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Journal of the American College of Nutrition, Vol. 19, No. 3, 383-391 (2000)
Published by the American College of Nutrition

Original Research

Fasting Lipoprotein and Postprandial Triacylglycerol Responses to a Low-Carbohydrate Diet Supplemented with n-3 Fatty Acids

Jeff S. Volek, PhD, R.D., Ana L. Gómez, MS and William J. Kraemer, PhD

The Human Performance Laboratory, Ball State University, Muncie, Indiana

Address reprint requests to: Jeff S. Volek, Ph.D., R.D., Assistant Professor, The Human Performance Laboratory, Ball State University, Muncie, IN 47306.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Background: The effects of a prolonged low-carbohydrate diet rich in n-3 fatty acids on blood lipid profiles have not been addressed in the scientific literature.

Objective: This study examined the effects of an eight-week ketogenic diet rich in n-3 fatty acids on fasting serum lipoproteins and postprandial triacylglycerol (TG) responses.

Design: Ten men consumed a low-carbohydrate diet rich in monounsaturated fat (MUFA) and supplemented with n-3 fatty acids for eight weeks. Fasting blood samples were collected before and after one week of habitual diet and on two consecutive days after 2, 4, 6 and 8 weeks of the intervention diet. Postprandial TG responses to a fat-rich test meal were measured prior to and after the intervention diet.

Results: Compared to the habitual diet, subjects consumed significantly (p <= 0.05) greater quantities of protein, fat, MUFA and n-3 fatty acids and significantly less total energy, carbohydrate and dietary fiber. Body weight significantly declined over the experimental period (-4.2 ± 2.7 kg). Compared to baseline, fasting total cholesterol, LDL cholesterol and HDL cholesterol were not significantly different after the intervention diet (+1.5%, +9.7% and +10.0%, respectively). Fasting TG were significantly reduced after the intervention diet (-55%). There was a significant reduction in peak postprandial TG (-42%) and TG area under the curve (-48%) after the intervention diet.

Conclusions: A hypocaloric low-carbohydrate diet rich in MUFA and supplemented with n-3 fatty acids significantly reduced postabsorptive and postprandial TG in men that were not hypertriglyceridemic as a group before the diet. This may be viewed as a clinically significant positive adaptation in terms of cardiovascular risk status. However, transient increases in total cholesterol and LDL cholesterol were also evident and should be examined further in regard to which particular subfractions are elevated.

Key words: lipids, cardiovascular disease, nutrition, ketogenic, fish oil


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 ACKNOWLEDGMENTS
 REFERENCES
 
There is now strong evidence indicating that elevated triacylglycerol-rich lipoproteins in the fasted [1] and postprandial [2–5] state are independent risk factors and strong predictors of future cardiovascular complications. A recent meta-analysis of prospective studies indicated that an increase in fasting triacylglycerols is associated with a significant increase in the incidence of cardiovascular disease in men and women, and the effect is independent of HDL cholesterol concentrations [1]. The atherogenicity of triacylglycerol-rich lipoproteins in the postprandial state may play a greater role than fasting concentrations in coronary artery disease (CAD) risk and has been the topic of several recent studies and review articles [6–10].

Abnormal lipemia resulting from an impaired metabolic capacity to remove triacylglycerol-rich lipoproteins from the circulation has recently been recognized as one of a host of abnormalities in plasma lipoproteins, which collectively are referred to as the atherogenic lipoprotein phenotype (ALP) [11]. This condition is also characterized by elevated fasting triacylglycerols, reduced HDL cholesterol and a predominance of the small dense LDL-III subfraction (i.e., Pattern B) as measured by density gradient ultracentrifugation and gradient gel electrophoresis. The prevalence of ALP has been estimated to be as high as one-third of North American middle-aged men [12] and these individuals exhibit a greater than three-fold risk of coronary heart disease [13,14]. Delayed lipolysis of triacylglycerol-rich lipoproteins is hypothesized to be the driving force underlying the lipoprotein abnormalities associated with the ALP. Thus, dietary strategies aimed at reducing cardiovascular risk should address the effects on both fasting and postprandial triacylglycerol-rich lipoproteins.

