Antioxidants in Food

Antioxidants are present in large amounts in several foods. However, actual amount of antioxidants in several plant products may differ due to several factors. These include:

• soil type and chemistry
• available nitrogen and levels of other plant nutrients
• moisture levels
• temperature
• pests

It has been seen that plants which are exposed to stress are driven to synthesize antioxidants and are richer in these polyphenols and flavonoids.

Phenolic antioxidants are present in plants at concentrations up to several grams per kilogram. In general, levels are higher in the rinds and skins of the fruits rather than within them. A number of chemical assays have been developed to measure different antioxidants. In vitro assays are designed to test antioxidant levels in foods, while other assays measure levels in blood, urine or blood cells.

Antioxidant food sources

Some food sources of antioxidants include:

In addition to the above:

• Linseed is the major source of lignans in the diet. Lignans are also found in other cereals, legumes and vegetables
• Stilbenes are found in only low quantities in the human diet. Resveratrol is the most widely studied stilbene and is found in grape seeds, peanuts or knotweed roots. These stilbenes have cancer protective effects.
• Flavonoids are divided into six classes including flavonols, flavones, isoflavones, Flavonols (catechins and proanthocyanidins), Flavanones and Anthocyanidins. Of these Flavonols are the most common flavonoids in foods.

Flavanols

The richest sources of flavonols are onions, kale, leeks, broccoli, and blueberries. Flavonol concentrations are highest in or near the peel or rinds of fruits since their biosynthesis is stimulated by light.

In green leafy vegetables, the outer leaves often contain flavonol concentrations more than 10 times the concentrations found in inner leaves. Also smaller fruits of the same species, compared to larger fruits, tend to have higher concentrations of flavonols due to the relationship between surface area and fresh weight.

Flavonols exist in both the monomer form (catechins) and polymer form (proanthocyanidins). Catechins are present in green tea and chocolate and in apricots. Red wine is another major source.

Flavones, isoflavanones and flavanones

Parsley and celery are important sources of flavones.

Isoflavones are found almost exclusively in legumes and soybeans.

Flavanones are found mostly in citrus fruits, tomatoes, mint etc.

Anthocyanins

Anthocyanins are pigments that give fruits and vegetables their color. Levels increase as fruits ripen and are highest in the skins and peels of fruits.

References

1. http://www.nutritionj.com/content/supplementary/1475-2891-9-3-s1.pdf
2. http://www.organic-center.org/reportfiles/Antioxidant_SSR.pdf
3. http://voh.chem.ucla.edu/vohtar/fall06/classes/153C/pdf/Best%20antioxidant%20foods.pdf
4. http://acudoc.com/Antioxidants.PDF
5. http://ocw.jhsph.edu/courses/humannutrition/PDFs/Lecture8.pdf
6. http://www.womenfirst.net/pdf/ADA/ADA_Antioxidants.pdf
7. http://class.fst.ohio-state.edu/fst821/Lect/AA.pdf
8. http://www.medlabs.com/Downloads/Antiox_acti_.pdf