Itt írjon a(z) fiber_diet-ról/ről

“The Beneficial Role of Diets Rich in Dietary Fibers Regarding Certain Diseases”

Sally Rosengren, Marlene Schmid, Meagan Scott

Supervisor: Dr. Barany Zoltan

Contents

1 Introduction. 3

2 Dietary Fibers and Disease Prevention. 4

2.1 Coronary Heart Disease. 4

2.1.1 Definition and Risk Factors of CHD.. 4

2.1.2 Effects on Lipid Concentration. 4

2.1.3 Effects on Blood Pressure. 4

2.2 Diabetes. 5

2.2.1 Definition and Risk Factors of Diabetes. 5

2.2.2 Effects of Soluble Fibers in Correlation with Glucose Index (GI). 5

2.2.3 Effects of Soluble and Insoluble Fibers on Insulin Resistance (IR). 6

2.2.4 Effects of Insoluble Fibers and Interference with Dietary Proteins. 6

2.3 The Role of Dietary Fibers in the Prevention of Colorectal Cancer. 6

2.3.1 Definition and Risk Factors of CRC.. 6

2.3.2 Scientific Background. 6

2.3.3 Effects of SCFA, butyrate. 6

2.3.4 Results of Experimentation with Sodium Butyrate, NaB. 7

3 Conclusion. 7

4 References. 8

Introduction

The physiological effects of macronutrients such as carbohydrates, fats, and fiber has become a focal point of research in the field of nutrition. Significant findings have proven that there is a strong correlation between dietary content and overall health of an individual, especially that of fiber. Dietary fibers are scientifically described as “carbohydrate polymers with 10 or more monomeric units, which are not hydrolyzed by the endogenous enzymes in the small intestines of humans” by the FAO and WHO (Satija and Hu, 2012). Plant non-starch polysaccharides such as cellulose, pectin, gums, hemicellulose, beta-glucans, oat and wheat bran fibers are all examples of dietary fibers (Institute of Medicine, 2005).

Dietary fibers are classified based on its digestibility, chemical, physical, and other functional properties. Within the digestive tract, some fibers absorb water to produce a viscous gel substance that can undergo fermentation. These are called soluble fibers which can be found in legumes, oats, nuts, broccoli, and bananas (Satija and Hu, 2012). Research has shown that its viscous gel can increase the sensation of fullness by delaying gastric flow into the small intestines (Institute of Medicine, 2005). It has also been found to decrease blood cholesterol and blood sugar concentrations, increasing insulin sensitivity (Institute of Medicine, 2005).

Dietary fibers that do not absorb water or ferment are known as insoluble fibers. Typical food sources of insoluble fibers include whole grains, avocado, celery, cauliflower and wheat bran (Satija and Hu, 2012). Consumption of insoluble fibers results in increased fecal matter, decreased constipation, and a reduction in the prevalence of colon cancer (Institute of Medicine, 2005). Physiological functions of both soluble and insoluble dietary fibers have obvious health benefits. Research has proven a decreased prevalence of certain diseases in individuals who consume the recommended amount of dietary fibers daily. Common diseases such as diabetes, coronary heart disease, and colorectal cancer will be discussed in relation to the effects of dietary fibers.

As the leading cause of death in adults within North America and Europe, prevention of coronary heart disease is of major concern amongst health professionals. Coronary heart disease (CHD) is defined as a condition “caused by atherosclerosis that reduces blood flow through the coronary arteries to the heart and typically results in chest pain or heart damage” (Merriam-Webster, 2019). Common risk factors associated with CHD include cigarette smoking, high blood pressure, high cholesterol, HDL- cholesterol, LDL- cholesterol, obesity, family history of CHD, and left ventricular hypertrophy (Wilson et al, 1998). Analysis of various studies have shown statistically significant relationship with a decrease in certain risk factors by increasing dietary fiber consumption.

Consumption of soluble fibers such as beta-glucan and pectin were studied in its effect on lipid concentrations, a major risk factor of CHD. It was determined that an increase in soluble fiber consumption of 2-10g/day decreased total cholesterol and LDL-cholesterol concentrations (Satija and Hu, 2012). Researchers are unsure of the exact mechanism, but a mode of action has been proposed. Dietary fibers affect the absorption of fat, cholesterol and bile acids in the digestive tract, therefore causing an increase in bile acid synthesis in the liver. Blood cholesterol therefore is reduced overall due to increased bile acid synthesis with decreased reabsorption (Satija and Hu, 2012).

Randomized controlled trials (RCT) were analyzed to determine the effects of dietary fiber intake on blood pressure. The first analysis of 25 RCT collectively proved that dietary fiber has a significant effect of reducing blood pressure in individuals. An average consumption of 11.5 grams of fiber a day resulted in a 1.13 mmHg decrease in systolic blood pressure and a 1.26 mmHG decrease in diastolic blood pressure (Satija and Hu, 2012). These statistics further prove the inverse relationship between risk of CHD and consumption of dietary fiber. Further studies are needed to determine the mechanism of action.

