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Original Research |
Energy Metabolism Lab, The Jean Mayer U.S.D.A. Human Nutrition Research Center on Aging at Tufts University, Boston, Massachusetts
Address reprint requests to: S.B. Roberts, Ph.D., Jean Mayer U.S.D.A. Human Nutrition Research Center on Aging at Tufts University, 711 Washington St., Boston, MA 02111. E-mail: sroberts{at}hnrc.tufts.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Methods: Subjects were 41 healthy men and women aged 18 to 78 years. Weight maintenance energy requirements were established over two weeks during consumption of a control diet with low soluble fiber content. Subjects then consumed a hypocaloric diet for six weeks, either consuming a low soluble fiber control diet or a diet containing 45 g/1000 kcal rolled oats, a whole grain cereal rich in soluble fiber (mean energy deficit -895 ± 18 kcal/day relative to weight maintenance energy requirements). Changes in body fat and fat-free mass were determined by underwater weighing, and dietary compliance was assessed using the urinary osmolar excretion rate technique. In a final phase of the study, subjects ate ad libitum for six months, and changes in body weight and composition were monitored.
Results: There was no significant effect of the oat-containing diet on body weight or composition changes during the hypocaloric regimen or in the subsequent ad libitum period. In addition, fecal energy excretion was not significantly different between groups. However, there were non-significant trends indicating reduced hunger in the oat group compared to controls (frequency of hunger 2.5±0.5 vs. 3.6±0.4, P=0.1). In addition, fewer oat subjects were non-compliant (four versus seven subjects dropped out or had urinary osmolar excretions greater than 130% of values predicted from dietary intake), but again the difference was not significant.
Conclusions: These results suggest that use of a cereal rich in soluble fiber in a closely monitored hypocaloric feeding regimen does not improve weight loss or dietary compliance. Further studies are needed to examine the possibility that cereals containing soluble fiber may have effects on hunger and dietary compliance that could be important in less tightly controlled protocols than the one described here.
Key words: energy intake, obesity, oats, soluble fiber, weight loss, body fat, compliance
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Several factors may potentially influence the effectiveness and acceptability of weight-loss diets, including the digestibility of food energy as well as hunger and satiety for a given level of energy intake. In particular, diets promoting some degree of energy malabsorption could theoretically enhance negative energy balance if they did not also promote increased hunger. Similarly, if dietary regimens could be identified that minimize hunger or promote satiety, they could potentially improve weight loss success by improving compliance. From these perspectives, diets high in soluble fiber might be expected to promote weight loss success, since soluble fiber reduces rate of macronutrient digestion and absorption from the small intestine and enhances short-term satiety [3, 4]. Soluble fiber also lowers the glycemic index of foods, a factor recently associated with suppression of hunger in a one-day study [5].
There is currently controversy over whether soluble fiber can aid weight loss [6], with some studies showing a positive effect of fiber on weight loss [3, 4, 7–12] and others showing no effect [4, 9, 13–16]. It is possible that these inconsistent results may be due to differences in study protocols, as well as the type, amount and physical form of fiber utilized. It is also possible that these inconsistencies may be due to differences in subject compliance, but this possibility has not been examined.
The study described here was designed to test the hypothesis that a hypocaloric diet high in oats, a whole-grain cereal rich in soluble fiber, would enhance weight loss by reducing both energy digestibility and hunger, thereby increasing both negative energy balance and subject compliance. This study was also designed to assess the influence of hypocaloric diets on energy regulation and subsequent weight regain in young and older persons. Compliance was assessed using the Osmol Excretion Rate Technique (OER) [17], a method developed previously by our group specifically for the assessment of dietary compliance in metabolic balance studies.
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METHODS |
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16 g/day (assessed at screening) for all subjects, as a habitually high intake of fiber was thought to potentially preclude further additional benefit of the study diet. Exclusion criteria also included vegetarian diet, self-reported strenuous physical activity of more than one hour a day, a dietary restraint score greater than 10 [18] and self-reported consumption of more than two alcoholic drinks a day. The protocol was approved by the Human Investigation Review Committee at New England Medical Center, and all subjects gave written informed consent before participating.
