Antioxidants and Phytoestrogens

Antioxidants
What are antioxidants?
Free radicals are highly reactive substances formed in the body's cells as a result of metabolic processes. Although free radicals play an essential role in the body, they can react with DNA, protein or lipids in the cell membrane and cause damage. Antioxidants protect cell membranes from damage by free radicals by acting as scavengers or by slowing down the rate at which free radicals are formed.
An imbalance between the formation of free radicals and antioxidant capacity can lead to disease development.

How do antioxidants function?
Grains and pulses contain a wide range of antioxidants, mainly in the bran layers and the germ.

Antioxidants found in grains and pulses include:

• Vitamin E
• Selenium
• Trace minerals such as copper, zinc and manganese
• Phenolic acids, particularly ferulic acid
• Phytic Acid.

Vitamin E

• Vitamin E is an intracellular antioxidant that protects polyunsaturated fatty acids in the cell membranes from oxidative damage (i.e. becoming "rancid"). Once oxidised, polyunsaturated fatty acids enter a chain reaction which produces more free radicals.
• It may also help to keep selenium in a reduced state so that it can perform its antioxidant activities.
• Vitamin E prevents the formation of nitrosamines, known carcinogens, from nitrites in food.

Selenium

• Selenium functions as a cofactor to glutathione peroxidase, an enzyme which prevents the formation of free radicals.
• It also prevents the development of cancerous cells when they have been exposed to a carcinogen.

Phenolic acids

• Phenolic acids found in grains and pulses include ferrulic acid, caffeic acid, tannic acid and catechins.
• They prevent the formation of endogenous nitrosamines.
• Phenolic acids also block the reaction of carcinogens with cells and in so doing, protect the cell from damage.

Phytic acid

• Phytic acid can bind free iron, making it unavailable for the formation of free radicals.
• Phytic acid may also bind free radicals produced by intestinal bacteria and in so doing, suppress oxidant damage to the cells of the colon.
• Phytic acid has been shown to reduce cell proliferation and tumour number in the colon of mice and rats.

Lignans and phytoestrogens

What are lignans and phytoestrogens?

Lignans are compounds in plant foods that are converted to estrogen-like compounds, by bacteria in the human intestine. Lignans appear to be present in the outer layers of the grain rather than in the endosperm. Lignin in the plant cell wall is made
from lignans.

Flaxseed has the highest lignan content of all foods but it is not widely consumed. Wholegrain cereals, legumes, oilseeds, vegetables and fruit may be more significant contributors of lignan because they are eaten frequently.

Isoflavones and coumestans are the main classes of phytoestrogens that occur in pulses. Phytoestrogens have been detected in other legumes but soybean appears to contain the highest concentration.

How do lignans and phytoestrogens function?
Lignans and isoflavones have a structure which is similar to estrogens.

In the human gut, plant lignans (matairesinol and secoisolariceresinol) are converted by bacteria to mammalian lignans (enterolactone and enterodiol). Similarly, isoflavones are transformed by colonic bacteria into daidzein and genistein.

Estrogens undergo enterohepatic circulation, a form of recycling where estrogen is conjugated in the liver, excreted through the bile duct, deconjugated by bacteria in the gut and then reabsorbed. Once converted in the gut, phyoestrogens also enter the same enterohepatic circulation.

Estrogens are transported though the blood to target tissues and the liver bound to sex hormone-binding globulin (SHBG). In the liver and tissues, estradiol, the most active estrogen, is converted to estrone or estriol.

In vitro studies suggest that enterolactone and oestrodiol stimulate the synthesis of SHBG, the protein that transports estrogens in the blood stream. Since only free or unbound estrogen is biologically active, the concentration of free-circulating estrogen is reduced as more SHBG is produced.

In the gut, lignans may prevent hormones from being deconjugated and therefore reabsorbed by the body. Instead, they are excreted in the faeces. Lignans may also prevent conversion of primary bile acids into secondary bile acids (which can act as carcinogens in the gut).

Due to their similarity in structure, mammalian lignans and phytoestrogen metabolites may have weak estrogenic activity and compete with oestrodiol, the most active estrogen, for estrogen receptors and hence prevent the growth of tumour cells.
By preventing the production of oestrone, they deny the tumour a source of
endogenous estrogen.

For Further Research