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Dietary Fiber and Lifestyle Influence Serum Lipids in Free Living Adult Men

Department of Human Nutrition, Centro de Investigación en Alimentación y Desarrollo, A.C. Hermosillo, Sonora, MÉXICO

Address reprint requests to: Martha Nydia Ballesteros, MS, Carretera a la Victoria Km 0.6., Hermosillo, Sonora, 83000 MEXICO. E-mail: nydia{at}cascabel.ciad.mx.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Objective: The aim of this study was to determine the effect of dietary fiber consumption and lifestyle on serum lipids in adult men with non-restricted diet and physical activity.

Methods: Two groups of 19 men were classified as high (48 g/day) and low fiber groups (27 g/day). Anthropometry, food frequency, daily weighed intakes and physical activity were done for a seven-day period. Fasting blood was collected and serum was analyzed for triglycerides, total cholesterol and lipoprotein cholesterol fractions.

Results: Crude correlation coefficients showed that total cholesterol was negatively associated with physical activity, total dietary fiber and P/S ratio (r = -0.52; p < 0.001, r = -0.44; p < 0.01, r = -0.51, p < 0.001). LDL-C was also correlated negatively with total dietary fiber and P/S ratio (r = -0.34, p < 0.03; r = -0.53, p < 0.01). It was also positively associated with dietary cholesterol and body weight (r = 0.34, p < 0.03; r = 0.31, p < 0.05). Serum triglycerides had an inverse association with total dietary fiber and physical activity (r = -0.30; p < 0.05; r = -0.45, p < 0.004). After controlling for energy intake, total fat, saturated fat, dietary cholesterol, physical activity and body mass index, LDL-C/HDL-C, and TC/HDL-C, remained significantly associated with dietary fiber (r = -0.34; p < 0.05 and r = -0.38; p < 0.02, respectively).

Conclusions: This study provides evidence in free living men that there is an association between dietary fiber intake and favorable lipid status and that lifestyle defined by socioeconomic status, physical activity and the quality of the dietary fat intake can play an important role. Public health nutrition advice and policy should continue to emphasize the importance of these factors.

Key words: dietary fiber, dietary fat, physical activity, blood lipids, diet

Abbreviations: BMI = body mass index • HDL-C = high-density lipoprotein cholesterol • LDL-C = low-density lipoprotein cholesterol • PAL = level of physical activity TC = total cholesterol • TEE = total energy expenditure • TG = triglycerides • VLDL-C = very-low density lipoprotein cholesterol • P/S = polyunsaturated-saturated ratio

	INTRODUCTION

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In 1954 Walker and Arvidsson [1], studying the Bantu people in South Africa, found that, apart from low fat intake, the high fiber content of the diet could bear some responsibility for the low serum cholesterol values observed. Currently it is thought that a low fiber intake increases the incidence of hyperlipidemia and cardiovascular diseases [2,3]. In most Western countries including Mexico, heart disease is a leading cause of death [4,5]. Age, smoking, diabetes mellitus, hypertension, obesity, high-fat diet, as well as high concentrations of total cholesterol and low-density lipoprotein (LDL) in the blood, are among the main risk factors associated with this disease [6,7].

Consumption of dietary fiber, specifically the soluble type, such a pectins and guar gum can result in a decrease of serum cholesterol levels in healthy and hyperlipidemic subjects [8,9]. A recent meta-analysis of 67 controlled trials was performed to quantify the cholesterol-lowering effect on blood lipids of pectin, oat bran, guar gum and psillium. Soluble fiber, 2 to 10 g/day, was associated with small but significant decreases in total cholesterol and LDL-cholesterol while triglycerides and HDL-cholesterol were not influenced by soluble fiber [10].

In the State of Sonora, located in the northern region of Mexico, cardiovascular diseases such as myocardial infarction are the main cause of death in adults [11] and the diet of this region is characterized as being high in fat and high in dietary fiber [12]. Therefore, the aim of this study was to determine the effect of dietary fiber consumption and lifestyle on serum lipids in adult men with a free living style diet and non-restricted physical activity.

	MATERIALS AND METHODS

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Subjects
Thirty-eight healthy men, living in the city of Hermosillo, Sonora, Mexico, were selected by intentional non-probabilistic sampling under the following selection criteria: 1) male subjects between the ages of 30 and 45 years, 2) no signs of diabetes mellitus nor arterial hypertension, 3) no signs of cardiovascular diseases, 4) not currently on blood lipid altering medication, 5) not taking any fiber supplements. The protocol was approved by the Ethics Committee of the Centro de Investigacion en Alimentacion y Desarrollo, A.C., and a consent form in accordance with the institutional guidelines was signed by the volunteer participants.

