The Journal of Nutrition Vol. 128 No. 7 July 1998, pp. 1144-1149

Plasma Lipid Response to Hypolipidemic Diets in Young Healthy Non-Obese Men Varies with Body Mass Index^1,2

Sergio Jansen, Jose Lopez-Miranda, Joaquin Salas, Pedro Castro, Juan A. Paniagua, Fernando Lopez-Segura, Jose M. Ordovas^*, Jose A. Jimenez-Pereperez, Angeles Blanco, and Francisco Perez-Jimenez³

Lipid Research Unit, University Hospital Reina Sofia, University of Cordoba Medical School, Cordoba 14004, Spain and ^* Lipid Metabolism Laboratory, U.S. Department of Agriculture, Human Nutrition Research Center on Aging at Tufts University, Boston, MA 02111

	ABSTRACT

Abstract Introduction Methods Results Discussion References

Lipid response to dietary fat is highly variable among individuals of a population. The aim of this study was to establish whether being overweight is one of the factors that determines this response. Forty-one non-obese healthy men were divided into two groups according to body mass index as follows: controls, <25 kg/m²; overweight, >25 kg/m² but <30 kg/m². After consuming a saturated fat-rich diet (SAT diet: 38% fat, 20% saturated) for 4 wk, subjects were switched to a low fat diet [National Cholesterol Education Program (NCEP)-I diet: 28% fat, 10% saturated] for 4 wk and then to a monounsaturated fat-rich diet (MUFA diet: 38% fat, 22% monounsaturated) for 4 wk. Data were analyzed by Student's t test and two-way ANOVA for repeated measures. After consuming the NCEP-I diet, the overweight subjects had a smaller decrease relative to the SAT diet period in plasma total cholesterol [0.30 vs. 0.67 mmol/L (7 vs. 16%), P < 0.02] and low density lipoprotein-cholesterol concentrations [0.24 vs. 0.55 mmol/L (9 vs. 21%), P < 0.04] than controls. However, in the overweight subjects, the MUFA diet produced a greater decrease in plasma triglycerides than in the controls relative to the SAT diet period [0.36 vs. 0.03 mmol/L (26 vs. 4%), P < 0.006] and to the NCEP-I diet period [0.29 vs. 0.01 mmol/L (22 vs. 1%), P < 0.01). Plasma cholesterol concentrations changed to a lesser extent, and triglyceride concentration to a greater extent, in overweight but non-obese young men than in those of normal weight in response to changes in dietary fat composition. Our data suggest that in the diet treatment of obese hyperlipemic subjects, it is more important for them to lose weight than to change the fat composition of their diets.

	INTRODUCTION

Abstract Introduction Methods Results Discussion References

Cardiovascular disease is the main cause of death in developed countries; in many cases, it is caused by arteriosclerosis. This process has a multifactorial etiology, and hypercholesterolemia is one of the most important factors involved. In turn, diet is the principal exogenous determinant of plasma cholesterol concentration. However, lipid response to a specific diet is highly variable among individuals of a population and is determined by a number of factors. It has been shown to be partly genetically controlled (Abbey 1992, Jansen et al. 1997, Lopez-Miranda et al. 1994) and also to be influenced by sex and age (Clifton and Nestel 1992, Cobb et al. 1993), and other clinical and biological determinants (Denke 1995).

Although the correlation between body weight or body mass index (BMI⁴; kg/m²) with plasma lipids levels has been shown in several studies (CARDIA Study 1991, Wattigney 1991), the influence of these variables on lipid response to diets of varying fat content has not been well studied. Therefore we decided to examine this relationship to determine whether overweight, without obesity, causes a different response to a specific diet change.

	SUBJECTS AND METHODS

Abstract Introduction Methods Results Discussion References

Subjects and diets.  Forty-one healthy young men aged 21 ± 2 y (means ± SD) were recruited from among students at the University of Cordoba, Spain. Before enrollment, subjects had a comprehensive medical history and physical examination that included measurement of blood pressure, weight, triceps skinfold thickness, and body mass index and a clinical chemistry analysis. Inclusion criteria were as follows: plasma total cholesterol and triglyceride concentrations <5.7 mmol/L and 3.4 mmol/L, respectively, BMI < 30 kg/m² and no evidence of chronic disease (renal, thyroid or hepatic).

Dietary information, including alcohol consumption, was collected for seven consecutive days. None of the subjects was using any medication that could affect lipid levels. Individual energy requirements were calculated by the Harris-Benedict equation, taking into consideration each subject's physical activity. All participants were encouraged to maintain their regular physical activity and lifestyle, and were asked to record in a diary any event that could affect the outcome of the study, such as stress, change in smoking habits and alcohol consumption, or intake of foods not included in the experimental design.

Subjects were allocated to one of two groups on the basis of BMI: those of normal weight with a BMI < 25 kg/m² (controls) and overweight subjects with a BMI > 25 kg/m², according to the definition of Bray (1989).

The study protocol was approved by the Human Investigation Committee at the University Hospital Reina Sofia. Informed consent was obtained from all participants.

The study design included an initial 28-d period during which all subjects consumed a saturated fat-rich diet (SAT diet), calculated to contain 15% of energy as protein, 47% as carbohydrate and 38% as fat (20% saturated, 12% monounsaturated and 6% polyunsaturated). The second diet period lasted 28 d, and all subjects consumed a low fat diet [National Cholesterol Education Program (NCEP-I) diet], containing 15% of energy as protein, 57% as carbohydrate and 28% as fat (10% saturated, 12% monounsaturated and 6% polyunsaturated). The third diet period also lasted 28 d, and all subjects consumed a monounsaturated fat-rich diet (MUFA diet) with 15% of energy as protein, 47% as carbohydrate and 38% as fat (22% monounsaturated, 10% saturated and 6% polyunsaturated). The cholesterol content was constant during the three periods, ~27.5 mg/1000 kJ. Subjects were weighed weekly and their energy intakes were modified such that they maintained their body weights.

A computerized database, created with the use of U.S. (Human Nutrition Information Service 1987) and European (Varela 1980) food-composition data, was used to calculate nutrient composition and to design the menus for each diet period. Palm oil and virgin olive oil were used for cooking, salad dressing, and spread during the SAT and MUFA periods, respectively. All meals were prepared in the hospital kitchen and were consumed in the dining room every afternoon and evening under the supervision of a dietitian. Breakfast and an afternoon coffee break were also prepared in the hospital kitchen and consumed by each subject at home. A homogenate of the meals of seven consecutive days in each dietary period, including breakfast and the afternoon coffee break, were collected, stored at 70°C and analyzed for protein, fat, cholesterol and carbohydrate content. Results of the analysis of dietary composition are shown in Table 1; results were in agreement with calculated values. To check for compliance with the diet treatments, fatty acids in LDL cholesteryl esters were determined at the end of each diet period by the method described by Ruiz-Gutierrez et al. (1993).

Lipid analysis.  Venous blood samples were collected into EDTA-containing (1 g/L) tubes from all subjects after a 12-h overnight fast at the beginning of the study and at the end of each dietary period. Plasma was obtained by low speed centrifugation at 789 × g for 15 min at 4°C within 1 h of venipuncture. To reduce interassay variation, plasma was stored at 70 °C and analyzed at the end of the study. Plasma total cholesterol (Allain et al. 1974) and triglyceride (Fossati and Prencipe 1982) were determined by enzymatic techniques. High density lipoprotein (HDL) cholesterol was determined after precipitation of apolipoprotein (apo) B-containing lipoproteins by phosphotungstic acid (Assman et al. 1983). Apo A-I and B were determined by immunoturbidimetry (Riepponen et al. 1987). The low density lipoprotein (LDL) cholesterol concentration was calculated by using the Friedewald formula (Friedewald et al. 1972).

Statistical analysis.  CSS (Statsoft, Tulsa, OK) was the statistical package used. Before further analyses, normal distribution of the variables was checked with the Kolmogorov-Smirnov test (Zar 1984). On the basis of this test, all of the continuous variables, except triglyceride, followed a normal distribution. Triglyceride data were transformed logarithmically to achieve distributions close to normal, and statistical tests were applied to the transformed variable. Student's t test was used to compare initial biochemical and anthropometric variables and increases in lipid levels after the different diet periods between the two groups. Two-way ANOVA for repeated measures was used to study the effects of BMI and diets on plasma concentrations of the different lipids. When significant F-tests were obtained, Tukey's test (Kleinbaum and Kupper 1978) was applied for the post-hoc comparison. Simple linear correlation analysis was also applied. Statistical significance was considered at P < 0.05. Values in the text are means ± SD.

	RESULTS

Abstract Introduction Methods Results Discussion References

Characteristics of the two groups of participants are shown in Table 2. All of the anthropometric variables used to quantify overweight were significantly higher in the group with the greater BMI. These patients also had significantly higher plasma total cholesterol and triglyceride levels, and lower HDL cholesterol concentrations than controls. Body weight was maintained throughout the study in both groups (control group: SAT, 69.9 ± 5.8 kg; NCEP-I, 69.5 ± 5.9 kg; MUFA, 69.6 ± 5.9; overweight group: SAT, 84.3 ± 8.1 kg; NCEP-I, 84.1 ± 8.1 kg; MUFA 84.4 ± 7.9 kg).