Dietary regimens that lower fasting triacylglycerols do not always result in reductions in postprandial lipemia [15]. Thus, dietary strategies may need to be combined with nutritional supplementation in order to produce the best overall effect on both fasting and postprandial triacylglycerol responses. High-fat diets have been shown to lower fasting triacylglycerols [16–20], primarily mediated via upregulation of lipoprotein lipase (LPL) activity [16,17,21]. Supplementation with n-3 fatty acids has also been shown to reduce triacylglycerols in the fasted [22] and postprandial [23–26] states, presumably mediated via a reduction in hepatic synthesis of triacylglycerol-rich lipoproteins [27–31]. To date, no studies have examined the combined effects of a high-fat/low-carbohydrate diet and n-3 fatty acid supplementation on fasting and postprandial triacylglycerols. The primary purpose of this investigation was to examine the influence of an eight-week low-carbohydrate diet supplemented with n-3 fatty acids on fasting and postprandial lipoproteins. The primary objective was to determine to what magnitude fasting and postprandial triacylglycerols could be modified using both dietary manipulations (i.e., low-carbohydrate) and nutritional supplementation (i.e., n-3 fatty acids) in healthy men with low-to moderate triacylglycerol concentrations.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Ten Caucasian men free of metabolic and endocrine disorders volunteered to participate in this free-living dietary intervention. The physical characteristics of the subjects and fasting serum lipoprotein concentrations are presented in Table 1. Subjects demonstrated serum total cholesterol concentrations ranging from 2.28 to 6.41 mmol/L and triacylglycerol concentrations from 0.69 to 2.72 mmol/L. The subjects had not lost or gained weight in the previous year, were not adhering to special diets or regular consumers of nutritional supplements and habitually consumed between 29% and 39% of energy as fat (assessed via a seven-day food diary). All subjects were nonsmokers and not prescribed any medication known to affect serum lipoproteins. Three subjects were sedentary, one subject ran two times per week (30 min), one subject swam three times per week, one subject performed push-ups/sit-ups five times per week, one subject performed resistance exercises four times per week (45–60 min), and the remaining three subjects performed a combination of running (4–5 times per week) and resistance exercise (2–5 times per week). Subjects had been performing these exercise routines for at least six months prior to the study and maintained their exercise program throughout the entire experimental period (assessed via activity records). All subjects were informed of the purpose and possible risks of this investigation prior to signing an informed consent document approved by the Institutional Review Board at Ball State University.


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  		Table 1. Baseline Physical Characteristics and Fasting Serum Lipoprotein Concentrations1

 
Experimental Design
All subjects participated in an eight-week free-living dietary intervention between the months of February and mid-April. Prior to the dietary intervention, each participant’s habitual diet was determined via a seven-day food diary. A 12-hour fasting blood sample was collected before and after one week of habitual diet and on two consecutive days after 2, 4, 6, and 8 weeks of the intervention diet. Postprandial responses to a fat-rich test meal were measured prior to and after the intervention diet. Subjects reported once per week to the laboratory to meet with a registered dietitian, collect food and supplements, review dietary records and monitor body weight.

Dietary Intervention
The aim of the intervention diet was to increase fat and reduce carbohydrate intake to 5% to 10% of energy. The diet was designed so that fat composed 65% to 70% of energy, primarily from MUFA (40% to 45% of energy), and was relatively low in SFA (<15% energy). Dietary cholesterol was also restricted to <500 mg/day. The actual diets consumed comprised mainly lean beef (e.g., hamburger, steak), poultry (e.g., chicken, turkey), fish, canola and olive oils, various nuts/seeds and peanut butter, moderate amounts of vegetables, salads with low-carbohydrate dressing, moderate amounts of cheese, egg substitute (one whole egg per day), protein powder and water or low-carbohydrate diet drinks. Foods avoided or consumed infrequently included fruits and fruit juices, most dairy products with the exception of hard cheeses and heavy cream, breads, cereals, beans, rice, desserts/sweets or any other foods containing significant amounts of carbohydrate. A portion of the foods consumed during the intervention diet was provided to subjects during weekly meetings to review compliance with the registered dietitian and included various nuts/seeds and an egg white protein powder (Egg FuelTM, Twin Laboratories, Hauppauge, NY), containing 460 kJ, 25 g protein, and 2 g carbohydrate per serving. Subjects were also provided with and required to consume 2.5 g/day of a fiber supplement blend (Fibersol® Capsules, Twin Laboratories, Hauppauge, NY), 2.5 g/day of n-3 fatty acids (1.8 g EPA and 0.7 g DHA) from fish oil concentrate (Dale Alexander® TwinEPA, Twin Laboratories, Hauppauge, NY) and a daily multi-vitamin/mineral complex (Daily One Caps With IronTM, Twin Laboratories, Hauppauge, NY).

Each subject received individual dietary instruction weekly by the same registered dietitian on how to consume meals within the specified nutrient goals and to assess achievement of the desired nutrient profile. To enhance dietary compliance, subjects were provided with a packet outlining specific lists of appropriate foods, recipes and sample meal plans that were compatible with their individual preferences and the nutrient profile goals of the intervention diet. Food measuring utensils and scales were provided to subjects prior to the study to assist in the documentation of all foods and beverages consumed during the experimental period. Each day of the intervention diet for all subjects was analyzed for nutrient content using Nutritionist IV, Version 4 nutrient analysis software (N-Squared Computing, First Databank Division, The Hearst Corporation, San Bruno, CA). All intervention foods and supplements were entered into the software database and included in the analysis of nutrient intake. Additionally, dietary compliance was monitored using ketostix® reagent strips (Bayer Corporation, Elkhart, IN), which determine qualitatively the presence of acetoacetic acid in urine. Each subject maintained a record of color changes on the reagent strips performed daily at approximately 8:00 PM.

