[/QUOTE]The idea that certain foods, which are required for life due to their nutrients, vitamins, and energy content, may cause cancer is a difficult one to swallow.
Foods are so essential; how can we even compare them with other known carcinogens like tobacco smoke or car exhaust? You may also notice that epidemiologic studies can attempt to link certain foods and behaviors with cancer, but without a full consideration of the mechanisms, any relationship identified is difficult to support.
A food could theoretically impact our risk of cancer only if it causes a tangible change within our body that would promote an environment that is conducive to a cell becoming cancerous.
The following mechanisms describe how foods could generally cause cancer:
The food and/or cooking process could contain a carcinogen or chemical that can damage our cells or a part of the body. This repetitive damage could eventually leave this area more prone to cancer (much like repeated injury from cigarette smoke in the lining of the lungs).
The food or cooking technique could contain a chemical or free radical that damages DNA, which can lead to the expression or mutations of genes that promote cellular replication, growth, and eventually cancer.
The food or cooking technique could contain free radicals that attack our cells (either in general, or the cell membranes, DNA, or other cellular components) in a way that promotes unchecked cellular replication, the formation of a rogue cell, and eventually leads to cancer.
The food could lead to a metabolic environment that makes the induction and growth of cancer cells more favorable, like obesity or type 2 diabetes. For example, in obesity, diabetes, or a diet that promotes elevated blood glucose and insulin, normal cells may receive messages that activate cellular growth and reproduction pathways that, over time, may call for their unrestrained growth and could increase the risk of eventual conversion to a cancerous cell. Such a growth stimulus has been described as a “Hallmark of Cancer.”16
These descriptions highlight how there is a lack of mechanistic connection between diet and cancer. This is important when considering the weak evidence pointing to a possible, but inconclusive, association between dietary patterns and cancer. A lack of credible mechanistic link means a causal relationship is less likely.
Furthermore, the fact that higher fat diets seem to decrease amounts of body fat, improve type 2 diabetes and insulin sensitivity, and improve overall metabolic status, it remains unclear how saturated fats could, at the same time, increase cancer. In a healthy low-carb, high-fat diet, there is no mechanistic explanation for fat contributing to cancer. But a mixed high-fat and high-carb diet certainly could start to have some mechanistic explanation as that mixed diet can lead to increased insulin, increased adipose, increased inflammation etc.
Unsaturated fats in the form of vegetable oils, on the other hand, have been used to promote cancer in animal and mouse models for decades.17Unsaturated fats often contain free radicals that can produce oxidative damage after consumption, which, in some cases, can impact lethal damage upon cells or DNA. This can lead to cancer.18
This has been shown to be the case in animal studies, when a switch from saturated to unsaturated fats in mice resulted in an increase in cancer rates.19 In fact, corn oil was a favorite dietary intervention in some of the older cancer studies due to its exquisite ability to promote cancer during animal experiments.20
Furthermore, randomized studies in humans have supported this link, as multiple randomized studies revealed increases in cancer deaths and reduction in survival within groups consuming higher amounts of vegetable oil.21
One randomized trial in humans revealed an increase in lung cancer and a doubling of cancer-related deaths in men randomized to a high vegetable oil diet.22 The differences were of borderline significance, but began to accelerate towards the end of the trial, potentially illustrating that the onset of cancer due to a lifestyle change can take years to develop.
Vegetable oils’ possible link to cancer is a great example of the requirement to demonstrate multiple mechanisms that explain a potential cancerous impact of a food. For more information about vegetable oils, please check out our guide:
Do fruits and vegetables fight cancer?
While red meat has had the reputation of the dangerous dietary component for the past several decades, fruits and vegetables have been the lauded darling of most modern dietary recommendations, receiving praise as an anticancer food. The studies, however, have been less consistent and do not completely confirm the impeccable reputation of fruits and vegetables.
As with red meat and most foods, the relationship between fruits and vegetables and cancer is plagued by epidemiologic studies utilizing often-inaccurate food-frequency questionnaires and all of the other issues that accompany observational associations.
For instance, smokers, heavy drinkers, and individuals who rarely exercise – all three of which are risk factors for cancer – eat fewer vegetables than nonsmokers, moderate drinkers, and those who exercise.23 Unsurprisingly, these folks are much less healthy, but we cannot be sure that it is due to diets that lack fruit and vegetables.24 Along these lines, similar studies point to a lower risk of all-cause mortality in the vegetable eaters, but again, these numbers are plagued by confounding social issues.25
When looking at all types of chronic diseases, the benefits point even more specifically to vegetables rather than fruit, but especially green, leafy vegetables.26 This more narrow anointing of certain vegetables would be consistent with potential mechanisms: green, leafy vegetables’ ability to feed bowel bacteria and the ability of cruciferous vegetables to stimulate our antioxidant defense system, along with the similar pathways that help detoxify potentially cancerous chemicals and hormones.27
Like most foods, studies are mixed, with some revealing a lower cancer risk, and others showing any difference to be minimal for the identical food. Furthermore, studies suggest that the anticancer benefit of vegetables is largest in heavy smokers and drinkers.28
From a mechanistic view this is also reasonable, as many vegetables enhance our cellular ability to detoxify potentially cancerous chemicals. In other words, these vegetables may be working hard to offset the massive damage from these individuals’ unhealthy behaviors, but the benefits are less robust or just aren’t there for the rest of us who follow a reasonably healthy lifestyle.
