What is cancer?
Our bodies are made up of cells, all which have a specific function, which is encoded in the cells DNA. Eventually, all cells will die and be replaced by new healthy cells, a process called apoptosis. Sometimes the DNA is damaged and genetic material is altered, so that the cell no longer performs the function it was supposed to, but gets out of control. Moreover, apoptosis fails to occur, so instead of each cell dying off and being replaced by new cells, the new cells continue to grow but the old cells remain. This causes a mass to grow, and eventually spread to other parts of the body. This mass has an enormous energy requirement, and spreads (metastasizes) throughout other tissues and organs of the body, taking over like weeds do in a garden. This mass, called a tumour, needs such an enormous energy supply, that it causes new blood vessels to grow in the area to supply it with extra nutrients. This growth of new blood vessels to the area is known as angiogenesis. In fact, when pathologists examine the blood supply of the area, they see a large mass surrounded by leg-like looking projections (the new blood vessels). This image is is said to resemble a crab, and the latin name for crab is cancer. Whilst it is thought that some people may be genetically predisposed to certain cancers, it is also known that many cancers are influenced largely by lifestyle factors such as smoking and diet. When trying to avoid cancer, it is important to look at 2 separate factors; both avoiding exposure to harmful influences and ensuring adequate exposure to protective influences.
Known and Suspected Carcinogens
Several well known influences are known to cause normal cells to become cancerous. These influences are known as carcinogens. Known carcinogens include Ultra Violet B band radiation from the sun and tanning machines (which causes skin cancer), a chemical in tobacco smoke called NNK (which causes cancer of the respiratory system and upper digestive tract), ionizing radiation such as that released in nuclear reactor malfunctions (which can cause thyroid cancer), asbestos used in building material (which causes mesothelioma) and certain viruses such as the Human Papiloma Virus, (warts) which can cause cervical cancer.
It is also known that a bacterium called Helicobacter pylori can cause stomach and duodenal ulcers which can lead to gastric cancer.
Certain fungi such as those from the Aspergillus genus produce mycotoxins which are known to be carcinogenic (which is why you should not eat mouldy food).
It is also known that certain professions that require employees to be exposed to carcinogenic chemicals have a higher rate of certain cancers such as bladder cancer.
Chronic alcoholism as well as long term hepatitis infections can cause severe liver damage which may eventually become cancerous. There are also many studies which have found that women who consume excessive amounts of alcohol are at greater risk of developing various cancers, most notably breast cancer. Other studies show that long term alcohol abuse may also increase the risk of developing digestive cancers such as esophageal, stomach, colon and rectal cancer.
These are all well known carcinogenic influences and there is little or no question about their cancer-causing effect. There are several other dietary influences which preliminary evidence suggests may increase the chance of certain cancers as well.
Delicatessen meats provide nitrosamines whilst other meats product heterocyclic amines when cooked. These substances are thought to be carcinogenic. Population studies have found modestly higher rates of various cancers among people who eat a lot of meat, particularly pork products.
Low intake of fruits and vegetables is strongly associated with an increased risk of many cancers, however it is thought that this is because these plant foods contain an abundance of beneficial and protective substances known as phytochemicals.
Some studies among Asian populations have have found higher rates of digestive cancers among people who eat raw seafood. This is because uncooked meats such as fish may contain harmful parasites which would have been killed simply by cooking the meat.
Some studies suggest that heavy consumption of salted or pickled foods may increase the risk of stomach cancer, however this evidence is not conclusive.
There is also at least some good evidence to indicate that consumption of burnt, charred, smoked or very well done meats may increase the risk of digestive cancers as well.
People who are overweight or obese also have a much higher risk of getting cancer, though it is not known whether excess body fact actually causes cancer or whether it is simply indicative of poor diet and unhealthy lifestyle (which consequently increases the risk).
Although it has caused much controversy over that past decade or so, there is no good evidence to suggest that use of mobile/cellular phones can cause brain tumours. Additionally, there is no evidence at all to suggest that radiation from microwave ovens or any other household electrical appliance can cause cancer.
There is very strong evidence that consumption of fruits and vegetables can protect against a wide variety of cancers, most notably lung cancer and cancers of the digestive system. People who consume the greatest variety of fruits and vegetables generally have the greatest protection. Fruits and vegetables contain an abundance of substances called phytochemicals, many of them showing the ability to inhibit cancer in laboratory experiments. Unfortunately, there are far too many of these phytochemicals to simply bottle or fit in a pill, so dietary supplements can not possibly have the same benefit as fruits and vegetable consumption has. Although it is best to consume a wide variety of plant foods, the most protective fruits are thought to include tomatoes, citrus fruits, red apples (particularly their skins), strawberries, blueberries, red grapes and other reddish/orange coloured fruits such as mangoes, apricots, plums, peaches etc. Among the vegetables thought to offer the most protection include dark green leafy vegetables such as kale and spinach, cruciferous vegetables such as broccoli, cabbage, brussel sprouts and cauliflower, allium vegetables such as garlic, onion and shallots, as well as red and orange coloured vegetables such as carrots and capsicum.