Blood Collection
Fasting blood samples were obtained after a 12-hour overnight fast and abstinence from alcohol and strenuous exercise for 24 hours prior to collection of blood. Subjects reported to the laboratory between 7:00 AM and 10:00 AM, rested quietly for 10 minutes in the supine position, and blood was obtained from an antecubital vein with a 20 gauge needle and vacutainers. To assess postprandial lipoprotein responses to a fat challenge, subjects reported to the laboratory after a 14-hour overnight fast between 6:00 AM and 9:00 AM and a flexible teflon cannula was inserted into a forearm vein. The cannula was kept patent with a constant saline drip (60 mL/hour). Subjects rested in a seated position for 10 minutes, and two baseline blood samples were obtained separated by 10 minutes with a 10 mL syringe. The first 3 mL of blood withdrawn was discarded to avoid dilution of the sample and approximately 10 mL was subsequently withdrawn and processed. The test meal was then consumed under supervision to ensure that the entire meal was ingested within a 15-minute period. The meal was designed to be rich in fat and low in carbohydrate, similarly to the intervention diet. The test meal was formulated with whipping cream, sugar-free vanilla pudding, canola oil and macadamia nuts and contained 5.44 MJ, 11% carbohydrate, 3% protein, 86% fat, 52 g SFA, 59 g MUFA, 12 g PUFA and 276 mg cholesterol. Postprandial blood samples were obtained immediately after the meal and hourly for a total of eight hours to assess the magnitude and time course of postprandial lipemia. Subjects rested quietly in a seated position and consumed exactly one liter of water only during the eight-hour postprandial period. Exactly the same protocol was performed after the eight-week intervention diet. All subjects completed the entire meal and no adverse side effects (stomach distress, nausea or the like) were reported.

Blood Analyses
Fasting and postprandial whole blood samples were collected into 10 mL vacutainer tubes with a clot activator. Within 15 minutes, whole blood was centrifuged at 3000g for 15 minutes at 10°C and the resultant serum divided into aliquots and immediately stored frozen at -80°C. Serum samples were later thawed (within one month) for determination of total cholesterol, LDL cholesterol, HDL cholesterol and triacylglycerols using automated techniques (Cobas Mira, Roche Diagnostics, Indianapolis, IN). Concentrations of LDL cholesterol were calculated according to the method of Friedewald et al. [32]. In addition, a routine clinical laboratory screening panel was performed biweekly to assess serum glucose, albumin, total protein, minerals (sodium, potassium, chloride, calcium, phosphorus), renal function (blood urea nitrogen, uric acid, creatinine, total bilirubin) and liver function (alkaline phosphatase, alanine aminotransferase, asparate aminotransferase, gamma glutamyl transferase, lactate dehydrogenase). Percent hematocrit was determined from baseline and eight-hour postprandial samples so that changes in plasma volume could be estimated [33]. Mean plasma volume shifts were less than 3% during the postprandial period. Serum ß-hydroxybutyrate concentrations were enzymatically determined in triplicate using a commercially available kit (Sigma Diagnostics) and spectrophotometric analysis (Spectronic 601, Milton Roy Co., Rochester, NY). Intra- and inter-assay variances were 5.9% and 3.2%, respectively.

Statistical Analyses
Two fasting samples were obtained for each blood variable, and the mean of these two values used for statistical analysis. A one-way repeated-measures analysis of variance (ANOVA) was used to evaluate changes in fasting biochemical indices over time. When a significant F value was achieved, the Fisher’s LSD test was used to locate the pairwise differences between means. Analysis of covariance (ANCOVA) was performed on serum lipid responses using the change in body weight as the covariate. Dependent t tests were used to evaluate differences in corresponding postprandial time points before and after the experimental diet. The total area (serum concentration x time) under the line connecting postprandial lipoprotein and glucose values was calculated using the trapezoidal method. In addition, postprandial triacylglycerol responses, normalized to the zero-hour (i.e., immediately after the meal) concentration, were calculated as defined by two lines (i.e., one connecting the 0-, 2-, 4-, 6-, and 8-hour concentrations and one originating from the zero-hour value parallel to the x-axis). Relationships between variables were examined using Pearson’s product-moment correlation coefficient. The level of significance was set at p < 0.05.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
ACKNOWLEDGMENTS
 REFERENCES
 
Daily dietary nutrient intake prior to the experimental period and during the intervention diet are presented in Table 2. Dietary energy and all dietary nutrients were significantly different during the intervention diet, with the exception of SFA and PUFA (g/day), cholesterol, and alcohol. Compared to the habitual diet, protein, fat, MUFA and n-3 fatty acids were significantly greater, whereas total energy, carbohydrate and dietary fiber were significantly lower during the intervention diet. There was a significant decrease in body weight evident after two weeks (-2.0 kg) that continued for the duration of the study. Mean weight loss after the intervention diet was -4.2 kg.


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  		Table 2. Daily Intake of Dietary Energy and Nutrients1

 
Serum ß-hydroxybutyrate concentrations were 0.02 mmol/L at baseline and significantly increased after two weeks (0.91 mmol/L) and remained significantly elevated for the duration of the study. All subjects demonstrated ß-hydroxybutyrate concentrations above 0.50 mmol/L, indicating compliance with the restricted carbohydrate diet. Ketosis was also supported by color changes recorded daily from reagent strips that detect for the presence of urinary acetoacetic acid (data not shown).

