Dietary antioxidants are mainly derived from plants, where they protect leaves, flowers and fruit from the ravages of strong sunlight, and from attack by viruses, fungi and bacteria. In plants, antioxidants help to discourage insect attacks, and some have a hot, peppery taste that discourages animals from eating them. Even the antioxidants obtained from marine sources, such as krill, are ultimately derived from algae and plankton at the bottom of the food chain.
When we consume foods high in antioxidants, we benefit from their protective effects, too. Antioxidants have anti-ageing effects by helping to neutralise damaging oxidation reactions that are associated with hardening and furring up of the arteries, raised cholesterol levels, rising blood pressure with age, and many if not most cancers.
While antioxidant supplements are available, it’s best to obtain them wherever possible by eating foods high in antioxidants.
Antioxidants and free radicals
Antioxidants neutralise chemicals known as free radicals. A free radical is a molecular fragment that carries an electrical charge in the form of a spare electron. This charge makes the free radical highly unstable, so it naturally loses this charge by passing on the spare electron during collisions with other molecules and cell structures. This process, known chemically as an oxidation reaction, automatically generates another unstable free radical from the molecule that absorbs the spare electron.
Rather like a game of pass-the-parcel, this new free radical also passes on the charge, and oxidation triggers a cascading chain reaction in which electrons are rapidly passed from one molecule to another with damaging results.
Body proteins, fats, cell membranes and genetic material (DNA) are constantly under attack from free radicals, with each cell undergoing an estimated 10,000 free radical oxidations per day. When genetic material is damaged, mutations can occur and this is thought to be the main mechanism that triggers cancer.
Lack of dietary antioxidants is associated with an increased risk of developing:
- hardening and furring up of the arteries
- high blood pressure
- coronary heart disease
- deteriorating vision due to cataracts and macular degeneration
- premature ageing of the skin
- chronic inflammatory diseases such as arthritis
- Alzheimer’s disease
- Parkinson’s disease
- poor sperm count and quality
- congenital birth defects.
What causes free radicals?
Free radicals are continuously produced in the body as a result of:
- muscle contraction during exercise
- normal and abnormal metabolic reactions (including those occurring in diabetes)
- smoking cigarettes
- drinking excessive amounts of alcohol
- exposure to excessive ultraviolet sunlight, especially if sunburned
- exposure to environmental pollutants
- exposure to x-rays
- taking some drugs – especially antibiotics or paracetamol.
The generation of a certain number of free radicals is unavoidable, and even desirable, as they are involved in important immune reactions, including fighting infections. Antioxidant protection against excessive free radical attack does appear to provide health benefits, however. This is especially true for people with diabetes, whose cells are exposed to significantly increased levels of oxidative stress due to oxidation reactions involving glucose, and for smokers.
Foods high in antioxidants
Foods high in antioxidants are our main defence against excessive free radical attack. Antioxidants work by quickly absorbing the spare electrons that create free radicals, and neutralising them – ideally before they can damage our cells.
Several vitamins and minerals act as antioxidants, of which the most important are vitamin A and the closely related carotenoids, vitamin C, vitamin E and the mineral, selenium. The greatest antioxidant activity comes from polyphenols, however, the chemicals that contribute to the vibrant colour, scent and taste of fruit – especially berries – and vegetables.
Herbs and spices are another rich source of antioxidants, and are so powerful in flavour that they’re usually only consumed in tiny quantities. Among the kitchen herbs, the highest levels of antioxidants are found in oregano, rosemary, thyme, parsley and basil. The highest levels in spices are found in ginger, turmeric, cloves, cinnamon, allspice and mustard.
Wine is another rich source of antioxidant polyphenols, especially resveratrol, which are derived from the grape skins and pips. White wine contains a different blend of antioxidant polyphenols to red wine.
Eating fruit and veg raw (where appropriate) is the best way to absorb dietary nutrients, including antioxidants, as heat can destroy some antioxidants such as vitamin C.
The health benefits of foods high in antioxidants
Dietary antioxidants protect circulating cholesterol from oxidation, so it is not absorbed by scavenger cells (macrophages). This reduces the extent to which overladen scavenger cells become trapped when trying to leave the circulation, which contributes to hardening and furring up of the arteries.
