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Effects of Dietary Inulin on Serum Lipids^{1, ,2}

Chicago Center for Clinical Research, Chicago IL

³To whom correspondence should be addressed.

	ABSTRACT

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Inulin is a carbohydrate belonging to a class of compounds known as fructans. Because inulin is resistant to digestion in the upper gastrointestional tract it reaches the large intestine essentially intact, where it is fermented by indigenous bacteria. Thus, it may be classified as a soluble dietary fiber. Soluble fibers have been shown to modulate serum lipids. A recent study examined the effect of consuming three servings per day of inulin-containing foods, compared with the effect of similar foods without inulin, on serum lipid profiles among hypercholesterolemic men and women. In addition, the practicality of including 18 g/d of inulin in a low fat diet was investigated. The recent study randomized, double-blind, crossover trial with two 6-wk treatment periods, separated by a 6-wk washout. Men and women (n = 21) with baseline LDL increased significantly (7.4 and 12.3%, respectively) during the control phase. There were small, nonsignificant declines in total (1.3%) and LDL-C (2.1%) during the inulin phase. Thus, differences in response between periods (inulin - control) were significant (P < 0.05) for LDL-C (-14.4%) and total cholesterol (-8.7%). Mild gastrointestinal discomfort was more common during the inulin than the control food phase; however, the gastrointestinal side-effect profile of inulin was similar to that of other soluble fibers. Although it was not possible to draw firm conclusions, inulin may have blunted the hypercholesterolemic effects observed during consumption of control foods. Additional research will be required to confirm the possible lipid-modulating properties of dietary inulin in humans.

KEY WORDS: • inulin • oligosaccharide • dietary fiber • hyperlipidemia • lipoproteins

	INTRODUCTION

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The prevalence of hypercholesterlemia in the United States remains a serious public health concern despite significant progress in both education and treatment of this important coronary risk factor since the creation of the National Cholesterol Education Program (NCEP) in 1985. Approximately 51% of the U.S. adult population have serum cholesterol levels above the desirable range (Sempos et al. 1993 ). Soluble fibers such as beta-glucoa in oats and psyllium have received FDA approval as a health claim supporting their role as dietary ingredients that if used in conjunction with a low fat diet may lower serum cholesterol and reduce the risk of heart disease. Oat fiber and psyllium have been demonstrated to lower LDL-C by 4–10% (Davidson et al. 1991 , Davidson et al. 1998 , Ripson et al. 1992 ). There is general agreement that consumption of certain types of soluble dietary fibers lowers serum total cholesterol and LDL-C. A recently published analysis of investigations on soluble fiber found that total cholesterol was reduced in 88% (68 of 77) of the studies reviewed (Glore et al. 1994 ). For studies that reported LDL-C values, reductions were observed in 84% (41 of 49). Clinically relevant hypocholesterolemic responses (5% reduction of total and/or LDL-C) were generally obtained with daily intakes of 6–40 g of pectin 8–36 g of guar or other gums, 10–30 g of psyllium fiber and 3–11 g of oat fiber (42–150 g of oat bran).

There are three characteristics of these fibers that are believed to be responsible for their hypocholesterlemic properties; 1) water solubility, 2) fermentability and 3)viscosity. Water solubility alone probably does not explain the hypocholesterolemic effects of these fibers because gum arabic, a highly soluble fiber does not lower LDL-C levels (Davidson et al., 1998 ). Hydroxy propyl methylcelluose, a highly viscous but nonfermentable fiber has been demonstrated to significantly lower LDL-C in a dose controlled fashion (Maki et al., in press ). Animal studies demonstrated that short chain fatty acids produced by bacterial fermentation and absorbed into the portal blood supply may have an inhibitory effect on hepatic synthesis. Inulin, a highly fermentable, but poorly absorbed saccharide may therefore mimic the hypocholesterolemic effects of some soluble fibers.

