentration declines with age. It is not known why plasma zinc declines, but weakened absorption and epigenetic dysregulation may be contributing factors. Several RCTs (the gold standard in clinical trials) suggest that supplementation with low to moderate doses of zinc (10–45mg zinc/day) in healthy elderly individuals improves immune function.
How much do we need a day?
In the UK, government guidelines for men and women aged 19–64 is 9.5mg and 7mg per day, respectively.
In the US, 11mg for men and 8mg for women are the recommended daily intakes.
You should be able to get all the zinc you need from your daily diet. However, as zinc is not stored in the body, regular dietary intake is important to maintain normal functioning.
What foods can zinc be found in?
Oysters contain more zinc per serving than any other food, but red meat, poultry and shellfish provide the majority of zinc in the average Western diet. Other good food sources include beans, nuts, wholegrains, pumpkin seeds and dairy products. Phytates that are present in wholegrain breads, cereals, legumes and other foods can inhibit its absorption. Thus, the bioavailability of zinc from grains and plant foods is lower than that from animal foods, although many grain and plant-based foods are still good sources of zinc.
Should you supplement?
The Department of Health (UK) recommends that zinc intake should not be over 25mg per day. Furthermore, high doses of zinc reduce the amount of copper the body can absorb, which can lead to anaemia and weakening of the bones. So, just make sure you are staying within the guidelines.
Selenium
Selenium is another mineral and antioxidant that is critical for immune function, and is involved in the production of antibodies. In addition, selenium helps to regulate the production of cytokines and eicosanoids, which are involved in immune response.
Selenium deficiency has been shown to impair aspects of both the innate and adaptive immunity, and has been reported to promote the severity or progression of some viral infections73.
An antioxidant is a molecule that prevents or lessens the damage caused by free radicals and other oxidants.
How much do we need a day?
In the UK, government guidelines for men and women aged 19–64 is 75mcg and 60mcg per day, respectively. If you eat meat, fish or nuts, you should be able to get all the selenium you need from your daily diet.
In the US, the RDA for individuals over the age of 14 is 55mcg, while 60mcg is recommended during pregnancy, and 70mcg for breastfeeding mothers.
What foods can selenium be found in?
Rich sources of selenium include: Brazil nuts, sesame seeds, wholegrains and seafood.
Should you supplement?
Selenium influences immune response in various ways, and whether to supplement entirely depends on your individual circumstances. Supplementation has been reported to enhance cell-mediated immunity in those who may be deficient, and also strengthen the response to viruses. In contrast, supplementation has been shown to exacerbate allergic asthma and weaken the immune response to parasites74. Nutrition and supplementation is completely personalized, so you may wish to speak with a nutritionist or dietician.
The deal with supplementation
Should I supplement or not? I get asked this question almost every single day!
The first step towards improving your vitamin and mineral status is to focus on including a good variety of the foods mentioned, which are rich in the essential vitamins and minerals that are crucial to our immunity and health. Whether to supplement depends on your individual circumstances – nutrition has to be personalized.
The one supplement I do recommend for people over 60 is vitamin B12, since this micronutrient becomes harder to absorb as we age. I also recommend B12 for those who follow a vegan diet. Additionally, if you are elderly, or under a lot of physical or emotional stress, you may wish to take an all-round, high-quality multivitamin supplement. In a randomized control trial, elderly individuals who took a multivitamin experienced half the number of infections. They also gained considerable improvements in the strength and integrity of their immunity75. Therefore, supplementation may help elderly individuals maintain stronger immunity.
However, for most people, I would advise focusing on getting your nutrients from your diet first. We know that an excess of certain micronutrients can result in harmful side effects, and several studies have confirmed that taking high-dose supplements of vitamins A, C, D, E and folic acid is not always helpful for disease prevention, and it can even be damaging to health76. Furthermore, the long term health consequences of vitamin and mineral consumption are still unknown.
Vitamins and minerals for strong immunity
These micronutrient recommendations are based on the daily UK government guidelines for an average adult aged 19–64.
