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American Journal of Clinical Nutrition, Vol. 70, No. 5, 839-846, November 1999
© 1999 American Society for Clinical Nutrition

Original Research Communications

Comparative lipid and lipoprotein responses to solid-food diets and defined liquid-formula diets1,2,3

Vikkie A Mustad, Satya S Jonnalagadda, Sarah A Smutko, Christine L Pelkman, Barbara J Rolls, Stephen R Behr, Thomas A Pearson and Penny M Kris-Etherton

1 From Strategic Research and Development, Ross Products Division, Abbott Laboratories, Columbus, OH; the Department of Nutrition, Georgia State University, Atlanta; the Nutrition Department, The Pennsylvania State University, University Park; Aeterna Laboratories, Quebec City, Canada; and the Department of Community and Preventive Medicine, University of Rochester Medical Center, Rochester, NY.

2 Supported in part by Ross Products Division, Abbott Laboratories, Columbus, OH, and NIH grant DK50156 (to BJR).

3 Address reprint requests to VA Mustad, Ross Products Division, Abbott Laboratories Medical Nutrition R&D, 625 Cleveland Avenue, Department 105670, RP3-2 Columbus, OH 43215. E-mail: Vikkie.Mustad{at}rossnutrition.com.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Background: Liquid-formula diets (LFDs) are useful in metabolic studies of the cholesterolemic effects of dietary lipids because they can be formulated with accuracy, facilitating precise delivery of fatty acids of interest. However, because of differences in composition and nutrient delivery between LFDs and solid-food diets (SFDs), there is a need to determine differences in their effects.

Objective: Our objective was to compare lipid and lipoprotein responses to changes in total fat, saturated fatty acids (SFAs), and cholesterol in subjects consuming an SFD or LFD.

Design: Twenty-one healthy subjects consumed controlled diets representative of an average American diet [AAD; 37% of energy from fat (15% from SFAs), and <50 mg cholesterol/MJ] or a National Cholesterol Education Program (NCEP) Step II diet [26% fat (5% from SFAs) and <25 mg cholesterol/MJ]. Other nutrients were similar between diets. Diets were consumed for 23 d in a randomized, crossover design.

Results: For the AAD and NCEP Step II diet, there were no significant differences in lipids and apolipoproteins when the LFD or SFD versions were consumed. In contrast, consumption of the SFD was associated with significantly lower total cholesterol and triacylglycerols than was consumption of the corresponding AAD or Step II LFD (P < 0.05). Subjective ratings of satiety, hunger, and quality of life between diet forms did not differ significantly.

Conclusions: Both LFDs and SFDs yield quantitatively similar cholesterolemic responses to changes in dietary fat, SFAs, and cholesterol. LFDs may offer advantages because they provide easily administered, complete, balanced nutrition without affecting satiety.

Key Words: Liquid-formula diets • fat • saturated fat • blood cholesterol • satiety • human food intake • solid food • lipids


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Many studies have examined the cholesterolemic effects of changes in dietary fat content and composition with whole-food experimental diets (1). Although progress has been made in our understanding of the effect of diet on blood cholesterol concentrations, there are still important questions that need to be answered to define the ideal diet for the prevention and treatment of hypercholesterolemia and other cardiovascular disease risk factors. Well-controlled feeding studies are needed to clarify the biological effects of individual fatty acids and other dietary components; the design of experimental diets is essential in the implementation of these studies (2). Of primary importance is that they be formulated to vary only in the individual nutrient of interest. Unfortunately, because of variations in food composition, it is often difficult to design experimental diets by using a mixture of solid foods that vary only in one or a limited number of nutrients. Whole-food diets can be designed to meet specified nutrient criteria; however, this usually requires that all foods be precisely measured and weighed for each individual subject.

An attractive alternative is the use of nutritionally complete liquid-formula diets (LFDs). These diets are well suited for use in controlled feeding studies because they provide accurate and constant intakes of energy, macronutrients, vitamins, and minerals. LFDs are convenient, easy to deliver, and homogeneous, facilitating precise delivery of fatty acids and other nutrients of interest.

