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Chin Med J (Taipei) 1997;59:341-7.

The Effect of Exercise Training on Physical Fitness and Plasma Lipids in Young Chinese Men and Women

Xiao-Rong Xi1, Imran Ahmad Qureshi2, Xiang-Dong Wu1, Ihsan-ul-Haq Khan3, Yang-Bin Huang1, Ekhtiar Shiarkar1

1Shanghai Medical University, Shanghai, People's Republic of China 2Rawalpindi Medical College, Rawalpindi-Pakistan 3Nishtar Medical College, Multan-Pakistan

Abstract

Background. The mortality from coron-ary heart disease (CHD) has been steadily increasing in many parts of the world. Measures to reduce the incidence of CHD were initially addressed mainly to middle aged and elderly population groups, but for effective prevention of CHD, measures addressed to younger age groups are also necessary. Moreover, differences between male and female plasma lipid changes after exercise training are not known exactly.

Methods. The concentrations of plasma lipids, before and after regular physical exercise training for 12 weeks, were analyzed in 25 male and 25 female medical students of the same age group. Physical fitness was evaluated by the measurement of maximum oxygen uptake.

Results. Results indicated that plasma tri-glyceride, HDL-cholesterol, LDL-cholesterol and VLDL-cholesterol differences between the before-and-after exercise were significant (p < 0.05) in males while the differences of plasma total cholesterol were found to be insignificant. In females there were no significant changes in the plasma lipids levels. In both males and females, the marked increase in maximal O2 uptake after exercise training indicated an improvement in physical fitness.

Conclusions. This study concludes that a short-term exercise program can improve plasma lipid and lipoprotein patterns more in males than in females. It also suggests that daily physical exercise may be an important prevention for cardiovascular diseases in later life, so it is essential to establish a life style with optimum nutrition and physical activity in both the young and in adults of all ages.

[Chin Med J (Taipei) 1997;59:341-7.]

Keywords: cholesterol, physical exercise, sex, triglyceride

Received: March 8, 1996.

Accepted: April 10, 1997.

Address reprint requests to: Dr. Xiao-Rong Xi, Room No. 702, Building No. 2905 Xie Tu Road, Shanghai-200030. People's Republic of China

Introduction

It is well known that endurance physical training such as running, swimming and cycling increases a person's maximum capacity for aerobic exercise and causes adaptations in different systems of the body. Research relating coronary heart disease (CHD) to lipids has grown extensively in this century [1-4]. Cumulative confirmatory evidence from animal experimental, clinical case series and epidemiologic population studies have established that lack of physical activity has been suspected to be associated with an increased incidence of CHD [4-5]. These findings indicate that physical exercise may reduce CHD by altering plasma lipid and lipoprotein levels [1-10]. The existing literatures regarding the effect of exercise upon lipid and lipoproteins are confusing and contradictory [2-3,6-8, 10-15]. Studies on males have shown consistent increase in high density lipoproteins (HDL) cholesterol during programs of moderate exercise [1-3], and have shown either a decrease [1,2] or no significant change in low density lipoprotein (LDL) cholesterol [3]. Even less is known about lipid variations in females. Ballantyne et al. [3] found no change in HDL-cholesterol and a decrease in LDL-cholesterol in females after moderate exercise, while Browned et al. [1] found a decrease in both measurements.

Mortality from CHD has been steadily increasing in many parts of the world. Measures have been already taken in China to reduce the incidence of CHD. These measures were initially addressed mainly to middle-age and elderly population groups. For effective prevention of CHD, however, measures addressed to younger age groups are also necessary because it is believed that the major risk factors in adult life are the result of behavior patterns established during childhood and adolescence [4,16]. As mentioned above, lack of physical activity has been suspected to be associated with an increased incidence of CHD; therefore, it is essential to evaluate the effect of physical exercise on the risk factors for CHD. Moreover, differences between males and females lipid changes after exercise training are not known accurately. This study compared the concentrations of plasma lipids of young males and females, before and after 12 weeks of regular physical exercise training.

Materials and Methods

All experimental procedures adhered to the Declaration of Helsinki of the World Medical Association. The effects of exercise training on plasma cholesterol and triglycerides were studied in 50 medical students of Shanghai Medical University (25 males and 25 females), whose age ranged between 21 and 30 years. Ages (mean +/- EM) were 24.6 +/- 0.5 years and 23.9 +/- 0.4 years in male and female subjects, respectively. Of the female group, none was using contraceptive in any form. All of these students met the following conditions of health: 1) were not under medical treatment; 2) did not have a past history of cardiac or hepatobiliary disease, and had no abnormal subjective symptoms; 3) did not exhibit abnormalities, such as in blood pressure, in routine physical examinations; 4) were not engaged in particularly demanding physical activities. These students were living in the same hostel, so they had been taking similar diet from the university mess for the past three years. For collection of data on types and quantities of food consumed, instruction sheet on form completions, with detailed examples were distributed to the subjects. The survey was conducted for three consecutive days, and the mean values obtained were used to reflect the food intake of each subject (Table 1). The nutrient content of the food table [17] was used to compute the energy and nutritional values of food.