Phytochemicals
Phytochemicals, also known as phytonutrients, are compounds produced by plants, which have been shown to be beneficial for our health. Studies have reported they can reduce inflammation and fight harmful free radicals, thus helping to nourish our immunity and maintain good gut health.
There are many phytochemicals providing lots of different health benefits:
Polyphenols such as carotenoids support immune function, eye health and have also been shown to decrease cancer risk. There are hundreds of different carotenoids, including beta-carotene, lutein and lycopene. It is best to eat a carotenoid-food with a fat source, since this will promote optimal absorption. Carotenoids can be found in sweet potatoes, cantaloupe melon, carrots and dark leafy greens, such as kale and spinach.
Ellagic acid is another potent polyphenol, which has antioxidant and anti-inflammatory properties. It has also been reported to reduce the risk of cancer. The best food source is raspberries. Other good food sources: strawberries, blackberries, grapes, walnuts and pecans.
Resveratrol is my favourite polyphenol, and is found predominantly in the skin of grapes and red wine! Resveratrol promotes cardiovascular health and also cognitive health. Good food sources: blueberries, strawberries, dark chocolate and peanuts.
Flavonoids are also part of the polyphenol class of phytonutrients, and are rich in antioxidant, anti-inflammatory and antimicrobial properties. Also in the flavonoid family we have flavones, anthocyanins, flavanones, isoflavones and flavonols. Good food sources: apples, onions, soy, coffee, legumes, ginger and green tea.
Glucosinolates are sulfur-containing compounds that may promote longevity by protecting against chronic disease. Good food sources: broccoli, cauliflower, cabbage, mustard and horseradish.
Variety is the spice of life
Herbs and spices are also high in phytochemicals, and including them in your daily diet not only benefits your immunity and health, but also provides excitement to food. I always advise clients to cook with herbs and spices regularly and – if possible – to use several at a time. Rich in phytochemicals, herbs and spices can help to combat harmful inflammation and promote good health. I use a lot of different herbs and spices in the recipes in Part 3, so let’s take a look at some of the main players:
Turmeric – this spice gets a lot of press. It’s a good source of curcumin, an antioxidant that eases inflammation, and studies also indicate that curcumin may help ease pain. In addition, further studies have demonstrated that frequently consuming even small amounts of turmeric may help prevent or slow down Alzheimer’s disease. It has also been shown to protect gut lining and inhibit the growth of harmful pathogens.
Cardamom – this is a sweet and pungent spice that has been reported to help combat inflammation. In addition, out of all the spices, cardamom is a great source of immunity-nourishing zinc.
Chilli pepper – this will certainly give your food a kick, which is down to capsaicin, the compound that makes chillies spicy. Chilli may also boost your metabolism and help maintain blood-vessel health.
Cinnamon – sweet and ultra-versatile, you can add it to so many different dishes. I use it a lot in porridge (oatmeal), baking, and main dishes such as curries. Research has shown that cinnamon may also help combat inflammation and oxidative stress, as well as helping to protect against harmful bacteria.
Ginger – anti-inflammatory with antioxidant properties, ginger can soothe the digestive tract and gut lining. Studies also show that ginger has anti-inflammatory and antioxidant properties, and may play a role in preventing diseases like cancer.
Rosemary – this is a beautiful and fragrant herb that is very rich in antioxidants, which prevent cell damage and protect against damaging free radicals.
Cocoa – a spice with many health benefits, the cocoa bean is loaded with flavonoids, which we know are ultra-powerful phytochemicals.
Garlic – a food that I include in my cooking almost every day, which has been used for centuries as both an ingredient and a medicine. Eating garlic offers numerous health benefits, including supporting strong immunity, as it contains compounds that help our immune system fight harmful pathogens. Whole fresh garlic contains alliin, but when garlic is chopped or crushed this converts to allicin, which isn’t stable, so quickly converts to other compounds that are said to give garlic its medicinal properties. Additionally, as a potent antibiotic, garlic has been shown to combat strains of Staphylococcus, the bacteria associated with staph infections. Tips for cooking with garlic: