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Cholesterol Lowering Abilities of Fiber: Fact, Fiction, or Somewhere In-between?
October 10, 2008
Off the coattails of the whole grains and pre-biotic active cultures movements in health foods has come the fiber health fad. While obviously related, one notices during recent trips to the grocery store that while it was once important to denote that a product was made with whole-grains, has no trans-fat, and is 100 calories or less, this alone is no longer sufficient. The use of a new label indicating the fiber content of food products has increased exponentially over the last year. While before I personally bought whole-wheat English muffins, I now make sure to buy the ones with 40% more fiber. Or, while before I would buy whatever flavor of low-fat yogurt I desired, I now opt for the Fiber One® flavors that have 20% of my daily value of fiber per serving. In addition, I now buy All Bran® Bran Buds® to add to my favorite cereals to increase the fiber content in my breakfast. And, just to clarify, I am not alone in my quest for more fiber in my daily diet. I live with five other women and know that this new craze is widespread. So, what is so appealing about fiber and is there truth in the advertising by the food companies that are upping the fiber content in their food products?
What is fiber?
Fiber, also called dietary fiber, roughage or bulk, is a collective term for the carbohydrates found in plants that cannot be digested or absorbed (http://www.mayoclinic.com/health/fiber/NU00033 and http://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/fiber-full-story/index.html). Fiber is usually classified into two categories, soluble and insoluble. Soluble fiber dissolves partially in water so that it forms a gel and can be found in oats, peas, beans, apples, citrus fruits, carrots, barley and psyllium. Insoluble fiber does not dissolve in water so it increases stool bulk and can be found in whole-wheat flour, wheat bran, nuts and many vegetables (http://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/fiber-full-story/index.html).
What is Cholesterol?
Cholesterol is “soft, waxy substance found among the lipids (fats) in the bloodstream and in all your body's cells” (http://www.americanheart.org/presenter.jhtml?identifier=4488). There are two types of cholesterol carrying lipoproteins the in blood, low-density lipoprotein (LDL) and high-density lipoprotein (HDL). LDL is considered the “bad” cholesterol because it can build up in the arteries as plaque that can clot the arties and cause a heart attack. However, LDL is the major cholesterol carrier in the blood. HDL is the “good” cholesterol because it is believed to remove excess cholesterol from the blood and thus prevents the formation of plaque (http://www.americanheart.org/presenter.jhtml?identifier=4488). When plaque builds up in the arteries they become hard and narrow through a process called atherosclerosis. If the arteries become too narrow, they block the flow of blood, which eventually can lead to a heart attack (http://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/fiber-full-story/index.html). Thus, maintaining a healthy cholesterol level is key to good health.
What are the advertisers saying about fiber and cholesterol?
How much fiber should you consume per day?
One of the main claims of the food industry in their advertising is that Americans on average do not get the daily recommended amount of fiber each day (http://www.fiberone.com/Benefits/Default.aspx). An article from the Mayo Clinic states that the National Academy of Sciences’ Institute of Medicine says men age 50 or younger should have 38 grams of fiber a day, and men age 51 and older should intake 30 grams a day. This article also says that women age 50 and younger should get 25 grams of fiber a day and women age 51 and older should get 21 grams of fiber a day (http://www.mayoclinic.com/health/fiber/NU00033). The Fiber One® website states that the “government experts from the National Academy of Science” state that most people should get at least 25 grams of fiber per day. The website also notes that women 25-55 years old on average only meet 56% of their daily fiber needs, and men only consume 47%, on average, of their daily fiber needs (http://www.fiberone.com/Benefits/Default.aspx). The tone of these advertisements is one of concern and calls for action by the American people. This information is used to get people worried about their fiber intake and then want to buy the products that have extra fiber added in to ensure that they are not one of those fiber-lacking Americans. Is there truth to this claim? Should Americans be actively trying to add more fiber to their diets? And, how much should we be adding—since everyone seems to offer a different daily value? Before we attempt to answer these questions, it is important to know what the supposed benefits of adding fiber to your diet are.
What are the health benefits from adding fiber to your diet?
The health benefits of fiber claimed by the advertisers are improved digestive health, decreased risk of type II diabetes, curbed hunger helpful for weight management, and lowered cholesterol levels (http://www.fiberone.com/Benefits/Default.aspx). Some sources add other health claims such as a decreased risk for colon cancer and diverticular disease (http://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/fiber-full-story/index.html and http://www.mayoclinic.com/health/fiber/NU00033). I have chosen to focus on the claim about fiber’s ability to lower cholesterol. According to the Fiber One® website, fiber is able to lower one’s cholesterol by blocking the absorption of cholesterol in the digestive tract (http://www.fiberone.com/Benefits/Default.aspx). But, the pertinent article on the Mayo Clinic website does not claim much about this benefit other than stating that fiber specifically lowers LDL and thus lowers one’s total cholesterol levels (http://www.mayoclinic.com/health/fiber/NU00033). What is the true mechanism? Does fiber intake really lower cholesterol?
