Cancer 3rd post

Diet and exercise

Main article: Diet and cancer

Diet, physical inactivity, and obesity are related to up to 30–35% of cancer deaths.^[5][39] In the United States excess body weight is associated with the development of many types of cancer and is a factor in 14–20% of all cancer deaths.^[39]Correspondingly, a UK study including data on over 5 million people showed higher body mass index to be related to at least 10 types of cancer, and responsible for around 12,000 cases each year in that country.^[40] Physical inactivity is believed to contribute to cancer risk, not only through its effect on body weight but also through negative effects on theimmune system and endocrine system.^[39] More than half of the effect from diet is due to overnutrition (eating too much), rather than from eating too few vegetables or other healthful foods.

Some specific foods are linked to specific cancers. A high-salt diet is linked to gastric cancer.^[41] Aflatoxin B1, a frequent food contaminate, causes liver cancer.^[41] Betel nut chewing causes oral cancer.^[41] The differences in dietary practices may partly explain differences in cancer incidence in different countries. For example, gastric cancer is more common in Japan due to its high-salt diet^[42] and colon cancer is more common in the United States. Immigrants develop the risk of their new country, often within one generation, suggesting a substantial link between diet and cancer.^[43]

Infection

Main article: Infectious causes of cancer

Worldwide approximately 18% of cancer deaths are related to infectious diseases.^[5] This proportion varies in different regions of the world from a high of 25% in Africa to less than 10% in the developed world.^[5] Viruses are the usual infectious agents that cause cancer but cancer bacteria and parasites may also have an effect.

A virus that can cause cancer is called an oncovirus. These include human papillomavirus (cervical carcinoma), Epstein–Barr virus (B-cell lymphoproliferative disease and nasopharyngeal carcinoma), Kaposi's sarcoma herpesvirus (Kaposi's sarcoma and primary effusion lymphomas), hepatitis B and hepatitis C viruses (hepatocellular carcinoma), and human T-cell leukemia virus-1 (T-cell leukemias). Bacterial infection may also increase the risk of cancer, as seen in Helicobacter pylori-induced gastric carcinoma.^[44] Parasitic infections strongly associated with cancer include Schistosoma haematobium(squamous cell carcinoma of the bladder) and the liver flukes, Opisthorchis viverrini and Clonorchis sinensis(cholangiocarcinoma).^[45]

Radiation

Main article: Radiation-induced cancer

Up to 10% of invasive cancers are related to radiation exposure, including both ionizing radiation and non-ionizingultraviolet radiation.^[5] Additionally, the vast majority of non-invasive cancers are non-melanoma skin cancers caused by non-ionizing ultraviolet radiation, mostly from sunlight. Sources of ionizing radiation include medical imaging and radon gas.

Ionizing radiation is not a particularly strong mutagen.^[46] Residential exposure to radon gas, for example, has similar cancer risks as passive smoking.^[46] Radiation is a more potent source of cancer when it is combined with other cancer-causing agents, such as radon gas exposure plus smoking tobacco.^[46] Radiation can cause cancer in most parts of the body, in all animals, and at any age. Children and adolescents are twice as likely to develop radiation-induced leukemia as adults; radiation exposure before birth has ten times the effect.^[46]

Medical use of ionizing radiation is a small but growing source of radiation-induced cancers. Ionizing radiation may be used to treat other cancers, but this may, in some cases, induce a second form of cancer.^[46] It is also used in some kinds ofmedical imaging.^[47]

Prolonged exposure to ultraviolet radiation from the sun can lead to melanoma and other skin malignancies.^[48] Clear evidence establishes ultraviolet radiation, especially the non-ionizing medium wave UVB, as the cause of most non-melanoma skin cancers, which are the most common forms of cancer in the world.^[48]

Non-ionizing radio frequency radiation from mobile phones, electric power transmission, and other similar sources have been described as a possible carcinogen by the World Health Organization's International Agency for Research on Cancer.^[49] However, studies have not found a consistent link between cell phone radiation and cancer risk.^[50]