Many of the classic studies that evaluated the effects of dietary fatty acids and cholesterol effects on serum cholesterol concentrations used LFDs. The landmark work of Ahrens et al (3, 4) and Beveridge et al (5, 6) evaluated serum lipid effects by using LFDs containing various amounts and types of fat. Connor et al (7) also used LFDs to investigate steroid and bile acid excretion in men fed an LFD containing different oils. More recently, Grundy's group (8–11) used LFDs extensively in studies designed to examine cholesterolemic effects of individual fatty acids. Such experiments using LFDs, however, have not been without criticism (12). Hegsted and Nicolosi (13) described "unusual findings," such as a drop in serum cholesterol, that have been observed in some studies even when a major part of the total fat in the formula was saturated. The unexpected effects of LFDs in some studies may have been due, in part, to the low concentration or absence of cholesterol in the formula; different amounts of complex carbohydrates, dietary fiber, or other micronutrients; or differences in digestion and metabolism of liquid compared with solid foods.

Although there is considerable precedent for the use of LFDs in studying the effects of dietary fats and fatty acids on serum cholesterol concentrations, there has been no direct comparison of the cholesterolemic response to solid-food diets (SFDs) and LFDs. Thus, the present study was conducted to directly compare the blood lipid and lipoprotein responses to changes in total fat, saturated fatty acids (SFAs), and cholesterol in healthy subjects consuming SFDs or LFDs. The influence of the form of the diet (solid or liquid) and composition (varying amounts of total fat) on subjective ratings of hunger and satiety were also assessed.


   SUBJECTS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Subjects
Twenty-four healthy men (n = 12) and women (n = 12) between the ages of 20 and 65 y were recruited for the study from The Pennsylvania State University and the local State College community. Subjects were selected if their initial baseline serum total cholesterol concentrations fell between 4.65 and 5.3 mmol/L (the {approx}25th and 50th percentiles from the third National Health and Nutrition Examination Survey reference population; 14), if their HDL-cholesterol concentration fell between the 10th and 90th percentiles, and if their triacylglycerol concentration was below the 90th percentile (percentiles adjusted for sex, age, and race) (15). Subjects were considered healthy on the basis of blood chemistry analysis and a self-administered health-screening questionnaire.

All potential subjects sampled the LFD during the screening phase. Those who passed the initial screening and otherwise qualified for the study participated in a 3-d run-in period, during which they consumed the LFD for 100% of their total energy needs. This run-in period was intended to help potential subjects determine whether they liked the LFD and would be able to comply with this diet condition. After the trial period, subjects confirmed their willingness to participate in the study. The study protocol was approved by the Institutional Review Board of The Pennsylvania State University and informed, written consent was obtained from each subject.

Design
Subjects were recruited in groups of 4 to maintain balance in this 4 x 4 period randomized crossover study. Each of the 4 test diets was consumed for 23 d with a 7-d washout period between experimental diet periods. Breakfast and dinner were consumed under the supervision of the study's staff at the Nutrition Department's Metabolic Diet Study Center. Subjects reported to the dining facility at preset times (ie, between 0630 and 0930 for breakfast and between 1630 and 1830 for dinner). Lunch and snacks were packed for each individual subject, which they consumed where convenient except on days when satiety assessments were made (see below). Overall, subjects consumed >75% of their daily intake under staff supervision. Subjects were allowed to consume unlimited quantities of non-energy-containing beverages (ie, water and diet drinks) throughout the day. During the study, body weight was monitored daily and energy intake adjusted so that each subject maintained a constant weight. Subjects were instructed to maintain their normal physical activity levels throughout the study. A trained phlebotomist drew fasting blood samples (20 mL) by venipuncture on days 22 and 24 of each experimental diet phase for lipid and lipoprotein analyses. Serum was isolated from whole blood by centrifugation at 3000 x g and 4°C for 20 min and aliquots were stored in cryovials at –80°C until analyzed.

Diets
The 2 study diets were designed to mimic an average American diet [AAD; 37% of energy from fat (15% from SFAs) and <50 mg cholesterol/MJ] and a National Cholesterol Education Program (NCEP) Step II diet [26% of energy from fat (5% from SFAs) and <25 mg cholesterol/MJ]. Both diets were provided either as whole-food diets (ie, SFDs) made up of commonly available foods or as defined LFDs (Ross Products Division, Abbott Laboratories, Columbus, OH). The LFDs were formulated to match the nutrient composition of the whole-food diets and were based on the estimated average nutrient content of 7 sample menus created by using the NDS programs (version 2.8/10/25; Nutrition Coordinating Center, University of Minnesota, Minneapolis).