Inulin is a carbohydrate belonging to a class of compounds known as fructans. It is composed mainly of linear chains of fructose units linked to a terminal sucrose molecule and varies in length from 2 to 60+ frutose moieties (Gibson et al. 1994 ). Some 36,000 plants from a wide variety of genera contain inulin as an energy reserve, or as an osmoregulator assuring cold resistance. Among those plants, several have been consumed by mankind for centuries. Recently, studies have demonstrated a daily per capita intake of 1–4 g of inulin in an average North American diet. Examples of commonly consumed foods containing inulin include wheat, onions, garlic, bananas, leeks, artichokes and asparagus (Van Loo et al. 1995 ).

Because inulin is resistant to digestion in the upper gastrointestinal tract (Knudsen and Hessov 1995 ), it reaches the large intestine essentially intact, where it is fermented by indigenous bacteria. Thus, it may be classified as a soluble dietary fiber (Roberfroid 1993 ). Little, if any inulin is detectable in the feces because colonic metabolism by fermenting anaerobic bacteria is nearly complete producing short-chain fatty acids (SCFA), lactic acid and gases including H₂, CO₂ and CH₄ (Roberfroid 1993 ). Because inulin has a bland neutral flavor and contributes a fat-like texture and mouth feel when added to some foods, commercial inulin can be used to replace sugar and fat in various food preparations such as chocolate, dairy products, table spreads, frozen desserts and baked goods.

Studies in experimental animals and limited data from human subjects suggest that dietary inulin, like other soluble dietary fibers, may modulate the concentration of serum lipids (Fiordaliso et al. 1995 , Tomomatsu 1994 , Yamashita et al. 1984 ). Supplemental inulin could prove to be useful adjunct in the dietary management of hypercholesterolemia by performing the following functions: 1) having a possible direct influence on serum lipids, 2) replacing certain cholesterol-raising fatty acids in some food formulations, and 3) reducing the caloric density of selected foods by substituting inulin for part of the fats or sugars in the foods [the caloric content of inulin is ~4.13 kJ/g (or 1 kcal/g)] (Roberfroid et al. 1993 ).

A recent study compared the serum lipid profile after 6 wk of consuming inulin-supplemented foods with the profile after 6 wk of consuming similar foods not containing inulin, in the diets of free-living persons with hypercholesterolemia.

This was a randomized, double-blind, crossover study in 25 adults with mild-to-moderate hypercholesterolemia. Eligible participants were randomly assigned to receive food products containing inulin or similar foods without inulin for 6 wk. This first treatment period was followed by a 6 wk washout after which subjects crossed over to receive the other treatment for the final 6 wk of the study. Inulin was used as a substitute for a portion of the sugar content of the study foods.

Inclusion criteria were age 30–75 y with LDL cholesterol (LDL-C) concentration between 3.36 and 5.17 mmol/L at the time of blood screening.

Serum LDL-C, HDL-C, total cholesterol and triglycerides at the beginning and end of each treatment period are shown in Table 1 . Unexpectedly, total and LDL-C were significantly lower at the start of the control phase than at the start of the inulin phase of the study (Fig. 1 , Table 1 ). This occurred regardless of whether subjects were assigned to receive inulin or control study foods during the first treatment period. Differences in serum lipids do not appear to have been secondary to dietary differences because body weight and intakes of cholesterol, saturated fatty acids, and unsaturated fatty acids, assessed by food record analysis, were similar at these time points.

View larger version (14K): [in this window] [in a new window]   Figure 1. Mean ± SEM total and LDL cholesterol concentrations at the beginning and end of each treatment period.

View larger version (12K): [in this window] [in a new window]   Figure 2. Mean ± SEM lHDL cholesterol concentrations at the beginning and end of each treatment period.

The total number of side effects reported by the volunteers was 44 during control and 77 during the inulin phase. Although most of the events recorded during the inulin period were mild, they did not generally reduce in frequency or severity during the 6 wk of treatment.

