Diet and Supplements 101: The 2022 Cancer Prevention Edition

Can foods and diet influence your risk of getting cancer? What types of foods reduce your risk and types that might increase your risk? What about vitamin and mineral supplements? Can supplements reduce your cancer risk or do they actually increase your cancer risk? And yes, there are many guides out there but most of them are product centric, biased and do not incorporate the latest and up to date evidence. 

Find out the answers below.


What is Cancer?

Cancer is the second leading cause of death in the United States, behind only heart disease. People with cancer also often experience physical effects (from the cancer itself and from treatment), distress, and a lower quality of life. Quality of life can also be affected for family members, caregivers, and friends of people with cancer.

For most Americans who do not use tobacco, the most important cancer risk factors that can be changed are body weight, diet, and physical activity. At least 18% of all cancers diagnosed in the US are related to excess body weight, physical inactivity, excess alcohol consumption, and/or poor nutrition, and thus could be prevented.

Genetic changes that cause cancer can be inherited from our parents. That said, environmental factors could contribute up to 95% of cancers. They can also arise during a person’s lifetime as a result of errors that occur as cells divide or because of damage to DNA caused by certain environmental exposures. Cancer-causing environmental exposures include substances, such as the chemicals in tobacco smoke, and radiation, such as ultraviolet rays from the sun. 

There are many types of cancer treatment. The types of treatment that you receive will depend on the type of cancer you have and how advanced it is. Surgery, radiation, and chemotherapy are the standard types of treatment for cancer. 

There are also many phoney “cancer treatment” with amazing claims but devoid of any scientific data to back them up, typical of health scams. Cancer is a life threatening disease and therefore many consumers fell prey to these phoney cancer treatments.

Diet and Cancer Prevention

diet and cancer prevention

Diet and nutrition are important determinants of cancer risk, both through their contributions to energy balance and via biological mechanisms that alter risk independent of body weight (R).

One major change in cancer prevention guidelines over time, which reflects the current and evolving scientific evidence, has been a shift from a reductionist or nutrient-centric approach to a more holistic concept of diet that is characterized as dietary patterns. A focus on dietary patterns, in contrast to individual nutrients and bioactive compounds, is more consistent with what and how people actually eat. People eat whole foods (not nutrients) that, in aggregate, represent an overall dietary pattern wherein dietary components often contribute additively or synergistically to modify cancer risk. Emerging evidence, largely epidemiological but also including a few controlled intervention trials, suggests that healthy (vs unhealthy) dietary patterns are associated with reduced risk for cancer, especially colon and breast cancer (R).

Randomized controlled trials (RCTs) of dietary interventions aimed at preventing cancer, conversely, are expensive and largely impractical. Therefore, most current evidence concerning diet and cancer prevention is derived from observational epidemiologic studies, in particular prospective cohort studies, mechanistic studies of food components in laboratory animals and cell culture, and RCTs when available.

Dietary patterns as a modern and more appropriate focus (R)

Because of accumulating evidence on healthy dietary patterns in relation to chronic disease risk reduction, an emphasis on dietary patterns is now highlighted in the 2015 to 2020 US DGA. This is particularly relevant because, although the associations of individual nutrients and foods with cancer may be small, additive and interactive effects could be important. Several comprehensive reviews support recommendations to follow healthy dietary patterns. The 2015 Dietary Guidelines Scientific Report concluded that there is moderate evidence that dietary patterns rich in plant foods and lower in animal products and refined carbohydrates are associated with a lower risk of postmenopausal breast cancer, and plant-based patterns low in red and processed meat and added sugars are associated with a lower risk of colorectal cancer. In addition, the WCRF/AICR (World Cancer Research Fund/American Institute for Cancer Research) concluded that a Mediterranean diet is “convincingly” associated with a lower risk of weight gain, overweight, or obesity, whereas a “Western”-type dietary pattern is “probably” associated with an increased risk of these outcomes. 