The nutrient composition of the diets was matched as closely as possible with respect to the estimated macro- and micronutrient contents, including total fat and fatty acid composition, cholesterol (mg/MJ), protein source (90% animal, 10% plant), and fiber distribution (33% soluble, 66% insoluble). Vitamin and mineral contents of all diets met recommended intakes. The nutrient composition of the 2 experimental LFDs was evaluated by using Association of Official Analytical Chemists (AOAC) methods for protein, carbohydrate, total fat, fatty acids, cholesterol, dietary fiber, vitamins, and minerals (Ross Products Division, Abbott Laboratories). Homogenates (two 50-mL aliquots) of the 7 full-day menus of the AAD and Step II SFD were analyzed at Covance Laboratories, Inc (Madison, WI) with standard AOAC procedures for protein, carbohydrate, total fat, fatty acids, cholesterol, and dietary fiber. The nutrient compositions of the experimental diets are shown in Table 1Go.


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  		TABLE 1. Nutrient composition of the solid-food (SFD) and liquid-formula (LFD) diets1
 
Energy needs for each subject were estimated at the start of the study according to the Harris-Benedict equation (with a moderate activity adjustment of 1.6; 16) and were adjusted to maintain body weight as needed throughout the study. During the LFD periods, subjects consumed approximately the same distribution of energy at each meal and snacks in the same pattern as in the SFD.

Compliance
In addition to the 3-d run-in period, which was used to identify noncompliers, the importance of compliance was stressed throughout the study by the staff nutritionists. Study subjects were asked to voluntarily disqualify themselves from the study if they did not comply with the regimen. A log of food and beverage intake was maintained daily by the study participants and monitored daily by study personnel; a weekly monitoring form was maintained for each participant by study personnel. Any food or beverages eaten outside of the Metabolic Diet Study Center were recorded, including caffeine-containing and alcoholic beverages. Liquid-formula products consumed off-site were accounted for by the return of all empty cans.

Blood lipid analyses
All serum lipid and lipoprotein analyses were performed at The Mary Imogene Bassett Research Institute, Lipid Research Center (Cooperstown, NY). Blood samples were analyzed for total cholesterol, LDL cholesterol, triacylglycerol, HDL cholesterol, apolipoprotein B-100 (apo B), and apolipoprotein A-I (apo A-I). All lipid assays were performed on a Roche MIRA random-access automated analyzer (Roche Diagnostics, Rotkreuz, Switzerland) and all samples were analyzed together at the end of the study to minimize assay variation. Non-HDL particles were precipitated with dextran sulfate (molecular weight: 50000). Total and HDL cholesterol were assayed by using an automated enzymatic method (Roche cholesterol esterase/oxidase and peroxidase/4 amino-antipyrene detection system). Triacylglycerols were assayed by using an enzymatic method (lipase and glycerol kinase coupled with glycerol-1-phosphate oxidation; Sigma Chemical Co, St Louis). LDL-cholesterol concentrations were calculated by using the Friedewald equation (LDL cholesterol = total cholesterol – HDL – triacylglycerol/5) with cholesterol concentrations as mg/dL (17). Apo B and apo A-I were assayed by rate nephelometry with use of specific polyclonal antisera (Beckman Instruments, Fullerton, CA). The Mary Imogene Bassett Lipid Research Center laboratory is a Centers for Disease Control and Prevention (CDC) Certified Regional Reference Center for lipid analyses; bimonthly calibration was done by using reference materials designated by the CDC and the International Immunological Society.

Satiety assessment
Diets differing in total fat content and form (solid or liquid) may influence hunger and subjective sensations of satiety. Visual analogue scales (VAS) were used to assess hunger, thirst, nausea, prospective consumption (how much individuals thought they could eat), and fullness. VAS have been validated for use in investigating appetite (18, 19). For example, hunger was rated on a 100-mm line preceded by the question, "How hungry are you right now?" and anchored on the left by "not at all hungry" (0 mm) and on the right by "extremely hungry" (100 mm). Other anchors consisted of the phrases "not at all" and "extremely" in combination with the adjectives "thirsty," "nauseated," and "full." To assess feelings of satiety throughout the dietary conditions, VAS were completed on 3 different days (days 2, 12, and 23) during each diet period. On these days, subjects consumed breakfast, lunch, and dinner in the Metabolic Diet Study Center and completed ratings before and after lunch, at hourly intervals throughout the day, and before and after dinner. Quality of life questionnaires (20) were also administered on these days to assess mood, energy level, and overall wellness.

