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Can polyphenols in fruit kill cancer cells?

Article at a Glance
  • Population studies show a correlation between increased fruit and vegetable intake and a reduced risk for cancer.
  • Polyphenols, antioxidants found in plants, modulate a variety of cellular signaling pathways, influencing the initiation, promotion, and progression of cancer. Specifically, polyphenols seem to induce cancer cells to, essentially, commit suicide, in a process called apoptosis.
  • Apoptosis is one of the body’s natural ways to get rid of damaged and diseased cells, including cancer cells.
  • A Cornell study found that of the eleven most commonly consumed fruits, cranberries have the highest level of polyphenol bioavailability, followed by apples. Both fruits inhibited the growth of liver cancer cells in vitro (in a petri dish).
Apples Nutrition

Polyphenols are currently all the rage in some scientific circles, but what are they and why are folks so excited? In short, polyphenols are a group of more than 10,000 compounds found in plants that appear to have antioxidant, anticarcinogenic, antiatherosclerotic, anti-inflammatory, spasmolytic, hepatoprotective, antiviral, antiallergic, antidiarrheal, antimicrobial, and estrogenic activity (R). Not surprisingly, then, one of the questions researchers are currently investigating is whether polyphenols in fruit kill cancer cells.

John has talked before about the general science supporting the old adage ‘an apple a day keeps the doctor away’ in his post on the health benefits of eating apples. In this post I focus more on the specific anti-cancer properties of polyphenols in general.

Thanks to evidence from multiple epidemiological studies, there’s no question that a diet rich in fruits and vegetables helps to protect against various types of cancer. As such, scientists have recently begun to hone in on how exactly a predominantly plant-based diet reduces your risk of cancer. And, lo and behold, polyphenols are a prime focus of study.

Modern farming has reduced polyphenol count

Polyphenols are part of the natural defenses of plants, but unlike lectins, they are good for us. Plants (and some microorganisms) produce phenols in response to ecological pressures. These include pathogens and attacks by insects, ultraviolet radiation, and injury. There is some conjecture that the rampant use of pesticides and herbicides in agriculture has diminished polyphenol concentrations in plants. Use of these external methods to eradicate pests, means plants no longer need to produce the same levels of polyphenols to protect themselves.

Given that polyphenols seem to have profound effects on human health, modern farming practices may well have backfired on us by reducing the nutritional value of food. One way to increase the polyphenol content of your diet may, therefore, be to eat more organic produce grown without the use of pesticides.

Types of polyphenols and where to find polyphenols in food

Polyphenols can be divided in to groups according to their chemical structure. There are four main groups of polyphenols: phenolic acids, flavonoids, polyphenolic amides, and miscellaneous polyphenols. Some of the best known and most studies polyphenols include (R):

  • Tannic acid (a phenolic acid)
  • Isoflavones – daidzein and genistein (flavonoids)
  • Flavonols – rutin and quercetin (flavonoids)
  • Flavanones – naringenin and hesperitin (flavonoids)
  • Flavanols – catechin and epigallocatechin gallate (flavonoids)
  • Proanthocyanidins – procyanidin B1, B2, A2, and C1, and theaflavin (flavonoids)
  • Anthocyanidins – cyanidin and petunidin (flavonoids)
  • Polyphenolic amides – capsaicin
  • Other polyphenols – resveratrol, curcumin, rosmarinic acid, gingerol, and ellagic acid.

Some of the richest food sources of polyphenols include spices and herbs, cocoa products, nuts and seeds, berries and vegetables (R). Top of the list are:

  • Cloves
  • Peppermint (dried)
  • Star anise
  • Cocoa powder
  • Mexican oregano (dried)
  • Celery seed
  • Black chokeberry
  • Dark chocolate
  • Flaxseed meal
  • Black elderberry.

Olives, artichoke, chestnuts, and hazelnuts are also good sources of polyphenols (R).

How polyphenols protect against cancer

To understand how polyphenols help to protect against cancer, we have to first understand how cancer occurs. For tumors to grow and cancer to spread, cancer cells have to resist cell death and evade anything that tries to suppress cell growth. They also must sustain proliferative signaling (i.e. send out and receive signals telling them to multiply. Cancer also relies on angiogenesis (the growth of blood vessels, to feed growing tumors), and cancer cells have to reprogram energy metabolism, avoid immune system cells that want to destroy them, and activate processes that facilitate invasion and metastasis. No small feat! And yet, cancer cells do this every day, with devastating effects.

Polyphenols modulate a variety of cellular signaling pathways, influencing the initiation, promotion, and progression of cancer. Specifically, polyphenols seem to induce cancer cells to, essentially, commit suicide, in a process called apoptosis.

Apoptosis is one of the body’s natural ways to get rid of damaged and diseased cells. In some types of cancer, genetic mutations disrupt apoptosis. This means that diseased cells can go on to reproduce, fuelling tumor growth and the spread (metastasis) of cancer. Confusingly, some oncogenic (cancer-causing) changes promote apoptosis, but, instead of being helpful, this is disruptive in its own way.

Most types of cytotoxic cancer treatments, such as chemotherapy, work by inducing apoptosis. Where these conventional cancer treatments fail, the blame is increasingly placed on defects in apoptotic programming. One major area of current cancer research focuses, then, on finding ways to enhance the sensitivity of cancer cells to these conventional drugs, so as to induce apoptosis in cancer cells (R). And, within that area of research, many scientists are focusing on polyphenols.