The Diet Patterns Methods Project, a multicenter study of dietary patterns and cause-specific mortality, reported an 8% to 17% lower risk of cancer mortality among women and a 17% to 24% lower risk among men whose diets were most (vs least) concordant with 4 healthy dietary pattern scores. 
  1. The dietary patterns examined included the Mediterranean Diet, 
  2. the Dietary Approaches to Stop Hypertension diet, 
  3. the US Department of Agriculture (USDA) Healthy Eating Index, and 
  4. the Harvard Alternate Healthy Eating Index. 
Although these and other healthful dietary patterns have unique features, they share a foundation of mostly plant foods (including non-starchy vegetables, whole fruits, whole grains, legumes, and nuts/seeds) and healthy protein sources (higher in legumes and/or fish and/or poultry, and lower in processed meats and red meat), and include unsaturated fats (eg, monosaturated and/or polyunsaturated fat); these patterns are also lower in added sugar, saturated and/or trans fats, and excess calories. These healthy dietary pattern scores have also been associated with a lower risk of colorectal cancer and total cancer incidence in meta-analyses of observational studies. Two randomized clinical trials found lower overall cancer or breast cancer risk among those randomized to follow the Mediterranean diet. Thus, these studies provide consistent and compelling evidence that healthy dietary patterns are associated with a decreased risk of cancer, all-cause mortality, and other chronic disease endpoints.

Healthy dietary patterns may reduce the risk of cancer and other diseases through multiple mechanisms. For example, plant-based diets are associated with lower levels of inflammation, improved insulin response, and less oxidative DNA damage. Plant-based diets are also associated with higher concentrations of beneficial gut bacteria compared with mostly animal-based diets high in saturated fat and sugar. Additional research on the relationship of dietary factors with these metabolic and microbial biomarkers and with health outcomes will continue to help in elucidating the role that diet plays in carcinogenesis.

Vegetables and fruit (R)

Several food and nutrient components of healthy dietary patterns are also independently associated with cancer risk. Although the relationship between vegetables and fruit intake with lowering cancer risk is weaker than previously believed, the 2018 WCRF/AICR (World Cancer Research Fund/American Institute for Cancer Research) report concluded that consuming nonstarchy vegetables and/or whole fruit “probably” protects against several aerodigestive cancers, including mouth, pharynx, larynx, nasopharynx, esophagus, lung, stomach and colorectal cancers. Promising research on molecularly defined tumor subtypes has shown that carotenoid-rich vegetables and fruit, and biomarkers of their consumption, are associated with a lower risk of more aggressive breast tumors, including estrogen receptor–negative breast tumors.

Vegetables (including beans) and fruits are complex foods, each containing numerous vitamins, minerals, fiber, carotenoids, flavonoids, and other bioactive substances, such as sterols, indoles, and phenols, that may help prevent cancer. There is ongoing research on the potential cancer chemopreventive properties of particular vegetables and fruits, or groups of these, including dark-green and orange vegetables, cruciferous vegetables (eg, cabbage, broccoli, cauliflower, brussels sprouts), soy products, legumes, allium vegetables (onions and garlic), and tomato products. Vegetables and fruits may also indirectly influence cancer risk through effects on energy intake or body weight. Many vegetables and fruits are low in energy, high in fiber, and have a high water content, which may increase satiety and decrease overall energy intake, and thus should contribute to weight loss and maintenance of that loss.

Vegetable and fruit consumption has also been associated with a reduced risk of other chronic diseases, particularly cardiovascular disease, an important contributor to overall morbidity and mortality in the United States. For cancer risk reduction, the ACS advises consistency with the DGA, which recommends consuming at least 2.5 to 3 cups of vegetables and 1.5 to 2 cups of fruit each day, depending on energy requirements.

Legumes are rich in protein, dietary fiber, iron, zinc, potassium, and folate, are low in saturated fat, and have a low glycemic index. This makes legumes a generally healthy addition to the diet, and good alternatives for those looking to reduce their consumption of red and processed meats. Legumes also are gluten-free, making them appropriate for people with celiac disease or gluten sensitivity (R). Legumes include kidney beans, pinto beans, black beans, white beans, garbanzo beans (chickpeas), lima beans (mature, dried), lentils, edamame (green soybeans) and other soy foods.

