Which foods give the nutrients for normal immune function?

Vitamin C, vitamin D, vitamin A, zinc, iron, folate, vitamin B6 and vitamin B12 contribute to the normal function of the immune system, and all eight come from everyday foods. Examples include red pepper, oranges, kiwifruit, salmon, eggs, sweet potato, oysters, beef, liver, beans, spinach, bananas, milk and seafood. No single food supplies all eight, so variety across food groups each day is what matters. The figures below come from the food tables of the US NIH Office of Dietary Supplements (ODS) and the Thai Reference Daily Intakes (Thai RDIs).

Which foods give the nutrients for normal immune function?

All eight nutrients contribute to the normal function of the immune system in the same way. What differs is where they come from. This table sums up the Thai RDI and the main food groups for each.

The Thai RDIs are single values for the general population aged 3 and over and are not split by sex or age.

What is the immune system made of?

MedlinePlus, from the US National Library of Medicine, describes the immune system as a network of cells, tissues and organs that work together. Its main parts include:

• the skin
• mucous membranes, the moist inner linings of some organs and body cavities, which make mucus and other substances
• white blood cells
• the organs and tissues of the lymph system, such as the thymus, spleen, tonsils, lymph nodes, lymph vessels and bone marrow, which produce, store and carry white blood cells

All eight nutrients in this article contribute to the normal function of the immune system.

How much of each nutrient is in food?

The table below picks foods you can find in Thailand from the ODS food tables, which draw on the US Department of Agriculture's food database. Amounts are per serving as stated. The last column compares each amount with the Thai RDI, rounded to a whole number. Thai varieties and cooking methods can give slightly different values.

Vitamin C: fresh is easiest

The ODS says the best sources of vitamin C are fruit and vegetables, especially citrus fruit, red and green peppers, kiwifruit, broccoli and strawberries, and that humans cannot make vitamin C themselves. Vitamin C is water-soluble and destroyed by heat, so long storage and cooking reduce it. Steaming or microwaving can lessen the losses. Fruit and vegetables you can eat raw, such as oranges, kiwifruit, tomatoes and sweet peppers, are the simple route.

Vitamin D: few foods have it naturally

The ODS says few foods naturally contain vitamin D. Oily fish such as salmon and trout are among the best sources, the vitamin D in eggs is in the yolk, and sunlight is a major source for many people, although how much sun is needed is hard to pin down because it depends on many factors. For more on vitamin D and Thai sunshine, see How does the body get vitamin D in a sunny country?.

Vitamin A: from animals and plants

The ODS says preformed vitamin A is found in animal foods such as dairy, eggs, fish and organ meats, while plants supply beta-carotene, which the body converts into vitamin A. Three ounces of pan-fried beef liver has 6,582 micrograms RAE of vitamin A, more than the US upper level for preformed vitamin A (3,000 micrograms RAE) in one serving, which is why liver is left out of the vitamin A rows above.

Zinc: animal foods go further

The ODS says oysters contain more zinc per serving than any other food. Beans and wholegrains contain zinc too, but their phytates make it less available than zinc from animal foods. Soaking beans and grains in water for several hours before cooking reduces this binding.

Iron: heme and nonheme

The ODS explains that food iron comes in two forms. Meat, seafood and poultry contain both heme and nonheme iron, while plants contain only nonheme iron. Heme iron is absorbed better. Vitamin C increases iron absorption, and meat, poultry and seafood also help the body absorb nonheme iron, while phytate in grains and beans reduces it.

Folate: dark leafy greens first

The ODS says folate occurs naturally in a wide variety of foods, including vegetables (especially dark leafy greens), fruit, nuts, beans, peas, seafood, eggs, dairy, meat, poultry and grains. Thai greens such as morning glory, Chinese kale and choy sum belong to that group.

Vitamin B6: spread across many foods

The ODS says vitamin B6 is found in a wide variety of foods. The richest sources include fish, beef liver and other organ meats, potatoes and other starchy vegetables, and non-citrus fruit such as bananas.

Vitamin B12: animal foods only

The ODS says vitamin B12 is found in foods of animal origin, including fish, meat, poultry, eggs and dairy products. It is not naturally present in plant foods, but some foods such as breakfast cereals and nutritional yeasts are fortified with it.

How can a Thai day of meals cover several of them?

You do not need to change your whole menu. Choosing dishes that combine food groups gives several nutrients at once. The examples below are about variety, not a set formula.

Pair plant iron with vitamin C

When a meal's iron comes from beans, tofu or leafy greens, having fresh fruit or vegetables with vitamin C in the same meal, such as an orange, kiwifruit, tomatoes or sweet pepper, fits what the ODS describes, because vitamin C increases iron absorption.

If you don't eat meat

People who do not eat meat can get vitamin C, vitamin A (from beta-carotene), folate and B6 from plants without much difficulty. B12 is the exception: it is not naturally present in plants. Eggs and dairy provide it for those who eat them. If you eat no animal products at all, look to foods fortified with B12 and talk to a doctor or dietitian.

I don't eat many vegetables. Can a multivitamin replace them?

The ODS says multivitamins cannot take the place of the variety of foods that are important to a healthy diet, because foods provide other nutrients and components with health benefits. The ODS also notes that five varied servings of fruit and vegetables a day can provide more than 200 mg of vitamin C, which is more than the Thai RDI of 100 mg.

Start with what you already eat

• add vegetables to dishes you eat often, such as fried rice, omelettes, clear soups or noodles
• pick easy fruit for snacks, such as oranges, guava, papaya or bananas
• order an extra plate of stir-fried greens when you eat out

If you eat at odd times or only a little

Choose foods that give several nutrients on one plate, such as eggs, milk, yoghurt, fish and a piece of fruit. One orange or kiwifruit gives about two-thirds of the Thai RDI for vitamin C. Not skipping meals is more important than relying on any one food.

Can you have too much?

Getting more than you need does not add extra benefit, and several of these nutrients have an upper level. The table shows US values for adults from the ODS.

The ODS notes that people who combine multivitamins, other supplements and fortified foods and drinks may get some nutrients above the upper level. If you use more than one product, add up every source. Some medicines can interact with these nutrients, so if you take prescription medication, check with a doctor or pharmacist.

For everyday habits such as sleep and moving during the day, see sleep and movement for office workers.

If you are pregnant, breastfeeding, or taking prescription medication, check with a doctor or pharmacist before taking supplements.

Dietary supplements are not intended to prevent, treat or cure any disease.