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During weeks 1 and 2 (Phase 1) all subjects consumed a provided control diet low in soluble fiber. Maintenance energy requirements were established for each subject during the first week of the phase and were generally maintained at that fixed level during the second week. The amount of dietary energy provided to the subjects on study day 1 was based on a dietitian’s estimate of energy requirements, taking into account age, gender, fasting body weight on day 1 and the self-reported level of physical activity. Body weight was monitored at least five times during the first week, and energy intake was adjusted as necessary to maintain weight within 500g of the value on day 1; diets were adjusted by the addition of muffins (each 250 kcal, with a macronutrient content similar to the control diet) or reduction of portions. Subjects were allowed to leave partial portions of food, which were re-weighed to estimate their energy content. During the second week of Phase 1, subjects were given the calculated average of energy consumed during the first week and were required to completely consume all portions of food and rinse and scrape food containers unless a noticeable trend in body weight occurred (in which case an adjustment to intake was again made). In the second week of Phase 1, a series of tests were performed including a three-day fecal collection for determination of fecal energy content. To obtain baseline information relative to Phase 2 (described below), fasting body weight was assessed at the end of the phase to ±100g (Toledo Weight Plate, Toledo Scale, Worthington, OH), and body fat and fat-free mass were determined by underwater weighing as described below.
During the following six weeks (weeks 3 to 8, Phase 2) subjects were randomly assigned within gender and age-group categories (ages 20 to 40 or 60 to 80 years) to receive either the control diet (Control group or Group 1) or an oat-containing diet (Oat group or Group 2). Both diets had the same amounts of fat, protein, carbohydrate and insoluble fiber, but had 45g/1000 kcal of rolled oats per day substituted for an equivalent amount of wheat products. Energy intake in Phase 2 was decreased by 1000 kcal/day relative to Phase 1 in all but seven subjects, who had initial energy intakes that were low and a 1000 kcal reduction was considered too severe (in these cases, subjects were typically older and had a lower BMI than the group average, and a 800–900 kcal/day deficit was generally employed). The energy and macronutrient composition of diets in Phases 1 and 2 are shown in Table 2. Subjects were not told that two different diets were being tested, rather that metabolic parameters were being examined over the course of weight loss. A three-day fecal collection was again obtained in Phase 2 to assess fecal energy losses, and in addition two three-day complete urine collections were obtained for measurement of osmolar excretion rate and assessment of dietary compliance [17]. Fasting body weight and body composition by underwater weighing were measured at the end of the phase in duplicate.
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Diets
All food and caloric beverages consumed by the subjects during Phases 1 and 2 and during the first 14 days of Phase 3 were provided by the research center. Except during the first week of Phase 1 and during Phase 3, subjects were required to consume all items and rinse and scrape containers. Meals consisted of normal food and beverage items divided between three meals and a snack each day, and three daily menus were provided on a rotating basis. Coffee and tea were included in the menus in fixed daily amounts if they formed part of the subject’s normal diet. Sodium intake was fixed in Phase 1, remained constant throughout Phases 1 and 2, and subjects were advised to add no additional salt.
The nutrient content of the diets provided during Phase 1 mimicked a typical American diet, providing 35% of energy from fat, 12% of energy from protein and 53% from carbohydrate. Total fiber intake was approximately 6 g/1000 kcal dietary energy, distributed as 1/3 soluble and 2/3 insoluble fiber as summarized in Table 2; this level of fiber intake was provided to minimize differences between average pre-study fiber intake and the control diet. During Phase 2, the Oat group was provided with a diet consisting of food items incorporating 45 g/1000 kcal of rolled oats ("quick cooking" Quaker® rolled oats, divided between all meals and snacks), and the Control group was provided with the control diet that all subjects received in Phase 1, which contained nearly identical macronutrient and insoluble fiber content to that of the Oat group.
Nutrient composition of the diets was calculated using standard dietary composition software (NDS 93 Version 2.4, Minnesota Nutrition Data System, Nutrition Coordination Center, University of Minnesota, Minneapolis, MN). Use of this software resulted in minor discrepancies between calculated total fiber and the sum of soluble and insoluble fiber (Table 2).
