Diet and supplements in cancer prevention and treatment: Clinical evidences and future perspectives

https://doi.org/10.1016/j.critrevonc.2018.01.002Get rights and content

Abstract

In recent years, calorie-restricted dietary regimens and compounds such as vitamins, curcumin, green tea extracts and omega-3 fatty acids have attracted attention for their potential anticancer effects. While definitive conclusions cannot be drawn in this field, many patients adopt complementary antitumor therapies aiming to improve efficacy or reduce toxicity of chemotherapy, with uncertain benefits and the risk of additional toxicities or antagonistic interactions with standard therapies.

In this manuscript, we review the different levels of available evidence to suggest or discourage specific dietary changes or supplement use in the context of cancer prevention, reduction of tumor recurrences and survival prolongation in advanced cancers. Preventing or treating obesity, as well as adhering to healthy dietary patterns, should be recommended to both the general population and cancer survivors because they are convincingly associated with reduced risk of primary or second cancers and, in some cases, with reduced cancer recurrences. On the contrary, the role of specific interventions or supplements in patients with advanced malignancies is much more uncertain and actually a highly debated topic. With some exceptions, such as melatonin, the use of most complementary therapies cannot be encouraged, or should be discouraged, because of the lack of sufficient safety and efficacy data.

Introduction

Obesity and excess intake of high-calorie foods, saturated and trans-unsaturated fatty acids, sugar-containing beverages and red or processed meats, are recognized risk factors for several chronic pathologies, including type II diabetes mellitus, cardiovascular and cerebrovascular diseases (Eyre et al., 2004; Hill et al., 2009; Lichtenstein et al., 2006). The same factors have been associated in several, but not all, conducted studies with an increased risk of different tumors, while diets rich in whole grains, fruit and vegetables are considered protective (American Institute for Cancer Research and World Cancer Research Fund, 2007). Parallel to diet, dietary supplements, including vitamins, minerals, herbal-derived extracts and other molecules that are present in our body, have also attracted attention for their presumptive cancer-preventing properties, but results have been disappointing in most conducted studies (The Alpha-Tocopherol, Beta Carotene Cancer Prevention Study Group, 1994; Blot et al., 1993; Cauley et al., 2013; Hennekens et al., 1996; Hercberg et al., 2004; Lippman et al., 2009; Omenn et al., 1996).

Even more debated is the possible role of specific diets or supplements in already established malignancies. Cancer survivors, as defined as subjects who have received a diagnosis of malignant tumor, form a large and heterogeneous clinical spectrum of patients that includes on the one hand subjects with limited-stage, surgically removed disease, and, on the other hand, patients with metastatic disease (2011). Approaching this heterogeneous population as a unique group is incorrect, because different tumor types and tumor stages require specific interventions with different therapeutic goals and expected results. In recent years, clinical trials investigating dietary interventions or supplements as adjuvants to standard antitumor therapies are increasing in number, stimulated by recent discoveries and pressure by the public opinion. Indeed, more and more cancer patients consider these complementary therapies as a safe way to play a more active role in the therapeutic process and to implement the efficacy of standard treatments (Richardson et al., 2000; Velicer and Ulrich, 2008). Future studies will hopefully clarify the actual efficacy of dietary interventions or supplements in reducing the risk of relapse in patients with limited-stage tumors, or in improving clinical outcomes in advanced cancers (Frenkel et al., 2013). To do so, it will be also crucial to investigate possible interactions between complementary and conventional anticancer treatments, including chemotherapy, radiotherapy or molecularly targeted therapies, because synergistic or antagonistic effects could arise from specific combinations.

Section snippets

Cancer prevention

Decades of research efforts tried to clarify the role of single nutrients, groups of nutrients and supplements in cancer development. Most of available evidence on diet derives from comprehensive meta-analyses of retrospective, case control, and prospective cohort studies. Prospective, randomized controlled trials (RCTs) are harder to perform in this context, mainly because it is difficult to guarantee and check long-term patient compliance to one specific dietary regimen (American Institute

Prevention of tumor recurrences

Patients who had their limited-stage tumor removed with surgery are exposed to the risk of tumor recurrence, which varies with tumor type, stage, and biological characteristics. Compared with the general population, cancer survivors are also exposed to a higher risk of second tumors, mainly due to the following reasons: exposure to risk factors that are common to different types of cancer; administration of cytotoxic, mutagenic adjuvant treatments, especially chemotherapy and radiotherapy;

Diet and supplements in advancer cancers

Some dietary regimens or supplements are considered therapeutic in the popular traditions of several countries, and preclinical evidences of antitumor activity also exist for some of them. Therefore, more and more patients with advanced malignancies are interested in combining complementary with standard therapies. However, as a consequence of the historically poor interest within the oncology community and the lack of solid clinical evidence in favor of or against these approaches, answers and

Conclusions and perspectives

Convincing evidence suggests that preventing obesity and discouraging “Western” dietary patterns, while promoting “prudent” patterns, such as the Mediterranean pattern, will result in a significant reduction of cancer incidence. These lifestyle recommendations should be extended to patients undergoing curative surgery for limited-stage cancers, because they could reduce primary tumor recurrences, and are undoubtedly protective against CVDs and second tumors, two prominent causes of death and

Conflicts of interest

None.

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