What are the common side effects of adding fiber? How can they be dealt with?
If you have ever consumed a lot of fiber, you may have experienced some of its more noticeable side effects such as increased flatulence, bloating, and defecation. While making bowel movements occur more readily is often a desired effect from taking fiber (for its bulk laxative qualities), this effect might not be so desirable if it also gives you gas and if you are more concerned with your heart health than digestive tract. Fiber One® and All Bran® address these issue on their respective websites with blurbs reassuring their consumers that if you increase your fiber intake slowly and are sure to drink 6-8 eight ounce glasses of water a day while doing so, you can avoid the side effect of getting gas associated with fiber consumption (http://www.fiberone.com/Benefits/Default.aspx and http://www.all-bran.com/). The reason you have to increase your fiber slowly is explained in a Mayo Clinic article, which states that the natural bacteria in your intestines need time to adjust to changes in your diet. Also, the increased water intake is key because without it soluble fiber can actually cause constipation (http://www.mayoclinic.com/health/fiber/NU00033). Does drinking water while increasing your fiber intake really help prevent the gassy side effect? How reasonable is it that people will be able to increase their fiber and drink 6-8 glasses of water a day?
Would it be better to take a daily fiber supplement instead of worrying about adding fiber rich foods to your diet?
Many of the websites claim that taking fiber supplements is not as beneficial as getting your fiber from natural, food sources. The All Bran® website claims that their “cereal delivers at least 40% of your daily fiber needs in one bowl—that’s 3 times more fiber than the leading fiber supplement” (http://www.all-bran.com/). The Mayo Clinic website claims that whole foods are generally better than fiber supplements because the supplements do not provide the vitamins, minerals, and other beneficial nutrients that high-fiber foods often have with them. But, they do acknowledge that food sources might not be enough for some people with specific conditions like irritable bowel syndrome, and thus, supplements can become necessary in those situations (http://www.mayoclinic.com/health/fiber/NU00033). Finally, the Harvard School of Public Health website states that “current recommendations suggest that children and adults consume at least 20 grams of dietary fiber per day from food, not supplements” (http://www.hsph.harvard.edu/nutritionsource/what-should-you-eat/fiber-full-story/index.html). So, are supplements not as good as the “real thing”? Which should you be using to help combat high cholesterol?
Summary of Our Questions in Relation to Advertising Claims
The following is a summary of the questions raised by public health officials’ and food producers’ claims about fiber: Are we getting enough fiber? Should we increase fiber? Does it lower cholesterol levels significantly? What is the mechanism? What causes the gas, and can it be avoided? Are supplements as good or not?
Motives and Evidence Quality of the Fiber Advocates
It seems clear that the two public health websites sited above are making this information available online to help spread the word about the health benefits of fiber to the public. The Harvard site makes a point of citing Harvard research to back up its claims about fiber, which could be seen as showing a secondary motive of spreading its own research findings as being the most notable. Furthermore, the two food websites mentioned above are seemingly motivated by their desire to make their products more desirable to consumers over other similar products. They explicitly use their products’ fiber content as an edge over the competition. So, given these motives, we must now turn to scientific research if we want to find reliable answers to the questions that have arisen from the claims being made about fiber and its effect on cholesterol by these two types of fiber advertisers.
All that being said, each of the websites does a fairly good job of citing evidence and credible sources for their information. However, on occasion a statement is made without credible source citation leaving a lack of evidence for the claim. For instance, on the Fiber One® website, it is stated that fiber works by blocking the absorption of cholesterol in the digestive system without citing how they know this to be true, possibly exhibiting a lack of sound evidence. The public health websites are better about citing research studies to back up their claims while the food producer websites just state “facts” and often do not cite where they came from.
What does the scientific literature have to say about the effects of fiber on cholesterol?