There were no changes in fasting serum glucose concentrations during the intervention diet. While transient changes occurred in several variables, fasting blood urea nitrogen (BUN), the BUN/creatinine ratio, alkaline phosphatase (ALP) and gamma glutamyl transferase (GGT) were the only variables that were significantly different from baseline values at the completion of the intervention diet. Both ALP and GGT were significantly decreased after two weeks (-9% and -12%, respectively) and remained below baseline values after the intervention diet (-11% and -29%, respectively). Serum asparate aminotransferase (AST) was significantly higher after two weeks (29%) and then returned to baseline values after the intervention diet. Serum sodium concentrations were significantly increased after two weeks (1%), remained elevated for six weeks and then returned toward baseline. Phosphorus, potassium and calcium concentrations were stable over the entire intervention diet except for a small increase observed at two weeks for phosphorus (8%) and at six weeks for potassium (5%). Serum albumin and total bilirubin concentrations were also stable except for an increase noted at week six (4% and 43%, respectively). Uric acid, BUN and the BUN/creatinine ratio were significantly elevated after two weeks (25%, 36% and 51%, respectively). Uric acid returned to normal values by the end of the intervention diet, whereas BUN and the BUN/creatinine ratio remained elevated for the duration of the study.

Fasting total, LDL and HDL cholesterol are presented in Table 3. There were no significant changes in fasting serum total cholesterol concentrations. Individual responses in total cholesterol were variable, decreasing in four subjects (range -0.19 to -1.25 mmol/L) and increasing in six subjects (range 0.10 to 1.16 mmol/L). Fasting LDL cholesterol concentrations significantly increased after two (12.5%) and four (20.9%) weeks and then started to decline back toward values that were not significantly different than baseline at the end of the intervention diet. LDL cholesterol decreased in four subjects (range -0.01 to -0.88 mmol/L) and increased in six subjects (range 0.44 to 1.24 mmol/L). Despite the apparent rise in mean fasting HDL cholesterol after eight weeks of the intervention diet (10.4%), no significant changes were detected (p = 0.077). Serum HDL cholesterol decreased in only one subject (-0.06 mmol/L) and increased in nine subjects (range 0.01 to 0.59 mmol/L). The ratio of total cholesterol to HDL cholesterol was not significantly affected. The ratio of LDL cholesterol to HDL cholesterol was significantly elevated after two and four weeks, but returned to values not significantly different than baseline after six weeks. Fasting serum triacylglycerols were significantly reduced after two weeks (-30.0%) and continued to significantly decline after four weeks (-31.9%) and six weeks (-46.2%). By the end of the intervention diet, fasting triacylglycerols decreased by -54.9% (Fig. 1). All ten subjects demonstrated a decrease in fasting triacylglycerols (range -0.01 to -2.20 mmol/L).


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  		Table 3. Fasting Serum Lipoprotein Responses1

 


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  		Fig. 1. Fasting serum triacylglycerol responses to an eight-week low-carbohydrate diet supplemented with n-3 fatty acids. Mean ± SE; n = 10. *p <= 0.05 from Wk 0 value.

 
All subjects tolerated the fat-rich test meal and there were no complaints of stomach distress or any other adverse symptomology. Postprandial serum glucose concentrations were relatively stable over the postprandial period (low 3.91 mmol/L and high 5.24 mmol/L). Postprandial glucose concentrations were lower at several time points during the postprandial period after the intervention diet except for a small elevation observed one hour after the fat-rich meal (Fig. 2). Compared to baseline, postprandial total area under the curve for serum glucose was significantly lower after the intervention diet (37.69 ± 2.70 vs. 34.66 ± 2.54 mmol/L x hrs). Postprandial serum triacylglycerol concentrations are shown in Fig. 3. Mean triacylglycerol concentrations peaked two or three hours postprandially and thereafter declined toward fasting values. Peak postprandial triacylglycerol concentrations after the intervention diet were significantly lower (3.15 vs. 1.85 mmol/L). Postprandial triacylglycerol area under the curve was also significantly lower after the intervention diet (22.61 vs. 11.86 mmol/L x hrs), even when normalized to zero-hour concentrations (8.82 vs. 6.22 mmol/L x hrs).



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  		Fig. 2. Serum glucose responses to a high-fat test meal before (closed circles) and after (open circles) an eight-week low-carbohydrate diet supplemented with n-3 fatty acids. Mean ± SE; n = 10. *p <= 0.05 between corresponding values.

 


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  		Fig. 3. Serum triacylglycerol responses to a high-fat test meal before (closed circles) and after (open circles) an eight-week low-carbohydrate diet supplemented with n-3 fatty acids (upper panel). Serum triacylglycerol concentrations normalized to the zero-hour value (lower panel). Mean ± SE; n = 10. *p <= 0.05 between corresponding values.

