Highlights
Different observational studies suggest that a high intake of foods rich in dietary fiber (soluble/insoluble) may be associated with a reduced risk of different cancer types such as colorectal, breast, ovarian, liver, pancreatic and kidney cancers. A study also observed that intake of dietary fiber (from foods/supplements) before the initiation of treatment may help in prolonging survival time in newly diagnosed head and neck cancer patients.
What is Dietary fiber?
Dietary fiber is a type of carbohydrate found in plant-based foods, which unlike other carbohydrates, cannot be digested by the enzymes in our body. Hence, these carbohydrates which are resistant to digestion and absorption in the human small intestine, reach the large intestine or colon relatively intact. These are also known as roughage or bulk and are found in a variety of plant-based foods including whole grains and cereals, legumes, nuts, fruits and vegetables, as well as supplements. Dietary fiber supplements are also commercially available in various forms.
Different Types of Dietary fiber
There are two major types of dietary fiber – soluble and insoluble.
Soluble Dietary Fiber
Soluble dietary fiber absorbs water during digestion and forms a gel-like material. It increases stool bulk and may decrease blood cholesterol levels. Soluble fiber including pectins and beta glucans can be found in oats, barley, psyllium, fruits such as apples, citrus fruits and grapefruit; vegetables; and legumes such as peas, beans and lentils.
Insoluble Dietary Fiber
Insoluble dietary fiber does not absorb or dissolve in water and remains relatively intact during digestion. It increases stool bulk and promotes the movement of intestinal material through the digestive system. A bulky stool is easier to pass and benefits people who struggle with constipation. Insoluble fibers can be found in whole grain products and foods including fruits, nuts, vegetables such as carrots, celery, and tomatoes. Insoluble fibers do not provide calories.
Health Benefits of Fiber-Rich Foods
Eating foods rich in dietary fiber has a variety of health benefits. Some of these include:
- Lowering the levels of bad cholesterol
- Reducing the risk of heart diseases
- Reducing the risk of stroke
- Normalizing bowel movements
- Controlling blood sugar levels, thereby reducing the risk of type 2 diabetes
- Aiding weight management
- Maintaining bowel health, in turn reducing the risk of bowel cancer.
High-fiber foods are hence good for our health. Including foods rich in dietary fiber also makes us feel fuller. Refined or processed foods and grains are lower in fiber. People often use dietary fiber supplements to manage weight, reduce cholesterol and blood sugar, and to prevent constipation. Psyllium (soluble) and Methylcellulose are some of the commonly used dietary fiber supplements.
Dietary Fiber, Fiber-Rich Foods and Cancer Risk
According to the American Institute for Cancer Research, unprocessed plant-based foods which are rich in fiber may help in reducing the risk of cancer. Different observational studies have been carried out by researchers worldwide to study the association between dietary fiber(soluble/insoluble) intake and the risk of cancer.
Association with Colorectal Cancer Risk
- In a study published by the researchers of South Korea and the United States in 2019, they carried out a dose–response meta-analysis to evaluate the association between different fiber sources (including cereals, vegetables, fruits and legumes) and the risk of colorectal cancer and adenoma. The data for the analysis was obtained from literature search in PubMed and Embase databases till August 2018 and included a total of 10 studies. The study showed that all fiber sources may provide benefits in colorectal cancer prevention, however the researchers found that the strongest benefit was found for dietary fiber from fiber-rich foods like cereals/grains. (Hannah Oh et al, Br J Nutr., 2019)
- Another study published in 2015 by the researchers in Queen’s University Belfast in Northern Ireland and National Cancer Institute, NIH, Bethesda in Maryland assessed the association between dietary fiber intake and the incidence of colorectal adenoma and cancer as well as the risk of recurrent colorectal adenoma. The study used dietary questionnaire based data from the study participants of the Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. The analysis of colorectal cancer, incident adenoma and recurrent adenoma were based on data from 57774, 16980 and 1667 participants, respectively. The study found that a high total dietary fiber intake may be associated with a significantly reduced incidence of distal colorectal adenoma and a reduced risk of distal colon cancer, however, no significant association was found for the risk of recurrent adenoma. Their findings also mentioned that these protective associations were most notable for dietary fiber from cereals/whole grains or fruit. (Andrew T Kunzmann et al, Am J Clin Nutr., 2015)
- Dr Marc P McRae from the National University of Health Sciences, Lombard, Illinois in the United States did a review of 19 meta-analyses published between January 1, 1980 and June 30, 2017 on the effectiveness of dietary fiber on reducing the incidence of cancer, which were obtained from Pubmed search. He found that those consuming the highest amounts of dietary fiber may benefit from a reduced incidence of developing colorectal cancer. He also mentioned that a small reduction in the incidence of breast cancer was also found in his review. (Marc P McRae, J Chiropr Med., 2018)
- In another study published in 2018, the researchers of Southeast University, Nanjing in China and Technical University Munich in Germany, evaluated the association between dietary fiber intake and subsite-specific colon cancer. They carried out meta-analysis on 11 cohort studies obtained through literature search in the PubMed database till August 2016. The study found that high dietary fiber intake may reduce the risk of both proximal and distal colon cancers. They also found that dietary fiber intake may reduce the risk of proximal colon cancer only in European countries, however, they found that this association may be observed for distal colon cancer in both European countries and the United States. (Yu Ma et al, Medicine (Baltimore)., 2018)
All these studies suggest that a high intake of dietary fiber may help in reducing the risk of colorectal cancer.