There is also at least come good evidence to indicate that whole grain cereal foods such as wholemeal wheat products, rye, oats, flaxseeds etc, may protect against cancers of the digestive system. These foods are high in certain fibres which speed the passage of stools through the colon, thus minimizing the time that potentially carcinogenic substances are exposed to intestinal tissues. Flaxseeds are also high in lignans which have estrogenic properties, and have shown to prevent hormone related cancers (breast cancer and prostate cancer) in animal studies. Flaxseed oil however loses the lignans and thus does not have the same beneficial effect.
There is some evidence to suggest that soy foods may also help to protect against certain cancers, most notably breast cancer and prostate cancer. Soy, like flax, contains phytoestrogens which may have hormonally related effects.
There is an abundance of evidence from laboratory studies (both in vitro and animal experiments) that green tea consumption may protect against a wide variety of cancers. Most case-control studies have confirmed this, but the results of prospective cohort studies are mixed, and intervention studies have shown disappointing results. There is at least preliminary evidence that frequent green tea consumption may at least have some protective effect, which would no doubt be enhanced by being part of a diet rich in plant foods such as fruits, vegetables, whole grains and legumes (including soy).
There is very preliminary evidence to suggest that olive oil, fish and moderate red wine consumption may reduce the risk of hormone related cancers such as breast and prostate cancer, however the evidence is far from conclusive and further studies are needed to confirm this.
There is no good evidence to suggest that dietary supplements can decrease the risk of any cancer, except perhaps selenium, however this is only the case in people whose selenium intake is very low to begin with. Some studies have even shown that certain supplements containing large doses of "anti-oxidant" vitamins such as vitamin E and especially beta carotene, can actually increase the risk of death and certain cancers.
As mentioned, there are many, many beneficial substances found in various plant foods which are known to have chemoprotective abilities. Vitamins are needed in the body in small amounts, but evidently do not protect against cancer by themselves.
Other Methods of Chemoprotection
There is evidence to suggest that non-steroidal anti-inflamatory drugs (NSAID's) such as Aspirin and Ibuprofen posses chemoprotective properties, however chronic over-use of NSAID's may have harmful side effects such as damage to the liver and digestive system. There is also some evidence to suggest that physically active people have a decreased risk of getting colorectal cancer, whilst those who lead sedentary lifestyles are at greater risk.
One of the most important cancer preventative strategies is early detection and intervention. It is very important that women over a certain age, particularly who have a family history of breast cancer, should receive regular mammogram. Likewise, men should receive regular prostate check-ups. All males and females, even during adolescence and early adulthood, should receive regular skin cancer check-ups. Skin cancer (melanoma) is the most common cause of death from cancer among young adults in Australia, yet melanoma is also the type of cancer that can be most easily detected and removed through regular check-ups. Everyone should become familiar with early warning signs of cancer, and not delay medical attention, even if uncertain. Many early symptoms of certain cancers are similar to symptoms of many other diseases, many of which are not life threatening. Nonetheless, it should be a doctor who decides whether symptoms are worth investigating further.
How Do We Know? - Experimental Methodologies
Obviously, one important step towards preventing cancer is to avoid known and suspected carcinogens wherever possible, however there are certain influences, most notably dietary influences, which have shown to protect against cancer (chemoprotective). There are several methods which scientists can use to identify dietary components which may have anti-cancer benefits. Because there are flaws in each method of experimentation, the evidence is generally only considered strong when similar results occur in all the different methodologies. These methods are described below, in order of weakest to strongest.
In vitro ("in glass") studies use cell cultures grown in a test tube or petri dish, and examine the effects that certain substances may have on those cells. For example, human lung cancer cells can be cultured in vitro, and scientists can expose these cells to various chemicals which may either have no effect on their growth, may cause them to grow and or spread quicker, or may even slow their growth. Scientists can also examine certain biochemical activities within the cells as well. This may help identify which substances may help to slow or inhibit cancer growth, but a major problem with in vitro studies is that the cells may not act the same way when part of whole tissues and organs etc. This is why it is necessary to then test such substances in vivo, which means to test their effects on cancers in live animals, not just in isolated cell cultures.
