活性酸素とは？酸化ストレス・抗酸化の仕組み、食品と栄養素をわかりやすく解説

What Are Reactive Oxygen Species? Oxidative Stress, Antioxidants, Foods & Nutrients Explained

OBARASOTARO

“Are reactive oxygen species bad for the body?” “Does antioxidant action simply mean removing ROS?” “Should we eat as many antioxidant-rich foods as possible?” “What do vitamin C and polyphenols actually do in the body?”

When reading about beauty, nutrition and health, you will often come across the terms reactive oxygen species, oxidative stress and antioxidants.

Reactive oxygen species are sometimes described simply as harmful substances that “oxidise the body”, but modern science gives us a more nuanced picture.

Some reactive oxygen species are naturally produced in the body as part of normal life and oxygen metabolism. At appropriate levels, they are also used in cell signalling, immune defence and other normal physiological processes.

When the balance between the production of reactive species and the body's antioxidant and regulatory systems shifts strongly toward oxidation, this is known as oxidative stress.

The body also has its own built-in systems for regulating reactive oxygen species, including enzymes such as SOD, catalase and glutathione peroxidase.

This guide explains what reactive oxygen species are, how they differ from free radicals, oxidative stress, the body's antioxidant systems, SOD, catalase, glutathione, vitamins C and E, polyphenols, carotenoids, foods, supplements and the story behind ORAC values.

Contents

What are reactive oxygen species?

Reactive oxygen species are commonly abbreviated as ROS.

The term refers to a group of chemically reactive oxygen-containing species.

We take oxygen into the body through breathing and use it in our cells to produce energy.

Reactive oxygen species are naturally generated during this process and through many other biological reactions.

In other words, ROS are not simply substances that enter the body from outside; they are also produced as part of normal life itself.

Modern research also shows that ROS are not only associated with cellular damage. At appropriate concentrations, they participate in normal cell signalling.

Are reactive oxygen species and free radicals the same?

The terms “reactive oxygen species” and “free radicals” are sometimes used as though they mean exactly the same thing.

Scientifically, however, they have slightly different meanings.

A free radical is an atom, molecule or other chemical species containing one or more unpaired electrons.

ROS include both free-radical species and non-radical species.

For example:

  • Superoxide anion radical: a free radical
  • Hydroxyl radical: a free radical
  • Hydrogen peroxide: an ROS, but not a free radical

Did you know? Hydrogen peroxide is also an ROS

Hydrogen peroxide (H2O2) is not a free radical.

It is nevertheless classified as a reactive oxygen species.

In modern redox biology, hydrogen peroxide is also intensively studied as an important molecule involved in intracellular signalling.

Rather than treating “ROS” and “free radicals” as identical terms, it is more accurate to understand that ROS include several chemically distinct species.

What types of reactive oxygen species are there?

Some representative ROS include:

Name Symbol Characteristics
Superoxide anion radical O2•− One of the major ROS generated from oxygen
Hydrogen peroxide H2O2 A non-radical ROS that also participates in cell signalling
Hydroxyl radical •OH A highly reactive free radical
Singlet oxygen 1O2 An excited form of oxygen with an electronic state different from ordinary molecular oxygen

These species differ in chemical behaviour, lifetime and the molecules they react with.

So even within the broad term “reactive oxygen species”, there are substances with very different properties.

Where are reactive oxygen species produced?

ROS are generated through many reactions and in several locations inside the body.

Important examples include:

  • energy production in mitochondria
  • enzymatic reactions involving NADPH oxidases
  • defence responses by immune cells
  • metabolism within cellular structures such as peroxisomes

External factors such as ultraviolet radiation, smoking and air pollution can also increase reactions associated with ROS production.

Mitochondria and reactive oxygen species

Mitochondria are cellular structures that use nutrients and oxygen to produce ATP, the major energy currency of cells.

Electrons move through this energy-production system, and some can contribute to the generation of ROS such as superoxide.

This means that the efficient use of oxygen for energy production and the generation of ROS are closely connected parts of normal biology.

Are reactive oxygen species always harmful?