Whole grains (R)

The evidence that whole grains, in which 100% of the original kernel is retained, lower colorectal cancer risk is considered “probable” by the WCRF/AICR. Each 30 g per day consumption of whole grains was estimated to lower the risk of colorectal cancer by 5%. In a separate meta-analysis, total cancer mortality risk was 6% lower with each 3 servings of whole grains daily. Rich in phytochemicals and dietary fiber, whole grains may lower colorectal cancer risk through modification of fatty acid production, lowered levels of proinflammatory bacterial species, and by accelerating gut transit time, thus reducing duration of exposure of the gut to carcinogens. In addition, the WCRF/AICR considers the evidence “probable” that whole grains and foods high in dietary fiber are associated with lower risk of weight gain, overweight, or obesity. The 2015 DGA recommends consuming at least one-half of grains as whole grains based on “moderate” evidence that dietary patterns rich in whole grains are associated with lower BMI, waist circumference, percentage body fat, and/or obesity. The ACS guideline recommendation to choose whole grains is consistent with these guidelines.

Dietary fiber (R)

Dietary fiber, which is found in plant foods, including legumes, whole grains, fruits and vegetables, and nuts and seeds, is considered “probably” associated with a lower risk of colorectal cancer as well as a lower likelihood of weight gain, overweight, and obesity. Dietary fiber has potent effects on bacterial species in the gut; and the relationship between gut microbial dysbiosis, body weight, and cancer risk is an active area of investigation. In RCTs of fiber supplements, including isphaghula husk (psyllium fiber) and wheat bran fiber, the supplements did not reduce the risk of recurrent adenomatous polyps. Thus, the ACS recommendation is to obtain most dietary fiber from whole plant foods, such as vegetables, fruits, whole grains, nuts, and seeds.

Red and processed meats (R)

Red meat refers to unprocessed mammalian muscle meat—for example, beef, veal, pork, lamb, mutton, horse, or goat meat—including minced or frozen meat, whereas processed meat is meat that has been transformed through curing, smoking, salting, fermentation, or other processes to improve preservation or enhance flavor, such as bacon, sausage, ham, bologna, hot dogs, and deli meats. Most processed meats contain pork or beef but may also contain other red meats, poultry, or meat byproducts.

Evidence that red and processed meat increases cancer risk has existed for decades, and health organizations recommend limiting or avoiding consumption of these foods. The 2015 DGA noted moderate evidence that eating patterns lower in red and processed meats were associated with lower risk of obesity, type 2 diabetes, and some types of cancer in adults. In 2015, the IARC expert panel concluded that processed meat is a group I carcinogen and red meat a “probable” (group 2A) human carcinogen based on evidence for increased risks of colorectal cancer in addition to evidence of biologically plausible mechanisms. The most recent WCRF/AICR report concluded that processed meat is “convincingly” related to colorectal cancer and that red meat “probably” increases colorectal cancer risk. Recent studies suggest a possible role of red and/or processed meats in increasing the risk of breast cancer and certain forms of prostate cancer, although more research is needed.

In contrast with these systematic reviews and guidelines, a 2019 review of prospective cohort studies considered the effects of red and processed meat intake on cancer mortality and incidence to be small, with certainty of evidence that is “low to very low certainty” based on review criteria that prioritized evidence from RCTs while downgrading evidence from observational studies. Therefore, the authors recommended that individuals continue current meat intake. However, the results of this group's meta-analyses found significant reductions in risk of cancer death with lower intake of red and processed meat as well as a lower risk of prostate cancer death and of incident colorectal, esophageal, and breast cancers with a reduction in processed meat intake, entirely consistent with the systematic evidence reviews from the WCRF/AICR (R) and other groups. Although imperfect, prospective cohort studies provide consistent evidence that individuals who consume higher amounts of red meat, and especially processed meat, are at higher risk of colorectal cancer. An RCT of red or processed meat and cancer outcomes is unlikely to take place for practical and ethical reasons. Even so, the authors point to the Women's Health Initiative dietary modification trial as evidence that does not support an association between decreased red meat intake and reduced risk of cancer, although that trial was focused on decreasing total fat intake and not on reducing red meat intake. The best available evidence continues to support recommendations to limit intake of red and processed meats for cancer prevention.