Study Protocol
All subjects evaluated were selected through home visits in different neighborhoods of the city of Hermosillo, Sonora, Mexico. Twenty-four neighborhoods of low, medium and high income levels were visited and 126 subjects screened. Income levels were defined by the number of minimum wage salaries. Only 38 subjects complied with the constraints of the protocol and were willing to participate in the study. During the first visit, which was considered a pre-trial, a brief clinical questionnaire was administered. This questionnaire was designed to eliminate any individuals who showed signs of heart disease, hypertension or diabetes mellitus. At this time, arterial blood pressure was measured [13]. The food frequency questionnaire used [14] explored only those food items in the regional diet that are important providers of dietary fiber. This allowed the separation of two groups of potential high fiber and low fiber consumption. This was later confirmed with the results from seven-day weighed intake diaries. The second home visit was conducted prior to the beginning of the study to train the subjects in the use of the seven-day weighed food record [15]. Both the subject and the person in charge of preparing the meals were given food scales (Ohaus Scale Corporation, Florham Park, NJ), one to be kept at home and the other for use outside home, and a form to record the subject’s daily intake. Analysis of food consumption was done using several food composition databases [16,17] and analytical data on traditional foods from the Northern Mexico region [18,19]. The subjects were trained to use a pre-coded form to register physical activity on a daily basis. All of these activities were supervised on a daily basis during the seven days of the study. Dietary fiber intake by the seven-day weighed food record was used to divide subjects into two groups of 19 subjects each using a cut-off point of 35 g/day for high and low intakes, according to the Instituto Nacional de la Nutricion (INN), which recommends an intake of 30 to 35 g/day for the Mexican population [20].

Physical Activity
Each subject kept an activity diary. The activity diary was divided into 15 minute periods, and the subjects were instructed to register activities every waking hour. The diary included the description of general and specific activities expressed as multiples of BMR coded by letters. Each diary was checked daily by a trained field technician to verify the information and find inconsistencies. This activity diary has been used in similar populations together with the doubly labeled water method [21]. Estimation of physical activity levels was based on the data from the activity diary in combination with the estimation of predicted basal metabolic rate from body weight based on Schofield equations [22].

Anthropometry
Body weight, to the nearest 50 grams, was measured using a portable Accu-Weigh beam balance (Health O Meter, Germany). Subjects were weighed without shoes and in a minimum of clothing. Height was measured, to the nearest 0.5 cm, using a portable steel measuring device [23]. From these two measurements body mass index (BMI) (kg/m²) was calculated [24].

Serum Lipids and Lipoproteins
After the dietary and physical activity evaluation, blood samples were obtained on an overnight fast (12 to 14 hours). A maximum of 15 mL of blood from the antecubital vein was collected into tubes (Becton Dickinson V.S. SST GEL) with a clot activator. The specimens were immediately packed in wet ice and transported to the laboratory within 30 minutes. The serum was obtained by centrifugation at 1600 g for 20 minutes at 4°C (CS-6R Centrifuge, Beckman, Instruments, Palo Alto, CA) and an aliquot of serum taken for immediate total cholesterol and triglycerides analysis.

High density lipoprotein (HDL) cholesterol was determined after precipitation of apo B-containing lipoprotein with sodium heparin and manganese chloride using the method of Warnick [25]. Low-density lipoprotein was estimated using the formula established by Friedewall [26], where very low density lipoprotein = triglycerides/5, and LDL-cholesterol = total cholesterol + VLDL-cholesterol + HDL-cholesterol).

Total cholesterol and triglycerides and high density lipoprotein (HDL) cholesterol were measured enzymatically by test kits ( Boehringer, Mannheim, Mannheim, Germany). For each analysis certified commercial serums Precinorm U, Precinorm L (Boehringer Mannheim, Mannheim, Germany) and Precilip L (Sigma Diagnostics, St. Louis, Missouri) were also used.

Statistical Analysis
Data were analyzed using the statistical program NCSS 1997 (Number Cruncher Statistical System for Windows, Kaysville, Utah). All results are expressed as mean ± standard deviation (SD), standard error of the mean (SEM) or range, and are indicated in footnotes to each table. After normality assessment, Student’s t test or the Mann-Whitney test were used to compare the groups of low and high fiber intakes. Analysis of covariance using the general linear model was used when adjustment for confounders was necessary and is indicated in footnotes to the tables. Correlation analyses between blood lipid variables and dietary fiber were done controlling individually for physical activity, BMI, energy, total fat, saturated fat and cholesterol intake as well as for all the variables together. From this, multiple regression was used to evaluate the impact of dietary fiber on LDL-C/HDL-C or TC/HDL-C adjusting for physical activity, BMI, dietary saturated fat and total cholesterol intakes.

	RESULTS

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Subjects in the low fiber group (Table 1) had higher body weight (p < 0.02) and were taller than subjects in the high fiber group (p < 0.01), but there were no differences in BMI. All of the subjects had normal blood pressure according to the criteria established by WHO [27].