Assessment of Dietary Compliance
Dietary compliance was assessed by comparison of the calculated urinary osmolar load, based on dietary intakes of sodium, potassium and nitrogen, with measured urinary osmolar loads in six 24-hour urine collections obtained in Phase 2; three consecutive 24-hour collections of urine were conducted in study week 5 and repeated in study week 7 or 8. The basis of this method is described by Roberts et al. [17]. Briefly, the urinary osmol load consists primarily of compounds derived from consumed foods and, in particular, nitrogen-containing compounds and sodium and potassium salts. Thus, the urinary osmol load can be predicted from dietary protein, sodium and potassium, taking into account obligatory non-urinary losses of these nutrients and correcting for other nutrients contributing to urine osmolality. A measured osmol load significantly greater than predicted indicates the consumption of additional or non-study foods, while a measured osmol load significantly less than predicted suggests either that provided foods are not being consumed or than the 24-hour urine collection is incomplete. The theoretical urinary osmolar load for the Phase 2 diet was calculated as:
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The measured urinary osmolar rate (mOER) in Phase 2 was calculated as the product of urine volume and urine osmolality (model 3MO Micro-osmometer, Advanced Instruments Inc., Needham, MA) for each of the six 24-hour periods when urine was collected. These values were then expressed as a percent of tOER, and values were then averaged over the six-day measurement period.
Weight and Body Composition
Body weight and height were measured to the nearest ±100 g and 0.1 cm, respectively. Body density was measured by hydrostatic weighing after an overnight fast as described elsewhere [19], and conducted twice during Phase 1 and twice at the end of Phase 2 and averaged for each phase. Body density was corrected for predicted residual lung volume [20]. Fat mass and fat-free mass were calculated using the equation of Siri [21] and the average of three tests within ±1% body fat was used for analysis.
Fecal Energy
Three-day stool collections were obtained during Phases 1 (study week 2) and 2 (study week 6 or 7) to determine energy excretion. The collections were weighed, freeze dried to constant weight (Virtis Model #6203-3006-OG freeze drier, The Virtis Co., Gardiner, NY), homogenized and analyzed in duplicate for fecal energy by isoperibol bomb calorimetry (Parr Instrument Co., Model 1261, Moline, IL).
Hunger and Satiety Ratings
A sub-group of subjects (n=19) completed a questionnaire at the end of Phases 1 and 2 to help quantify changes in hunger, satiety, thirst (a control variable) and constipation. Subjects were asked to quantify the frequency of satiety, hunger, constipation and thirst using a 5-point scale where 0=never or no change, 1=rarely once or twice only, 2=occasionally or 3 to 6 times in the phase, 4=frequently or 2 to 3 times per week, 5=very frequently or more than once a day.
Data Analysis
Values are expressed as means ± SEM except where stated. Statistical analysis was performed using SPSS® software, Version 7.5 (SPSS, Inc., Chicago, IL). Differences between groups in Phase 1 were assessed by using Student’s t test for unpaired data. Within-subject changes over time were calculated as Phase 2—Phase 1, and changes in weight were expressed as absolute changes or as a percentage of initial weight. Analysis of covariance was used to compare differences between groups over time, while controlling for values in Phase 1, and to assess interactions of diet with initial BMI, age and gender. All tests were considered significant at p <0.05.
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RESULTS |
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Table 2 shows the dietary intake and fecal energy excretion of the subjects who completed the study. As shown, the primary difference between the groups was the increase in soluble (and therefore total) fiber in the Oat group during Phase 2. Although there was a statistically significant difference in soluble fiber between the groups in Phase 1, the difference was extremely small and of no practical importance. Fecal energy excretion tended to be greater in the Oat group, but the difference was not significant.
