The importance of nutrition for immunity

There’s a lot of worry about getting sick right now. Being proactive, empowered and feeling protected is a bit of an antidote to all that anxiety. So where do we start by thinking about how to power up our immunity?

Says nutritionist Kaytee Boyd, “Your diet is CRUCIAL for a rock-solid immune system.” We asked her to elaborate and talk us through some of the key food sources of important vitamins and minerals that can help us stay well and protect ourselves and our families against infection.

Nutrients to support immunity: from A to Zinc

Vitamin A

Vitamin A is top of the list for practitioners working with immunity. This fat-soluble vitamin does wonders for skin, immunity, lung health, vision and cellular repair. There are two different types of Vitamin A. Preformed Vitamin A is found in meat, poultry, fish, and dairy products. Provitamin A is in red and orange coloured fruits, vegetables, and other plant-based products. The most common type of Provitamin A in foods and dietary supplements is beta carotene – a precursor to active Vitamin A. It’s good to get Vitamin A from a range of sources, as some people have mutations in a gene that enables us to convert beta carotene in veges into a form that our body can use. Not sure about you guys, but my mum used to give us cod liver oil as kids. Now I know why!

Sources of Preformed Vitamin A:

  • Cod liver oil
  • Liver in general (beef, chicken, sheep, etc)
  • Eggs
  • Some types of fish, such as salmon
  • Dairy products

Sources of Provitamin A (beta carotene):

  • Green leafy vegetables including kale and spinach (Cos lettuce is also a good source)
  • Green, orange, and yellow vegetables, such as peppers, broccoli, sweet potatoes, peas, carrots, and squash
  • Fruits, including rock melon, apricots, and mangos
  • Herbs and spices, including coriander, cayenne, paprika, parsley, sage and marjoram
  • Pairing the above with a healthy fat like avocado, olive oil, nuts or seeds can help you absorb the beta carotene better

Vitamin C

Our bodies don't make Vitamin C. And while high doses of Vitamin C might not stop you getting sick, we do need it for everyday immune function, bone structure, iron absorption, making immune cells and keeping skin healthy. It’s a good idea to try and get this vitamin from a variety of foods, as foods that are rich in Vitamin C contain other nutrients and valuable fibre (which supports a healthy weight range and good gut flora).

Here are some Vitamin C rich foods to add into your diet daily – providing 150 milligrams of Vitamin C. Each serve = 100 grams:

  • One serve: blackcurrants, yellow or green bell peppers, green chilli peppers.
  • Two servings: kale, broccoli, kiwifruit, red bell peppers, jalapeno peppers, red chilli peppers, Tahitian taro, or mustard spinach.
  • Three to five servings: oranges, strawberries, pineapple, papayas, lemons, peas, cabbage, green cauliflower, Brussels sprouts, banana pepper, red or cayenne pepper, mustard greens, persimmons, kohlrabi, turnip greens, taro leaves, or lychees.

Mix and match these foods to get 150 milligrams of Vitamin C into your day. If this is too inconvenient or if you have reasons to avoid all of these foods, you can supplement with 500 milligrams per day of Vitamin C as a starting point. If you have a sensitive gut, look for one labelled lipospheric, esterified or buffered.

Vitamin D

This vitamin is super important for preventing and fighting infections such as flu and other viruses, particularly so for seniors. Vitamin D has important roles for healthy immunity, as well as helping to keep bones strong by telling the cells in your gut to absorb calcium and phosphorus. Deficiency in Vitamin D is connected with increased risk of autoimmunity as well as greater susceptibility to infection, sepsis and even cancer.

There’s a strong case for the theory that rates of illness increase over the winter because of widespread Vitamin D deficiency. Studies have shown that people supplemented with adequate levels of Vitamin D3 during the cold and flu season had significantly lower rates of infection. Vitamin D3 increases our body’s production of cathelicidin, an antimicrobial compound, which helps fight viral and bacterial infections. The Vitamin D Council recommends a maintenance dosage of 1000IU of Vitamin D3 per 11kg of body weight. People with Vitamin D deficiency may need more: this can be measured through an easy blood test.