 
Analysis of covariance using the decrease in body weight as the covariate did not significantly influence the fasting or postprandial serum lipoprotein responses, nor were the changes in body weight significantly associated with fasting or postprandial lipoproteins. Relationships between variables are shown in Table 4. There was a significant relationship between baseline values in fasting and postprandial triacylglycerols and the corresponding changes in these variables (those with higher starting levels decreased more). Postprandial lipemia was significantly positively related to fasting triacylglycerol concentrations before and after the intervention diet. The changes in fasting triacylglycerols and postprandial lipemia were significantly related. The only dietary nutrient significantly related to any triacylglycerol value was intake of n-3 PUFA and the change in postprandial lipemia.


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  		Table 4. Pearson Product Moment Coefficient Correlation Matrix among Serum Triacylglycerols and n-3 Fatty Acids

 

   DISCUSSION
 
The results of this study demonstrate that adaptation to a hypocaloric ketogenic diet rich in n-3 fatty acids resulted in significant reductions in fasting (-55%) and postprandial (-48%) serum triacylglycerols. The design of the study did not allow us to isolate what part of the dietary intervention (low-carbohydrate or n-3 fatty acids) contributed to this finding. Increasing dietary fat to between 45% and 50% of total energy has been shown to reduce fasting triacylglycerols up to 30% [17,18]; increasing dietary fat further (65% to 70% of total energy) does not offer additional benefits [16,34]. Supplementation with n-3 fatty acids also lowers fasting triacylglycerols. Harris [22] summarized the results from 29 parallel and 36 cross-over studies, examining the influence of moderate n-3 PUFA supplementation on TG. The results showed a 25% to 30% reduction in fasting triacylglycerol concentrations. Thus, the reductions in fasting and postprandial triacylglycerols observed in this study are greater than would be predicted from either carbohydrate restriction or n-3 PUFA supplementation alone, suggesting a composite effect of the intervention on triacylglycerol metabolism.

Weight loss may have also impacted the triacylglycerol response in this study. A meta-analysis of over 70 studies concluded that a kilogram reduction in body weight is associated with a 0.015 mmol/L decrease in serum triacylglycerols [35]. Extrapolated to this study, the -4.2 kg decrease in body weight would be predicted to reduce triacylglycerols by approximately -0.063 mmol/L, accounting for only 8% of the actual -0.756 mmol/L reduction in triacylglycerols. This is consistent with our finding that the change in body weight, when used as a covariate, did not influence the triacylglycerol response.

In a study very similar to the diet intervention in this investigation, moderately overweight normolipidemic subjects consumed a hypocaloric very low-carbohydrate (<10 g/day)/high-fat (rich in SFA) diet for eight weeks [34]. The caloric deficit was almost exactly the same as this study (-2.25 vs. -2.23 MJ/day); however, weight loss was less in this study (-4.2 vs. -7.7 kg). Fasting triacylglycerols, total cholesterol, LDL cholesterol and HDL cholesterol changed -33%, +6%, +18% and -6%, respectively [34]. Corresponding changes in this study were -55%, +2%, +10% and +10%. Thus, in comparison to the findings of Larosa et al. [34], subjects consuming the hypocaloric diet intervention in this study (which was rich in MUFA and supplemented with n-3 PUFA) demonstrated a greater reduction in fasting triacylglycerols, less of an increase in total cholesterol and LDL cholesterol and a greater increase in HDL cholesterol.

Studies ranging in duration from 14 to 112 days have shown that increases in n-3 PUFA (1 to 28 g/day) not only reduce fasting triacylglycerols, but also postprandial lipemia in healthy subjects [23–26,36,37]. Roche and Gibney [37] examined the effects of including fish oil supplementation (1 g n-3 PUFA/day) in a group of normolipidemic subjects consuming either a low-fat (16% to 18% of total energy)/low-energy diet or a moderate-fat (33% to 38% of total energy)/eucaloric diet for 112 days. Fish oil-supplemented subjects consuming the low-fat/low-energy diet demonstrated a significant decrease in BMI (4%) and a significant increase in postprandial triacylglycerol AUC (12%). In contrast, subjects consuming the moderate-fat/eucaloric diet maintained body weight and demonstrated a significant decrease in postprandial triacylglycerol AUC (-32%). Thus, the beneficial effects of fish oil on postprandial lipemia may be negated if combined with a high-carbohydrate diet (even if accompanied by weight loss), lending support to a synergistic effect of low-dose n-3 fatty acid supplementation and a high-fat diet.