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Association with Head and Neck Cancer
In a recent study published in 2019, the researchers from the United States evaluated the association between dietary fiber and recurrence or survival after head and neck cancer diagnosis. The data was obtained from a cohort study including 463 participants who were newly diagnosed with head and neck cancer. A total of 112 recurrence events, 121 deaths, and 77 cancer-related deaths were reported during the study period. (Christian A Maino Vieytes et al, Nutrients., 2019)
The study found that dietary fiber intake before the initiation of treatment may prolong survival time, in those with a new head and neck cancer diagnosis.
Association with Endometrial Cancer
In a meta-analysis done by the researchers of China, they evaluated the association between dietary fiber intake and endometrial cancer risk. Data for the study was obtained from 3 cohort and 12 case⁻control studies through literature search in the PubMed and ISI Web databases through March 2018. (Kangning Chen et al, Nutrients., 2018)
The study found that higher total dietary fiber intake and higher vegetable fiber intake may be associated with a reduced risk of endometrial cancer risk in the case⁻control studies. However, results from the cohort studies suggested that higher total fiber intake and higher cereal fiber intake may marginally increase the endometrial cancer risk.
The association between dietary fiber intake and endometrial cancer risk is hence inconclusive.
Association with Ovarian Cancer
In a study published in 2018, the researchers from China conducted a dose-response meta-analysis to evaluate the association between dietary fiber intake and ovarian cancer risk. Data was obtained from 13 studies, with a total of 5777 ovarian cancer cases and 1,42189 participants found through literature search in PubMed, EMBASE, and the Cochrane Library databases till August 2017. (Bowen Zheng et al, Nutr J., 2018)
The meta-analysis found that a high dietary fiber intake may significantly reduce the risk of ovarian cancer.
Association with Liver Cancer
In a study published in 2019, the researchers evaluated the association between dietary fiber intake and liver cancer based on 2 cohort studies – the Nurses’ Health Study and the Health Professionals Follow-up Study – with 125455 participants in the United States, which included 141 patients with liver cancer. The average follow-up for the study was 24.2 years. (Wanshui Yang et al, JAMA Oncol., 2019)
The study found that an increased intake of whole grains and cereal fiber and bran may be associated with a reduced risk of liver cancer among adults in the United States.
Association with Pancreatic Cancer
In a study published in 2017, the researchers evaluated the association between dietary fiber intake and pancreatic cancer risk. Data was obtained from 1 cohort and 13 case-control studies found through literature search in the PubMed and Embase databases up to April 2015.(Qi-Qi Mao et al, Asia Pac J Clin Nutr., 2017)
The study found that a high intake of dietary fiber may reduce the risk of pancreatic cancer. However, the researchers suggested further well-designed prospective studies to confirm these findings.
Association with Kidney Cancer
A study published by the researchers in China assessed the association between dietary fiber intake and the risk of kidney cancer/renal cell carcinoma (RCC). Data for the analysis was obtained from 7 studies, including 2 cohort studies and 5 case-control studies found through literature search in the electronic databases including MEDLINE, EMBASE and Web of Science. (Tian-bao Huang et al, Med Oncol., 2014)
The study found that fiber intake, especially from fiber rich foods like vegetable and legume fiber (not fruit and cereal fiber intake), may be associated with a reduced risk of kidney cancer. However, the researchers recommended more well-designed prospective studies to confirm these findings.
Association with Breast Cancer
In a study published in 2016, the researchers from Hangzhou Cancer Hospital, Zhejiang in China conducted a meta-analysis to determine the effectiveness of dietary fiber intake in reducing breast cancer risk. Data was obtained from 24 studies found through literature search in the PubMed, Embase, Web of Science, and Cochrane Library databases. (Sumei Chen et al, Oncotarget., 2016)
The study found a 12% decrease in breast cancer risk with dietary fiber intake. Their dose-response analysis showed that for every 10 g/day increment in dietary fiber intake, there was a 4% reduction in breast cancer risk. The study concluded that dietary fiber consumption may be significantly associated with a reduced risk of breast cancer, especially in postmenopausal women.
Many other observational studies also supported these findings. (D Aune et al, Ann Oncol., 2012; Jia-Yi Dong et al, Am J Clin Nutr., 2011; Yikyung Park et al, Am J Clin Nutr., 2009)
Conclusion
These studies suggest that a high intake of dietary fiber (soluble/insoluble) rich foods may be associated with a reduced risk of different types of cancers such as colorectal cancer, breast cancer, ovarian cancer, liver cancer, pancreatic cancer and kidney cancer. The association between dietary fiber intake and endometrial cancer risk is inconclusive. A study also found that dietary fiber intake before the initiation of treatment may prolong survival time, in newly diagnosed head and neck cancer patients.
However, dietary fiber rich foods and supplements should be taken in the right amounts. The American Institute of Cancer Research recommends a daily intake of at least 30 gms of dietary fiber as part of a healthy diet to lower cancer risk. The AICR report also showed that every 10 gm increase in dietary fiber is associated with a 7% decrease in the risk of colorectal cancer.
Most adults, especially Americans, take less than 15 gm of dietary fiber every day. Hence, we should start including foods rich in dietary fiber to our daily diet. However, please note that sudden addition of too much dietary fiber (from foods or supplements) to our diet can promote intestinal gas formation and also lead to bloating and stomach cramps. Hence, add dietary fiber through foods or supplements to your daily diet gradually.
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