Antioxidants
"Look and feel younger!" "Reverse the aging process!"
"Boost your energy!" What could possibly do all that? --
antioxidants, devoted pill-poppers will tell you, despite
the fact that the Food and Drug
Administration (FDA) has prohibited manufacturers from
claiming that consuming their antioxidant products will
reduce disease risk.
Each year, countless hopeful Americans shell out billions of dollars on
antioxidant supplements (nearly $2 billion, in fact, just on beta carotene and
vitamins C and E), believing they will dramatically lower their risk of cancer,
heart disease, and memory loss. Most experts agree, however, that taking
antioxidants is not a shortcut to good health or the answer to staying young.
So, where does that leave us?
In this section, you'll learn the truth about antioxidants: what they are,
how they function, how much of them you need, where to find the best dietary
sources, and what the latest scientific research shows.
What Are Antioxidants?
As the name implies, antioxidants are
substances that are capable of counteracting
the damaging, but normal, effects of the
physiological process of
oxidation in animal tissue. Antioxidants are nutrients (vitamins
and minerals) as well as enzymes
(proteins in your body that assist in
chemical reactions). They are believed to
play a role in preventing the development of
such chronic diseases as cancer, heart
disease, stroke, Alzheimer's disease,
Rheumatoid arthritis, and cataracts.
Oxidative stress occurs when the production of harmful molecules called
free radicals is beyond the protective capability of the antioxidant
defenses. Free radicals are chemically active atoms or molecular fragments that
have a charge due to an excess or deficient number of electrons. Examples of
free radicals are the superoxide anion, hydroxyl radical, transition metals such
as iron and copper, nitric acid, and ozone.
Free radicals containing oxygen, known as reactive
oxygen species (ROS), are the most biologically
significant free radicals. ROS include the radicals
superoxide and hydroxyl radical, plus derivatives of oxygen
that do not contain unpaired electrons, such as hydrogen
peroxide, singlet oxygen, and hypochlorous acid.
Because they have one or more unpaired electrons, free radicals are highly
unstable. They scavenge your body to grab or donate electrons, thereby damaging
cells, proteins, and DNA (genetic material). The same oxidative process also
causes oils to become rancid, peeled apples to turn brown, and iron to rust.
It is impossible for us to avoid damage by free radicals. Free radicals arise
from sources both inside (endogenous) and outside (exogenous) our bodies.
Oxidants that develop from processes within our bodies form as a result of
normal aerobic respiration, metabolism, and inflammation. Exogenous free
radicals form from environmental factors such as pollution, sunlight, strenuous
exercise, X-rays, smoking and alcohol. Our antioxidant systems are not perfect,
so as we age, cell parts damaged by oxidation accumulate.
The Antioxidant Process
Antioxidants block the process of
oxidation by neutralizing free
radicals. In doing so, the antioxidants
themselves become oxidized. That is why
there is a constant need to replenish our
antioxidant resources.
How they work can be classified in one of two ways:
- Chain-breaking - When a free radical releases or steals an
electron, a second radical is formed. This molecule then turns around and
does the same thing to a third molecule, continuing to generate more
unstable products. The process continues until termination occurs -- either
the radical is stabilized by a chain-breaking antioxidant such as
beta-carotene and vitamins C and E, or it simply decays into a harmless
product.
- Preventive - Antioxidant enzymes like superoxide dismutase,
catalase and glutathione peroxidase prevent oxidation by reducing the rate
of chain initiation. That is, by scavenging initiating radicals, such
antioxidants can thwart an oxidation chain from ever setting in motion. They
can also prevent oxidation by stabilizing transition metal radicals such as
copper and iron.
The effectiveness of any given antioxidant in the body
depends on which free radical is involved, how and where it
is generated, and where the target of damage is. Thus, while
in one particular system an antioxidant may protect against
free radicals, in other systems it could have no effect at
all. Or, in certain circumstances, an antioxidant may even
act as a "pro-oxidant" that generates toxic oxygen
species.
Types of Antioxidants
Antioxidant Nutrients
Antioxidants from our diet appear to be of great
importance in controlling damage by free radicals. Each
nutrient is unique in terms of its structure and antioxidant
function.
Vitamin E is actually a generic term that refers to all entities
(eight found so far) that exhibit biological activity of the isomer tocopherol
(an isomer is one of two or more molecules that have the same chemical formula
but different atomic arrangements). Alpha-tocopherol, the most widely available
isomer, has the highest biopotency, or strongest effect in the body. Because it
is fat-soluble (and can only dissolve in fats), alpha-tocopherol is in a
unique position to safeguard cell membranes -- largely composed of fatty acids
-- from damage by free radicals. Alpha-tocopherol also protects the fats in
low-density lipoproteins (LDLs, or the "bad" cholesterol) from oxidation.
Vitamin C, also known as ascorbic acid, is a water-soluble vitamin. As
such, it scavenges free radicals that are in an aqueous (watery) environment,
such as inside your cells. Vitamin C works synergistically with vitamin E to
quench free radicals. Vitamin C also regenerates the reduced (stable) form of
vitamin E.
Beta-carotene, also a water-soluble vitamin, is the most widely
studied of the 600 carotenoids identified to date. It is thought to be the best
quencher of singlet oxygen (an energized but uncharged form of oxygen that is
toxic to cells). Beta-carotene is also especially excellent at scavenging free
radicals in low oxygen concentration.
Selenium is a trace element. It is a mineral that we need to consume
in only very small quantities, but without which we could not survive. It forms
the active site of several antioxidant enzymes including glutathione peroxidase.
Similar to selenium, the minerals manganese and zinc are trace
elements that form an essential part of various antioxidant enzymes.
Antioxidant Enzymes
The antioxidant enzymes superoxide dismutase
(SOD), catalase (CAT) and glutathione peroxidase
(GPx) serve as your primary line of defense in destroying
free radicals.
SOD first reduces (adds an electron to) the radical superoxide (O2-)
to form hydrogen peroxide (H2O2)
and oxygen (O2).
2O2- + 2H --SOD--> H2O2
+ O2
Catalase and GPx then work simultaneously with the protein glutathione to
reduce hydrogen peroxide and ultimately produce water (H2O).
2H2O2 --CAT--> H2O
+ O2
H2O2 + 2glutathione
--GPx--> oxidized glutathione + 2H2O
(The oxidized glutathione is then reduced by another antioxidant enzyme --
glutathione reductase.)
Together, they repair oxidized DNA, degrade oxidized protein, and destroy
oxidized lipids (fat-like substances that are a constituent of cell membranes).
Various other enzymes act as a secondary antioxidant defense mechanism to
protect you from further damage.
Other Antioxidants
In addition to enzymes, vitamins, and minerals, there
appear to be many other nutrients and compounds that have
antioxidant properties. Among them is
coenzyme Q10 (CoQ10, or ubiquinone), which is essential
to energy production and can also protect the body from
destructive free radicals. Also, uric acid, a product
of DNA metabolism, has become increasingly recognized as an
important antioxidant. Additionally, substances in plants
called phytochemicals are being investigated for
their antioxidant activity and health-promoting potential.