Overall, a strong dose-response relationship between coronary heart disease and dietary fiber has been determined. A ‘pooling project of cohort studies on diet and coronary disease’ combined data from 10 studies that were conducted within the United States and Europe. This pooled analysis included the Nurses’ Health Study (NHS) on female nurses in the US and the EPIC-Norfolk case control study in the UK. The NHS showed results of a 19% decrease in CHD risk for every 10g/day dietary fiber consumed amongst female nurses 37-64 years old (Satija and Hu, 2012). The EPIC-Norfolk study results showed a 14% decrease in CHD risk for every 6g/day consumed by men and women (Satija and Hu, 2012). The Pooling Project concluded an average increased consumption of 10 grams per day of dietary fiber decreased risk of all coronary events by 14% and coronary death by 27% (Satija and Hu, 2012). Therefore, dietary fiber has a preventative effect in regard to coronary heart disease.

Similarly to CHD, diabetes is a serious health problem which can cause a long-list of complications if not treated properly. If left untreated diabetes can lead to other health issues such as hypertension and nephropathy as well as coronary heart disease. Especially in the case of type 2 diabetes (T2D) dietary modifications are an essential step in diabetes management (Zhang et al, 2018). One addition to the diet that can help reduce the symptoms of diabetes is the consumption of dietary fibers. When applying dietary fibers as a tool to manage type 2 diabetes, it is important to note the differences in soluble and insoluble fiber types. Here, table 1 depicts some of the major effects consumption of dietary fibers has on diabetes. After analysis of various studies, results showed an inconsistent effect of soluble fibers on diabetes while insoluble fibers proved to reduce the risk of developing type 2 diabetes.

Figure 1: Effects of soluble and insoluble fibers

• Consumption of soluble-type fibers resulted an overall reduction in both total and LDL-cholesterol concentrations as well as a decreased glucose index. Russell WR et al, defines glucose index (GI) as “measure of the blood glucose-increasing ability of the available carbohydrates in food sources” (Russell et al. 2016). The concept of GI can be correlated to the effects of consumption of soluble dietary fibers which delays or prevents the absorption of dietary carbohydrates, therefore reducing postprandial glucose fluctuations (Weickert and Pfeiffer, 2018). However, results were found to be inconsistent in human trials comparing the effects of soluble dietary fibers on both high-GI and low-GI diets. This can be partially explained by the fact that it is difficult to control cofounding factors in the applied diet that may also affect the glucose index (Wickert and Pfeiffer, 2018).

One of the strongest predictors of developing type two diabetes is insulin resistance (IR). Insulin resistance occurs due to consistent excessive energy intake which leads to adiposity, or obesity. Therefore, insulin resistance may be controlled or reduced simply by implementing nutritional modifications which may result in weight loss. Consumption of dietary fibers have proven to increase post-meal satiety, therefore reducing subsequent sensation of hunger. However, the results showed only a moderate effect with no distinguishable differences amongst soluble, insoluble, fermentable or unfermentable types of dietary fiber (Weickert and Pfeiffer, 2018).

Further studies revealed that high intake of insoluble fibers may improve insulin resistance independently of weight loss. This may occur due to the ability of insoluble fibers to interfere with the absorption of dietary protein (Weickert and Pfeiffer, 2018). Prospective cohort studies based on high-protein intake during weight loss therapies have shown improvements in insulin resistance in post-menopausal women (Smith et al, 2016). In addition to this, it was revealed that replacing 1% of the dietary energy sources from carbohydrates with energy from protein is associated with an increased 5% risk type two diabetes. However exchanging 1% of energy from animal protein with 1% energy from plant protein has resulted in a 18% decreased risk of developing type 2 diabetes(Virtanen HEK et al, 2017).

In conclusion, consumption of dietary fibers has proven to be beneficial in the prevention of diabetes. Whether soluble or insoluble-type fibers are consumed in the diet, both share beneficial properties in reducing the prevalence of diabetes development. However it is evident that increased consumption of insoluble type fibers resulted in stronger, more consistent effects in reducing the risk of developing type two diabetes.

Amongst adults in the United States, colorectal cancer is ranked as the third most prevalent cancer for both men and women (Zeng et al., 2017). It is hypothesized that implementing certain dietary modifications could reduce the risk of developing colorectal cancer (CRC). Navarro et al, states that “lifestyle factors such as diet, physical activity, and body weight contribute substantially to CRC’’. The consumption of dietary fibers have previously been recorded to reduce the risk of colorectal cancer development amongst human and animal populations. Therefore, the beneficial effect of dietary fibers in regards to colorectal cancer prevention will be considered (Zeng, et al., 2017). Supporting evidence recorded by the World Cancer Research Fund/American Institute for Cancer Research Expert Panel classified the consumption of fiber-containing foods against CRC as “convincing”. The expert panel noted that for every 10g of fiber consumed per day, a 10% decrease in the risk of CRC resulted (Navarro et al., 2016).

