232 Produce Degradation: Reaction Pathways and their Prevention
The proposed mechanisms by which phytonutrients act to protect human health
include serving as antioxidants, enhancing the immune response, enhancing cell-to-
cell communication, altering estrogen metabolism, converting to vitamin A (α-, β-,
and γ-carotene are metabolized to retinal), causing cancer cells to die, repairing
DNA damage caused by smoking and other environmental toxicants, and detoxifying
carcinogens through the activation of the cytochrome P450 and Phase II enzyme
systems (Gillman et al., 1995; Voorrips et al., 2000; Riboli and Norat, 2003). Table
8.2 shows some of the common phytonutrients, their food sources, and some asso-
ciated health benefits.
Evidence that consuming phytonutrients in fruits and vegetables protects human
health is accumulating from large epidemiological studies, human feeding studies,
and cell culture studies (Gillman et al., 1995; Van-Duyen and Pivonka, 2000; Voor-
rips et al., 2000; Riboli and Norat, 2003). For example, in a study by Gillman et al.
(1995), fruits and vegetables were linked to decreased risk of stroke — both hem-
orrhagic and ischemic. They observed that each increment of three daily servings
of fruits and vegetables equated to a 22% decrease in the risk of stroke, including
transient ischemic attack. In other studies by Gaziano et al. (1995) and Colditz et
al. (1985), elderly men whose intake of dark green and deep yellow vegetables put
them in the highest quartile for consumption of these vegetables had 46% decrease
in the risk of heart disease relative to men who ranked in the lowest quartile. Men
in the highest quintile had about 70% lower risk of cancer than did their counterparts
in the lowest quintile. Men in the highest quartile or quintile consumed more than
two servings (> 2.05 and > 2.2) of dark green or deep yellow vegetables a day; those
TABLE 8.2
Common Phytonutrients, Potential Food Sources and Associated Postulated
Health Benefits
Phytonutrient Health benefit Potential food sources
Flavonoids Fights oxidation and blood clots Apples, citrus fruits, cranberries, grapes,
broccoli, celery, onions, tea, red wine
Carotenoids Fights oxidation Yellow/red fruits and vegetables: papaya,
carrots, peppers, tomatoes, dark green
leafy vegetables (e.g., spinach)
Allyl sulfides May reduce blood cholesterol, helps
liver detoxify carcinogens
Chives, garlic, leeks, onionsIsothiocyanates May block carcinogens from
damaging DNA
Cruciferous vegetables: broccoli, cabbage,
sunflower
Indoles May convert estrogen into less cancer-
promoting form of the hormone
Cruciferous vegetables: broccoli, cabbage,
sunflower
Terpenes May help the liver to detoxify
carcinogens
Citrus fruits: oranges, tangerines, limes,
lemons
Isoflavones May block entry of estrogen into cells,
reducing the risk of breast, colon, or
ovarian cancers; may alleviate
menopausal symptoms
Soy