Heredity

Main article: Cancer syndrome

The vast majority of cancers are non-hereditary ("sporadic cancers"). Hereditary cancers are primarily caused by an inherited genetic defect. Less than 0.3% of the population are carriers of a genetic mutation that has a large effect on cancer risk and these cause less than 3–10% of all cancer.^[51] Some of these syndromes include: certain inherited mutations in the genes BRCA1 and BRCA2 with a more than 75% risk of breast cancer and ovarian cancer,^[51] andhereditary nonpolyposis colorectal cancer (HNPCC or Lynch syndrome), which is present in about 3% of people withcolorectal cancer,^[52] among others.

Physical agents

Some substances cause cancer primarily through their physical, rather than chemical, effects on cells.^[53] A prominent example of this is prolonged exposure to asbestos, naturally occurring mineral fibers that are a major cause ofmesothelioma, which is a cancer of the serous membrane, usually the serous membrane surrounding the lungs.^[53] Other substances in this category, including both naturally occurring and synthetic asbestos-like fibers, such as wollastonite,attapulgite, glass wool, and rock wool, are believed to have similar effects.^[53] Non-fibrous particulate materials that cause cancer include powdered metallic cobalt and nickel, and crystalline silica (quartz, cristobalite, and tridymite).^[53] Usually, physical carcinogens must get inside the body (such as through inhaling tiny pieces) and require years of exposure to develop cancer.^[53]

Physical trauma resulting in cancer is relatively rare.^[54] Claims that breaking bones resulted in bone cancer, for example, have never been proven.^[54] Similarly, physical trauma is not accepted as a cause for cervical cancer, breast cancer, or brain cancer.^[54] One accepted source is frequent, long-term application of hot objects to the body. It is possible that repeated burns on the same part of the body, such as those produced by kanger and kairo heaters (charcoal hand warmers), may produce skin cancer, especially if carcinogenic chemicals are also present.^[54] Frequently drinking scalding hot tea may produce esophageal cancer.^[54] Generally, it is believed that the cancer arises, or a pre-existing cancer is encouraged, during the process of repairing the trauma, rather than the cancer being caused directly by the trauma.^[54]However, repeated injuries to the same tissues might promote excessive cell proliferation, which could then increase the odds of a cancerous mutation.

It is controversial whether chronic inflammation can directly cause mutation.^[54][55] It is recognized, however, that inflammation can contribute to proliferation, survival, angiogenesis and migration of cancer cells by influencing the microenvironment around tumors.^[56][57] Furthermore, oncogenes are known to build up an inflammatory pro-tumorigenic microenvironment.^[58]

Hormones

Some hormones play a role in the development of cancer by promoting cell proliferation.^[59] Insulin-like growth factors and their binding proteins play a key role in cancer cell proliferation, differentiation and apoptosis, suggesting possible involvement in carcinogenesis.^[60]

Hormones are important agents in sex-related cancers, such as cancer of the breast, endometrium, prostate, ovary, andtestis, and also of thyroid cancer and bone cancer.^[59] For example, the daughters of women who have breast cancer have significantly higher levels of estrogen and progesterone than the daughters of women without breast cancer. These higher hormone levels may explain why these women have higher risk of breast cancer, even in the absence of a breast-cancer gene.^[59] Similarly, men of African ancestry have significantly higher levels of testosterone than men of European ancestry, and have a correspondingly much higher level of prostate cancer.^[59] Men of Asian ancestry, with the lowest levels of testosterone-activating androstanediol glucuronide, have the lowest levels of prostate cancer.^[59]

Other factors are also relevant: obese people have higher levels of some hormones associated with cancer and a higher rate of those cancers.^[59] Women who take hormone replacement therapy have a higher risk of developing cancers associated with those hormones.^[59] On the other hand, people who exercise far more than average have lower levels of these hormones, and lower risk of cancer.^[59] Osteosarcoma may be promoted by growth hormones.^[59] Some treatments and prevention approaches leverage this cause by artificially reducing hormone levels, and thus discouraging hormone-sensitive cancers.^[59]