In this monograph I write about phytochemicals, ie, complex
chemicals found in plants, notably in fruits and vegetables — with the focus
on the phytochemicals rather than on their sources. This review is not
comprehensive, it is simply a first attempt at classification of phytochemicals
that have attracted my attention. Many phytochemicals have not been included.
properties tend to be
because they contain chromophores,
ie, a series of alternating single-bonded and double-bonded carbons.
Isoprene is often the building block of such units. The darkest green
vegetables contain the most chlorophyll, and vegetables with the most
chlorophyll require the most antioxidants. Green will mask the other colors,
when other-colored antioxidant phytochemicals are present.
Many phytochemicals have an anti-carcinogenic (anti-cancer) action by:
slowing cell proliferation (division) by interfering with the cell cycle
inducing apoptosis (cell suicide)
inhibiting phase 1 enzymes (enzymes that convert harmless substances
inducing phase 2 enzymes (enzymes that can attach carcinogens to
molecules that facilitate speedy excretion).
Phytochemicals are not classified as vitamins with official RDA values, but they
can contribute greatly to health and well-being. We are adapted to a world that contains
phytochemicals in our diet. The macula of the eye is adapted to concentrate the
yellow caroteniods lutein and zeaxanthin to protect against harmful blue light.
Although the emphasis is on the positive effects of phytochemicals, note
that very many can be toxic and harmful. Plants containing the most harmful
phytochemicals are usually not treated as foods. Oxalic acid is included here
because it is primarily harmful and is found in plant foods.
Carotenoids make corn yellow, carrots orange and tomatos red.
Carotenoids also give color to salmon, goldfish, flamingos and autumn leaves (when
the green chlorophyll has gone, the carotenoids and phenols remain). Bell peppers
of different colors offer a selection of carotenoids.
Orange Carotenoids — alpha, beta and gamma carotene
Red Carotenoids — lycopene and astaxanthin
Yellow Carotenoids — lutein and zeaxanthin
More than 600 carotenoids have been found in plants. About half of the roughly 50
carotenoids in the human diet are absorbed into the blood stream. Lycopene and beta-carotene
each constitute about 30% of plasma carotenoids. Only alpha, beta and and a few other carotenes
(not lycopene or lutein) can be converted to Vitamin A. Hypervitaminosis of
Vitamin A cannot be caused by excessive alpha or beta carotene intake because the conversion
and absorption rates are too slow. Both alpha-carotene and beta-carotene are protective against
liver cancer and lung cancer in cell culture and animal studies.
Heating, chopping and/or crushing of vegetables frees-up carotenoids, especially beta-carotene &
lycopene. Carotenoids are nearly insoluble in water and are best absorbed when associated with oils. In the blood
stream carotenoids are transported in the most lipid-rich (LDL) cholesterol particles. Tissues with the most LDL
receptors receive the most carotenoid.
Although epidemiological studies have shown reduced lung cancer incidence among those with
high plasma serum levels of beta-carotene, some large intervention studies showed an increased
incidence of lung cancer among smokers taking beta-carotene supplements. (For more details, see
General AntiOxidant Properties.)
weak antioxidant, but strong against singlet oxygen
supplements can enrich LDL cholesterol ß-carotene content without affecting other carotenes
can boost the activity of NK (Natural Killer) immune cells
can stimulate DNA repair enzymes
gives better cornea protection against UV light than lycopene
Flavonoids are flavone-like substances that are usually
antioxidants and sometimes anti-inflammatory. Flavonoids scavenge free
radicals by forming a stable radical that can react with another flavanoid
radical to produce two non-radicals. The citrus flavanoids include rutin,
hesperidin and naringin. Flavanoids and resveratrol are present in red wine,
but are largely absent from white wine because white wine is made by pressing
juice away from the solids, whereas red wine is made by fermenting the pulp
along with the skin and seeds (although ultrafiltration is sometimes
used to reduce astringency and bitterness). For detailed chemistry of the
flavonoids, see Flavonoid AntiOxidants.
Anthocyanins are water-soluble glycosides and acyl-glycosides of
Anthocyanins make roses red and violets blue. They make cherries &
strawberries red and blueberries blue. Blueberries increase anthyocyanin
content as they ripen. Anthocyanins have anti-inflammatory effects.
Anthocyanins are easily damaged by heat (cooking). Up to 30 different
anthocyanins have been found in wild blueberries and Concord grapes.
Proanthocyanidins (colorless substances sometimes called "pycnogenols")
are short-chained polymers of anthocyanidins that
release anthocyanins with heat and/or acidic hydrolysis.
in many berries, especially blueberries, blackberries and black raspberries
white grapes lack color because they have no anthocyanins
Tannin is a functional term rather than a distinct chemical group. Tannins
have been used to tan and protect leather since the 18th century. Tannins are polyphenolics
that make cranberries and pomegranates bitter. Tannins, along
with Vitamin C, help build and strengthen collagen. Tannins
prevent urinary tract infection by preventing bacteria from adhering to the walls.
Combination of tannin plus anthocyanins (as in pomegranate juice) can break-down
oxidized cholesterol in the bloodstream and in atherosclerotic plaques. Most of the
active compounds in black tea are tannins which are 90% catechins. Epicatechin is the
major component of natural tannin in grapes. The hydrolyzable tannins in aged wines come from
the oak barrels, and are mainly composed of gallic acid and ellagic acid esters.
Cranberry juice is rich in phenolic acids, which reduce adherence of bacteria
to teeth and the cells lining the bladder — thereby reducing urinary tract infections
and dental caries. Sweetening reduces the anti-adhesion properties of phenolic acids.
Phenolic acids reduce oxidation of LDL cholesterol. Phenolic acids reduce the formation
of cancer-promoting nitrosamines from dietary nitrates and nitrites. The most important
phenolic compounds in grapes (red wine, grape juice, raisons) are proanthocyanidins,
resveratrol and ellagic acid.
Curcumin is a phytochemical in the spice tumeric which is used to make curry. (The spice
"cumin" contains no curcumin, despite the similar name). Curcumin
inhibits the gene that makes inflammatory COX-2 enzymes, preventing their production.
(Celebrex simply inhibits COX-2 enzymes.) Curcumin is both strongly anti-inflammatory and
strongly anti-oxidant. Curcumin inhibits release of the pro-inflammatory cytokine TNF-alpha.
Curcumin is a more effective anti-clotting agent than aspirin, without the ulcer-inducing
stomach irritation caused by aspirin.
Resveratrol shows the strongest sirtuin-like deacetylase
action of any known phytochemical. Sirtuins have been shown to extend the lifespan of yeast and fruit flies. Contrary
to media representations, there are other sources of resveratrol besides
alcoholic beverages (red wine) — such as purple grape juice.
Glucosinolates convert to isothionates (contain sulfur) and indoles
(contain no sulfur) when vegetables containing them are cut. They are
high in cruciferous vegetables, particularly cauliflower & cabbage — and to a
lesser extent in broccoli & brussel sprouts. They act against cancer by
phase 2 enzyme induction. Broccoli and cabbage show the greatest protection
against bladder cancer.
For a good non-technical introduction to phytochemicals — with emphasis on
plants of origin rather than chemistry — see THE COLOR CODE by James A.
Joseph, et.al. (2002), and EAT YOUR COLORS by Marcia Zimmerman (2001).