View this table: [in this window] [in a new window]   Table 4. Lipid and Lipoprotein Serum Concentrations in all Subjects

View larger version (29K): [in this window] [in a new window]   Fig. 1. The association of blood lipids and dietary fiber and P/S ratio.

	DISCUSSION

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Dietary fiber intake is high in this region of Mexico, both rural and urban, primarily due to the high intake of the basic staples: pinto beans, corn tortillas and wheat flour tortillas [12]. Cereals and legumes are known to have a high proportion of non-cellulose polysaccharides, which are considered beneficial in preventing cardiovascular disease [8,9].

Dietary fiber intake was defined as low and high using a cut-off point of 35 g/day for this study since intakes below 25 g/day were not found in this population. This resulted in a mean consumption of 48 and 27 g/day for the high and low groups respectively. A value of 25 g/day is higher than those reported as low fiber diets in other countries [28].

Energy intake was higher in the high fiber group, but this group also had a higher level of estimated total energy expenditure and physical activity. The energy coming from fat was 35% in both groups and surpassed the recommended 30% suggested by the National Cholesterol Education Program (NCEP) [29] and can be considered as a high fat diet. Similar values in energy intake from fat have been reported in Mexican-American and Caucasian populations [30].

After adjusting for energy intake, no differences were found between the groups for total fat, cholesterol, saturated and polyunsaturated fatty acid intake, while monounsaturated fatty acids and P/S ratio were significantly higher in the high fiber group. In this respect, the diet of the high fiber group had a better profile in terms of lipid content than the diet of the low fiber group (Table 2). In the low fiber group, most of the fat was provided by animal products like ground beef, ham, sausages, dairy products and vegetable oils, whereas in the high fiber group, the fat was provided mainly by vegetable oils, vegetable hydrogenated solid fats used in the preparation of wheat flour tortillas and some pork lard used for frying beans. The high fiber group also consumed more complex carbohydrates.

The main sources of fatty acids from vegetable oils in both groups were corn and safflower oils. These oils have a lower amount of monounsaturated fatty acids than olive oil, which has been associated with a low prevalence of atherosclerosis in the population of the Mediterranean [31]. In relation to polyunsaturated fatty acids, corn and safflower oils were the main sources. The consumption of sea food products in the diet of both groups was quite small, and a low contribution of omega-3 fatty acids could be expected. The effect of elevated cholesterol consumption on serum cholesterol is still a controversial subject [32,33]. In this study, both groups showed consumption above the suggested intake by the NCEP [29]. A positive relationship was found between dietary cholesterol intake and LDL-C.

After adjusting for energy intake, total fat, saturated fat, dietary cholesterol and physical activity, the strongest lipid status indicators related to dietary fiber were the LDL-C/HDL-C and total cholesterol/HDL-C ratios rather than individual values (Table 5). As evidenced by the multiple regression models tested, dietary fiber remained unchanged in its significance in terms of its relationship to these ratios, after adjusting for dietary confounders and physical activity. Physical activity by itself was strongly and inversely associated with total cholesterol and LDL-C, although no relationships were observed with HDL-C.

In the present study, considering the low and high fiber intake groups of free living men and the limitations in sample size, it is difficult to establish that dietary fiber affects lipid levels. However, the strong significant correlations found after controlling for important confounders suggest the relative importance of this dietary component in lipid status. We observed important differences in diets between groups, not only in total dietary fiber and soluble fiber, but also in monounsaturated fatty acids and P/S ratio, which were better in the high fiber group.

Finally, we also must consider the importance of lifestyle differences (diet and physical activity) in the general health and lipid status of the subjects studied. The low dietary fiber group was also from lower socioeconomic levels, mainly low and medium income. These subjects have occupations that are less sedentary and presumably have higher levels of occupational physical activity. Also, the diet of the low income population in Mexico can be equally high in fat to that of the higher income groups, although lower in consumption of animal products, and richer in dietary fiber and complex carbohydrates, but also poorer in consumption of fruits and vegetables.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
This study provides evidence in Mexican men living in an urban environment that there is an association between dietary fiber intake and favorable lipid status. Lifestyle related to socioeconomic status, physical activity and the quality of the dietary fat intake can play an important role. Public health nutrition advice and policy should continue to emphasize the importance of increasing the intake of dietary fiber in the form of complex carbohydrates (e.g., beans and tortillas), fruits and vegetables, increasing physical activity and reducing the consumption of high fat foods such as animal products high in fat.

	ACKNOWLEDGMENTS

 
We thank all those volunteers who took part in the study. We appreciate the assistance of Dr Jane Wyatt and Dr Elaine Rush for revision and valuable comments on the manuscript.

Received January 22, 2001. Accepted September 5, 2001.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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