This is one of the most important points for understanding ROS.

Modern redox biology recognises that appropriate levels of ROS are part of normal physiological function.

ROS such as hydrogen peroxide can interact with proteins inside cells and help regulate signalling pathways.

This physiological range of oxidative signalling is sometimes described as oxidative eustress.

When ROS production rises substantially or exceeds the capacity of the body's regulatory systems, oxidative changes to lipids, proteins and DNA can become more prominent.

This state is sometimes distinguished as oxidative distress.

Reactive oxygen species need to be produced and regulated in the right balance

The body produces ROS where and when they are needed, and it also has systems that break them down and regulate their activity.

For this reason, antioxidant biology is best understood not simply as reducing ROS, but as maintaining appropriate redox balance within the body.

What is oxidative stress?

Oxidative stress generally refers to a state in which the balance between oxidant production, including ROS, and antioxidant or regulatory defences shifts strongly toward oxidation.

In modern redox biology, oxidative stress can also involve:

  • disruption of redox signalling and regulation
  • oxidative modification of lipids, proteins and DNA

Oxidative stress is widely studied in relation to age-related biological changes and many medical conditions.

These studies are important for understanding how redox balance works in the body. When considering foods and supplements, however, findings from biological research should be considered separately from claims about the effects of individual commercial products.

What can change during oxidative stress?

When oxidative stress becomes pronounced, several types of biological molecules can undergo oxidative reactions.

Lipids

Polyunsaturated fatty acids in cell membranes and other structures can undergo oxidation, which may lead to chain reactions known as lipid peroxidation.

Proteins

Amino acid residues within proteins can also undergo oxidative modification.

These changes can influence protein structure and function.

DNA

DNA is another biological molecule that can be affected by oxidative reactions.

Cells also contain repair systems that respond to this type of molecular change.

In other words, oxidation, defence and repair are all taking place continuously within the body.

The body's own antioxidant systems

When people hear the word “antioxidant”, vitamin C or polyphenols in food often come to mind first.

The human body itself, however, also has powerful endogenous antioxidant and redox-regulating systems.

Major examples include:

  • SOD (superoxide dismutase)
  • catalase
  • glutathione peroxidase
  • glutathione
  • the thioredoxin system

These systems work through multiple interconnected pathways to regulate ROS and the cellular redox environment.

What are SOD, catalase and glutathione peroxidase?

SOD (superoxide dismutase)

SOD is an enzyme that converts superoxide into other molecules.

This reaction produces hydrogen peroxide, which can then be further processed by enzymes such as catalase and glutathione peroxidase.

Humans have several forms of SOD, including types that use copper and zinc, and another type that uses manganese.

Catalase

Catalase is an enzyme that breaks hydrogen peroxide down into water and oxygen.

It has particularly important functions in cellular structures such as peroxisomes.

Glutathione peroxidase

Glutathione peroxidases are a family of enzymes involved in the processing of hydrogen peroxide and lipid hydroperoxides.

Some glutathione peroxidases contain the trace mineral selenium in the form of the amino acid selenocysteine.

Did you know? Minerals also contribute to antioxidant enzyme systems

Vitamins C and E are well known in discussions of antioxidants, but antioxidant enzymes in the body also depend on minerals such as:

  • copper
  • zinc
  • manganese
  • selenium

Meeting appropriate nutritional requirements for these minerals helps support normal enzyme function.

What is glutathione?

Glutathione is an important molecule in the body's redox system.

It is a small peptide made from three amino acids: glutamate, cysteine and glycine.

In the body, glutathione exists in forms including:

  • reduced glutathione (GSH)
  • oxidised glutathione (GSSG)

and is involved in maintaining the cellular redox environment.

Reduced glutathione is also used when glutathione peroxidase processes peroxides.

This illustrates an important point: antioxidant defence is not carried out by one single “antioxidant substance”. It is a network involving enzymes, peptides, vitamins, minerals and other systems.

Nutrients involved in antioxidant defence

Several nutrients obtained from food are involved in redox reactions or antioxidant systems in the body.