Potential biologic mechanisms underlying these associations include consumption of nitrates and nitrites in processed meats, with oxidative DNA damage from the formation of nitrosamines in the gut catalyzed by heme iron and the formation of heterocyclic aromatic amines and polycyclic aromatic hydrocarbons during high-heat cooking of meat, such as cooking meat over flames or grilling. It is not known whether there is a safe level of consumption for either class of meat products, since the risk of colon cancer increases 23% with each additional serving (almost 2 ounces) of processed meat and 22% per 3 ounces serving of red meat). In the absence of such knowledge, while recognizing that the magnitude of increased risk has some uncertainty, the ACS recommends choosing protein foods such as fish, poultry, and beans more so than red (unprocessed) meat, and, for individuals who consume processed meat products, to do so sparingly, if at all.

Added sugars (R)

White (processed) sugar, raw and brown sugar, corn sweetener, high-fructose corn syrup, and other added sugars in sugar-sweetened beverages and energy-dense foods (eg, traditional “fast food” or heavily processed foods) are associated with risk of weight gain, overweight, or obesity, (R) which itself is considered a cause of 13 types of cancers. In addition, the WCRF/AICR notes that diets with high “glycemic load”—reflecting their blood sugar-raising potential—are probably associated with higher endometrial cancer risk. Energy-dense and highly processed foods are often higher in caloric sweeteners, refined grains, saturated fat, and sodium. The 2015 DGA recommends limiting calories from added sugars and saturated fat and specifically consuming <10% of energy per day from added sugars. Likewise, global health organizations note that limiting sugar-sweetened beverages should be a high priority, and recommend instead choosing water and unsweetened beverages.

Processed foods (R)

The health impact of highly processed foods has become an area of heightened public health interest. Some types of processing, such as peeling, cutting, and freezing fresh vegetables and fruit for later consumption, have important health benefits that increase the safety, convenience, and palatability of foods. It is useful to consider the spectrum of food processing, from less processed foods such as whole grain flour and pasta to highly processed foods that include industrially produced grain-based desserts, ready-to-eat or ready-to-heat foods, snack foods, sugar-sweetened beverages, candy, and other highly palatable foods that often do not resemble their original plant or animal sources. Highly processed foods tend to be higher in fat, added sugars, refined grains, and/or sodium and have been associated with adverse health outcomes, including cancer, in a small number of studies. It is notable that up to 60% of energy consumed per day in US households is from highly processed foods and beverages. The 2018 WCRF/AICR report recommends limiting consumption of “fast foods” and other processed foods high in saturated fat, starches, or added sugars (R) because of their association with body weight.

Calcium, vitamin D, and dairy products (R)

In addition to dietary patterns and foods, certain nutrients may modify cancer risk. The WCRF/AICR considers the evidence “probable” that diets high in calcium and dairy products lower colorectal cancer risk. The evidence that diets high in calcium may lower breast cancer risk is considered “limited/suggestive.” Also “limited/suggestive” according to the WCRF/AICR is evidence that calcium and dairy products increase prostate cancer risk. For each 400 grams of dairy intake (equivalent to almost 2 cups of milk per day), prostate cancer risk was 11% higher, and a long-term diet that included higher doses of calcium (>2000 mg calcium) was associated with a greater risk of prostate cancer, including lethal, advanced, and high-grade cancers. The Recommended Dietary Allowance for calcium for adults ranges from 1000 to 1200 mg daily. Because the intake of dairy foods may decrease the risk of some cancers and possibly increase the risk of others, the ACS does not make specific recommendations regarding dairy food consumption for cancer prevention.

