The common belief is that the less fat you consume, the lower your risk of getting breast and colon cancer. But is this true? To find out, the PURE researchers reviewed all available evidence on diet and cancer, and looked at the evidence from multiple sources. The conclusion was that you can’t make a blanket statement that eating less fat will reduce your risk of getting cancer.
Diet and cancer: What we know and what we don’t —”
2. Does dietary fat cause breast cancer?
Dr. Walter Willett, MD, PhD, of the Harvard School of Public Health, sought to prove the link between dietary fat and breast cancer using the Doll study, which used a dietary questionnaire with 61 questions in a large group of nearly 90,000 nurses. The results were analyzed, total fat, saturated fat, linoleic acid (a polyunsaturated fat found in vegetable oil) and cholesterol in the diet of the nurses were calculated and compared with indicators of diseases such as cardiovascular disease and cancer.
Willett’s group then published three sets of results on the link between dietary fat and breast cancer. The first publication, Dietary Fat and Breast Cancer Risk, published in 1987, showed a trend toward a lower risk of breast cancer with higher dietary fat intake, but it was not statistically significant. A study published five years earlier also found no correlation.
Willett’s group will then combine data from seven studies and analyze data from more than 337,000 women and 4,980 cases of breast cancer. However, the results again showed no association between dietary fat and the risk of developing breast cancer.
The researchers cautioned that the length of follow-up in their original study may have been too short to identify an increased risk of cancer, or that even the lowest fat intake in the diet was too high to reduce the risk.
After a follow-up period of nearly 15 years, a new analysis that now included women whose diets contained less than 20% fat (recall that this analysis followed decades of endless recommendations for low-fat diets) found no significant change in breast cancer incidence.
Since Willett’s first large-scale study, many other studies have attempted to establish a link between dietary fat and breast and other cancers. The research remains negative, and even Willett has publicly stated that support for the primary link between fat intake and breast cancer risk has weakened considerably as the results of large prospective studies have become available.
Based on the evidence available to date, it is unlikely that reducing fat intake will be beneficial in reducing breast cancer risk.
3. Do red meat and processed meat cause cancer?
Meat bears the lion’s share of responsibility for foods that can cause cancer. Meat consumption in particular is associated with an increased risk of colorectal cancer. However, as in the global epidemiological study by Doll and Armstrong, this association has been inconsistently demonstrated in population studies.
Many sources discussing health issues related to red meat and its possible link to cancer often refer to the World Health Organization (WHO) International Agency for Research on Cancer (IARC) report. What is often underestimated is that this WHO report is not really a study, but rather an interpretation of general scientific findings by a certain group of people. In particular, the authors of the WHO report have repeatedly stated that the evidence for an association between red meat and colorectal cancer is weak and contradictory, and that the magnitude of any real effect may be small.
Moreover, some randomized studies have shown that a diet low in red meat has little or no effect on the risk of colorectal cancer. Several randomized studies have examined whether a low-fat, high-fiber diet low in red and processed meat can reduce the number of pre-cancerous adenomas called colorectal polyps. All showed that there was no reduction in polyps in the dietary intervention groups. In addition, these low-fat groups were randomized compared to the Western diet group, but still there was no benefit.
The fact that red meat and processed meat are completely different entities and should not be mixed together only makes this potential connection more confusing. Even the WHO acknowledges that red meat and processed meat do not pose the same risk.
Moreover, some compelling mechanisms identified in animal studies may explain why charring or burning meat (which releases potentially carcinogenic chemicals such as polycyclic aromatic hydrocarbons and heterocyclic aromatic amines) may increase the risk of colon cancer. However, this suggests that the carcinogen is the charred meat, and not necessarily the red meat itself.
The potential of burnt food to become a carcinogen in the colon is the main reason why the link between colorectal cancer and red meat has been so intensively studied. Nevertheless, even this association revealed a possible increased overall risk of colorectal cancer that is very small, and the reliability of the evidence is low to very low.
Although we know that meat in the diet provides many vitamins and minerals, the link to cancer is still supported by weak and contradictory evidence. Meat is also a source of high-quality protein that helps the body repair and regenerate. Thus, for those pursuing a nutrient-rich diet, specifically avoiding meat to reduce cancer risk is not supported by substantial evidence and may even lead to an inadequate diet. The strongest conclusion we can draw from the available data is that consumption of charred or processed red meat may increase risk more than consumption of red meat in general (although in absolute terms and based on weak data, the risk is still very low).