Diet, detoxification, and cancer

Inflammation and instability in the genome underpin many of the processes behind cancer development. These processes increase the risk of random genetic mutations that result in cancer. It makes sense, then, that dietary interventions that help reduce inflammation, promote proper immune function, and facilitate transcription of genetic material to new cells, can help reduce the risk of cancer. Diet may also influence detoxification processes to reduce carcinogen activation and promote their safe removal from the body.

Unfortunately, some dietary interventions actually increase Phase I enzymes, including cytochrome P450 enzymes. These enzymes convert potential carcinogens into their reactive forms that can go on to damage DNA, thus beginning or progressing cancer development. Far preferable are dietary interventions that increase the activity of Phase II enzymes that help to safely remove carcinogens from the body.

Happily, polyphenols not only increase the activity of Phase II enzymes glutathione reductase, glutathione peroxidase, glutathione S-reductase, catalase, and quinone reductase (R, R), they have also been seen to inhibit CYP1A enzymes, creating a dual action chemopreventive effect (R). Polyphenolic compounds also effectively scavenge for free radicals and singlet oxygen, thereby reducing DNA damage (R).

See also: glutathione supplements: what you need to know

Apple polyphenols and colon cancer

Apple-derived polyphenols seem to be especially helpful in this regard. In one study, these polyphenols modulated gene expression to inhibit the growth of human colon cancer cells. The genes in question were GSTP1, GSSTT2, MGST2, CYCP4F3, CHST5, CHST6, and CHST7 (R). Other studies have also found that polyphenols sourced from apples, namely procyanidins, inhibit a signaling mechanism involved in the progression of colorectal cancer, the second most major cause of mortality and morbidity among females and the third among males (R, R).

Tea polyphenols and cancer

It’s not just apples that offer benefits from polyphenols, of course. Tea polyphenols are relatively well studied and have been found to have a range of anticancer properties, including guarding against epigenetic errors that can occur during DNA methylation, histone modifications, and in micro-RNAs (R). One particular polyphenol found in abundance in green tea (epigallocatechin-3-gallate, or EGCG) has been seen to inhibit a signaling pathway involved in colorectal cancer development (R) and to increase the expression of a p53 gene, thereby inhibiting the progression of colorectal adenoma to carcinoma (R).

Three factors for apoptosis (technical)

Three key factors regulate apoptosis. The p53 genes are one factor. The other two factors are caspase signaling proteins and the Bcl family of proteins. Polyphenols in general have been seen to influence these factors to induce apoptosis in cancer cell lines including lung, skin, and prostate cancer cells (R, R, R, R, R). In colon cancer HT-29 cells, flavone polyphenols have been found to increase the production of superoxide radical, leading to apoptosis in the cancer cells (R). Other studies have also found that some polyphenols act as prooxidants, enhancing the production of reactive oxygen species to trigger cancer cell death (R, R, R).

In addition to their anticancer properties, polyphenols may help protect against cardiovascular and neurodegenerative diseases and affect immune function by influencing white blood cell proliferation and the production of cytokines (R).

Bioavailability of polyphenols

Of course, all this talk of polyphenols is redundant if we can’t absorb the compounds from plant foods. Fortunately, a Cornell University study looked at just that, and found that of eleven commonly consumed fruits, cranberries have the highest level of polyphenol bioavailability, followed by apples. This study also looked at the effects of the polyphenols on liver cancer cells. The researchers found that the polyphenols had antiproliferative activity, i.e. they inhibited the growth of new cancer cells.

We have documented the data with two charts that I have included below.

Impact of different fruits on cancer

The takeaway – polyphenols and cancer

This was a deep dive into the science of polyphenols, but what does it mean for treating and preventing cancer? Well, there certainly seems to be support for the prevailing advice to eat a predominantly plant-based diet rich in fruits, vegetables, nuts, seeds, legumes, pulses, and whole grains to reduce your risk of various types of cancer. This type of diet is naturally high in polyphenols. It also provides plenty of fiber, lean protein, and healthy fats, and essential vitamins and minerals. Together, these all help to keep your body healthy and your cells in good working order.

What can’t be recommended from the current state of evidence is the adoption of anything extreme, such as Gerson therapy for cancer.

What about Gerson Therapy?

Gerson therapy is characterized by a strict low sodium, high potassium diet based on organic fruit and vegetables. Gerson therapists claim that cancer can be cured by consuming vast amounts of fruit and vegetable matter, along with potassium and vitamin B12 supplements, pancreatic enzymes, and thyroid supplements, and three or four coffee or castor oil enemas every day.

Although films like the Food Cure offer an enticing picture of success for some people, there is scant evidence that Gerson therapy cures cancer. Indeed, while such treatment (at home or in a clinic) may provide temporary solace and comfort to individuals with terminal cancer, this kind of extreme diet can cause harm. Coffee enemas deplete the body of potassium and may cause infections, dehydration, seizures, heart and lung issues, and constipation and colitis. In some people, the effects of this regimen can prove fatal. Other side effects can include diarrhea and vomiting, loss of appetite, abdominal cramps, cold sores, dizziness, weakness, and flu-like symptoms such as aches, fever, and sweats.

As with so many things in nutrition, cautious enthusiasm and moderate consumption may be key for polyphenol intake. Focusing too much on these compounds could decrease dietary variety, and polyphenols aren’t the be all and end all of good health. That said, there’s unlikely to be any harm (and may well have some benefits) in including polyphenol rich foods such as apples, cranberries, cocoa products, herbs, spices, and a refreshing cup of peppermint tea in your everyday diet.

Leigh Matthews

Leigh Matthews, BA Hons, H.Dip. NT, is a health and wellness writer specializing in plant-based nutrition. A long-time vegan, Leigh is interested in nutriepigenetics, diet as preventative medicine, and the politics of food justice.

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