Studies reviewing tomato lycopene
and the incidence of prostate cancer risk
Prostate
cancer is the second leading cause of cancer death in men, with an estimated
31,500 deaths in the United States in 2001.
According to the American Cancer Society, there will be approximately
198,100 new cases of prostate cancer in the U.S. in 2001 (1). An estimated 72% of men diagnosed with
prostate cancer will survive 10 years; 53% will survive 15 years. Prostate cancer rates are significantly
higher in black men than in white men and mortality rates are twice as high for
black men than for white men. The
incidence of prostate cancer increases with age, as more than 70% of all
prostate cancers are diagnosed in men over age 65 (1).
Dietary
factors are considered an important risk factor for the development of prostate
cancer in addition to age, genetic predisposition, environmental factors, and
other lifestyle factors such as smoking.
Some studies have found a lower incidence of prostate cancer in
populations that consume large amounts of tomatoes and tomato products; thus,
consuming tomatoes and tomato products may decrease the risk for developing
prostate cancer. Lycopene, one of over
600 carotenoids, is one of the main carotenoids found in human plasma. It is responsible for the red pigment found
in tomatoes and other foods (guava, red grapefruit, and watermelon) and is
absorbed well into the human body. Lycopene is a natural pigment synthesized by
plants and microorganisms but not animals.
It is one of the most potent antioxidants, with a
singlet-oxygen-quenching ability twice as high as that of beta-carotene and 10
times higher than that of alpha-tocopherol (2).
Studies
based either on (a) dietary/dietary supplement intake or (b) blood/tissue
measurements of carotenoids have been the primary methods to determine
lycopene’s role in lowering the risk of developing or enhancing the growth of
prostate cancer cells. The following
studies provide an historic review of lycopene and prostate health as well as
offering a look at more recent studies.
An
important study examining specific dietary and lifestyle habits and the risk of
developing prostate cancer was published by Mills et al. in 1989. In the six-year study involving 14,000 men,
only tomato, bean, lentil and pea consumption were related to lowered prostate
cancer risk. Intake of beta-carotene-rich foods, such as carrots, was unrelated
to risk (3). In a large, comprehensive
study at the Harvard School of Medicine involving 47,894 prostate cancer-free
male health professionals, dietary intake of various carotenoids was assessed
using a detailed questionnaire (4). Of all of the carotenoids (including
beta-carotene), only high lycopene consumption had a statistically significant
21 percent risk reduction. Of the 46 food items that contained carotenoids,
three of the four significantly associated with a lower risk of prostate cancer
contained lycopene -- tomato sauce, tomatoes and pizza. Those subjects that
consumed more than 10 servings of tomatoes and tomato-based products per week
-- accounting for an estimated 82 percent lycopene intake -- had a 35 percent
reduced risk of prostate cancer compared to those that consumed fewer than 1.5
servings per week. For more advanced or aggressive prostate cancers, which are
more likely to cause death, the apparent protective effects of lycopene were
even higher (47 percent). Of all the food items analyzed, tomato sauce provided
the maximum protection (66 percent).
A population-based, case-control study carried out in
Auckland, New Zealand during 1996-1997 also investigated associations between
prostate cancer risk and dietary intake of the carotenoids beta-carotene and
lycopene and their major plant food sources, including carrots, green leafy
vegetables, and tomato-based foods. The study reviewed 317 prostate cancer
cases and 480 controls. The authors found that dietary intake of beta-carotene
and its main vegetable sources was largely unassociated with prostate cancer
risk, whereas intake of lycopene and tomato-based foods was weakly associated
with a reduced risk. These results suggest that in contrast to the findings
regarding many types of cancers, vegetables rich in beta-carotene are not
protective against prostate cancer. However, lycopene from tomato-based foods
was found to be associated with a small reduction in risk (5).
Researchers
also examined the protective effect of vegetables, fruits, and legumes against
prostate cancer in a multicenter case-control study of 1619 African-American,
white, Japanese, and Chinese men with histologically confirmed cases of
prostate cancer and 1618 controls.
Intake of legumes, yellow-orange vegetables, and cruciferous vegetables
were inversely associated with prostate cancer. However, in this study, intake of tomatoes and fruits was not
related to risk (6),
A
recent study suggests dietary supplementation with lycopene in a pill may
decrease the growth of prostate cancer (7).