More information on this topic can be found in our guide :
Guide to red meat – is it healthy?
GuidelinesHere’s our guide to what we currently know about red meat, so you can make an informed decision about whether you should include it in your diet and, if so, how much you can eat per week.
4. Are there foods that cause cancer?
With the amount of data currently available, it is difficult to prove that a particular food causes cancer. Most studies are epidemiological, which suggests associations but does not demonstrate causality. Moreover, these studies use food frequency questionnaires, which are known to be unreliable. Although some data are of acceptable quality, the magnitude of the association found between diet and cancer is generally very small, making it impossible to determine whether the association is real, random or due to an unreported variable.
The problems associated with epidemiological research and the fact that it generates frenzied media attention and misinformation were scientifically addressed in the study Is Everything We Eat Linked to Cancer? A systematic review of cookbooks. This study shows that evidence of positive or negative effects can be found for most foods in population and cancer studies. However, the vast majority of these claims are based on very weak statistical evidence.
To demonstrate a strong link between certain foods or diets and cancer, the following four levels of support are needed:
- Aid for demographic/epidemiological research
- Mechanistic support (i.e., there is a mechanism to explain the relationship).
- Aid for research on animals
- Support for randomised controlled trials in humans
Surprisingly, these four values have never been demonstrated for any diet or type of cancer.
5. How can food cause cancer?
Food is essential for life; how can it be compared to other known carcinogens such as tobacco smoke or automobile exhaust?
In theory, foods can increase the risk of developing cancer if they cause significant changes in the body that contribute to a favourable environment for the development of cancer cells.
The following mechanisms describe how food can cause cancer:
- The food and/or cooking process may contain or produce a carcinogenic substance that damages our cells or parts of the body. This repeated damage can ultimately make this area more vulnerable to cancer (like the repeated damage caused by cigarette smoke in the lining of the lungs).
- The food or cooking method may contain/produce a chemical or free radical that damages DNA, which can lead to the expression or mutation of genes that promote uncontrolled cell replication and growth, and ultimately cancer.
- Food can create a metabolic environment that promotes the induction and growth of cancer cells, as is the case with obesity or type 2 diabetes. For example, with obesity, diabetes or a diet that promotes high levels of sugar and insulin in the blood, normal cells may receive messages that activate cell growth and proliferation pathways, which over time can lead to uncontrolled growth and increase the risk of eventual transformation into a cancerous cell. This growth stimulus is known to be the hallmark of cancer.
In evaluating the mechanisms by which a food may be injurious, consideration shall also be given to possible beneficial effects of that food. For example, high-fat diets reduce body fat, improve type 2 diabetes progression and insulin sensitivity, and improve overall metabolic health, all risk factors for cancer. By reducing these risk factors for cancer, it can be assumed that a healthy, low-carbohydrate, high-fat diet can protect against cancer. This is in contrast to the so-called standard American diet, which is high in carbohydrates, high in fat, and highly processed and refined foods.
When considering broad categories of macronutrients, it is also important to remember that there may be considerable heterogeneity within a given category. As for fats, for example, unsaturated fats in the form of vegetable oils have been shown to promote cancer in animal and mouse models for decades. These types of fats often contain free radicals that can cause oxidative damage after consumption, which in some cases can lead to fatal cell or DNA damage. This can lead to cancer.
But does this mean a potentially increased risk to humans? Some observational studies show an association between high consumption of omega-6 polyunsaturated fatty acids and cancer. However, the analysis of all observational studies shows that there is no correlation between the two.
As for randomized trials in humans, a study from the 1960s showed a non-statistically significant trend of increased cancer mortality in a group of men prescribed a diet rich in polyunsaturated vegetable oils. However, since no statistical significance was achieved and there were more non-diabetics in the experimental group, no serious conclusions can be drawn from these results. Prior to this study, all other similar studies were negative.
The possible link between vegetable oils and cancer is an excellent example of the need to interpret all the data, bearing in mind that some positive observational studies do not show a causal relationship.
