Phytochemicals are compounds produced by plants that serve a variety of functions, including deterring predators or in response to tissue damage or infection.1 These compounds frequently provide pigment or result in the plant’s aroma or taste.
Phytochemicals are also believed to offer beneficial health effects to humans due to antioxidant, anti-inflammatory, or antiproliferative activity.2 Some of the more well-known phytochemicals thought to have anticancer properties include genistein from soybean, curcumin from turmeric root rhizomes, resveratrol from grapes, and lycopene from tomatoes, but there are multiple others found in vegetables, fruits, and herbs.
Evidence for Anticancer Properties
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Chemoprevention is defined as the use of pharmacologic or natural compounds to prevent tumor development, promotion, or progression.2 Epidemiologic data suggest that a diet high in fruits and vegetables, and therefore, theoretically, phytochemicals, can reduce the risk of cancer. For example, a meta-analysis of observational studies found that higher consumption of fruit and vegetables reduced the risk of renal cell carcinoma.3 Another meta-analysis found that higher fruit and vegetable intake reduced the risk of lung cancer, including among smokers.4,5 A meta-analysis of colorectal cancer studies found that higher fruit intake, but not vegetables, was associated with decreased colorectal cancer.6
In vitro and animal studies have also evaluated specific phytochemicals. Multiple animal studies have demonstrated that different phytochemicals decrease the multiplicity or incidence of induced tumors.7 Supporting these findings, numerous in vitro studies demonstrate that phytochemicals inhibit or downregulate many factors involved in cell signaling important for cell proliferation and survival or cell cycle progression, and enhance signaling important for apoptosis.2 Common pathways affected by phytochemicals in these studies include the tumor promoting pathways, MAPK/ERK and PI3K/Akt, and the caspase-dependent apoptotic pathway.
In-human studies have been conducted as well, but to a lesser extent. The findings from in vitro and animal studies have generally not been demonstrated in humans to date. There are multiple challenges that may contribute to this, such as poor bioavailability of phytochemicals, trial design, and dosing.3 A review of the epidemiologic evidence of various specific phytochemicals and cancer risk found that the strongest evidence is for carotenoids such as lycopene or beta carotene and reduction in risk of prostate, breast, lung, or pancreatic cancer; soybean isoflavones and reduction in risk of breast, colon, prostate, and lung cancers; isothiocyanates and reduced risk of breast, colorectal, prostate, renal, gastric, and bladder cancer; and allyl sulfur compounds and reduced risk of upper aerodigestive tract, gastric, prostate, colorectal, and kidney cancers.8 This review concluded that there is weak evidence for resveratrol and curcumin and reduction in risk of cancer.
Some studies have shown that specific phytochemicals can modulate of biomarkers associated with cancer. For example, a study of 20 patients with colorectal cancer who received resveratrol demonstrated that resveratrol metabolites were measurable in resected colorectal tissue, and cell proliferation markers were significantly reduced compared with before and after the intervention.9 A study of 79 patients with prostate cancer who ingested tomato products or a control diet demonstrated that tomato products with the highest level of lycopene reduced prostate-specific antigen levels.10
Conclusions
Though there is evidence to suggest that phytochemicals, particularly those that have been well studied, may have anticancer properties, there is insufficient evidence to specifically recommend use of phytochemicals to prevent cancer. However, given the purported positive health effects with the majority of phytochemicals and a diet high in phytochemicals, there is likely no harm in patients adopting a diet high in phytochemicals or taking phytochemical supplements, provided that they discuss this with their oncologist.
References
1. Budisan L, Gulei D, Zanoaga OM, et al. Dietary intervention by phytochemicals and their role in modulating coding and non-coding genes in cancer. Int J Mol Sci. 2017;18(6):1178.
2. Iqbal J, Abbasi BA, Ahmad R, et al. Potential phytochemicals in the fight against skin cancer: Current landscape and future perspectives. Biomed Pharmacother. 2019;109:1381-1393.
3. Zhang S, Jia Z, Yan Z, Yang J. Consumption of fruits and vegetables and risk of renal cell carcinoma: a meta-analysis of observational studies. Oncotarget. 2017;8(17):27982-27903.
4. Vieira AR, Abar L, Vingeliene S, et al. Fruits, vegetables and lung cancer risk: a systematic review and meta-analysis. Ann Oncol. 2016;27(1):81-96.
5. Wang M, Qin S, Zhang T, Song X, Zhang S. The effect of fruit and vegetable intake on the development of lung cancer: a meta-analysis of 32 publications and 20,414 cases. Eur J Clin Nutr. 2015;69(11):1184-1192.
6. Ben Q, Zhong J, Liu J, et al. Association between consumption of fruits and vegetables and risk of colorectal adenoma: A PRISMA-compliant meta-analysis of observational studies. Medicine (Baltimore). 2015;94(42):e1599.
7. Maru GB, Hudlikar RR, Kumar G, Gandhi K, Mahimkar MJ. Understanding the molecular mechanisms of cancer prevention by dietary phytochemicals: From experimental models to clinical trials. World J Biol Chem. 2016;7(1):88-99.
8. Ruiz RB, Hernandez PS. Cancer chemoprevention by dietary phytochemicals: Epidemiological evidence. Maturitas. 2016;94:13-19.
9. Patel KR, Brown VA, Jones DJ, et al. Clinical pharmacology of resveratrol and its metabolites in colorectal cancer patients. Cancer Res. 2010;70(19):7392-7399.
10. Paur I, Lilleby W, Bøhn SK, et al. Tomato-based randomized controlled trial in prostate cancer patients: effect on PSA. Clin Nutr. 2017;36(3):672-679.