Examples include:

  • vitamin C
  • vitamin E
  • selenium
  • zinc
  • copper
  • manganese

In addition to essential nutrients, plant compounds such as polyphenols and carotenoids are also widely studied.

Rather than treating all of these simply as “antioxidants”, it is more useful to recognise that each has different chemical properties and biological roles.

Vitamin C and antioxidant activity

Vitamin C is a water-soluble vitamin also known chemically as ascorbic acid.

It acts as an antioxidant in the body and is also an essential nutrient involved in processes such as collagen synthesis.

Under Japan's Foods with Nutrient Function Claims system, the authorised nutrient-function statement for vitamin C includes:

“Vitamin C is a nutrient that helps maintain the health of skin and mucous membranes and also has antioxidant activity.”

Recommended daily intake in Japan

According to the Dietary Reference Intakes for Japanese (2025), the recommended intake of vitamin C for men and women aged 18 years and over is 100 mg per day.

Foods containing vitamin C include:

  • red and yellow capsicum
  • kiwifruit
  • citrus fruits
  • strawberries
  • broccoli
  • potatoes

Food provides vitamin C together with many other nutrients, making a varied diet the basic foundation of intake.

Vitamin E and antioxidant activity

Vitamin E is a fat-soluble vitamin that includes several tocopherols and tocotrienols.

The form considered most important nutritionally is alpha-tocopherol.

Vitamin E functions in lipid-rich environments and participates in reactions that help limit lipid oxidation.

Under Japan's Foods with Nutrient Function Claims system, the authorised statement for vitamin E includes:

“Vitamin E is a nutrient that, through its antioxidant action, helps protect lipids in the body from oxidation and supports the maintenance of healthy cells.”

Adequate intake for adults in Japan, 2025

Age Men Women
18–29 years 6.5 mg/day 5.0 mg/day
30–49 years 6.5 mg/day 6.0 mg/day
50–64 years 6.5 mg/day 6.0 mg/day
65–74 years 7.5 mg/day 7.0 mg/day
75 years and over 7.0 mg/day 6.0 mg/day

Foods containing vitamin E include:

  • nuts such as almonds
  • vegetable oils
  • seeds
  • avocado
  • fish

What are polyphenols?

Polyphenols are a large family of compounds widely found in plants.

“Polyphenol” is not the name of one single ingredient; it is a collective term covering many different compounds.

Examples include:

  • catechins
  • anthocyanins
  • quercetin
  • hesperidin
  • cocoa flavanols
  • isoflavones

Foods and drinks containing various polyphenols include:

  • black tea and green tea
  • berries
  • grapes
  • apples
  • citrus fruits
  • cocoa
  • soybeans
  • coffee

Polyphenols are absorbed and metabolised in the body

An important concept when understanding polyphenols is bioavailability.

After being consumed, polyphenols may be absorbed and metabolised in the intestine and liver.

Some are also transformed into different metabolites by gut microorganisms.

For this reason, rather than looking only at antioxidant activity measured for the original compound in a laboratory test, research also examines what happens after absorption and metabolism inside the body.

What are carotenoids?

Carotenoids are yellow, orange and red pigments found in plants and other organisms.

Examples include:

  • beta-carotene
  • lycopene
  • lutein
  • zeaxanthin

Beta-carotene is also a provitamin A carotenoid, meaning the body can convert it to vitamin A when required.

Different carotenoids vary in absorption, metabolism and distribution within the body.

Understanding these individual characteristics gives a more accurate picture than treating every carotenoid as if it behaves in exactly the same way.

Foods containing antioxidant-related compounds

In everyday eating, it is more practical to combine a variety of plant foods than to focus on a single “antioxidant ranking”.

Food group Examples of notable compounds
Berries Anthocyanins, vitamin C and other compounds
Kiwifruit and citrus fruits Vitamin C, flavonoids and other compounds
Colourful vegetables Carotenoids, vitamin C and other nutrients
Tomatoes Lycopene and other compounds
Nuts and seeds Vitamin E, polyphenols and other nutrients
Green tea and black tea Polyphenols including catechins and theaflavins
Cocoa Cocoa flavanols and other polyphenols
Soybeans Isoflavones and other compounds

These foods provide more than compounds commonly described as antioxidants. They also supply vitamins, minerals, fibre, fats, protein and many other nutrients.