A meta-analysis of 26 studies assessing the risk of breast cancer in women from 1982-1997 found no benefit of fruit consumption in reducing the risk of breast cancer, while vegetables were associated with a 25% lower relative risk.29 Additionally, an analysis specifically in premenopausal women revealed a similar potential benefit with vegetables, but not fruit.30
In men, cruciferous vegetable consumption is associated with a decreased risk of prostate cancer.31
Healthy fresh fruits and vegetables heap isolated on white background
When we look at a newer study, no benefit of fruits or vegetables were seen when scientists looked at over 350,000 women and their risk of breast cancer.32 Multiple other studies echoed these findings, revealing no reduction in breast cancer or any cancer with fruit or vegetable consumption.33
Other data may suggest that when a food is eaten earlier in life it may be protective against breast cancer, but I think you are getting the point; the data is an inconsistent mess, and most studies don’t indicate any conclusive link between diet and cancer.34
When moving downstream to the gastrointestinal tract, things do not seem to clear up. Epidemiologic studies reveal a potential association with a higher risk of colon cancer in those who eat less than 1.5 servings of vegetables per day, but the association is weakly positive with a hazard ratio of just 1.65.35 Even less optimistic, a pooled analysis of 14 studies revealed no reduction in colon cancer risk seen in people who reported high fruit and vegetable consumption.36
Much like meat, the issues of the different types of vegetables and preparation of these vegetables may impact many of these studies. For instance, just as the negative impact of burnt meat may be offset by a benefit of meat consumption, the same could be possible for vegetables. Studies, however, have yet to assess this relationship, limiting data. As discussed above, randomizing individuals to a high fruit and vegetable diet did not reduce the incidence of precancerous colon polyps.37
Additionally, the type of vegetable and growing conditions can largely affect the potentially beneficial chemicals present within the vegetable. For example, organosulfurs, compounds that promote cellular detoxification and antioxidant production, are found in onions, garlic, broccoli, cabbage, and other vegetables, but amounts vary considerably between varieties.38
From a cancer standpoint, we can view vegetables as generally consisting of an array of vitamins, minerals, soluble and insoluble fiber, and an array of defensive chemicals to ward off predators. Assessing each of these components may provide some clues as to whether (and how) vegetables could decrease the risk of cancer.
In greater detail, I would suggest the following possibilities:
Fibrous material in vegetables feeds and nurtures our bowel bacteria.
Our normal bowel bacteria help to fight inflammation, detoxify potentially cancerous chemicals, and protect the lining of the bowels.39 Thus, aiding in their health could improve our health and lower the risk of cancer forming in the lining of our gastrointestinal tract.40
Bowel bacteria bind and metabolize potentially dangerous chemicals.41 For instance, the hydrocarbons in burnt food are metabolized by bowel bacteria, which may protect us from cancer.
Feeding these bowel bacteria will help to increase their presence in our gut, further potentiating the breakdown of harmful chemicals like heterocyclic amines and polycyclic aromatic hydrocarbons, while converting the antioxidant promoting organosulfurs in cruciferous vegetables to their cancer-fighting byproducts.42
Bowel bacteria create butyrate from the fiber in vegetables.43 Animal studies have revealed that this conversion can reduce the risk of colon cancer via apoptosis, the systematic destruction and pruning of damaged cells that can become cancerous if left unchecked.44 Studies in humans, however, are limited.
Many vegetables contain defensive chemicals like sulforaphane that work to ward off or even kill potential prey like animals and insects.
These same chemicals signal a warning sign to our cells, but instead of being fatal, simply increase our immune system and antioxidant response and also activate our detoxification systems. Animal studies have revealed the ability of sulforaphane from broccoli sprouts to block chemically-induced cancer.45 Studies in humans have revealed that cruciferous extracts can aid in the detoxification of carcinogenic tobacco smoke.46
In summary, the research evidence for vegetables being protective from cancer is minimal. Some studies may suggest that green and cruciferous vegetables provide a potential benefit, but this relationship has not been fully elucidated.
Non-starchy vegetables provide a plethora of vitamins and nutrients. When eaten as part of a diet low in simple sugars or other harmful foods, these vegetables likely contribute to our health with very little, if any, risk. Furthermore, from a purely mechanistic point of view, there are plenty of ways in which vegetables could, at least in theory, lower our risk of cancer, especially for those of us living in urban settings or polluted settings with unavoidable exposure to carcinogenic chemicals on a daily basis.
Please see DD for the entire content.
Trying to wrap my head around info that contradicts Fiber Menace. Im suspecting it is the amounts and the kinds of fiber. "Fiber" becomes a catchall, and as a catchall it really is a wide variety of types of material, and uses different microbes to process it. IMHO dropping brans and hulls fits fiber menace, while eating real fruits and veg and nuts fills the supply of other fibers.
In feed terms, fiber is lignan. Even the cattle and sheep cannot use this. At least not directly.