in an attempt to overcome the gap between recommended levels of fiber consumption versus what is actually consumed in a typical american diet , fiber supplements have become a popular option as a convenient , concentrated source of fiber . given that a fiber supplement is an isolated fiber source consumed in addition to an existing diet , and many of the additional benefits described above for meal replacement are not provided by adding an isolated fiber source , it becomes essential to have a more in - depth understanding of the unique physiochemical characteristics of each fiber supplement and how these characteristics are , or are not , associated with 1 or more clinically meaningful health benefits . the term fiber supplement implies that regular ( daily ) consumption will provide health benefits that may be missing from a low - fiber diet , but for most fiber supplements , this implication is not supported by clinical data . health benefits derived from fiber supplements are primarily a function of the fiber s physical effects in the small bowel ( eg , cholesterol lowering , improved glycemic control , satiety / weight loss ) and large bowel ( improved stool form and reduced symptoms in constipation , diarrhea , and irritable bowel syndrome [ ibs ] ) . there are 4 main characteristics of fiber supplements that drive clinical efficacy : solubility , degree / rate of fermentation , viscosity , and gel formation . solubility defines whether a fiber supplement will dissolve in water ( soluble ) or remain as discreet insoluble particles.3,11 for soluble fibers , viscosity refers to the ability of some fibers to  thicken  when hydrated , in a concentration - dependent manner ( figure 1).3,1115 gel formation refers to the ability of a subset of soluble viscous fibers to form cross - links , resulting in a viscoelastic gel when hydrated ( figure 2).3,1113 linear versus branched polymers . swept out  by a fully extended linear fiber is much greater than a fiber with an equal number of sugar units ( same molecular weight ) but with a  bush - like ,  highly branched configuration . because the volume occupied by a polymer molecule is a function of the radius cubed , even small increases in effective hydrodynamic size can translate into a large increase in viscosity for linear fibers . drawings representing viscous linear polymers ( top ) and gel - forming linear polymers ( bottom ) . long - chain linear polymers orient parallel to adjacent fibers and increase viscosity in a concentration - dependent manner . some long - chain linear polymers also can form cross - links that create a gel in a concentration - dependent manner ( behave as a viscoelastic solid ) . gel formation is an important driver of several metabolic health benefits for dietary fiber supplements , including cholesterol lowering , improved glycemic control , satiety , weight control , and stool normalization ( soften hard stool in constipation and firm loose / liquid stool in diarrhea ) . ( drawings recreated with permission from john d. keller , jr , keller konsulting llc , freehold , nj . ) fermentation refers to the rate and degree to which a dietary fiber , after resisting enzymatic digestion in the small bowel , can be degraded by gut bacteria , producing fermentation byproducts such as short chain fatty acids and gas.3 most fibers are not exclusively soluble or insoluble , so for the purposes of this review , the predominant characteristic will be discussed ( eg , a fiber that is 70% soluble will be considered a soluble fiber ) . using the 4 fiber characteristics described above , fiber supplements can be divided into 4 clinically meaningful categories : insoluble , poorly fermented ( eg , wheat bran ) : when you think of  insoluble fiber ,  think of plastic ( clinical studies described later actually used plastic particles to mimic effects of wheat bran ) : does not dissolve in water ( no water - holding capacity ) ; poorly fermented ; can exert a laxative effect by mechanical irritation / stimulation of gut mucosa if particles are sufficiently large and coarse (  plastic effect  ) ; small smooth particles ( eg , wheat bran flour / bread ) have no significant laxative effect ; insoluble fiber does not gel or alter viscosity and thus does not provide other ( gel - dependent ) fiber health benefits . soluble , nonviscous , readily fermented ( eg , inulin , wheat dextrin , oligosaccharides , resistant starches ) : dissolves in water ; no increase in viscosity ; rapidly and completely fermented ( once fermented , the fiber is no longer present in stool , rapid gas formation , increased flatulence , energy harvest [ calorie uptake ] from fermentation by - products ) ; may alter the numbers of specific bacteria in the gut ( eg , effect ) ; no laxative effect at physiologic doses ; does not gel or alter viscosity and thus does not provide any of the gel - dependent fiber health benefits . readily fermented fibers are part of an emerging area of science relating to their effects on the gut microbiome , but to date , the marketed fiber supplements have no established clinically meaningful health benefits . soluble viscous / gel forming , readily fermented ( eg , -glucan [ oats , barley ] , raw guar gum ) : dissolves in water , forms a viscous gel ( eg , oatmeal ) , increases chyme viscosity to slow nutrient absorption and improve glycemic control , lowers elevated serum cholesterol , readily fermented ( gas formation , energy harvest [ calorie uptake ] from fermentation by - products ) , fermentation results in loss of gel and water - holding capacity , and thus , no significant laxative effect and no retained gel to attenuate diarrhea . soluble viscous / gel forming , nonfermented ( ie , psyllium ) : dissolves in water ; forms a viscous gel ; increases chyme viscosity to slow nutrient absorption and improve glycemic control , lowers elevated serum cholesterol ; not fermented ( no gas production , no calorie harvest from fermentation by - products ) ; because it is not fermented , it remains gelled throughout the large bowel , providing a dichotomous  stool - normalizing  effect : softens hard stool in constipation ( relieves / prevents constipation ) and firms / forms loose / liquid stool in diarrhea ( relieves / prevents diarrhea ) , and normalizes stool form in ibs . the small intestine is 7 m long and divided anatomically into 3 regions : duodenum , jejunum , and ileum . the mucosa of the small intestine is studded with millions of small villi , each covered with 1000 microvilli per 0.1 m , making the small intestine the largest body surface exposed to the outside world ( approximately 250 m , roughly the size of a tennis court).3,16 delivery of acidic nutrients into the duodenum ( proximal small bowel ) stimulates pancreatic secretions ( inorganic = water , bicarbonate , and electrolytes ; organic = digestive enzymes ) as well as the release of bile from the gall bladder . the total quantity of fluid absorbed by the small bowel each day is a combination of fluids consumed ( 1.5 l / d ) and the digestive juices secreted ( 6 - 7 l / d ) . in the fed state , the motor activity of the small bowel predominantly consists of segmental ( mixing ) contractions.3,16 these segmental contractions mix chyme back and forth , exposing food particles to digestive enzymes and bile and facilitating exposure of digested nutrients to the absorptive brush border of the mucosa for absorption . chyme , the liquid content of the small intestine , is normally very low in viscosity and is easily mixed with digestive enzymes for degradation and easily exposed to the villi for absorption of nutrients . the very large surface area of the mucosa normally results in efficient absorption of nutrients , which occurs early in the proximal small bowel.3,16 introduction of insoluble fiber ( eg , wheat bran ) or soluble nonviscous fiber ( eg , inulin , wheat dextrin ) has no significant effect on the rate of nutrient absorption in the small bowel because neither type forms a gel to alter the viscosity of chyme . in contrast , introduction of a soluble viscous , gel - forming fiber ( eg , guar gum , psyllium , high - molecular - weight -glucan ) will significantly increase the viscosity of chyme in a dose - dependent manner , which will slow the mixing of chyme with digestive enzymes . this will lead to a slowing of the degradation of complex nutrients into simple , absorbable components and also slow turnover of chyme at the villi , all of which will slow the absorption of glucose and other nutrients.3 this slowing of nutrient degradation and absorption lowers peak serum glucose concentration after a meal and delivers nutrients further into the small bowel for absorption . an effective gelling fiber can delay the absorption of nutrients long enough to deliver nutrients to the distal ileum , where they are not normally present . nutrients delivered to the distal ileum stimulate mucosal receptors to initiate a cascade of metabolic responses , 1 of which is the release of glucagon - like peptide-1 into the blood stream . glucagon - like peptide-1 is a short - lived ( 2-minute half - life ) peptide that significantly decreases appetite , increases insulin secretion , decreases glucagon