For more information on vegetable oils, see our guide :
Vegetable oils: What we know and what we do not know
TipsGrove oils are becoming one of the main sources of calories in our diet. Is that a good thing? Let’s see what we know and what we don’t know.
6. Do fruits and vegetables fight cancer?
While red meat has had a reputation for being a dangerous food in recent decades, fruits and vegetables have been lauded for their cancer-fighting abilities. However, the studies are contradictory and do not fully support the impeccable reputation of fruits and vegetables.
As with red meat and cancer, the link between fruits and vegetables and cancer is difficult to establish. This is again due to the fact that the epidemiological data suffer from all the problems already mentioned in this guide.
Moreover, it is often difficult to control for the effects of confounding variables. Smokers, heavy drinkers and people who exercise little – all risk factors for cancer – eat fewer vegetables than non-smokers, moderate drinkers and people who do exercise. It is not surprising that these people are much less healthy, but we cannot be sure that this is due to a diet that lacks fruits and vegetables. Similar studies indicate a lower risk of all-cause mortality in people who eat vegetables, but these figures are also associated with social problems.
For all types of chronic diseases, the benefits are greater for vegetables than for fruits, but especially for green and leafy vegetables. This closer anointing of certain vegetables is consistent with the proposed mechanisms: leafy vegetables can nourish and stimulate the growth of certain gut bacteria, and these bacteria can contribute to the metabolism of chemicals or carcinogens that subsequently cause harm. Additionally, cruciferous vegetables can boost our antioxidant defense system, as well as similar pathways that help detoxify potentially carcinogenic chemicals and hormones.
Like the studies on cancer and other foods, the studies on fruits/vegetables and cancer are mixed, with some showing lower cancer risk and others showing little difference for the same foods. Moreover, studies show that the cancer-fighting benefits of vegetables are greater in smokers and heavy drinkers.
Mechanically, this makes sense because many vegetables increase our cells’ ability to detoxify potentially cancer-causing chemicals. In other words: These vegetables may attempt to offset the significant damage caused by the unhealthy behaviors of these individuals, but the benefits will likely be less or nonexistent for those who live a fairly healthy lifestyle.
A meta-analysis of 26 studies on breast cancer risk in women between 1982 and 1997 found that fruit consumption did not reduce breast cancer risk, while vegetables were associated with a 25% reduction in relative risk. In addition, an analysis conducted specifically for premenopausal women found that vegetables can provide similar benefits, but not fruits.
A recent article that conducted a joint analysis of several cohort studies found that fruits and vegetables had no beneficial effect on the risk of breast cancer in more than 350,000 women. These results have been confirmed by many other studies that have found no reduction in the incidence of breast cancer or other cancers by eating fruits and vegetables.
There is further evidence that eating certain foods at an early age may protect against breast cancer. Still, all things considered: The data are inconsistent, making it impossible to establish a definitive link between higher consumption of fruits or vegetables in adults and a lower risk of breast cancer.
There is also no conclusive evidence that fruits and vegetables reduce the risk of colorectal cancer. A pooled analysis of 14 studies found no reduction in the incidence of colorectal cancer in people who reported eating lots of fruits and vegetables. A positive epidemiological study showed an increased risk of colorectal cancer in people who ate less than 1.5 servings per day compared to those who ate more than 2.5 servings per day. The increase in relative risk was statistically significant, although small (1.65).
As with meat, the issue of different vegetables and how they are prepared can affect many of these studies. Were they barbecued? A couple? What were they cooked with? Research has not yet evaluated this relationship, which limits the conclusions we can draw.
In addition, the type of vegetable and the growing conditions can have a great influence on the potentially beneficial chemical content of the vegetable. For example, sulfur compounds that promote cellular detoxification and antioxidant production are found in onions, garlic, broccoli, cabbage, and other vegetables, but the amount varies considerably from variety to variety.
Overall, the results of clinical trials suggesting that vegetables protect against cancer are inconclusive. Some studies suggest that green vegetables and cruciferous vegetables have potential benefits, but this link has not been conclusively proven.
Non-starchy vegetables contain many vitamins and nutrients. When eaten as part of a diet low in simple sugars and other unhealthy foods, these vegetables likely contribute to our health with little or no risk. Moreover, purely mechanically, vegetables can reduce cancer risk in many ways, especially for those of us who live in urban or polluted environments where daily exposure to carcinogenic chemicals is inevitable.
