Natural products for cancer prevention: a global perspective

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Abstract

The control of cancer, the second leading cause of death worldwide, may benefit from the potential that resides in alternative therapies. The primary carcinogens stem from a variety of agricultural, industrial, and dietary factors. Conventional therapies cause serious side effects and, at best, merely extend the patient's lifespan by a few years. There is thus the need to utilise alternative concepts or approaches to the prevention of cancer. This review focuses on the many natural products that have been implicated in cancer prevention and that promote human health without recognisable side effects. These molecules originate from vegetables, fruits, plant extracts, and herbs.

Introduction

Mortality that results from the common forms of cancer is still unacceptably high. Despite many therapeutic advances in the understanding of the processes in carcinogenesis, overall mortality statistics are unlikely to change until there is a reorientation of the concepts for the use of natural products as new chemopreventive agents. Natural or semisynthetic compounds may be used to block, reverse, or prevent the development of invasive cancers. Cellular carcinogenesis forms the biologic basis for the identification of preventive products, the assessment of their activity, and ultimately the success or failure of a therapy.

As long ago as 480 BC, Hippocrates recognised that several aspects of what we now call “lifestyle” must come together to produce a healthy body. He said, “Positive health requires a knowledge of man's primary constitution and the powers of various foods, both those natural to them and those resulting from human skill.” What Hippocrates called “man's primary constitution,” we today call “genetics,” and we can infer that foods “resulting from human skills” can be equated with today's diet.

Cancers may be caused in one of three ways, namely incorrect diet, genetic predisposition, and via the environment. At least 35% of all cancers worldwide are caused by an incorrect diet, and in the case of colon cancer, diet may account for 80% of the cases. When one adds alcohol and cigarettes to their diet, the percentage may increase to 60%. Genetic predisposition to cancer lends itself to ∼20% of cancer cases, thus leaving the majority of cancers being associated with a host of environmental carcinogens. Doll and Peto (1981) reported that in the United States the major environmental carcinogens include air and water pollution, radiation, and medication.

Section snippets

Carcinogens

The majority of human cancers result from exposure to environmental carcinogens; these include both natural and manmade chemicals, radiation, and viruses. Carcinogens may be divided into several classes, as shown in Table 1. (1) Genotoxic carcinogens, if they react with nucleic acids. These can be directly acting or primary carcinogens, if they are of such reactivity so as to directly affect cellular constituents. (2) Alternatively, they may be procarcinogens that require metabolic activation

Epidemiological studies

A recent United States study was conducted involving 628 men under the age of 65 years with newly diagnosed prostrate cancer. They were placed on a trial of fruit and vegetables for 5 years. It was found that while fruit was not protective, vegetables, especially cruciferous vegetables (cabbage, broccoli, brussel sprouts, and cauliflower), reduced risk. Tomatoes containing lycopene are protective against prostrate cancer, and when α-tocopherol (a variety of vitamin E) is added to lycopene,

Natural products and defense against carcinogenesis

The literature indicates that many natural products are available as chemoprotective agents against commonly occurring cancers occurring worldwide. A major group of these products are the powerful antioxidants, others are phenolic in nature, and the remainder includes reactive groups that confer protective properties. These natural products are found in vegetables, fruits (Table 3), plant extracts, and herbs (Table 4). Although the mechanism of the protective effect is unclear, the fact that

Mechanisms of action of natural products on carcinogenesis

In the last decade, advances in cancer research have enhanced our understanding of cancer biology and genetics. Among the most important of these is that the genes that control apoptosis have a major effect on malignancy through the disruption of the apoptotic process that leads to tumor initiation, progression, and metastasis. Therefore, one mechanism of tumor suppression by natural products may be to induce apoptosis (Table 4), thereby providing a genetic basis for cancer therapy by natural

Summary

The top two causes of cancer are related to dietary habits and tobacco smoke, and as such, it is largely a preventable disease. The incidence of cancer can thus be substantially reduced by diet modification. Diets rich in vegetables, fruits, and legumes contain large quantities of antioxidants that protect against the deleterious action of free radicals that may lead to cancer development. Consumption of reduced amounts of red meats, saturated fat, salt, and sugar and the avoidance of tobacco

Acknowledgements

We are particularly grateful to the Medical Research Council and the National Research Foundation, South Africa for research grant support.

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