Oat group subjects tended to have both a lower dropout rate (Oats five subjects vs. Control three subjects) and better compliance as measured by fewer subjects providing urine collections with osmolality greater than 130% (Oats 2 vs. Control 1) and less than 70% of expected (Oats 2 vs. Control 1), but the numbers were small and were not significantly different between groups. Note that data from subjects with urinary osmolar excretions of >130% expected were not included in the primary study analyses, but those with expected excretions <70% were included because of the possibility that low excretion rates were due to incomplete urine collection rather than dietary noncompliance.
Table 3 shows differences between the groups in weight and body composition changes over the course of the study. Weight loss in Phase 2 was very similar in the two groups, as was weight regain during Phase 3 and weight change over the entire study, and differences were not significant. There were no significant major effects of BMI, age or gender on weight loss in kg or as a percent of initial, nor were there any significant interactions between diet and these variables on weight loss. Trends in body weight over the course of the study are shown in more detail in Fig. 1 and again emphasize the similarity between the groups.
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DISCUSSION |
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Despite fermentation of some soluble fiber to metabolizable forms of energy, fecal energy loss due to reduced fat (and possibly other macronutrient) absorption is thought to persist following consumption of soluble fiber [26, 27], sometimes exceeding the energy value of the fiber itself.
Based on these effects of soluble fiber, we tested the hypothesis that soluble fiber as a component of food would enhance compliance with hypocaloric diets by reducing both digestible energy intake and hunger and, through these mechanisms, would enhance compliance and total weight loss. There are no known published trials investigating these issues in which soluble fiber has been provided as an intrinsic component of food during a hypocaloric diet. As summarized elsewhere [6], previous studies on soluble fiber and weight loss [3, 4, 7–16] have yielded inconsistent results, possibly due to the effect of uncontrolled differences between studies such as study duration, protocol, compliance with study diets, amount and type of fiber and how incorporated into the diet, and dietary macronutrient content. There are insufficient numbers of trials to establish if soluble fiber as a food component benefits body weight, and the trials of soluble supplements do not support a clear dose-response relationship. In trials where soluble fiber was provided as a supplement (often as a preload prior to meals), beneficial effects on weight have been noted with ad libitum feeding with soluble fiber between 3 and 20 g/day [11, 12, 28] and with fixed energy deficits with at 18 g/day [10]. However, other investigators report no weight-loss benefit of supplemental soluble fiber, such as in recently diagnosed diabetics, who were provided 15 g/day of cereal fiber or guar vs. no supplement over 20 weeks while consuming a 1200 kcal/day diet [29] or in children supplemented with glucomannan 2 g/day vs. placebo for two months [30].
In some trials investigating effects of mixed fibers on body weight when subjects consume ad libitum diets, significant effects have been observed when 14 to 35 g/day are provided [31, 32], while no benefit was observed in other trials with 10 to 20 g/day [33–35]. When mixed fibers are provided with hypocaloric diets of fixed energy deficit, weight loss benefits have been noted with 6 g/day [36], 15 g/day [37] and 30 g/day [38], while no effect on weight loss was observed with 5 g/day [39]. Addition of approximately 20 g/day of bran or ispaghula before meals was not associated with weight benefits in dieting subjects over two weeks [14]. Astrup [40] also found no weight loss benefit of adding predominantly insoluble fiber 30 g/day to a very low energy diet. Thus, there appear to be factors beyond fiber type or dose that influence effects on weight.
In this study of adult men and women spanning a wide range of adult ages and body composition, there was no significant difference between the groups in weight loss or body composition change during the hypocaloric phase of the study (Phase 2). Using standard factors of 9.25 kcal/g and 1 kcal/g for the energy content of fat and fat-free mass, respectively [41], body composition data indicated that there was a mean body energy loss of 621 kcal/day during Phase 2 in both groups combined. The mean decrease in energy intake during Phase 2 was 896 kcal/day, indicating that body energy mobilization was equivalent to 69% of the deficit in energy intake. Previous work from our group has suggested that energy expenditure decreases during underfeeding by approximately 1/3 of the energy deficit and that body energy mobilization is equivalent to 2/3 of the decrease in energy intake [42]. Thus, our finding in this study of body energy mobilization equal to 69% of the decrease in energy intake is completely consistent with previous research and is also consistent with our removal of non-compliant subjects, as determined by higher-than-expected urinary osmolar excretion.
