Diet and Cancer

Natural mechanisms protecting against cancer

Carcinogenesis is a multistage process. At each step of this process, there are natural mechanisms protecting against development of cancer. The majority of cancers in humans is induced by carcinogenic factors present in our environment including our food. However, some natural substances present in our diet or synthesized in our cells are able to block, trap or decompose reactive oxygen species (ROS) participating in carcinogenesis. Carcinogens can also be removed from our cells. If DNA damage occurs, it is repaired in most of the cases. Unrepaired DNA alterations can be fixed as mutations in proliferating cells only and mutations of very few strategic genes can induce tumor formation, the most relevant are those activating proto-oncogenes and inactivating tumor suppressor genes. A series of mutations and/or epigenetic changes is required to drive transformation of a normal cell into malignant tumor. The apparently unrestricted growth has to be accompanied by a mechanism preserving telomeres which otherwise shorten with succeeding cell divisions leading to growth arrest. Tumor can not develop beyond the size of 1-2mm in diameter without the induction of angiogenesis which is regulated by natural inhibitors. To invade the surrounding tissues epithelial tumor cells have to lose some adhesion molecules keeping them attached to each other and to produce enzymes able to dissolve the elements of the basement membrane. On the other hand, acquisition of other adhesion molecules enables interaction of circulating tumor cells with endothelial cells facilitating extravasation and metastasis. One of the last barriers protecting against cancer is the activity of the immune system. Both innate and adaptive immunity participates in anti-tumor effects including the activity of natural killer (NK) cells, natural killer T cells, macrophages, neutrophils and eosinophils, complement, various cytokines, specific antibodies, and specific T cytotoxic cells. Upon activation neutrophils and macrophages are able to kill tumor cells but they can also release ROS, angiogenic and immunosuppressive substances. Many cytokines belonging to different families display anti-tumor activity but their role in natural anti-tumor defense remains largely to be established.

The influence of dietary vitamin E and calcium status on intestinal tumors in rats

Male Sprague-Dawley rats were fed from weaning low (1-5 ppm) and normal (26-50 ppm) vitamin E diets for 30-34 weeks. Dietary fat was also varied from 5% (Experiment 1) to 20% (Experiments 2 and 3). Intestinal tumors were induced by 1,2-dimethylhydrazine given subcutaneously as 10 weekly doses at 20 mg/kg body wt. Tumor incidence was lower by 30% and burden was 25%-50% lower for low vitamin E rats than for vitamin E-replete rats. This result was independent of the fat content of the diet. In Experiment 3, vitamin E and calcium were assessed for their influence on intestinal tumors at two levels, with dietary vitamin E at 5 and 50 ppm and calcium at 0.2% and 1.0% in a 2 x 2 factorial experiment. The high calcium-low vitamin E diet produced the greatest fall in tumor incidence and burden relative to the other treatments. In this experiment, vitamin E deficiency reduced tumor incidence and calcium supplementation reduced tumor burden, with a significant interaction of the two. However, this group also showed evidence of reduced food intake and kidney change (calcification), which may have confounded the result. This points to a risk associated with this combination of nutrients at these levels in long-term experiments.

Effects of dietary fat and protein on DMH-induced tumor development and immune responses

Although in three different mouse tumor systems with corn oil as dietary fat we previously found that milk protein decreased tumor development compared with beef, the results were reversed in 1,2-dimethylhydrazine (DMH)-injected mice. The purpose of this study was to determine if the latter result was due to the protein source. BALB/c mice (n = 280) were divided into five diet groups and injected 10 times at weekly intervals with DMH (20 mg/kg wt) or saline. Four diets contained 11% protein (casein, milk, or beef) and 5% fat (corn oil or beef tallow), and the AIN-76A diet was used as a control diet. The source of fat was a significant modulator of tumor development. Corn oil markedly increased total tumor volume and the number of tumors per mouse compared with beef tallow. Its tumor-enhancing effects were evident when it was combined with milk but not with casein. In addition, significantly lower lymphoproliferation and T-cell cytotoxicity against colon tumor cell targets was associated with corn oil consumption, whereas nonfat milk as the protein source was related to normal oxidative burst capacity of phagocytes. These results demonstrate that the source of dietary fat, in addition to the protein source, has a profound effect on both tumor development and immune responsiveness in this animal tumor system.