Study investigating the effect of fiber in an already low-fat, low cholesterol diet:
Jenkins, Wolever, Rao, Hegele, Mitchell, Ransom et al (1993) investigated the effect on cholesterol levels when very high levels of fiber were added to an already low fat, low cholesterol diet. The question being asked was whether there would be any additional benefit to adding fiber when cholesterol was already significantly lowered by an established and maintained low fat and low cholesterol diet. They also were curious as to what the mechanism was behind the cholesterol lowering effects of fiber. As the Fiber One® website claimed earlier, there had been studies that suggested the way fiber helped reduce cholesterol was by displacing the saturated fat and cholesterol from one’s diet, but these researchers wondered if they saw a reduction in cholesterol when the diet was low in fat and cholesterol how this would could be explained given this proposed mechanism. To answer these questions, they performed a crossover study with 43 subjects (15 men and 28 women) with hyperlipidemia.
The subjects were randomly assigned to receive either soluble or insoluble fiber diets during two four month studies. Neither the subjects nor study personnel were blinded to the conditions the subjects were placed in. The participants were given personalized two-week repeating menus that were planned out based on their personal food preferences. A dietitian monitored their compliance with the menus, and all diet foods were delivered to their homes weekly. Participants’ blood samples were taken about every two weeks. Also, samples of the participants feces were taken at various points in the study to investigate the mechanism. For both diets, the blood lipids feel by the fourth week and were maintained throughout the remainder of the study at that level. Overall, soluble fiber decreased LDL and total cholesterol levels greater than insoluble fiber. And interestingly, the study found a noteworthy gender difference in percent reduction of total and LDL cholesterols levels with men having a greater reduction (7.5%) than women (3.4%). Furthermore, the journal article states, “it has been suggested that a 5 to 10 gram increase in dietary soluble fiber will reduce serum total cholesterol by approximate 5 percent” and such a reduction is “comparable to that observed in” this study (Jenkins, 1993). Finally, the article addresses that prior to this article the mechanism by which fiber helped lower cholesterol was unclear, though “it was proposed that fiber increased the binding and fecal loss of bile acids”. They also added to their report the finding from this study that there was a larger loss of sterol due to the increased fiber intake which they believe could contribute to the reduction of cholesterol in the blood as they found these two entities to be related from their data.
Meta-analysis of how significantly or not dietary fibers reduce cholesterol:
In an attempt to quantify how significantly fiber reduces cholesterol levels, a meta-analysis was performed by Brown, Rosner, Willett, and Sacks (1999) using 67 controlled trials. These investigators noticed how a wide range of effect sizes were being submitted about fiber’s ability to lower cholesterol and that different effect sizes were seen with different types of fibers. Through the meta-analysis, they sought to standardize the studies so as to determine whether the various fibers were really all that different in their effects on cholesterol, and if the effect size was really as large as it was often published as being. They did this by accounting for the variations in sample size, dosage of fiber, background diets of samples, types of subjects, etc. The 67 trials were selected based on the criteria of whether there was a control condition (insoluble fiber or low fat condition used to compare to the fiber condition), where lipid changes recorded for the different conditions, was a single source of soluble fiber used, etc. To limit the analysis, they only compared studies that used oat product, psyllium, pectin and guar gum as their source of soluble fiber for comparison. Overall, the soluble fiber reduced both cholesterol and LDL, which was consistent with the findings in the individual studies, as 60 to 70% of them found this also to be true.
Higher fiber diets also reduced HDL but by a smaller amount than soluble fiber at any dose reduces total cholesterol and LDL. Also, the type of soluble fiber used was insignificant; all types reduced total cholesterol and LDL by similar amounts with no significant difference between them. In contrast, difference in dose did account for some significant variance in effectiveness to reduce cholesterol. However, at higher doses, there was significant nonlinearity which suggests “diminished adherence or a biological maximum being reached” (Brown, 1999). As to the mechanism, the study had this to say, “the mechanism by which fiber lowers blood cholesterol remains undefined”. Hypothesizes cited by the study include: (1) that “soluble fibers bind bile acids or cholesterol during the intraluminal formation of micelles” which causes a “reduction in cholesterol content of liver cells” leading “to an up regulation of LDL receptors and this increased clearance of LDL cholesterol. However, increased bile acid excretion may not be sufficient to account for the observed cholesterol reduction”, (2) “inhibition of hepatic fatty acid synthesis by products of fermentation”, (3) “changes in intestinal motility”, (4) “fibers in high viscosity causing slowed absorption of macronutrients”, and (5) “increased satiety, leading to lower overall energy intake” (Brown, 1999).