Among the 21 subjects who completed the study, 13 reported no side effects during the control phase vs. 5 during the inulin phase. Seven persons noted some mild side effects during the control period vs. thirteen during the inulin treatment (mainly flatulence, but also bloating, cramping and loose stool, Table 2 ). The total incidence of reported gastrointestional symptoms was significantly higher during the inulin period (P < 0.003).

On the basis of previously reported data showing a lipid-lowering effect of inulin, it may be hypothesized that inulin consumption prevented the increase in total and LDL-C observed during the control period. However, because of the lower than expected values for total and LDL-C before both control periods, regression to the mean should be considered as a possible explanation for the differences in lipid response.

This study, due to the variable lipid responses in the crossover treatment groups, cannot be used to confirm the lipid lowering effects of inulin. Additional research is required in order to determine possible lipid modulating effects of dietary inulin in humans.

	FOOTNOTES

 
¹ Presented at the conference Nutritional and Health Benefits of Inulin and Oligofructose held May 18–19, 1998 in Bethesda, MD. This symposium was supported in part by educational grants from the National Institutes of Health Office of Dietary Supplements, the U.S. Department of Agriculture and Orafti Technical Services. Published as a supplement to The Journal of Nutrition. Guest editors for the symposium publication were John A. Milner, the Pennsylvania State University, and Marcel Roberfroid, Louvain University, Brussels, Belgium.

² A similar version of this manuscript was published in Nutrition Research, vol. 18, no.3, pp 503–517, "Effects of dietary inulin on serum lipids in men and women with hypercholesterolemia," M. H. Davidson, K. C. Maki, C. Synecki, S. A. Torri, K. B. Drennan. Reprinted with permission from Elsevier Science.

	REFERENCES

TOP ABSTRACT INTRODUCTION REFERENCES

 

1. Davidson, M. H., Dugan, L. D., Burns, J. H., Bova, M., Story, K. & Drennan, K. (1991) The hypochesterolemic effects of ß-Glucan in oatmeal and oat bran. JAMA. 265:

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3. Davidson et al. (1998) .

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5. Gibson G. R., Willis C.L., Van Loo J. Non-digestible, oligosaccharides and bifidobacteria - implications for health. Int. Sugar Journal. 1994;96:381-387

6. Glore S. R., Van Treeck D., Knehans A. W., Guild M. Soluble fiber and serum lipids: a literature review. J. Am. Diet Assoc. 1994;94:425-436[Medline]

7. Knudsen K.E.B., Hessov I. Recovery of inulin from Jerusalem artichoke (Helianthus tuberosus L.) in the small intestine of man. Br. J. Nutr. 1995;74:101-113[Medline]

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9. Ripsin, C. M., Keenan, J. M., Jacobs, D. R., Elmer, P., Welch, R. R., Van Horn, L., Liu, K., Turnbull, W. F., Thye, F. W., Kestin, M., Hegsted, M., Davidson, D. M., Davidson, M. H., Dugan, L. D., Demark-Wahnefried, W. & Beling, S. (1992) Oat products and lipid lowering. JAMA. 267:

10. Roberfroid M. Dietary fiber, inulin, and oligofructose: a review comparing their physiological effects. Crit. Rev. Food Sci. Nutr. 1993;33:103-148[Medline]

11. Van Loo J., Coussement P., DeLeenheer L., Hoebregs H., Smits G. On the presence of inulin and oligofructose as natural ingredients in the Western diet. Crit. Rev. Food Sci. Nutr. 1995;35:525-552[Medline]

12. Sempos. 1993

13. Tomomatsu H. Health effects of oligosaccharides. Food Tech 1994;48:61-65

14. Yamashita K., Kawai K., Itakura M. Effects of fructo-oligosaccharides on blood glucose and serum lipids in diabetic subjects. Nutr. Res. 1984;4:96l

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