Vitamin D, which is synthesized in the skin with exposure to ultraviolet radiation, is recognized for its role in maintaining bone health. Dietary sources include a few foods (eg, fatty fish, some mushrooms) in which this vitamin is found naturally, as well as fortified foods (milk, some orange juice and cereals) and supplements. Laboratory and observational studies indicate a potential role of vitamin D in the prevention of cancer. To date, the most consistent evidence for a cancer risk–lowering effect of vitamin D is for colorectal cancer. However, evidence from RCTs for the prevention of colorectal adenomas or cancer have not supported an association. The Vitamin D and Omega-3 Trial (VITAL) supplement trial (R) of 2000 IU of vitamin D per day found no association of vitamin D supplementation with all incident cancers combined; however, the trial reported overall lower cancer mortality from vitamin D supplementation. No association was seen for colorectal cancer specifically, but the study was not powered to test colorectal cancer outcomes. The study reported no adverse events with taking 2000 IU daily over the 6-year trial. Based on current evidence, the US Preventive Services Task Force does not recommend widespread screening of vitamin D levels. However, most Americans have inadequate vitamin D intake, and, despite recent improvements, >25% of US teens and adults have insufficient (<50 nmol/L) vitamin D blood concentrations. Although the role of vitamin D in cancer prevention remains an area of research interest and debate, avoiding deficient levels is recommended. People at higher risk of vitamin D insufficiency include individuals with dark skin, those living in Northern latitudes, and those who stay indoors and who do not consume sources of vitamin D.

Dietary supplements (R)

Dietary supplements are a heterogeneous group of products defined under current US laws and regulations as containing vitamins and minerals as well as amino acids, herbs/botanicals, and other kinds of ingredients. Vitamin and/or mineral supplements are truly “dietary” because they contain micronutrients that are also present in foods. They are also “supplemental” because they have important health benefits for people whose intake of these micronutrients from foods is not sufficient or for those with malabsorption disorders. In contrast, many other products that are marketed as dietary supplements are not truly “dietary” because many come from sources other than foods and contain substances not found in foods, and they are not “supplemental” because they do not increase intake of micronutrients that have been scientifically shown to be important for human health. Furthermore, current laws and regulations do not guarantee that products sold as dietary supplements actually contain substances in the quantities claimed on their labels or that they are free from undeclared substances that can be harmful to human health.

For reasons other than cancer prevention, some vitamin and/or mineral supplements may be beneficial for some people to prevent nutrient deficiency, such as in pregnant women, women of childbearing age, and people with restricted dietary intakes. Dietary supplementation may also be indicated to correct a documented clinical deficiency or insufficiency, such as supplementation with vitamin D in those with low circulating concentrations or vitamin B12 supplementation in those with vitamin B12-associated anemias.

Although a diet rich in vegetables, fruits, and other plant-based foods may reduce the risk of cancer, there is limited and inconsistent evidence that dietary supplements can reduce cancer risk.4 Whereas 2 RCTs showed reductions in cancer risk among men taking low-dose antioxidants or low-dose multiple micronutrients, evidence for women is lacking. Furthermore, evidence exists that some high-dose supplements containing nutrients such as β-carotene and vitamins A and E can increase the risk of some cancers (R). For individual nutrients, an exception may be calcium, in which supplemental calcium may reduce the risk of colorectal cancer. However, people who have excessive calcium intake (mostly from supplements) may have a higher risk of death from all cancer types combined compared with those who have a recommended level of dietary calcium. The same study also reported no overall benefit to longevity from all dietary supplements considered together. Nonetheless, more than one-half of US adults use one or more dietary supplement(s).

Many healthful compounds are found in vegetables and fruits, and it is likely that these compounds work synergistically to exert their beneficial effect. There are likely to be important, but as yet unidentified, components of whole food that are not included in dietary supplements. Some supplements are described as containing the nutritional equivalent of vegetables and fruits. However, the small amount of dried powder in such pills frequently contains only a small fraction of the levels contained in the whole foods, and there is a lack of evidence supporting a role of these products in cancer prevention. Food is the best source of vitamins, minerals, and other bioactive food components. If a dietary supplement is used for general health purposes, the best choice is a balanced multivitamin/mineral supplement containing no more than 100% of the “daily value” of nutrients, and the ACS does not recommend the use of dietary supplements for cancer prevention, consistent with WCRF/AICR guidelines (R).

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