All-in-all, the mechanism is still a mystery. Otherwise, the study found no difference in responsiveness between subjects with high and healthy cholesterol levels to the cholesterol lowering effects of fiber. Overall, increase in fiber intake does lower cholesterol but by a small amount. The study concludes its findings by warning that publication bias, where only studies that show positive results are published, could be a factor, and if it is, it is possible that the small effect of fiber on cholesterol is in actuality even smaller. Finally, the study cites that the “major benefit from eating fiber-rich foods may be a change in dietary patterns, resulting in a diet that is lower in saturated and trans-unsaturated fats and cholesterol and higher in protective nutrients such as unsaturated fatty acids, minerals, folate, and antioxidant vitamins” (Brown, 1999).
Study investigating the effect of a fiber supplement in reducing blood cholesterol levels:
The Knopp, Superko, Davidson, Insull, Dujovne, Kwiterovich, et al (1999) study sought to explore the effects of a fiber supplement (Choltrol, a mixture of soluble and insoluble fibers) on lowering cholesterol and how effective this treatment is in the long-term. The 15-week study used 125 subjects who were randomly assigned to either the test condition, which entailed taking 20g/day of the fiber supplement, or the control condition, which entailed taking a placebo. All subjects were stabilized with a 9 week National Cholesterol Education Program Step I Diet prior to the study. Eighty-five of the subjects continued the study to a total of 36 weeks in a noncomparative extension phase. Overall, the fiber group had a greater decrease in LDL cholesterol (-12.1% as compared to only -1.3%), total cholesterol (-8.5% as compared to -0.8%), and LDL to HDL ratio (-9.4% as compared to -1.5%) (P<0.001). There was no significant effect on HDL levels and similar results were found at the end of the 36 week extension period. But, there was an issue with side effects in this study. A total of 15 subjects dropped out along the course of the study due to gastrointestinal side effects such as diarrhea, gas, and loose stools. The prevalence of these side effects seemed to decrease with the duration of the study.
Study investigating the FDA approved dosage of fiber necessary to reduce cholesterol:
Study investigating the difference in effects between soluble and insoluble fiber on cholesterol:
In a study by Solá et al (2007), the effects of soluble
fiber (Plantago ovata husk) on
cholesterol were compared with the effects of insoluble fiber (Plantago ovata seed) in a randomized,
crossover, controlled, single blind study.
The study was performed in
The study found that the soluble fiber had a more favorable effect on cholesterol than insoluble fiber. Specifically, the soluble fiber from P. ovata husk increased HDL levels 6.7% relative to the insoluble fiber. This is a significant finding because often LDL lowering diets decrease HDL levels as well, which is an unwanted side effect. The study recognizes that this effect is different from the effect of most other soluble fibers that have been found to lower LDL levels. Despite this finding the journal article reads, “the mechanism by which P. ovata husk modifies lipid, lipoprotein, and apolipoprotein concentrations remains, as yet, undefined” (Solá, 2007). Speculative reasons offered as to why P. ovata husk did not lower LDL are that the dose was not as high as what other previous studies had used and that the patients in this particular study had low-moderate LDL concentrations in contrast to previous studies, which could be significant since it is thought that the change in LDL concentration is proportional to the severity of the concentration (meaning, if LDL concentration is higher, then the effect of soluble fiber on lowering the LDL concentration will be greater). Finally, the study makes note that although the National Cholesterol Expert Program Adult Treatment Panel III states people should consume 20-30g of dietary fiber each day, people should be sure that that amount of fiber consumed be 10-25g of soluble fiber if your goal is to lower your LDL cholesterol in order to prevent heart disease (Solá, 2007).
How do these studies answer our questions about the advertisers’ claims about fiber?
How much fiber should you consume per day?
The consensus on this seems to be in the 20-30 g per day range based on the previously discussed studies and websites (Solá, 2007 and http://www.mayoclinic.com/health/fiber/NU00033). Both the Fiber One® and All Brand® websites have daily fiber calculators that you can use to see how if you need to add more fiber to your daily diet or not (http://www.fiberone.com/Counter.aspx and http://www.all-bran.com/). If your goal is to reduce your total cholesterol and LDL levels, it is necessary that most of it be soluble fiber, not insoluble fiber, because only soluble fiber has a significant effect on your cholesterol levels (Jenkins, 1993; Brown, 1999; Jenkins, 2002; Solá, 2007). Insoluble fiber helps with other issues like regularity.
Does adding fiber to your diet lower your cholesterol levels significantly? If so, how?