The protective effect of dietary fiber in regards to colorectal cancer is associated with the colonic fermentation of soluble fibres into short-chain fatty acids (SCFA), particularly butyrate (Navarro et al., 2016). Zeng et al., states that diets which include moderate levels of fiber can result in a concentration of colonic-luminal SCFA of up to 10 mmol/L. Additionally, dietary fibers have shown to increase stool bulk and reduce passage time within the gastrointestinal tract, which results in less exposure to possible carcinogens (Navarro et al., 2016).

Through regulation of cell proliferation as well as eliminating the S-phase checkpoint of cancerous cells, butyrate may prevent colon cancer development. Butyrate application may also induce apoptosis, therefore is considered to be a chemopreventive agent in regards to colon cancer development (Zeng, et al., 2017). “A successful chemoprevention agent should have a minimal effect on normal cells but a strong inhibitory effect on cell proliferation and carcinogenic pathways in cancer cells’’ (Zeng et al., 2017).

Butyrate is also classified as a histone-acetylase inhibitor. Inhibition of histone acetylase results in a strong activation of p21, a cyclin-dependent kinase inhibitor with tumor suppression capabilities (Zeng et al., 2017). For successful progression of cell proliferation, cell signalling pathways such as extracellular-regulated kinase ½ (ERK1/2) and myelocytomatosis (c-Myc) are both required. The c-Myc pathway is also capable of modulating the kinase inhibitor, p21 (Zeng et al., 2017). Therefore, the effect of butyrate on the levels of signalling molecules must be taken into consideration.

A study on the effects of butyrate on cancerous colon cells hypothesized that “butyrate inhibits cancerous cell proliferation but to a lesser extent in non-cancerous colon cells through signaling pathways regulating apoptosis and cellular survival’’ (Zeng et al., 2017). Two human epithelial cell lines, NCM460 non-cancerous cell line and the HCT116 cell line, derived from colon mucosa were used for the purpose of this study. To observe the effects of butyrate, the cell lines were treated with dose-dependent amounts of sodium butyrate (Zeng et al., 2017).

Zeng et al., (2017) provided detailed figures to illustrate the effect of NaB on differential cell lines. Figure 2 is a summarized form of the results obtained in the experiments.

Figure 2 Experimental results illustrating the effect of NaB on cancerous/ non-cancerous cell lines.

The NaB treatment resulted in an amplified inhibitory effect on cell growth rate in cancerous (HCT116) cell lines, whereas that of cell growth rate in normal (NCM460) cell lines was observed to a lesser extent. In comparison to non-cancerous cell lines, application of sodium butyrate showed a significantly stronger apoptosis effect on the cancerous cell lines. It was also observed that there was an increased prevalence of DNA fragmentation amongst cancerous cell lines with no effect on NaB treated non-cancerous cell lines.

The effect of butyrate on certain signalling molecules was also observed in both cell lines. Within cancerous cell lines, the ERK1/2 signaling molecule which is described as a ‘pro-survival’ signal that is often activated by growth factors, lower concentrations were measured. Meanwhile in non-cancerous cell lines, higher concentrations of ERK1/2 were found. Therefore, it is possible to conclude a greater pro-survival signal amongst non-cancerous cell lines.

From the data recorded in the above mentioned experiments, some conclusions can be made. The inhibition of cell proliferation in cancerous cells was more effective than the inhibition of cell proliferation in non-cancerous cells treated with 0.25-2 mmol/L butyrate (Zeng et al., 2017). Also, butyrate effectively induced apoptosis and DNA-fragmentation in HCT116 cells but not in NCM460 cells (Zeng et al.,2017)

The nutritional content of an individual's diet has significant physiological effects on the body. It is evident that through proper nutrition, it is possible to maintain a good health status. Epidemiological research has confirmed that through consumption of dietary fibers, an individual may prevent the development of chronic diseases such as diabetes, coronary heart disease, and colorectal cancer. Guidelines posted by the United States Department of Agriculture (USDA) state dietary recommendations for various nutrients such as dietary fibers. The adequate intake for fiber-rich whole grains is 14 grams per 1000 calories consumed, or 38 and 25 grams per day for men and women respectively (Satija and Hu, 2012). These recommendations are only guidelines which may be adjusted based on individual needs.

• Through prevention and reduction of certain risk factors such as blood pressure, cholesterol concentrations, and insulin resistance, dietary fibers work to evade the development of diabetes and coronary heart disease. As a metabolite of dietary fibers, butyrate proves to efficiently deteriorate cancerous cell lines. This strengthens its role as a successful chemopreventive agent through affecting cell growth, cell apoptosis, DNA-fragmentation and certain signalling molecules. It can be concluded that an increased dietary fiber intake escalates the production of colonic fermented fibers into short chain fatty acids such as butyrate, a dose-dependent protective agent against colorectal cancer. Further studies should be conducted to determine the exact mechanism of action of dietary fibers within the digestive tract in which the reduction of disease risk factors is seen.

“Coronary heart disease.” Merriam-Webster, 20 April 2019. Web. 03 May 2019. https://www.merriam-webster.com/dictionary/coronary%20heart%20disease#medicalDictionary

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