The hypothesis that inclusion of oats would improve weight loss and satiety was based on several factors. The additional fiber provided in the Oat diet was within, but in the lower end, of ranges employed in other trials. Provision of a whole food that contains fiber, as opposed to an isolated supplement, has the potential benefit of ease and may result in phenomena such as delayed digestion and absorption of macronutrients on the basis of physical form as well as fiber content. The oat dose of 45 g/1000 kcal was chosen as this represents a readily achievable goal for most persons and is also consistent with doses demonstrated to benefit blood lipids [43]. It should be noted that while the energy deficit in our study was fixed, the addition of oats was based on energy intake and encompassed a wide range; however, no dose response relationship was observed for weight loss, satiety or compliance measures (data not shown). It is possible that the dose of oats in this study was too low to bring about a beneficial effect on weight loss or that the energy deficit achieved by subjects may have been too low to be influenced by the addition of oats in this amount. Further trials are needed to establish if a higher dose would be effective at this or other dietary energy levels.
The additional finding that there was no significant difference in the frequency of reported symptoms of hunger, satiety or constipation between the groups suggests that the oat-containing diet used in this study was ineffective in minimizing adverse symptoms associated with consuming a hypocaloric diet. This may be because the amount of oats was insufficient to modify symptoms, because the provision of soluble fiber as a component of food is less effective than provision as a separate supplement, because the degree of energy restriction was too great to be influenced by a singular dietary factor such as inclusion of oats or because oats were an ineffective modulator of these symptoms. Other investigators have reported salutary effects of fiber on hunger in studies lasting hours or days or during weight loss efforts. The provision of soluble fiber supplements ranging from five to twenty grams prior to meals has been associated with decreased hunger or increased satiety [3, 44–46]. In two trials examining effects of different doses of guar or pectin as preloads, no dose-response relationship was observed in effects on hunger or satiety [44, 46]. It should be noted, however, that the mean frequency of hunger reported by the Oat group tended to be lower than reported ratings of subjects consuming the control diet (p=0.1), suggesting the possibility of an effect that might have been detected with a bigger supplement and/or a larger number of subjects and suggesting in turn that further research in this area is needed. On the other hand, the fact that the control diet was used in both Phase 1 and Phase 2 for the Control group, while the Oat group had different diets in Phase 1 and Phase 2, may have influenced the results inasmuch as continued consumption of the control diet may have predisposed to a sense of monotony regarding the diet. The fact that there was no significant difference in measured compliance or drop-out between the groups is consistent with the finding of no significant difference between the groups in hunger and satiety, though again non-compliance tended to be worse in the Control group, whose members also tended to report more frequent hunger. Further studies are needed to resolve the question of whether there may be an effect of an oat-enriched diet on the frequency of hunger and subject dropout in studies involving the use of hypocaloric diets and whether diets containing more oats or soluble fiber may be more effective. In a less tightly controlled study, or in the free-living situation, the effect of an oat or soluble fiber enriched diet may be more evident since subjective factors (such as hunger or satiety) would have substantially greater importance.
Concerning the ad libitum phase of the study (Phase 3) subjects tended to gain back some but not all of the weight lost in Phase 2, with the result of a mean weight loss in both groups over the entire study period. However, there was no difference in weight regain and body composition change between the groups. Thus, at the end of the eight-month study period, both groups had very similar body composition changes relative to Phase 1.
In summary, a weight-loss diet enriched with whole grain oats did not measurably improve weight loss relative to a control diet without oats in this study of highly compliant, carefully monitored subjects. These results suggest that cereals containing soluble fiber may be of limited utility in promoting weight loss in highly controlled programs. However, further studies are needed to examine the possibility that soluble-fiber-containing foods or supplements may reduce hunger and thereby enhance compliance during less intensively supervised weight loss programs, which represent the majority of weight loss efforts.
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ACKNOWLEDGMENTS |
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Received June 16, 2000. Accepted November 5, 2000.
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REFERENCES |
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