There is a significant decrease in cholesterol levels found with adding fiber to one’s daily diet, but it is not a huge difference (Jenkins, 1993; Brown, 1999; Jenkins, 2002; Solá, 2007; Knopp, 1999). Also, each study stressed that it was still important to pair the addition of fiber to one’s diet with a low-fat, low-cholesterol diet for the best effect on lowering one’s cholesterol and decreasing one’s risk for heart disease (Jenkins, 1993; Brown, 1999; Jenkins, 2002; Solá, 2007). There were many different hypotheses offered as to how fiber helps to lower cholesterol but no one mechanism has been proven to be the real way it works (Jenkins, 1993; Brown, 1999; Jenkins, 2002; Solá, 2007), despite the claim on the Fiber One® website that the added fiber prevents the absorption of cholesterol (http://www.fiberone.com/Benefits/Default.aspx).
What are the common side effects of adding fiber? How can they be dealt with?
Many of the studies I found looked into the gastrointestinal effects of fiber while running their experiments, and these studies found it to be an insignificant issue (Solá, 2007; Jenkins 2002). However, one study did observe a problem with these side effects and subjects dropping out of the study because of them (Knopp, 1999). It is also important to note that in the Solá et al (2007) study the subjects were instructed to take 250 mL (roughly 8.45 fluid ounces) of water before taking the fiber. Since the fiber was taken three times a day, this only equates to 3-4 eight ounce glasses of water. So, it seems that the 6-8 ounces of water suggested by the websites is a bit extreme (http://www.fiberone.com/Benefits/Default.aspx and http://www.all-bran.com/). The Knopp et al (1999) study noted that over time the gastrointestinal side effects subsided, which is consistent with the claim by Mayo Clinic website that it is a matter of the bacteria in your intestines getting used to the change in your diet (http://www.mayoclinic.com/health/fiber/NU00033). Nevertheless, the increase in water consumption with the increase in soluble fiber consumption seems to be the key to avoiding getting gassy when consuming soluble fiber since no problems occurred in the Solá et al (2007) study with water intake necessary but there were problems in the Knopp et al (1999) study where water intake was not specified and adjustment to the increase in fiber took considerably longer.
Would it be better to take a daily fiber supplement instead of worrying about adding fiber rich foods to your diet?
The Brown et al (1999) meta analysis made a point of saying that the “major benefit” of adding fiber is that fiber-rich foods have a lot of other healthy goodies in them that are also beneficial to good health. Obviously, these goodies, like vitamins, minerals, etc., are absent in fiber supplements, so it does seem that fiber rich foods have an overall greater health benefit. But in relation to ability to reduce cholesterol, supplements get the job done just as well (Knopp, 1999). Also, some people need to take supplements due to other health problems, like irritable bowel syndrome (http://www.mayoclinic.com/health/fiber/NU00033).
Conclusion: Should you increase your fiber intake to prevent heart disease?
The effect of soluble fiber on cholesterol is small, often cited as a matter of less than 10 percent. Thus it certainly should not be used as a sole or primary method of lowering ones cholesterols levels. However, I am a firm believer that every little bit helps. It seems easy enough to get wheat bread instead of white when having a sandwich or eat an apple instead of apple pie for dessert to increase your daily fiber intake and slightly decrease cardiac risk in the process. Additionally, soluble and insoluble fibers’ health benefits are not limited to soluble fiber’s ability to lower cholesterol; as mentioned before, there are many health benefits to consuming fiber, which make it a worthwhile addition to everyone’s daily diets.
Brown, L., Rosner, B., Willet, W. W., & Sacks, F. M. (1999). Cholesterol-lowering effects of dietary fiber: a meta-analysis. The American Journal of Clinical Nutrition, 69, 30-42.
Jenkins, D., Kendall, C. W., Vuksan, V., Vidgen, E., Parker, T., Faulkner, D., et al. (2002). Soluble fiber intake at a dose approved by the US Food and Drug Administration for a claim of health benefits: serum lipid risk factors for cardiovascular disease assessed in a randomized controlled crossover trial. The American Journal of Clinical Nutrition, 75, 834-839.
Wolever, T., Rao, A. V., Hegele, R. A., Mitchell, S. J., Ransom, T., et al.
(1993). Effect on blood lipids of very high intakes of fiber in diets low in
saturated fat and cholesterol. The
Knopp, R. H., Superko, H. R., Davidson, M., Insull, W., Dujovne, C. A., Kwiterovich, P. O., et al. (1999). Long-term blood cholesterol-lowering effects of a dietary fiber supplement. American Journal of Preventative Medicine, 17, 18-23.
Solá, R., Godás, G., Ribalta, J., Vallvé, J., Girona, J., Anguera, A., et al. (2007). Effects of soluble fiber (Plantago ovata husk) on plasma lipids, lipoproteins, and apolipoproteins in men with ischemic heart disease. The American Journal of Clinical Nutrition, 85, 1157-1163.
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