Rather than evaluating a food by one antioxidant score, looking at the diversity of the whole diet provides a more useful nutritional perspective.

Reactive oxygen species, oxidative stress and skin

Reactive oxygen species and oxidative stress are also widely studied in skin biology.

ROS are generated during normal metabolism in the skin, and external factors such as ultraviolet radiation can increase reactions associated with ROS generation.

The skin also contains defence systems involving SOD, catalase and glutathione-related pathways that help regulate redox balance.

Oxidative stress is widely studied in relation to biological changes caused by ultraviolet exposure and changes associated with ageing skin.

This research helps scientists understand biological processes taking place in the skin. For foods, supplements and cosmetics, claims should be considered within the appropriate product category and permitted scope of use.

From a beauty perspective, it is also useful to consider sun protection, a varied diet, adequate sleep and other everyday habits as part of the overall picture.

Did you know? How to understand ORAC values

For a time, ORAC became widely promoted as a way of ranking foods according to “antioxidant power”.

ORAC stands for:

Oxygen Radical Absorbance Capacity

and is one laboratory method used to measure the radical-scavenging capacity of foods and other materials under specific test conditions.

Foods such as blueberries, prunes and spices were once frequently promoted for their high ORAC values.

ORAC values are laboratory measurements

ORAC measures chemical activity under specific in-vitro conditions.

When a food is actually eaten, its compounds may be:

  • digested
  • absorbed to different degrees
  • metabolised in the liver and other tissues
  • modified by gut microorganisms
  • converted into different metabolites

Different laboratory methods such as ORAC, FRAP, DPPH and ABTS can also produce different rankings.

The United States Department of Agriculture once published an ORAC food database but removed it from its online resources in 2012.

This is a useful example of why antioxidant capacity measured in a test tube needs to be distinguished from what happens after a person eats a food.

Research today places greater emphasis on how food compounds are absorbed and metabolised and what they do within biological systems, rather than relying only on a single ORAC number.

How should we think about antioxidant supplements?

When comparing supplements containing antioxidant-related nutrients, it is useful to look beyond the word “antioxidant” and check exactly which ingredients are included and in what amounts.

ROS themselves have normal physiological roles, and vitamins and plant compounds also have different intake ranges and biological characteristics.

Food intake and high-dose supplements are different situations

Eating fruit and vegetables is very different from consuming a single isolated compound in a high-dose supplement.

For example, large clinical trials using high-dose beta-carotene supplements in smokers reported an increased risk of lung cancer in some study populations.

This illustrates why nutrients consumed as part of ordinary foods and high-dose single-ingredient supplements should be considered separately.

Think of antioxidant supplements as foods or nutritional supplements

Oxidative stress is studied in relation to many medical conditions.

Large intervention trials investigating antioxidant supplements for disease prevention, however, have produced different results depending on the ingredient, dose and population studied.

Supplements are therefore best viewed as foods that may help supplement the everyday diet, rather than as medicines.

How to compare supplements containing antioxidant-related nutrients

1. Look at the actual ingredients rather than a general “antioxidant power” claim

When comparing products, check:

  • which ingredients are included
  • how much is provided per daily serving
  • whether recognised reference intakes exist for the nutrients

This provides more useful information than an isolated marketing number such as “antioxidant power ×10” or an ORAC value.

2. Check daily amounts of vitamins and minerals

For products containing vitamin C, vitamin E, selenium, zinc or other nutrients, check the amount provided in the recommended daily serving.

If you also use a multivitamin or other supplements, checking for overlapping ingredients makes product comparison easier.

3. Check the ingredient name and amount of botanical extracts

For products containing berry extracts, grape seed extract, green tea extract or other botanicals, useful information includes:

  • the source ingredient
  • extract amount
  • any standardised constituent
  • the recommended daily serving

4. Focus on the type and amount of ingredients rather than the number of ingredients

Some products contain 10, 20 or more antioxidant-related ingredients.