secretion ( a peptide that stimulates glucose production in the liver ) , increases pancreatic -cell growth ( cells that produce insulin ) , improves insulin production and sensitivity , and slows gastric emptying and small bowel transit via a feedback loop called the phenomenon.3 all of the above metabolic responses are therapeutic targets for treating type 2 diabetes mellitus . taken together , the sum of these phenomenon leads to a gel - dependent improvement in glycemic control for patients with type 2 diabetes and those at risk for developing the disease ( eg , metabolic syndrome).3,14,1722 there are 2 primary methods for assessing the effects of fiber supplements on glycemic control : an acute postprandial study and a long - term assessment of glycemic control . the acute postprandial test ( glucose tolerance test ) provides a glucose load ( eg , 50 g glucose solution ) with and without fiber supplementation . blood glucose concentrations are drawn at frequent , predetermined intervals over a few hours to assess the rate of glucose absorption . glucose is normally very rapidly absorbed in the most proximal region of the small intestine , resulting in a relatively fast rise in blood glucose and a high peak concentration . because of a short lag in insulin response , this is typically followed by rapid decline in glucose with a transient excursion below the baseline level ( figure 3a , c ) . this transient hypoglycemia is caused by the lag in insulin response , which tends to stay elevated past the point where the blood glucose concentration has returned to baseline . introduction of a viscous , gel - forming fiber ( eg , guar gum , psyllium , high - molecular - weight -glucan ) can delay nutrient absorption to more distal regions of the small bowel ( b ) . rapid nutrient absorption ( c : grey line , corresponds with a ) is reflected by the higher peak concentration of blood glucose followed by a transient hypoglycemic trough below baseline . with the introduction of a viscous , gel - forming soluble fiber , the delay in nutrient absorption ( c : black line , corresponds with b ) results in an attenuation of glucose excursions : lower peak concentration of blood glucose , and attenuated hypoglycemic trough . ( drawings recreated with permission from thomas wolever , phd , university of toronto . ) it has been established for more than 3 decades that the viscosity of a gel - forming fiber supplement is highly correlated with minimizing the high and low excursions of postprandial glucose . in a study published in 1978,23 volunteers underwent glucose ( 50 g ) tolerance tests with and without the addition of several fiber supplements , including guar gum . native guar gum is a highly viscous , gel - forming fiber , and it exhibited a clinically meaningful decrease in both postprandial blood glucose and insulin concentrations . this beneficial response , however , was abolished when the guar gum was hydrolyzed to a nonviscous form , as is typically marketed today  partially hydrolyzed guar gum . the study showed that a reduction in postprandial blood glucose was highly correlated with the viscosity of a gel - forming fiber ( r = 0.926 ; p < .01 ) , as was a slowing of small bowel transit ( r = 0.885 ; p < .02 ) . taken together , a high - viscosity , gel - forming fiber supplement ( eg , raw guar gum , high - molecular - weight -glucan , psyllium ) can provide a clinically meaningful effect on elevated blood glucose level in a viscosity / dose - dependent manner , but nonviscous soluble fiber supplements ( eg , wheat dextrin , inulin ) do not provide a gel - dependent , clinically meaningful glycemic benefit.1,3 the second method , long - term assessment of glycemic control , entails multimonth , well - controlled , randomized clinical studies that assess a fiber s effects on glycemic control in the target population . these long - term studies provide evidence of a sustained effect and should be the standard for establishing a clinically meaningful therapeutic response for a given fiber supplement . numerous multimonth ( 26 months ) clinical studies have demonstrated that consumption of a soluble , gel - forming fiber supplement with meals can improve glycemic control ( lowers fasting blood glucose , insulin , and hemoglobin a1c concentrations ) in subjects at risk for type 2 diabetes ( eg , metabolic syndrome ) and in patients being treated for type 2 diabetes.1,3,2438 an example is an 8-week , placebo - controlled clinical study that evaluated psyllium ( 5.1 g bid ) for improved glycemic control in 49 patients being treated for type 2 diabetes ( fasting