Several mechanisms could explain the significant reduction in fasting and postprandial triacylglycerol concentrations observed in this study. The two most likely are an attenuation of triacylglycerol secretion and an acceleration of triacylglycerol clearance via LPL. The lower postprandial triacylglycerol concentrations observed in the study by Harris et al. [23] after chronic intake on n-3 PUFA was associated with no change in LPL activity; however, recent work indicated lipase activity may be enhanced with fish oil and contribute to the lower postprandial lipemia [38]. The extremely low-carbohydrate/high-fat nature of the diet in this study may have upregulated LPL activity and triacylglycerol clearance from the circulation, but this was not measured directly. Although Harris et al. [23] failed to detect a significant increase in LPL activity with fish oil supplementation, subjects consumed much less fat compared to this study (30% to 40% vs. 64% of energy). Thus, quantity of dietary fat in the background diet may impact the postprandial triacylglycerol response to n-3 fatty acid supplementation and the mechanism(s) by which postprandial triacylglycerol-rich lipoproteins are metabolized. In support of this hypothesis, several studies have indeed shown that post-heparin LPL activity and skeletal muscle LPL activity are increased after consumption of diets rich in fat (46% to 65% of total energy) in humans [16,17,21]. Further, a diet rich in unsaturated fat, which was the predominant fat in this study, has been shown to increase post-heparin LPL activity more than SFA in the rat [39].

Subjects with higher initial fasting and postprandial triacylglycerols demonstrated the largest reductions in response to the diet. Thus, a logical population that may respond more favorably to a low-carbohydrate diet supplemented with fish oil are individuals with hypertriacylglycerolemia. A significant correlation between n-3 fatty acids intake and the decline in postprandial lipemia was observed, suggesting the possibility of a dose-response relationship. Perhaps, higher levels of n-3 fatty acids may have resulted in even greater improvements in triacylglycerol concentrations.

HDL cholesterol was not significantly increased after the intervention diet (+10%, p = 0.077), and LDL cholesterol was transiently elevated. Although the increase in LDL cholesterol would not be considered positive, there were likely changes in the structure, composition and distribution of both HDL and LDL cholesterol that have been shown to be protective in terms of cardiovascular disease. Regarding HDL cholesterol, high fat diets (50% to 65% of total calories) have been shown to increase larger lipid-rich HDL2 cholesterol concentrations in humans [16,17], and this is the fraction negatively related to cardiovascular disease [40,41]. When switching to a fat-rich diet, the decrease in triacylglycerol concentrations is correlated with the increase in cardio-protective HDL2 cholesterol concentrations [16]. Relative to LDL cholesterol, switching to a fat-rich diet (46% vs. 24% of total calories) was shown to increase large LDL I mass and decrease small dense LDL III cholesterol [17], which has also been shown to be cardio-protective [13,14]. Although ultracentrifugation techniques were not employed to quantify LDL and HDL subfractions, the significantly lower postprandial lipemia observed in this study would be predicted to alter these lipoprotein fractions in a manner that would decrease cardiovascular risk status (i.e., increase lipid-rich HDL2 and LDL I cholesterol subfractions and decrease HDL3 and small dense LDL III).

A significant decrease in energy intake and body weight occurred during the intervention diet, which has also been reported in previous free-living low-carbohydrate diet interventions [34,42]. A reduction in voluntary dietary energy intake may be due to the higher satiety value of fat and protein [43] or the anorectic effect of ketosis [44]. Alternatively, subjects may have simply had difficulty switching from a diet comprising greater than 45% carbohydrate to one less than 10% carbohydrate. Certainly, fewer food choices were available than in their habitual diet.

As expected, there was a small elevation in serum uric acid concentrations at two weeks that gradually returned toward baseline values by the end of the intervention diet. Increased uric acid is a normal response to a low-carbohydrate diet [20,34], resulting from competition between ketones and uric acid for excretion in the renal tubules [45]. Serum BUN concentrations and the BUN/creatinine ratio were elevated during the entire intervention diet, whereas creatinine concentrations remained stable. A disproportionate rise in BUN relative to creatinine suggests dehydration. In addition, there may have also been a greater amount of dietary protein available for hepatic catabolism, leading to increased urea formation during the intervention diet. Krehl et al. [43] observed progressively higher BUN concentrations in healthy men consuming a low-carbohydrate diet that was gradually increased in protein, so that the protein to fat ratio was raised in increments from 30%:70% to 70%:30%. Also, the higher protein content of the diet may have placed an extra solute load on the kidneys necessitating an increase in excretion of water, possibly leading to dehydration and elevated BUN concentrations.

This study shows that eight weeks of a hypocaloric low-carbohydrate (<40 g/day) diet rich in MUFA and supplemented with n-3 fatty acids (2.5 g/day) significantly reduced fasting and postprandial triacylglycerol concentrations in response to a fat-load challenge. The fact that these reductions in triacylglycerols occurred in men who were not hypertriglyceridemic as a group to begin with should also be noted. The significant reductions in fasting and postprandial triacylglycerols observed in this study are probably not the result of a single element of the intervention. Rather, the triacylglycerol response was likely a composite effect of multiple variables, namely n-3 PUFA supplementation and the very-low carbohydrate and high-MUFA content of the diet. This non-drug approach to improving triacylglycerol tolerance may have profound clinical significance in terms of attenuating the development of atherosclerosis; however, prospective studies to support this hypothesis are currently lacking.


   ACKNOWLEDGMENTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 ACKNOWLEDGMENTS
REFERENCES
 
We thank P.M. Kris-Etherton and C.R. Denegar for their assistance in this study. A special thanks to Twin Laboratories (Hauppauge, NY) for providing the nutritional supplements for the study.