Statistical methods
All data were examined for normality by fitting a one-way, randomized block design analysis of variance (ANOVA) to the data and examining the residuals for normality. Normally distributed data were analyzed with a one-way randomized block ANOVA. If there were significant differences among the diet groups, Tukey's honestly significant difference test and contrast (food, dose, and interaction) were carried out to determine where the differences existed. Those variables (triacylglycerol and apo B) for which the residuals showed evidence of not being normally distributed (P < 0.05 with the Shapiro-Wilk test) were analyzed with nonparametric methods (Friedman's two-way ANOVA). Hourly ratings of hunger and satiety and mean daily values were analyzed by ANOVA using the general linear models procedure in which sex was tested as a between-subjects factor and condition (form of diet), fat intake, and time (day 2, 12, or 23) as withinsubjects factors. Tukey's honestly significant difference test was used for post hoc comparisons of significant effects with the appropriate error terms for between- and within-subjects factors specified. SAS software was used for the analysis (SAS Institute Inc, Cary, NC).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Enrollment and weight maintenance
Twenty-four healthy men and women aged 31 ± 2 y (21–51 y) were enrolled in the study. Three subjects (2 women and 1 man) discontinued participation in the study within the first week, during which time they were consuming the LFD (2 because of boredom with the diet and 1 for personal reasons); consequently, their data were not included in the analyses. Baseline characteristics of the 21 subjects who completed the entire study are shown in Table 2Go. Acceptance of the LFDs and SFDs by the subjects who completed the study was very good, and compliance with both test diet forms was judged to be excellent on the basis of subject feedback and daily records. Because energy intakes were adjusted as needed throughout the study, body weight during each experimental diet phase did not change by >1 kg. Mean energy intakes during the SFD (11.06 ± 0.52 and 11.48 ± 0.58 MJ/d for the AAD and Step II diet, respectively) and LFD (11.66 ± 0.61 and 11.35 ± 0.54 MJ/d for AAD and Step II diet, respectively) phases did not differ significantly among diet forms or fat intakes.


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  		TABLE 2. Characteristics of study subjects at baseline1
 
Effect of experimental diets on blood lipids
Mean (±SEM) lipid and lipoprotein endpoints are presented in Table 3Go. Significant effects of diet composition and form on total cholesterol, LDL cholesterol, triacylglycerol, apo B, and apo A-I were observed. As expected, differences were observed in mean concentrations of serum lipids as dietary SFA and cholesterol intakes decreased when subjects switched from the AAD to the Step II diet; however, differences were also observed in plasma lipids after subjects consumed LFDs compared with after they consumed SFDs. Specific effects of the dietary treatments are described below.


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  		TABLE 3. Serum lipid and lipoprotein concentrations in subjects after consumption of solid-food (SFD) or liquid-formula (LFD) diets1
 
Effects of reducing dietary fat and cholesterol on blood lipids
Regardless of the physical form of the diet, reduction in dietary SFAs and cholesterol resulted in significant reductions in total cholesterol, LDL-cholesterol, and apo B concentrations (Table 3Go and Figure 1Go). In Figure 1Go, individual and mean changes in both total cholesterol and LDL-cholesterol concentrations emphasize the consistency of the individual responses. Compared with the AAD, lower dietary total- and saturated fat and cholesterol reduced total cholesterol (by 14% and 16%) and LDL cholesterol (by 19% and 23%) when consumed as either an LFD or SFD, respectively. In contrast, no significant changes in HDL-cholesterol or triacylglycerol concentrations were observed in response to reductions in total dietary fat content (Figure 2Go). Interestingly, there was an interaction between dietary form and fat content in the apo A-I response in that the decrease in this apolipoprotein was greater when subjects consumed the Step II SFD (0.14 ± 0.02 g/L) than when they consumed the Step II LFD (0.06 ± 0.02 g/L) when compared with the respective AAD (Table 3Go).



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  		FIGURE 1. Individual serum total and LDL-cholesterol responses to an average American diet (AAD) and a National Cholesterol Education Program Step II diet consumed for 23 d as either solid food or liquid formula. Each point is an average of 2 fasting serum cholesterol values as described in Methods. The thick line represents the average response for the 21 participants. *Significant difference between the AAD and Step II diets and between the solid-food and liquid-formula diets, P < 0.05 (Tukey's honestly significant difference test and contrast).

 


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  		FIGURE 2. Individual serum HDL-cholesterol and triacylglycerol responses to an average American diet (AAD) and a National Cholesterol Education Program Step II diet consumed for 23 d as either solid food or liquid formula. Each point is an average of 2 fasting serum cholesterol values as described in Methods. The thick line represents the average response for the 21 participants. *Significant difference between the AAD and Step II diets and between the solid-food and liquid-formula diets, P < 0.05 (Tukey's honestly significant difference test and contrast).

