Foods and Nutrition 1070A/B Lecture 9: Lecture 9 Note - Antioxidants
27 Mar 2021
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Lecture 9Antioxidants
The AntioxidantNutrients
Free Radicals
Vitamin A
March 22nd, 2021
The nutrients that act as antioxidants to protect our cells from oxidative damage are the fat-soluble vitamins A (including beta-carotene) •
and E, the water-soluble vitamin C, and the trace mineral selenium
The vitamin antioxidants donate electrons to quench free radicals that cause damage and can lead to cancer and heart disease ◦
The trace mineral antioxidants are cofactors required by enzyme systems that exert protective effects◦
Oxidation occurs as a part of normal metabolism. During metabolic reactions when atoms lose electrons, oxidation occurs, and when •
atoms gain electrons, reduction occurs. Most often, when molecules are broken down, bonds break and re-form with a redistribution of
atoms and a rearrangement of bonds to form new, stable compounds
Occasionally, an atom loses an electron that is not paired with another electron, resulting in a highly unstable and reactive atom. An •
atom with an unpaired electron in its outer orbital is called a free radical. Free radicals cause damage by attacking lipids in cell
membranes and lipoproteins, our DNA and RNA, and body proteins resulting in widespread damage and increasing our risk for disease
Antioxidants donate one of their own electrons to pair up with the unpaired electron in the outer orbital of free radicals, thus •
neutralizing the damaging effects. A key feature of antioxidants is their stability in both oxidized and reduced forms, so they do not
create new free radicals
All naturally occurring compounds with the biological activity of retinol, the alcohol form of vitamin A •
Vitamin A was the first fat-soluble vitamin to be recognized •
Three different forms of vitamin A are active in the body: retinol (stored in the liver), retinal, and retinoic acid. Collectively, these •
compounds are known as retinoids
The cells can convert retinol and retinal to the other active forms of vitamin A as needed. The conversion of retinol to retinal is ◦
reversible, but the further conversion of retinal to retinoic acid is irreversible. This irreversibility is significant because each form
of vitamin A performs a function that the others cannot
Foods derived from animals provide compounds (retinyl esters) that are readily digested and absorbed as retinol in the intestine •
Foods derived from plants provide carotenoids, some of which can be converted to vitamin A •
The most studied of the carotenoids with vitamin A activity is beta-carotene, which can be split to form retinol in the intestine and ◦
liver. Beta-carotene’s absorption and conversion are significantly less efficient than those of the retinoids
Good food sources of vitamin A: •
The richest sources of the retinoids are foods derived from animals—liver, fish liver oils, milk and milk products, butter, and eggs ◦
Because vitamin A is fat-soluble, it is lost when milk is skimmed. To compensate, 2%, 1%, and skim milks are often fortified ‣
to supply 6-10% of the Daily Value per cup. In Canada, margarine is fortified to provide about the same amount of vitamin A
as butter
Plants contain no retinoids, but many vegetables and some fruits contain vitamin A precursors—the carotenoids, red and yellow ◦
pigments of plants
Only a few carotenoids have vitamin A activity; the carotenoid with the greatest vitamin A activity is beta-carotene ‣
The bioavailability of carotenoids depends in part on fat accompanying the meal. More carotenoids are absorbed when salads ‣
have regular dressing than when reduced-fat dressing is used, and essentially no carotenoid absorption occurs when fat-free
dressing is used
Dark leafy greens (like spinach, bok choy, and broccoli—not celery or cabbage) and rich yellow or deep orange vegetables ‣
and fruits (such as winter squash, cantaloupe, carrots, mango and sweet potatoes—not corn or bananas) help people meet
their vitamin A needs
Beta Carotene
Vitamin ARoles in theBody
The most abundant carotenoid; an orange pigment and vitamin A precursor found in plants •
It is a dietary anti-oxidant •
Beta-carotene in excess – not toxic compared to retinol •
The body will only convert as much beta-carotene as it needs over to vitamin A ◦
If you have an overabundance of beta carotene, your skin's just going to turn a little bit yellow/orange (due to a build up in fat ◦
tissues beneath the skin)
It doesn't have the detrimental effects that a true toxic level of vitamin A would◦
Rich sources of beta-carotene include carrots, sweet potatoes/yams, pumpkins, mango, cantaloupe, spinach, broccoli, etc. •
Other colourful vegetables like red cabbage, beets, yellow corn, and white yam are poor sources of beta-carotene ◦
Note: Beta-carotene is one type of carotenoid, but there are many carotenoids •
Alpha-carotene and beta-cryptoxanthin are two other carotenoids with pro-vitamin A properties—that is, like beta-carotene they ◦
can be converted into retinol, the most active form of vitamin A
Common carotenoids with no vitamin A activity include lycopene, lutein, and zeaxanthin. Although not considered essential ◦
nutrients, they have shown positive health effects
A focus of research for lycopene (high in tomatoes - red) has been on its role in reducing risk for cancer—in particular, ‣