If a substance has been identified in vitro as having a chemoprotective effect, then it should be tested in vivo, usually in rodents. There are two main methods of testing the effect of certain substances of dietary components on animals. The first is to inoculate many of them with actual cancer cells. Half the animals will receive a normal diet whilst the other may receive the same diet only with one added component. After several months, all the rodents will be sacrificed (killed) and will be examined to see how much the cancers grew in each animal. If there is a significant difference between the rodents receiving the added component compared with those that didn't (known as "controls"), then it can be ascertained that this component had an effect on the cancer growth. For example, 100 rats may be implanted with colon cancer cells, and receive either normal drinking water or drinking water with green tea for 20 weeks. Several experiments have shown that the rate of tumour growth in those given green tea was approximately 30% less compared with those not given green tea. This kind of experiment may give scientists an indication of whether a certain dietary component may slow cancer growth, but it does not indicate whether it can totally prevent cancer; that is, whether it can stop normal cells from becoming cancerous cells (carcinogenesis). The other type of experiment used in animals is where all will be exposed to a certain carcinogenic chemical known to cause a specific cancer. Half will be given a normal diet, (controls) the other half given the added component. Similarly, after a certain period of time, all will be killed and examined to see whether there was a difference in cancer rates between the two groups. An example may be where 100 rats are exposed to a chemical known to cause breast cancer. half are given an ordinary diet, whilst the other half are given a similar diet, but with added soybeans. Several experiments have shown that rats given soybeans had significantly less incidence of cancer than control rats. Whilst this type of experiment can determine whether a certain dietary component may or may not prevent cancer from occurring or not, the major flaw is that at best, it would only prove that the dietary component may only protect against cancers caused by that specific carcinogen. In humans, breast cancers may not be caused by that same chemical, therefore it can know be known whether a similar diet would protect against other causes of the same cancer. Importantly, it should be noted that different species of animals have very different dietary requirements and may respond different to different diets due to their own unique physiological characteristics. Therefore it should not be assumed that the effects seen in animal experiments would be the same in humans. Consequently, human experiments are necessary.
Usually, the most effective means for determining what effect a certain drug may have on humans is by conducting controlled trials. This involves half the participants taking the drug, whilst the other half receive a placebo, neither group knowing which one they have been given. After a certain period of time, the effects of the drug will be compared with the effects of the placebo (control). When determining whether certain dietary components may prevent cancer or not, this type of experiments is virtually impossible, because it may take a lifetime of dietary habits to have the desired effect. Therefore, there controlled trials can measure bio markers or reoccurrence of cancer among high risk groups, though determining whether or not cancer prevention is possible is very difficult.
Bio markers are certain biological or biochemical indications that a person may or may not be at a high risk of getting a certain cancer. For example, high levels of Prostate Specific Antigen (PSA) may indicate an increased risk of prostate cancer, but it is not definite. Some trials have tested the effects of different diets on PSA levels. Diets shown to decrease PSA levels are thought to be protective against prostate cancer, but this is only a very vague estimate.
The other type of controlled trial which is used to examine the effects of different diets on cancer rates may involve giving different diets to patients already known to be at high risk (intervention studies), usually those whose cancer has gone into remission. The best kind of intervention study should be large and use people with no known preexisting cancer risk. These kinds of studies are very expensive and take many years, but provide us with the more reliable source of information on what works when it comes to cancer prevention.
Population Studies (Epidemiology)
As apposed to controlled studies which involve intervening in a groups lives, epidemiology is a type of observational studies, which aim to look at what groups are doing on their own and what effects are occurring. It usually involves surveying the dietary habits of very large numbers of people, and comparing those of people who get cancer with those who don't. These types of studies are more common but considered less reliable, as it is difficult to separate cause from effect when it comes to observational studies. For example, population studies have shown us that those who eat a lot of meat are more likely to get bowel cancer. Because people who eat a lot of meat are less likely to eat a lot of protective substances (such as fruits and vegetables) it is difficult to know whether the meat itself is the causative factor or whether it is simply an indication of poor vegetable intake, which may actually be the causative factor. Observational studies attempt to 'control; for confounding factors, thereby attempting to differentiate cause from effect. Ultimately, if laboratory studies can provide an explanatory mechanisms, it will be more likely to be considered a causative factor (for example, we know that meat produces carcinogenic chemicals in the laboratory, so it may well be plausible to suggest it is a causative factor.)
The most common type of epidemiological study is the case-control study. When examining the effect of diet on cancer, this involved surveying a large number (several hundred) of people who have been diagnosed with a specific cancer, to analyze their dietary habits as well as their medical history, their family history as well as other lifestyle factors such as smoking, drinking, exercise etc. The same surveys will then analyses these factors among people of similar age, that do not have cancer, known as "age-matched controls". The results of these surveys are then compared to see if there are any significant differences between the two groups. Whilst this is considered a relatively reliable method for identifying chemoprotective foods, case-control studies generally only involve a few hundred people, and can not accurately determine long-term food frequency and life time dietary habits. It is also said that case-control studies are biased because they study people who have already been diagnosed. The most reliable type of Epidemiological experiment is known as the prospective cohort study.
Prospective cohort studies involve registering a very large number (usually several thousand) healthy people who agree to be surveyed every few years. In regards to identifying dietary components and cancer, all participants will be cancer free at the beginning of the study. The participants are then studied for many years, throughout which time researchers keep track of which ones are diagnosed with certain cancers and other diseases. Every few years, the results of follow-up surveys will look at significant differences in dietary habits between the people who get a certain disease and those that don't. Whilst prospective cohort studies are considered to have the strongest design in regards to identifying the effects of certain dietary components on cancer, they are generally very expensive to conduct and take many years before any significant results can be obtained.
Because each type of experimental methodology has its own strengths and weaknesses, when identifying foods with chemoprotective abilities, it is generally considered necessary to obtain similar results from all types of studies. Moreover, it is necessary to obtain similar results from the majority of studies to examine the same factors. The strength of the evidence depends upon how many studies have demonstrated the same result, and the strength of those studies design.