 
Effect of liquid-formula or solid-food diet on blood lipids
Regardless of the amount and composition of the fat in the diet, consumption of SFDs resulted in significantly lower serum total cholesterol and triacylglycerol concentrations than did the corresponding AAD or Step II LFD (Table 3Go and Figures 1 and 2GoGo). Compared with the corresponding AAD or Step II LFD, consumption of the SFD was associated with an 8–10% lower total cholesterol and 16–20% lower triacylglycerol concentration (P < 0.05). The reduction in total cholesterol was due exclusively to a reduction in LDL cholesterol; there was no independent effect of the SFD on HDL-cholesterol concentration.

Effects of experimental diets on ratings of satiety
Ten men and 7 women provided data for this portion of the study. Overall, there were no consistent effects of time (day 2, 12, or 23) on any ratings; thus, mean values were calculated for each subject within each condition. Daily average ratings of hunger and prospective consumption did not differ significantly between diet forms (SFD compared with LFD) or fat intakes (Step II diet compared with AAD). The average daily ratings for hunger were 31 ± 3, 33 ± 3, 35 ± 4, and 32 ± 3 mm for the low-fat (Step II) and AAD LFD and the low-fat and AAD SFD, respectively. A significant effect of fat intake on average fullness rating was found for the SFD only (P < 0.04); however, the difference between conditions was not large (55 ± 4 and 50 ± 4 mm for the AAD and Step II SFD, respectively). Ratings of fullness for the LFD were 53 ± 4 mm for the AAD and 51 ± 3 mm for the Step II diet (NS). Ratings of nausea also differed by fat intake for the SFDs only (P < 0.02); these differences were small, however, and overall nausea ratings were low in all conditions (2 ± 1, 1 ± 0.5, 1 ± 0.5, and 3 ± 1 mm for the Step II diet and AAD LFD and Step II diet and AAD SFD, respectively). No significant differences were observed with respect to hunger, mood, energy level, or overall wellness assessed by the quality of life questionnaire. No systematic differences were found between men and women for any ratings.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The results of this study clearly showed that the physical form of the diet does not differently affect the serum cholesterol response to changes in dietary fat, SFAs, or cholesterol. Furthermore, these results also show that when energy needs are met, ratings of hunger, fullness, and quality of life are comparable between the solid and liquid diet forms. Taken together, these data indicate that the use of nutritionally complete LFDs in clinical studies is justified.

Numerous studies using both solid food and liquid formulas conducted since the 1950s confirmed the potent hypercholesterolemic effects of SFAs (12:0–16:0) and dietary cholesterol (1). The results from the current study support the validity of the observations from well-controlled studies using LFDs and show that, given an appropriate experimental design in which similar diet forms are compared, LFDs can be used to assess the cholesterolemic response to changes in dietary lipids. In fact, the magnitude of the average effects on total cholesterol concentrations observed as a result of the changes in dietary fatty acid composition from either diet form were quantitatively similar to those estimated by the Keys et al (21) and Hegsted et al (22) predictive equations (Table 4Go), which were developed from metabolic studies with SFDs. Note that the equations slightly underestimate the difference in the actual response between the AAD and Step II SFD by {approx}0.23 mmol/L, whereas the difference between predicted and observed total cholesterol between the 2 LFDs was {approx}0.09 mmol/L. The magnitude of these differences could be related to the consistency of intakes provided by the solid or liquid diet forms.


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  		TABLE 4. Comparison of the observed changes in serum total cholesterol concentrations in subjects consuming solid-food (SFD) and liquid-formula (LFD) diets with expected changes in dietary fatty acid and cholesterol composition as estimated by predictive equations
 
Other studies using LFDs have not always yielded predictable effects on blood cholesterol. From a review of studies (published predominantly between 1950 and 1960) in which LFDs were used to study the cholesterolemic effects of fatty acids, Hegsted and Nicolosi (13) concluded that the responses to dietary lipids are attenuated when compared with results obtained with whole-food diets and suggested that data obtained with the use of LFDs should not be extrapolated to other conditions. In a meta-analysis of metabolic ward studies, Clarke et al (23) reported that the overall effects of SFAs on blood cholesterol were less in the studies using LFDs than in those using SFDs, although, in contrast with the conclusions of Hegsted and Nicolosi (13), the cholesterolemic responses to poly- and monounsaturated fatty acids did not differ substantially. Review of these early studies using LFDs indicates that several aspects of their design suggest that the "unusual findings" may not have been the result of the liquid formula per se. For example, some studies were complicated by the use of stearic acid as a major source of SFAs; thus, the results obtained should not be expected to predict a response similar to that of whole-food diets containing other cholesterol-raising SFAs (24). In addition, most of the LFDs used in these early studies were essentially cholesterol-free and high in fat (40–60% of energy) and many contained 15–20% of energy as linoleic acid, which may attenuate the cholesterolemic response to SFAs (25).