As well as nutrition, sunlight is a great source of this brilliant vitamin. When your skin is exposed to sunlight (specifically UVB rays), your body makes Vitamin D from cholesterol in the skin cells. In winter, aim for 10–30 minutes of sunlight, around midday if possible, several times per week. The darker your skin, the more sunlight you need to generate adequate levels. Particularly during short, rainy days, combine sunshine with these food sources:

  • Fatty fish, like tuna, mackerel, and salmon – around 100 grams of cooked salmon contains more than 450 international units (IU) of Vitamin D
  • Foods fortified with Vitamin D, like some cereals, dairy products, orange juice, protein powders and nut milks
  • Beef liver
  • Cheese
  • Egg yolks

Vitamin K

Vitamin K plays key roles in healthy blood clotting, bone metabolism, and regulating some enzyme systems. It can also affect immune and inflammatory responses, particularly involving T cells. Vitamin K is found in these foods:

  • Green leafy vegetables, such as silver beet, spinach, kale, Swiss chard, mustard greens, parsley, romaine, collard greens, rocket, and green leaf lettuce
  • Vegetables such as Brussels sprouts, broccoli, cauliflower, and cabbage
  • Fish, liver, meat, eggs, and cereals (these contain smaller amounts) 


It’s nature’s chill-pill, but many people aren’t getting their daily recommended doses. Magnesium is required for energy production, the structural development of bone and synthesis of DNA, RNA, and the antioxidant glutathione. Magnesium also plays a role in the active transport of calcium and potassium ions across cell membranes, vital to nerve impulse conduction, muscle contraction, and normal heart rhythm. Magnesium also keeps the immune system strong and is a key hormone regulator for women. Magnesium is available in avocados, green leafy vegetables, potato skins, almonds, whole grains, and many legumes.


Our soils are low in selenium, which is critical for many important functions, including a healthy immune response. I ensure everyone is taking at least three Brazil nuts a day or supplementing with 150-200 mcg of liquid selenium daily.


Zinc can help us fight colds and flu, and reduces the inflammation that can often come with illness. Zinc has been shown in clinical studies to shorten the duration of a cold. Zinc is also important for the normal functioning of white blood cells. Supplementing with just 15mg of Zinc per day in adults has been found to improve our immune cells’ ability to ward off infection.

Oysters are a very high source of zinc, but red meat, poultry, beans, nuts and shellfish are good sources. Vegans may need extra Zinc, as vegan and raw diets may be higher in copper, which competes with zinc. Phytates —which are present in wholegrain breads, cereals, legumes, and other foods —bind zinc and inhibit its absorption. Glyphosate (Round Up spray) depletes zinc in our soils, so choosing some organic foods may be a good option if you’re zinc deficient. Don’t rely on the ‘zinc taste test’ to know what your zinc level is like, research has proven this ineffective as a measuring tool.

Other nutritional support for immunity

As well as vitamins and minerals, there are other aspects to your daily diet and lifestyle that support immunity. Omega-3 essential fatty acids have a host of immune benefits too long to list! Think fatty fish, flaxseed oil, walnuts, chia seeds…

I can’t emphasise enough how important your gut and its microbiome are to immunity. A 2016 study showed a reduction in symptoms for children with upper respiratory tract infections who took a daily probiotic with Lactobacillus and Bifidobacterium throughout the cold and flu season. I am a firm believer in fermented foods and the addition of a solid probiotic to keep our gut and immune system healthy.

Melatonin is an important hormonal immune support. Your body naturally makes Melatonin in the pineal gland in the evening, but it makes less as we age or when we directly expose our eyes to light (particularly blue light from our devices) at night. Melatonin is best known for its ability to control circadian rhythm, but it affects immunity as well. It’s important to make sleep a priority, because sleep itself has a direct effect on immunity.


Kaytee Boyd has been involved in the health and wellness industry for more than 25 years. With a double degree from Otago University in Human Nutrition and Sports Science, Kaytee is a member of the Nutrition Society of NZ and an MINND foundation practitioner (specialising in Autism), as well as a previous advisor on the Breast Cancer Network Foundation board. Kaytee splits her time across two busy Auckland clinics, as well as heading cancer masterclass workshops.