Plasma LDL cholesterol ester composition after each dietary period is shown in Table 3. Significant enrichment was observed in palmitic acid during the SAT diet period compared with the NCEP-I and MUFA diet periods. There was also a significant enrichment in oleic acid during the MUFA diet period compared with the NCEP-I diet. These differences suggest good dietary compliance.

Lipid and apoprotein concentrations in control and overweight subjects after the three dietary periods are shown in Table 4. Changes in diet produced significant changes in plasma total cholesterol, LDL cholesterol, HDL cholesterol, triglycerides and apo A-I and B concentrations. BMI classification affected plasma HDL cholesterol, triglyceride and apo A-I concentrations. The interaction between BMI classification and diet was significant for total cholesterol, LDL cholesterol and triglyceride concentrations. After consumption of the SAT diet, overweight individuals had higher plasma triglyceride (P < 0.0002) and lower HDL cholesterol (P < 0.0002) and apo A-I (P < 0.003) concentrations than control subjects. After the NCEP-I diet, overweight subjects had higher concentrations of total cholesterol (P < 0.004), triglycerides (P < 0.0002) and apo B (P < 0.003), and lower HDL cholesterol concentrations (P < 0.0003) than controls. After the MUFA diet, the overweight group had higher triglyceride (P < 0.009) and apo B levels (P < 0.03) and lower concentrations of HDL cholesterol (P < 0.02) and apo A-I (P < 0.003).

In controls, replacement of saturated fats (SAT diet) by carbohydrates (NCEP-I diet) produced a significant decrease in total cholesterol (P < 0.0002), LDL cholesterol (P < 0.0002), HDL cholesterol (P < 0.05) and apo A-I (P < 0.0002) and B (P < 0.0002). In contrast, in overweight subjects, only plasma total cholesterol and apo B levels decreased significantly (P < 0.008 and P < 0.02, respectively). After consuming the MUFA, compared with the SAT diet, controls exhibited significant decreases in total cholesterol (P < 0.0002), LDL cholesterol (P < 0.0002) and apo A-I (P < 0.05) and B (P < 0.0002). In the overweight group, the MUFA diet produced significant decreases in total cholesterol (P < 0.002), triglycerides (P < 0.0008) and apo B (P < 0.05), compared with the SAT diet, and a significant decrease in triglycerides (P < 0.008) compared with the NCEP-I diet.

The differences in lipid level response in subjects with BMI greater and less than 25 kg/m² and the percentage change in these levels are shown in Table 5. When subjects switched from the SAT to the NCEP-I diet, overweight subjects had significantly smaller decreases in plasma total and LDL cholesterol concentrations. The MUFA diet produced a significantly greater decrease in triglyceride levels in the overweight group than in controls compared with values in the groups when they consumed the SAT and NCEP-I diets.

We analyzed the correlation between anthropometric measures (BMI, weight, triceps skinfold thickness and waist-hip ratio) and changes in lipid levels after consumption of the different diets. After the NCEP-I diet, compared with the SAT diet, there were significant (P < 0.05) inverse correlations between weight and the decrease in total cholesterol (r = 0.43), LDL cholesterol (r = 0.33) and apo B (r = 0.42) concentrations, and between BMI and the decrease in total cholesterol (r = 0.35) and apo B levels (r = 0.41). BMI (r = 0.38) and the waist/hip ratio (r = 0.45) were correlated significantly with the decrease in triglyceride concentrations that occurred when subjects switched from the NCEP-I to the MUFA diet. The waist/hip ratio was also correlated with the changes in triglyceride (r = 0.38) and LDL cholesterol levels (r = 0.33) that occurred when subjects switched from the SAT to the MUFA diet. The triceps skinfold thickness was not correlated with changes in plasma lipid changes.

	DISCUSSION

Abstract Introduction Methods Results Discussion References

The main objective of this study was to assess the influence of overweight, without obesity, on the degree of response to hypolipemic diets. We have shown that subjects with a greater BMI have a lower decrease in plasma total and LDL cholesterol than normal weight individuals when consuming hypolipemic diets.