The present study, however, showed a significant effect of diet form on serum total cholesterol and triacylglycerol that appears to be independent of dietary fat composition. As shown in Table 4Go, the Keys and Hegsted equations predict a negligible difference in total cholesterol (–0.098 to –0.034 mmol/L for LFDs compared with SFDs) when considering only the small mean differences in fatty acids and cholesterol between diet forms; however, total cholesterol was elevated by 0.46–0.54 mmol/L when subjects consumed LFDs. The reasons for this independent effect of diet form on blood lipids are not known. Although an attempt was made to match the macronutrient compositions of the SFD and LFD as closely as possible, the 2 diet forms obviously differed in their physical characteristics. A complex whole-food diet varying in the proportion of nutrients delivered in meals and snacks, the texture of individual food components, and the presence of nonnutritive constituents (eg, fiber) and other naturally occurring phytochemicals can delay gastric emptying and prolong the absorption time of nutrients, whereas liquid diets made from purified ingredients are more rapidly emptied from the stomach (26). Some or all of these differences can affect fat digestion and absorption, lymphatic transport, bile acid metabolism, and postprandial responses of plasma lipoproteins (27–29). For instance, viscous or fermentable fibers can interfere with the enterohepatic circulation of bile acids and increase their fecal excretion, which can increase hepatic conversion of cholesterol to bile acids and lower the hepatic cholesterol content, resulting in an increase in LDL-receptor activity and uptake of serum LDL cholesterol (30). These dietary fibers have well-documented cholesterol-lowering effects, which, in some cases, are equal in magnitude to those of fat (31). Although the carbohydrates in the experimental LFD were provided as a mixture of sources and included both soluble and insoluble fibers, the SFD provided a complex variety of carbohydrates. The efficiency of dietary cholesterol absorption can also be influenced by its physical form (32); the LFD provided primarily free cholesterol whereas a variable proportion (8–20%) of that in whole foods is esterified in adipose triacylglycerol or found as a structural component of cellular membranes. Thus, greater availability and absorption of cholesterol from the LFD may have contributed to some of the difference in blood lipids noted when the 2 diet forms were consumed.

Higher concentrations of triacylglycerol in subjects consuming LFDs also may be explained by their higher content of readily digestible carbohydrate than of the more complex carbohydrates in SFDs (33). The biological mechanism may be a result of the ability of insulin to stimulate VLDL and apo B production and secretion (34, 35). It is of interest that insulin also can stimulate b-hydroxy-b-methylglutaryl-CoA reductase activity (36), a rate-limiting step in cholesterol synthesis, which suggests another possible mechanism contributing to the higher total cholesterol concentrations in subjects consuming the LFD.

Finally, the present study also showed that there were no consistent differences between diets among the many subjective measures of satiety. An effect of fat intake on fullness was detected for the SFD only, although the difference was small, and no significant differences were found between ratings of hunger or prospective consumption. Note that the study subjects may not be representative of the general population because they agreed to consume a liquid diet for extended periods of time. Furthermore, it was not possible to determine the effects of energy density on satiety because subjects had unlimited access to non-energy-containing beverages and the diets were adjusted throughout the study to meet individual energy needs. Keep in mind, too, that LFDs may not be appropriate in many experimental study designs because the major disadvantage that limits their widespread use is that most people prefer eating solid foods. The lack of variety, chewing, and change in taste and texture with LFDs may be drawbacks to their use over extended periods of time. Despite these limitations, these data suggest that LFDs can be used in clinical settings without compromising subjective sensations of satiety, and thus limit feelings of deprivation and promote adherence to the experimental diet.

In conclusion, the results show that experimental diets that have a comparable nutrient profile but differ in physical form elicit a similar cholesterolemic response to changes in dietary total fat, SFAs, and cholesterol. Thus, because LFDs offer complete and balanced nutrition, adequate satiety, and safety and ease of administration, they greatly simplify the delivery of well-controlled experimental diets and can be used in clinical studies without compromising results.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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Received for publication August 21, 1998. Accepted for publication April 11, 1999.




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