For more information on the potential anti-cancer mechanisms of fruits and vegetables, click Read More.
- The fiber-rich material in vegetables nourishes and maintains our gut bacteria.
- Our normal gut bacteria help fight inflammation, detoxify potentially cancer-causing chemicals, and protect the intestinal lining. Thus, caring for their health can improve our health and reduce the risk of cancer of the lining of the gastrointestinal tract.
- Gut bacteria bind and metabolize potentially harmful chemicals. For example, the hydrocarbons in the food we burn are metabolized by gut bacteria that can protect us from cancer.
- Feeding them increases the presence of these bacteria in our guts, which help break down harmful chemicals like heterocyclic amines and polycyclic aromatic hydrocarbons, while the antioxidant organosulfides found in cruciferous vegetables are converted into cancer-fighting by-products.
- The intestinal bacteria form butyrate from the dietary fibers found in the vegetables. Animal studies have shown that this transformation can reduce the risk of colon cancer through apoptosis, the systematic destruction and pruning of damaged cells that can begin to form cancer if not controlled. However, studies in humans are limited.
- Many vegetables contain protective chemicals, such as sulforaphane, that deter or even kill potential prey and insects.
- These same chemicals send a warning signal to our cells, but instead of being lethal, they merely stimulate the immunity and antioxidant response and activate the detoxification system. Animal studies have shown that sulforaphane from broccoli sprouts can block chemically induced cancer. Human studies have shown that cruciferous extracts can help detoxify carcinogenic tobacco smoke.
7. What can we say with certainty?
Unfortunately, not as much as I would have liked. Based on population studies, limited controlled trials, studies of cellular mechanisms and animal studies, there is evidence of an association between dietary intake and cancer risk.
As discussed earlier in this article, it can be assumed that certain foods (e.g., processed meat) and cooking methods (e.g., grilling meat at a high temperature) may slightly increase the risk of developing certain cancers. On the other hand, foods that promote cell and DNA repair, help detoxify carcinogens, and boost immune system health (such as certain vegetables) may reduce cancer risk. Finally, many foods and macronutrients previously associated with the development of cancer (e.g., fats) may have no effect on cancer risk.
As mentioned earlier, dozens of randomized studies have shown that a high-fat, low-carb diet can lead to weight loss, lower insulin levels, and improved insulin sensitivity in people with type 2 diabetes or pre-diabetes. Therefore, a low-carbohydrate, plant-based diet to nourish gut bacteria and detoxify potentially carcinogenic chemicals seems to make the most sense, given the conflicting results of the available research.
But back to Richard Dole: Let us remember the important warning he left us about the link between cancer and nutrition before his death in 2005 at the age of 92:
Given the many shortcomings of this method [of nutritional epidemiology] in terms of data quality, consideration of latency and uncertainty….. it is clear that these and other correlations should be considered only as a spur to further research and not as a proof of causality or as a basis for preventive action.
/ Colin Champ, M.D.
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Frequently Asked Questions
What diet is related to cancer?
A diet that is high in fat and sugar can increase the risk of cancer.
What are the 11 cancer causing foods?
1. Beef 2. Bacon 3. Butter 4. Cheese 5. Chicken (fried) 6. Coffee 7-9: Corn, potatoes, sugar, wheat, and white breads/pastries/cakes/cookies/donuts 10-11: Alcohol and red meat 1. Cheese 5-6: Chicken (fried) and red meat 7-9: Corn, potatoes, sugar, wheat, and white breads/pastries/cakes/cookies/donuts 10-11: Alcohol and red meat
What should you not eat if you have cancer?
Some foods that are not recommended for people with cancer include: -Raw or undercooked meat, poultry, fish, or eggs. -Unpasteurized dairy products such as milk and cheese. -Raw fruits and vegetables. -Unpasteurized juices. -Unpasteurized fruit or vegetable juice cocktails. -Raw sprouts, such as alfalfa, clover, radish, and watercress. -Raw or undercooked beans, lentils, and peas. -Raw sprouted seeds. -Unpasteurized honey. -Unpasteurized cider or wine. -Unpasteurized fruit or vegetable juices. -Raw or undercooked eggs. -Raw or undercooked meat, poultry, fish, or eggs.
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