blood glucose and hemoglobin a1c at baseline : psyllium group , 208 mg / dl and 10.5 ; placebo group , 179 mg / dl and 9.1).38 after 8 weeks of treatment , fasting blood glucose for the psyllium group showed a significant decrease ( 89.7 mg / dl ; p < .05 ) versus placebo . hemoglobin a1c also showed a significant decrease ( 3.0 ) versus placebo.38 note that the improvement in glycemic control observed with psyllium was above that already conferred by a restricted diet and stable doses of a sulfonylurea and/or metformin . the long - term effects of an effective gel - forming fiber on fasting blood glucose concentrations are proportional to baseline glycemic control : there is no significant effect on normal blood glucose concentrations in healthy subjects.39 a moderate effect in patients with prediabetes and metabolic syndrome ( eg , 19.8 mg / dl for psyllium 3.5 g bid ; 9 mg / dl for guar gum 3.5 g bid),26 and a larger effect in patients with type 2 diabetes ( eg , psyllium , 35.0 to 89.7 mg / dl).34,38 note that gel - forming fiber supplements will not directly cause hypoglycemia ( suppression of glucagon by glucagon - like peptide-1 does not occur at hypoglycemic levels ) , but for patients being treated for type 2 diabetes , fasting blood glucose should be closely monitored with the initiation of an effective fiber therapy as the fiber supplement may reduce the required dose of hypoglycemic drugs . it is well established that reducing elevated serum low - density lipoprotein ( ldl)-cholesterol concentration reduces the risk of coronary artery disease.40 it has been estimated that a 1% reduction in ldl - cholesterol concentration reduces the risk of coronary artery disease by 1.2% to 2.0%.41 similar to the gel - dependent nature of improved glycemic control with fiber supplements , lowering elevated serum cholesterol concentrations is also a gel - dependent phenomenon in the small bowel , and the effectiveness of cholesterol lowering is highly correlated with the viscosity of the gel - forming fiber : the higher the viscosity of a gel - forming fiber is , the greater the effect on lowering elevated cholesterol concentrations.37 clinical studies have shown that the viscosity of a gel - forming fiber is actually a better predictor of cholesterol - lowering efficacy than is the quantity of fiber consumed.42 the primary mechanism by which gel - forming fibers lower serum cholesterol is by trapping and eliminating bile . bile is secreted by the liver ( normally 6001000 ml / d ) to emulsify large fat particles into many small particles for digestion by lipase enzymes and absorption across the mucosa.16 bile is normally highly conserved , recovered in the distal ileum , and recycled up to several times within a single meal . when bile is trapped in a gel - forming fiber and eliminated via stool cholesterol is a component of bile , and the liver uses serum stores of cholesterol to generate more bile , effectively lowering serum ldl - cholesterol and total cholesterol concentration , without affecting high - density lipoprotein cholesterol.43 to assess the importance of viscosity for a gel - forming fiber in lowering elevated serum cholesterol concentration , a clinical study compared the cholesterol - lowering effects of a medium - viscosity blend of gel - forming fibers ( psyllium , pectin , guar gum , and locust bean gum ) with those of an equal amount ( three 5-g servings per day for 4 weeks ) of low - viscosity gum arabic ( acacia gum , highly branched ) in 26 patients with hypercholesterolemia.44 the medium - viscosity gel - forming blend exhibited a 10% reduction in total cholesterol concentration ( p < .01 ) and a 14% reduction in ldl - cholesterol concentration ( p < .001 ) , with no significant change in high - density lipoprotein or triglyceride levels . in contrast , the low - viscosity gum arabic - treated group showed no change in any plasma lipid characteristics.44 a second publication with 4 studies ( duration 412 weeks ) explored the plasma lipid - lowering effects of a variety of soluble dietary fibers.45 the studies were randomized , double - blind , placebo - controlled trials involving men and women with hyperlipidemia ( plasma cholesterol > 200 mg / dl ) . low - viscosity gum arabic ( acacia gum ) ( 15 g / d for 4 weeks ) did not produce a significant lipid - lowering effect versus placebo . in contrast , 15 g / d of a medium - viscosity blend of soluble fibers ( psyllium , pectin , guar gum , and locust bean gum ) consumed for 4 weeks yielded significant reductions in total cholesterol ( 8.3% ) and ldl - cholesterol ( 12.4% ) concentrations ( p < .001 ) , similar to the 10 g / d high - viscosity raw guar gum . the lipid - lowering benefit of the medium - viscosity blend of soluble fibers ( psyllium , pectin , guar gum , and locust bean gum ) also showed a dose - response effect for reducing ldl - cholesterol concentration : placebo , + 0.8% ; 5 g / d , 5.6% ; 10 g / d , 6.8% , and 15 g / d , 14.9% ( all doses p < .01 vs placebo ) . the effects of the gel - forming fibers on plasma lipids were similar for both men and women . the authors concluded that the findings support the usefulness of medium- and high - viscosity gel - forming fibers as a cholesterol - lowering therapy but cautioned against ascribing cholesterol lowering benefits solely on a classification of solubility.45 as with improved glycemic control , the viscosity of the gel - forming fiber is the key driver of efficacy for lowering cholesterol in patients with hyperlipidemia . also similar to the gel - dependent effect glycemic control , the potential for a cholesterol - lowering benefit is highly influenced by the baseline cholesterol level : gel - forming fibers have no appreciable effect on cholesterol concentrations in healthy subjects with normal cholesterol concentrations but exhibit a progressively greater benefit as baseline cholesterol exceeds normal concentrations . the cholesterol - lowering benefit of gel - forming fiber supplements is observed in addition to the benefits conveyed by the prescription drugs in patients already being treated for hyperlipidemia . eight clinical studies have shown that a gel - forming fiber ( psyllium ) enhanced the cholesterol - lowering benefit of prescription drugs when dosed as a cotherapy to statin drugs and bile sequestrants.4653 there are only 2 fiber supplements approved by the food and drug administration to claim a reduced risk of cardiovascular disease by lowering serum cholesterol : -glucan ( oats and barley ) and psyllium , both gel - forming fibers.54 an effective gel - forming fiber supplement can actually lower the required dose of a prescription statin drug . for example , in a 12-week randomized , double - blind study including 68 patients with hyperlipidemia , a low dose of simvastatin ( 10 mg ) combined with psyllium ( 15 g / d , divided doses before meals ) was superior to the low dose of simvastatin alone ( 63 vs 55 mg / dl , respectively ; p = .03 ) and identical to a high dose of simvastatin alone ( 20 mg , 63 mg / dl ) for lowering elevated serum ldl - cholesterol concentration.55 gel - forming fibers can provide an effective cotherapy for hypoglycemic and cholesterol - lowering drugs to help reduce required doses and potentially reduce side effects . as described above , low - viscosity fibers ( gum arabic / acacia gum , methylcellulose ) and nonviscous supplements ( eg , inulin , wheat dextrin ) do not exhibit a cholesterol - lowering benefit.37,44,45,56,57 it is important to recognize the difference between replacement with dietary fiber that is intrinsic and intact in whole foods and supplement with an isolated fiber source . fiber supplements can not be presumed to have the same health benefits that are associated with dietary fiber that is intact and intrinsic in whole foods . the clinically proven health benefits for fiber supplements are associated with specific characteristics ( eg , viscous gel ) , and only a minority of marketed fiber products provide health benefits ( summarized in the table ) . health benefits associated with fiber effects in the small bowel ( eg , cholesterol lowering , improved glycemic control , satiety , weight loss ) are a gel - dependent phenomenon , and the degree of benefit is proportional to the viscosity of the gelling fiber . health benefits associated with fiber effects in the large bowel ( eg , relief from constipation , diarrhea , ibs ) will be discussed in detail in part 2 of this 2-part series . briefly , large bowel effects are derived from 2 mechanisms : an insoluble fiber provides a mechanical stimulus proportional to particle size ( eg , wheat bran  softens hard stool in constipation but can exacerbate diarrhea and ibs ) and a nonfermented gel - forming fiber that remains intact / retains its high water - holding capacity throughout the large bowel can provide a stool - normalizing effect ( ie , psyllium  softens hard stool in constipation , firms loose / liquid stool in diarrhea , normalizes stool form in ibs ) . when recommending a fiber supplement , only a soluble nonfermenting , gel - forming fiber has been clinically proven to provide all of the health benefits typically associated with a fiber supplement .