prostate cancer
Research into the roles of lutein and zeaxanthin (high in corn - yellow) in eye health has indicated these carotenoids may be ‣
involved in preventing age-related macular degeneration
Even though over 600 carotenoids exist in nature, about 50 are found in the foods humans eat. Past and emerging research strongly ◦
suggests an adequate daily intake of vegetables and fruit offers the potential to yield substantial health benefits
Vision
Vitamin A plays two indispensable roles in the eye: it helps maintain a crystal-clear outer window, the cornea, and it participates in the •
conversion of light energy into nerve impulses at the retina
The Cornea •
Keratinization of the Cornea ◦
If we do not get enough vitamin A, the cornea is compromised ‣
Keratin, the hard, inflexible protein of hair and nails, accumulates in the cornea ‣
Keratinization also occurs on other surfaces of the body ‣
This can be reversed through supplementation ‣
Xerophthalmia ◦
Total blindness—failure to see at all ‣
Blindness due to vitamin A deficiency develops in stages. At first, the cornea becomes dry and hard because of inadequate ‣
mucus production—a condition known as xerosis. Then, corneal xerosis can quickly progress to keratomalacia, the softening
of the cornea that leads to irreversible blindness
Severe vitamin A deficiency is the major cause of childhood blindness in the world, causing more than half a million ‣
preschool children to lose their sight each year
The Retina •
Some of the photosensitive cells of the retina contain pigment molecules called rhodopsin; each rhodopsin molecule is composed ◦
of a protein called opsin bonded to a molecule of retinal
When light passes through the cornea of the eye and strikes the retina, rhodopsin responds by changing shape and becoming ◦
bleached. As it does, the retinal shifts from a cis to a trans configuration
The bleached trans-retinal cannot remain bonded to opsin. When retinal is released, opsin changes shape, thereby disturbing the ◦
membrane of the cell and generating an electrical impulse that travels along the cell’s length. At the other end of the cell, the
impulse is transmitted to a nerve cell, which conveys the message to the brain
Much of the retinal is then converted back to its active cis form and combined with the opsin protein to regenerate the pigment ◦
rhodopsin. Some retinal, however, may be oxidized to retinoic acid, a biochemical dead end for the visual process. Visual activity
leads to repeated small losses of retinal, necessitating its constant replenishment either directly from foods or indirectly from
retinol stores
Night Blindness ◦
One of the first detectable signs of vitamin A deficiency ‣
In night blindness, the retina does not receive enough retinal to regenerate the visual pigments bleached by light ‣
The person loses the ability to recover promptly from the temporary blinding that follows a flash of bright light at night or to ‣
see after dark
Gene Expression
Retinoic acid regulates the activities of genes that direct the synthesis of proteins and enzymes (thus, affecting the metabolic activities of •
tissues)
It’s not enough to just have the machinery to make genes, we also need retinoic acid to activate and deactivate genes as required •
Analogy: A car alone can’t take you across town. You need a key in the ignition to start the motor in order to go anywhere ◦
Cell Differentiation
Vitamin A participates in protein synthesis and cell differentiation, the process by which immature cells develop specific functions •
different from those of the original that are characteristic of their mature cell type (e.g., goblet cells produce mucus to protect them from
respiratory infections)
Keratinization can occur in the wrong place with a lack of vitamin A •
Makes the surface hard/dry/cracked and increases the risk of infection ◦
Links to cancer - especially the role of retinoic acid in regulating genes that may suppress or even reverse malignant cell changes •
Immunity and Epithelial Health
Vitamin A has an “anti-infective” property in the body due to its many roles in the body’s defences •
Plays a role in keeping our epithelial cells (cells on the surface of the skin and mucous membranes) and epithelial tissues (the layer of •
the body that serves as a selective barrier between the body’s interior and the environment - e.g., the cornea of the eyes, the skin, the
respiratory lining of the lungs, and the lining of the digestive tract) healthy
Helps to protect against skin damage from sunlight •
Regulation of genes that produce proteins involved in immunity •
Growth
To convert a small bone into a large bone, the bone-remodelling cells must “undo” some parts of the bone as they go, and vitamin A •
participates in the dismantling. The cells that break down bone contain sacs of degradative enzymes. With the help of vitamin A, these
enzymes eat away at selected sites in the bone, removing the parts that are not needed
Deficiency in children is evidenced in poor growth •
Major Antioxidant
Not all dietary beta-carotene is converted to active vitamin A. Some beta-carotene may act as an antioxidant capable of protecting the •
body against disease. A diet rich in vegetables and fruits containing beta-carotene and other carotenoids helps to defend against some
cancers