The independent effect of obesity on the risk of coronary heart disease is controversial, because of its frequent association with other risk factors (Kaplan 1989). However, it has been demonstrated that obesity, and in particular abdominal obesity, is clearly related to hyperinsulinemia, glucose intolerance, noninsulin-dependent diabetes mellitus, hypertension and dislipoproteinemia, and especially to high triglyceride and low HDL cholesterol concentrations (Després 1991, Foster et al. 1987). All of these factors have been related to insulin resistance and increased flux of free fatty acids to the liver (Björntorp 1990, Després et al. 1990). In our study, overweight subjects had significantly higher total cholesterol concentrations than normal weight individuals when consuming their usual diets. However, there were no differences between groups in this variable after consumption of the SAT diet. This lack of difference is difficult to clarify because basal diet data were collected when participants consumed their usual (not controlled) diets, whose exact composition was unknown to us. After consumption of the SAT diet, overweight subjects had significantly higher concentrations of triglycerides and lower levels of HDL cholesterol than controls. In addition, after the NCEP-I diet, the decreases in total and LDL cholesterol concentrations were lower in overweight subjects. The predicted decrease in total cholesterol levels, according to the equation in Keys et al. (1965), in subjects with a BMI < 25 kg/m² was as expected (0.67 mmol/L observed vs. 0.7 mmol/L expected), whereas in subjects with BMI > 25 kg/m², this was lower than expected (0.31 mmol/L). These results are in agreement with those of another study in obese individuals, who switched from a SAT diet to an isocaloric diet that was low in saturated fats (Leenen et al. 1993). It has been suggested that obesity could reduce the response to dietary changes (Cole et al. 1992, Goff et al. 1993). This is also supported by the simple correlation analysis of our data in which BMI was inversely correlated with decreases in total cholesterol and apo B concentrations after the consumption of the NCEP-I diet compared with the SAT diet. Additionally, only in subjects with normal weight did the consumption of hypolipemic diets decrease LDL cholesterol in comparison with the SAT diet.

During the three dietary periods, overweight subjects had higher triglyceride and lower HDL cholesterol concentrations than normal weight subjects. Higher triglyceride levels in obese subjects have been attributed to increased flux of fatty acids to the liver, producing increased synthesis and secretion of very LDL (VLDL) (Björntorp 1990). The higher plasma VLDL levels resulting from this increased production and the decreased catabolism of the triglyceride-rich particles lead to an increased exchange of VLDL triglycerides with LDL and HDL cholesterol esters (Egusa et al. 1985). This exchange can induce an enrichment in LDL and HDL triglycerides with reduced levels of HDL cholesterol and formation of dense and atherogenous LDL particles (Després et al. 1989).

In overweight, but not in normal subjects, the MUFA diet marked decreased plasma triglyceride levels, compared with the SAT diet (26%) and the NCEP-I diet (22%). It has been suggested that overweight subjects could be hypersensitive to carbohydrate-rich diets such that these diets increase their triglyceride levels to a greater extent than in thinner individuals (Cole et al. 1992). Both BMI and the waist/hip ratio were directly correlated with the decrease in triglyceride levels between the NCEP-I and the MUFA diet periods, and the waist/hip ratio was also correlated with the decrease in this variable between the SAT and MUFA diet periods. These data suggest that overweight subjects may be more sensitive to decreases in triglyceride concentrations due to dietary monounsaturated fats than subjects with normal weight. This sensitivity may be related to changes in insulin levels and insulin resistance, because it has been reported that monounsaturated fat-rich diets decrease insulin levels (Espino-Montoro et al. 1996). In our study, overweight subjects had decreased total and LDL cholesterol concentrations, whereas those of control subjects increased when they switched from the NCEP-I to the MUFA diet, although differences were not significant in any case.

Our results suggest that overweight subjects have a lower response in total and LDL cholesterol concentrations after consuming hypolipemic diets. However, these subjects appear to be more susceptible to changes in their triglyceride levels when they consume a monounsaturated fat-rich diet. The beneficial effect of weight loss on plasma cholesterol levels is greater than that produced by a decrease in dietary saturated fats (Leenen et al. 1993). Similarly, it has been suggested that in obese subjects, a decrease in dietary saturated fats is even more beneficial for serum cholesterol levels after weight loss (Goff et al. 1993), and several large-scale dietary trials have shown that reductions in serum (Caggiula et al. 1981) and plasma (Gordon et al. 1982) total cholesterol induced by dietary changes were enhanced by weight reduction. Consequently, in the diet treatment of obese hyperlipemic subjects, it is more important for them to lose weight than to change the fat composition of their diets. Our data suggest that even in overweight, but not obese, young men, the plasma cholesterol response to hypolipemic diets is less than that in subjects with normal weight. Furthermore, our data also suggest that overweight young men benefit more from a diet rich in monounsaturated fats than a NCEP-I type diet, given that both diets reduce plasma total cholesterol and apo B concentrations, but the latter also significantly decrease triglyceride levels.

	FOOTNOTES

Manuscript received 7 July 1997. Initial reviews completed 18 September 1997. Revision accepted 4 March 1998.

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