Received December 1, 1999. Revised March 1, 2000. Accepted March 1, 2000.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 ACKNOWLEDGMENTS
 REFERENCES
 

  1. Austin MA, Hokanson JE, Edwards KL: Hypertriglyceridemia as a cardiovascular risk factor. Am J Cardiol 81: 7B–12B, 1998.[Medline]
  2. Patsch JR: Triglyceride-rich lipoproteins and atherosclerosis. Atherosclerosis 110(Suppl): S23–S26, 1994.
  3. Patsch JR, Miesenbock G, Hopferwieser T, Muhlberger V, Knapp E, Dunn JK, Gotto AM, Patsch W: Relation of triglyceride metabolism and coronary artery disease: studies in the postprandial state. Thrombosis 12: 1336–1345, 1992.
  4. Ebenbichler CF, Kirchmair R, Egger C, Patsch JR: Postprandial state and atherosclerosis. Curr Opin Lipidol 6: 286–290, 1995.[Medline]
  5. Cohn JS: Postprandial lipemia: emerging evidence for the atherogenicity of remnant lipoproteins. Can J Cardiol 14(Suppl): 18B–27B, 1998.
  6. Karpe F, Hamsten A: Postprandial lipoprotein metabolism and atherosclerosis. Curr Opin Lipidol 6: 123–129, 1995.[Medline]
  7. Karpe F, Steiner G, Uffelman K, Olivecrona T, Hamsten A: Postprandial lipoproteins and progression of atherosclerosis. Atherosclerosis 106: 83–97, 1994.[Medline]
  8. Krauss RM: Atherogenicity of triglyceride-rich lipoproteins. Am J Cariol 81: 13B–17B, 1998.
  9. Hanefeld M, Temelkova-Kurktschiev T: The postprandial state and the risk of atherosclerosis. Diabet Med 14(Suppl): S6–S11, 1997.
  10. Kirchmair R, Ebenbichler CF, Patsch JR: Post-prandial lipemia. Clin Endocrinol Metab 9: 705–719, 1995.
  11. Griffin BA, Zampelas A: Influence of dietary fatty acids on the atherogenic lipoprotein phenotype. Nutr Res Rev 18: 1–26, 1995.
  12. Austin MA, King MC, Vranizan KM, Krauss RM: Atherogenic lipoprotein phenotype. A proposed genetic marker of coronary heart disease risk. Circulation 82: 495–506, 1990.[Abstract/Free Full Text]
  13. Austin MA, Breslow JL, Hennekens CH, Buring JE, Willett WC, Krauss RM: Low density lipoprotein subclass patterns and risk of myocardial infarction. J Am Med Assoc 260: 1917–1921, 1988.[Abstract]
  14. Griffin BA, Freeman DJ, Tait GW, Thomson J, Caslake MJ, Packard CJ, Shepherd J: Role of plasma triglyceride in the regulation of plasma low density lipoprotein (LDL) subfractions: relative contribution of small, dense LDL to coronary heart disease risk. Atheroscler 106: 241–253, 1994.
  15. Zoppo A, Maggi FM, Catapano AL: A successful dietary treatment fails to normalize plasma triglyceride postprandial response in type IV patients. Atheroscler 146: 19–23, 1999.
  16. Jackson RL, Yates MT, McNerney CA, Kashyap ML: Relationship between post-heparin plasma lipases, triglycerides and high density lipoproteins in normal subjects. Horm Met Res 22: 289–294, 1990.[Medline]
  17. Campos H, Dreon DM, Krauss RM: Associations of hepatic and lipoprotein lipase activities with changes in dietary composition and low density lipoprotein subclasses. J Lipid Res 36: 462–472, 1995.[Abstract]
  18. Yost TJ, Jensen DR, Haugen BR, Eckel RH: Effect of dietary macronutrient composition on tissue-specific lipoprotein lipase activity and insulin action in normal-weight subjects. Am J Clin Nutr 68: 296–302, 1998.[Abstract]
  19. Newbold HL: Reducing the serum cholesterol level with a diet high in animal fat. S Med J 81: 61–63, 1988.
  20. Phinney SD, Bistrian BR, Wolfe RR, Blackburn GL: The human metabolic response to chronic ketosis without caloric restriction: physical and biochemical adaptations. Metabolism 32: 757–768, 1983.[Medline]
  21. Kiens B, Essen-Gustavsson B, Gad P, Lithell H: Lipoprotein lipase activity and intramuscular triglyceride stores after long-term high-fat and high-carbohydrate diets in physically trained men. Clin Physiol 7: 1–9, 1987.[Medline]
  22. Harris WS: n-3 Fatty acids and serum lipoproteins: human studies. Am J Clin Nutr 65(Suppl): 1645S–1654S, 1997.[Abstract/Free Full Text]
  23. Harris WS, Conner WE, Alam N, Illingworth DR: Reduction of postprandial triglyceridemia in humans by dietary n-3 fatty acids. J Lipid Res 29: 1451–1460, 1988.[Abstract]