Some antioxidants, especially polyphenols, also improve the elasticity of artery walls, and have an effect on nitric oxide, a chemical released in blood vessel linings to promote blood vessel dilation. This helps to protect against high blood pressure.
Other antioxidants interact with cell signalling systems and have regulating effects on cell growth and functions.
In people with diabetes, a good intake of antioxidants helps to prevent oxidation damage associated with raised levels of glucose, and provides some protection against diabetes complications.
The ORAC score for foods high in antioxidants
The antioxidant potential of fruit and vegetables is assessed by measuring their ORAC (Oxygen Radical Absorbance Capacity) score.
Researchers have estimated that the average person following a Western diet obtains around 5700 ORAC units per day. Ideally, you need at least 7000 ORAC units for health and the optimum level to aim for is 20,000 ORAC units per day, or more.
The following table shows which fruit and vegetables have the highest ORAC score to help maximum your antioxidant intake. Interestingly, dark chocolate (at least 70% cocoa solids) which is obtained from the cocoa bean, contains the highest level of antioxidants found in any common food item.
Fruit, Nuts, Cacao
ORAC score* per 100g
Average serving size (grams)
ORAC score per average serving
|Capers (dry weight)||65,598||10g||6,559|
|Red Delicious apple||4,275||138g||5,900|
|Golden Delicious apple||2,670||138g||3,685|
|Pears (green varieties)||1,911||166g||3,172|
|Pears (Red Anjou)||1,773||166g||2,943|
ORAC score per 100g
Average serving size (grams)
ORAC score per average serving
|Red kidney beans||14,413||92g||13,259|
|Red cabbage (cooked)||3,146||75g||2,359|
|Red leaf lettuce||1,785||68g||1,213|
|Russet potatoes (cooked)||1,555||299g||4,649|
|Red potatoes (cooked)||1,326||173g||2,294|
|Red bell pepper||901||119g||1072|
|Sweet potatoes (cooked)||766||156g||1,195|
|Green bell pepper||558||119g||664|
*ORAC score = micromol of TE/100g. Source: Wu X et al. 2004. J. Agric. Food Chem. 52:4067-4037
How to increase your antioxidants
The higher a fruit or vegetable’s ORAC score, the higher its ability to neutralise free radicals and suppress unwanted oxidation reactions. By balancing your intake of foods with high, medium and low ORAC scores you can maximum the antioxidant potential of your diet on a daily basis.
If you select five average servings made up of: blueberries, plums, red kidney beans, spinach and red peppers, for example, you can rack up an astonishing 33,000 ORAC units in a single day. Add a square or two of dark chocolate and your antioxidant intake will be more than optimal.
On the other hand, if your five servings consist of a kiwi, a slice of watermelon, a mixed salad, cauliflower and carrots you would obtain less than 2000 ORAC units – even though you have achieved the recommended five servings a day.
Some guidelines recommend that you eat more than five fruit and veg a day – even as much as ten servings per day.
Fruit versus vegetables
As well as supplying antioxidants, fruit and vegetables also provide other important phytochemicals and nutrients, including vitamins, minerals, trace elements and fibre. Vegetables tend to contain less water than fruit (so are a more concentrated source of nutrients) and less sugar, as well as providing more fibre. Because of this, the majority of the recommended five to ten servings of fruit and vegetables you eat per day (eg 3 out of 5, or 7 out of 10) should ideally be in the form of vegetables rather than fruit, even though vegetables tend to have a lower ORAC score. This is because
How the ORAC test works
The ORAC test measures how well a particular food prevents the breakdown of a chemical (fluorescein) after it is mixed with a strongly oxidant substance (peroxyl radical).
Fluorescein is used because it is luminescent, and the intensity of light it emits decreases as it breaks down. This provides an easy way of measuring how much fluorescein remains intact at set intervals after the fruit or vegetable extract, and the oxidant, are added.
If the food has a low ORAC value, it provides little protection and the mixture’s luminosity rapidly decreases.
If the food has a high ORAC value, it protects the fluorescein from degradation and the sample remains luminescent for longer.
By measuring the intensity of fluorescence in the mixture every 35 minutes after adding the oxidant, scientists develop graphs which are compared with the results from different concentrations of a standard antioxidant related to vitamin E (trolox). The final results are given as ‘trolox equivalents’ or TE.
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