In the study, 26 men with newly-diagnosed, clinically-localized prostate
cancer were randomly assigned to receive 15 mg of lycopene (n=15) two times a
day or no supplementation (n=11) for three weeks before a radical
prostatectomy. Researchers found that
lycopene supplementation may increase prostate tissue levels of lycopene,
positively affect biomarkers of growth and differentiation, and decrease
clinical signs for aggressiveness of the prostate cancer. These findings suggest that lycopene may
have a role in the prevention of prostate cancer.
Blood and
Tissue Measurement Studies
Lycopene
has been shown to concentrate in prostate tissues (8). Among the carotenoids
present in the prostate gland, lycopene levels appeared to be highest. Thus, it
has been hypothesized that lycopene may lower the risk of prostate cancer.
Studies at the University of Toronto found that prostate cancer patients have
lower serum and prostate tissue lycopene levels compared to control subjects
(9). In a cell culture study, lycopene,
when combined with Vitamin E, prevented the growth of prostate cancer cells (10).
These data provide further evidence that increased consumption of tomato
products and other foods containing lycopene could lower the risk of prostate
cancer.
One
recent study evaluated how prostate levels of antioxidants relate to plasma
levels and self-reported usual dietary intake (11). Levels were measured in 47 men undergoing radical prostatectomy
or transurethral prostatectomy at Loyola University Medical Center in Chicago. The levels of tocopherols and carotenoids in
the prostate were significantly correlated with plasma levels; the strongest
correlations were associated with lycopene, beta-carotene, and
gamma-tocopherol. The researchers note
that this finding supports their potential to provide better estimates of
internal dose, and thus target organ exposure, than reported intake.
In
a study conducted on 65 patients with prostate cancer and 132 cancer-free
controls, significant inverse relationships with prostate cancer were observed
with plasma concentrations of lycopene.
An 83% reduction of prostate cancer risk was observed in the group with
the highest plasma concentration of lycopene in comparison with individuals
with the lowest concentration. The
strength of the association was directly related to dose; with increasing
concentrations of plasma lycopene, the risk of prostate cancer was decreased
(12).
Studies
suggest that lycopene from various tomato products is indeed associated with
the lowered risk of several types of cancers (13). This further supports the
current dietary recommendations to increase consumption of fruits and
vegetables, including tomatoes and tomato products, as part of a healthy diet
to possibly reduce the risk of prostate cancer. Additional studies are needed
to continue to investigate the relationship between dietary lycopene, levels of
lycopene in human tissues, and the prevention and treatment of prostate
cancer. In particular, more large-scale
studies must be conducted before any definitive conclusions can be made.
References
1. American
Cancer Society, Cancer Facts & Figures 2001: 5; 16-17
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A and Rao, AV. Tomato lycopene and its
role in human health and chronic diseases. Canadian Medical Association
Journal, Volume 163(6):739-744, September 19, 2000
3.
Mills PK, Beeson WL, Phillips RL, Fraser GE. Cohort study of
diet, lifestyle and prostate cancer in Adventist men. Cancer 1989; 64:598-604
4. Giovannucci E, Ascherio A, Rimm EB, Stampfer MJ, Colditz GA, Willett WC. Intake of carotenoids and retinol in relation to risk of prostate cancer. Journal of the National Cancer Institute 1995; 87:1767-1776
5.
Norrish, Alan E.; Jackson, Rodney T.;
Sharpe, Susan J.; Skeaff, C. Murray, Prostate Cancer and Dietary Carotenoids.
American Journal of Epidemiology. 151(2):119-123, January 15, 2000
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LN, Hankin JH, Whittemore AS, Wu AH, Gallagher RP, Wilkens LR, John EM, Howe
GR, Dreon DM, West DW, and Paffenbarger, Jr. RS. Vegetables, Fruits, Legumes, and Prostate Cancer: A Multiethnic
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August 2000
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SK, Emenhiser C, Schwartz SJ, Bostwick DG, Williams AW, Moore BJ, Erdman JW Jr.
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M, Pfander H, Boscoboinik D, Azzi A. Lycopene in association with
alpha-tocopherol inhibits at physiological concentrations proliferation of
prostate carcinoma cells. Biochemical and Biophysical Research Communications
1998; 250:582-585
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VL, Mohsen M, Yong S, Pyle J, Wan Y, Arvizu-Durazo R, Liao Y. Prostatic Levels
of Tocopherols, Carotenoids, and Retinol in Relation to Plasma Levels and
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