  24. Brown AJ, Roberts DCK: Moderate fish oil intake improves lipemic response to a standard fat meal. Arterioscler Thromb 11: 457–466, 1991.[Abstract/Free Full Text]
  25. Argen JJ, Hanninen O, Julkunen A, Fogelholm L, Vidgren H, Schwab U, Pynnonen O, Uusitupa M: Fish diet, fish oil and docosahexaenoic acid rich oil lower fasting and postprandial plasma lipid levels. Eur J Clin Nutr 50: 765–771, 1996.[Medline]
  26. Tinker LF, Parks EJ, Behr SR, Schneeman BO, Davis PA: (n-3) fatty acid supplementation in moderately hypertriglyceridemic adults changes postprandial lipid and apoliporotein B responses to a standardized test meal. J Nutr 129: 1126–1134, 1999.[Abstract/Free Full Text]
  27. Harris WS, Conner WE, Illingworth DR, Rothrock DW, Foster DM: Effect of fish oil on VLDL triglyceride kinetics in man. J Lipid Res 31: 1549–1558, 1990.[Abstract]
  28. Nestel PJ, Conner WE, Reardon MR, Conner SJ, Wong SH, Boston R: Suppression by diets rich in fish oil of very low density lipoprotein production in men. J Clin Invest 74: 82–89, 1987.
  29. Harris WS, Conner WE, Illingworth DR, Rothrock DW, Foster DM: The mechanism of the hypotriglyceridemic effect of dietary n-3 fatty acids in man. Clin Res 32: 560a, 1984.
  30. Harris WS, Conner WE, Inkeles SB, Illingworth DR: Dietary fish oil inhibits carbohydrate-induced hypertriglyceridemia. Metabolism 33: 1016–1019, 1984.[Medline]
  31. Illingworth DR, Harris WS, Conner WE: Inhibition of low density lipoprotein synthesis by dietary n-3 fatty acids in man. Arteriosclerosis 4: 270–275, 1984.[Abstract/Free Full Text]
  32. Friedewald WT, Levy RI, Fredrickson DS: Estimation of the concentration of low-density lipoprotein cholesterol in plasma without use of the preparative ultracentrifuge. Clin Chem 18: 499–502, 1972.[Abstract]
  33. Van Beaumont W, Underkofler S, van Beaumont S: Erythrocyte volume, plasma volume, and acid-base changes in exercise and heat dehydration. J Appl Physiol 50: 1255–1262, 1981.[Abstract/Free Full Text]
  34. Larosa JC, Fry AG, Muesing R, Rosing DR: Effects of high-protein, low-carbohydrate dieting on plasma lipoproteins and body weight. J Am Diet Assoc 77: 264–270, 1980.[Medline]
  35. Dattilo AM, Kris-Etherton PM: Effects of weight reduction on blood lipids and lipoproteins: a meta-analysis. Am J Clin Nutr 56: 320–328, 1992.[Abstract/Free Full Text]
  36. Weintraub MS, Zechner R, Brown A, Eisenberg S, Breslow J: Dietary polyunsaturated fats of the w-6 and w-3 series reduce postprandial lipoprotein levels. J Clin Invest 82: 1884–1893, 1988.
  37. Roche HM, Gibney MJ: Postprandial triacylglycerolaemia: the effect of low-fat dietary treatment with and without fish oil supplementation. Eur J Clin Nutr 50: 617–624, 1996.[Medline]
  38. Harris WS, Lu G, Rambjor GS, Walen AI, Ontko JA, Cheng Q, Windsor SL: Influence of n-3 fatty acid supplementation on the endogenous activities of plasma lipases. Am J Clin Nutr 66: 254–260, 1997.[Abstract/Free Full Text]
  39. Coffier E, Paris R, Lecerf J: Effects of dietary saturated and polyunsaturated fat on lipoprotein lipase and hepatic triglyceride lipase activity. Comp Biochem Physiol 88B: 187–192, 1987.
  40. Gofman JW, Young W, Tandy R: Ischemic heart disease, atherosclerosis, and longevity. Circulation 34: 679–697, 1966.[Free Full Text]
  41. Katzel LI, Busby-Whitehead J, Rogus EM, Krauss RM, Goldberg AP: Reduced adipose tissue lipoprotein lipase responses, postprandial lipemia, and low density lipoprotein2 subspecies levels in older athletes with silent myocardial infarction. Metabolism 43: 190–198, 1994.[Medline]
  42. Yudkin J, Carey M: The treatment of obesity by the "high-fat" diet. The inevitability of calories. Lancet 2: 939–941, 1960.
  43. Krehl WA, Lopez-S A, Good EI, Hodges RE: Some metabolic changes induced by low carbohydrate diets. Am J Clin Nutr 20: 139–148, 1967.[Abstract]
  44. Bray GA, Davidson MB, Drenick EJ: Obesity: a serious symptom. Ann Int Med 77: 779–795, 1972.
  45. LeCocq FR, McPhaul JJ: The effects of starvation, high fat diets, and ketone infusions on uric acid balance. Metabolism 14: 186–197, 1965.[Medline]



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