What are free radicals and antioxidants?

Free radicals are highly reactive substances, often containing oxygen, that your cells make during normal activity and that also come from outside the body, for example from cigarette smoke and air pollution. When there are more of them than the body can handle, the result is a state called oxidative stress. Antioxidants are partly made by the body and partly come from food such as fruit, vegetables and grains. This article focuses on vitamin C (Thai Reference Daily Intake, or Thai RDI, 100 mg a day) and zinc (11 mg a day), which contribute to the protection of cells from oxidative stress.

What are free radicals?

The US National Center for Complementary and Integrative Health (NCCIH) explains that during their normal activities, the body's cells produce highly reactive substances that contain oxygen. Scientists often call this group free radicals or reactive oxygen species (ROS). They are not always something foreign: they are a by-product of the body using oxygen every day.

Where do free radicals come from?

A review by Pizzino and colleagues (2017) splits the sources of free radicals into two groups.

The same review explains that when these outside substances enter the body, they are broken down or metabolised, and free radicals form as by-products.

Free radicals are not only harmful

Pizzino and colleagues note that at low or moderate levels, free radicals play several useful roles. They are needed to build some cell structures, and certain immune cells make and store them for use. The issue is balance, not getting rid of free radicals altogether.

What is oxidative stress?

Pizzino and colleagues define oxidative stress as an imbalance between the production and build-up of reactive oxygen species in cells and tissues and the body's ability to clear them. The NCCIH says that overproduction of these substances leads to this condition, which may damage cells.

The body already has a defence system

The NCCIH says the body has preventive and repair mechanisms that regulate these processes, and that research has shown antioxidant enzymes are the most important part of this defence. Pizzino and colleagues give examples of these enzymes, such as superoxide dismutase (SOD) and catalase, which cells use to protect themselves.

What are antioxidants, and where do they come from?

MedlinePlus, from the US National Library of Medicine, describes antioxidants as natural or man-made substances found in many foods, including fruit and vegetables. The NCCIH says the body makes some of the antioxidants it needs, and additional antioxidants come from foods such as fruit, vegetables and grains.

They are not interchangeable

The NCCIH says each of the many antioxidants in the body has different properties, and different types may not be interchangeable. Getting them from a varied diet is the foundation, rather than relying on any single food.

Which nutrients help protect cells from oxidative stress?

This article covers two nutrients with this role:

• Vitamin C contributes to the protection of cells from oxidative stress
• Zinc contributes to the protection of cells from oxidative stress

Vitamin C

The ODS says vitamin C acts as an antioxidant in the body and that humans cannot make it themselves, so it has to come from food. Besides protecting cells from oxidative stress, vitamin C contributes to the regeneration of the reduced form of vitamin E, to normal collagen formation for the normal function of cartilage, bones, gums, skin and teeth, and to the normal function of the immune system, and it increases iron absorption. For how the forms of vitamin C differ, see What forms does vitamin C come in, and how do they differ?.

Zinc

Zinc is a mineral you need from food. Besides protecting cells from oxidative stress, it contributes to normal DNA and protein synthesis, the maintenance of normal skin, hair, nails and vision, and the normal function of the immune system. The ODS says oysters contain more zinc per serving than any other food. For more on zinc, see What is zinc, and what does it do for the body?.

How much vitamin C and zinc is in food?

The figures below come from the ODS food tables, which draw on the US Department of Agriculture's FoodData Central. 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 and cooking

The ODS says vitamin C is water-soluble and destroyed by heat, so long storage and cooking reduce the amount in food. Steaming or microwaving can lessen the losses. Helpfully, many of the best sources are usually eaten raw. The ODS also notes that five varied servings of fruit and vegetables a day can provide more than 200 mg of vitamin C.

Zinc from animal foods goes further

Beans, nuts and wholegrains contain zinc too, but the ODS notes that their phytates make zinc less available than zinc from animal foods. Soaking beans, grains and seeds in water for several hours before cooking reduces this binding. Fruit and vegetables contain very little zinc.

How much do you need a day, and can you have too much?

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

More is not better

The NCCIH says antioxidants, especially in larger-than-usual amounts, may not always be beneficial, and large amounts may interfere with important functions in the cell, including its defence mechanisms and normal signalling. Getting more than you need does not add extra benefit. MedlinePlus also notes that antioxidant supplements may interact with some medicines, so tell your health care providers about any you use.

Taking several products at once

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 vitamin C and zinc from every source. Zinc can interact with some medicines, so if you take prescription medication, check with a doctor or pharmacist.

How can a Thai menu cover vitamin C and zinc?

The two come from different food groups. Vitamin C comes from fresh fruit and vegetables; zinc comes from animal foods, seafood and seeds. Having both groups each day is straightforward.

• breakfast: rice soup or porridge with pork and egg, plus an orange
• lunch: stir-fried pork or beef with holy basil and sweet pepper, with side vegetables
• dinner: tom yum with prawns or stir-fried crab, with stir-fried broccoli or cabbage
• snacks: kiwifruit, strawberries, cherry tomatoes or a handful of pumpkin seeds

For vegetables rich in vitamin C, a quick stir-fry or steaming rather than long boiling fits what the ODS says about heat losses.

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.