When comparing them, it is more useful to check what each ingredient is and how much is provided per daily serving.

Clear ingredient amounts also make it easier to compare one product with another and to identify overlap with other supplements.

5. Check ingredients carefully if you take medication

High-dose vitamin E products and some botanical extracts may require additional consideration when used together with certain medicines.

If you are receiving medical treatment or take prescription medicines, check with a doctor or pharmacist before use.

Summary: balance matters when understanding reactive oxygen species and antioxidants

Reactive oxygen species are often presented in beauty and health discussions as something purely harmful, but modern science recognises them as molecules naturally generated during life that also participate in normal physiological processes such as cell signalling.

The key points are:

  • Reactive oxygen species (ROS) are a group of reactive oxygen-containing chemical species.
  • ROS include both free-radical and non-radical species.
  • Appropriate levels of ROS participate in normal processes such as cell signalling.
  • Oxidative stress develops when the balance between ROS production and regulatory or defence systems shifts strongly toward oxidation.
  • The body has its own antioxidant systems involving SOD, catalase, glutathione and other pathways.
  • Vitamins C and E and several minerals contribute to antioxidant or redox-related systems.
  • Polyphenols and carotenoids differ in absorption, metabolism and biological behaviour.
  • When considering antioxidant compounds in foods, absorption, metabolism and overall dietary patterns are more informative than laboratory antioxidant scores alone.

A useful way to understand antioxidants is to think in terms of maintaining appropriate redox balance while allowing ROS to perform their normal physiological roles.

The body already has sophisticated antioxidant and redox-regulating systems, while nutrients and plant compounds from food interact with those systems in many different ways.

For everyday nutrition, a varied diet containing vegetables, fruit, legumes, nuts, fish, tea and other nutrient-rich foods is a practical foundation for obtaining a wide range of vitamins, minerals and plant compounds.

When choosing supplements, look beyond the word “antioxidant” and check which ingredients are included and how much is provided per daily serving.

Related Articles

Main References

  • Ministry of Health, Labour and Welfare, Japan. Dietary Reference Intakes for Japanese, 2025.
  • Consumer Affairs Agency, Japan. Foods with Nutrient Function Claims.
  • Sies H, Berndt C, Jones DP. Oxidative Stress. Annual Review of Biochemistry. 2017;86:715–748.
  • Sies H. Hydrogen peroxide as a central redox signaling molecule in physiological oxidative stress: Oxidative eustress. Redox Biology. 2017;11:613–619.
  • Averill-Bates DA. Reactive oxygen species and cell signaling. Biochimica et Biophysica Acta - Molecular Cell Research. 2024.
  • NIH Office of Dietary Supplements. Vitamin C – Fact Sheet for Health Professionals.
  • NIH Office of Dietary Supplements. Vitamin E – Fact Sheet for Health Professionals.
  • NIH Office of Dietary Supplements. Vitamin A and Carotenoids – Fact Sheet for Health Professionals.
  • El-Saadony MT, et al. Polyphenols: Chemistry, bioavailability, bioactivity, nutritional aspects and human health benefits: A review. International Journal of Biological Macromolecules. 2024.
  • National Cancer Institute. Antioxidants and Cancer Prevention.
  • USDA Agricultural Research Service. Historical Oxygen Radical Absorbance Capacity research and food data.

Important Information

This article is intended to provide general educational information about reactive oxygen species, oxidative stress, antioxidants, nutrition and health.

It is not intended to diagnose, treat, cure or prevent disease, and it does not claim that any particular food, supplement or cosmetic prevents or treats disease.

Research discussed in this article was conducted under specific study conditions and should not be interpreted as guaranteeing the same results for any individual commercial product.

If you are receiving medical treatment, take medication, are pregnant or breastfeeding, or have concerns about your health, consult a doctor, pharmacist or other qualified healthcare professional before using supplements or health products.

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Beauty & Wellness Guide

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