The subjects participated in a voluntary program of 12 weeks of exercise. The percentage of maximum heart rate (%HRmax) was used to determine the difficulty level of the exercise [l 8]. The %HRmax is expressed as follows:

%HRmax = (HR during exercise - HR at rest) x 100 / (Maximum HR - HR at rest)

Maximum heart rate is obtained from 190 minus age [18]. The laboratory temperature was 20OC. In the present study riding a bicycle ergometer was chosen for two reasons: the feasibility of examining the heart rates during exercise, and because this form of activity is well accepted as a means for calibrated levels of physical exercise [19]. Each subject performed exercises at the levels of 60% HRmax for 30 minutes, at least four times per week. During exercise, heart rates were measured by palpation for 10 seconds and exercise load was increased until the individually prescribed heart rate was achieved. During the exercise, heart rates were confirmed three times. Blood pressures and heart rates were measured immediately after exercise. Weight loss was not encouraged. It has been reported that consumption of alcohol raises plasma HDL-cholesterol level [20], while, cigarette smoking has an opposite action [21]. Therefore, the subjects were advised to avoid alcohol consumption during the exercise program. None of them was a smoker.

The subjects were asked to fast overnight 12-16 hr before the test and they were requested not to do any exercise during the fasting period. Adherence to the requirement for abstinence from recent exercise, fasting, or both were checked by carefully questioning the participants at the time of their test; any nonfasting individuals were rescheduled. All participants were recumbent for no more than a few minutes during blood drawing. Venous blood was drawn into Vacutainer tubes containing 1 mg/ml disodium EDTA. Plasma was prepared from blood within 2 hours and was kept at 4OC. Total cholesterol, HDL-cholesterol, LDL-cholesterol, very low density lipoproteins (VLDL) cholesterol, and triglycerides were determined using the procedures of the Lipid Research Clinics [22]. The analyses were performed on the AutoAnalyzer II (Technicon Instruments, Tarrytown, New York, U.S.A.). The laboratory procedures were standardized according to the criteria for the Lipid Research Clinics Program [22]. The Liebermann-Burchard reaction [23] was used for cholesterol measurement. Lipoproteins were separated by ultracentrifugal flotation at saline density (d=1.006 g/ml), to yield a supernatant fraction containing VLDL-cholesterol and an infranatant fraction containing both LDL-cholesterol and HDL-cholesterol. HDL-cholesterol was estimated in total plasma after precipitation of the apo-B-lipoproteins by means of heparin-manganese chloride. After direct estimation of HDL-cholesterol, LDL-cholesterol was observed by the formula: LDL-cholesterol = cholesterol in the 1.006 infranatant minus HDL-cholesterol. The VLDL-cholesterol was determined by the formula: VLDL-cholesterol = total plasma cholesterol minus 1.006 infranatant cholesterol. In instances of incomplete precipitation of VLDL-cholesterol and LDL-cholesterol, the procedure was repeated on the infranatant fraction after ultracentrifugation [24]. Kessler and Lederer's method [25] was used for triglycerides.

Maximum oxygen uptake (Vo2max) represents the maximal circulatory transport of oxygen from the lungs to metabolically active tissues. Vo2max is the best physiological reference for functional capacity of the circulation and is a standard measure of cardiovascular fitness [18]. In this study physical fitness was evaluated by the measurement of maximum oxygen uptake (ml/kg/min) with a Beckman O2 analyzer (Beckman Instruments Inc. Irvine, Calif. U.S.A.). Maximal oxygen uptake was measured in the laboratory by using a bicycle ergometer as described by Astrand and Rodahl [19]. Oxygen uptake was determined at progressively increasing workloads until a workload was reached at which oxygen uptake failed to increase significantly over that of the previous workload. This highest level of oxygen uptake represented maximal oxygen uptake.

Statistical Analyses: For all variables descriptive statistics (mean, standard deviation, standard error of mean) were calculated by Statistical Analysis System 76 [26]. All data are expressed as means and standard error of mean. Significance of the difference between before and after exercise was calculated by applying the two-tailed paired Student's t-test. Differences are regarded as significant when the P value was less than 0.05. Actual p values are also given.

Results

The results of this study are summarized in Tables 1-3. As shown in Table 1, both male and female subjects were taking the optimal nutrition which includes high complex carbohydrate, high fiber, low fat and low cholesterol. Moreover, in both male and female subjects, energy intake was found to be sufficient according to their age and body weight [17]. The main source of carbohydrates was fruits, of which most common fruits were apple, pear, tangerine, watermelon, grapes and banana. At least one vegetable was always present in their diet. Chinese cabbage, spinach, celery, aubergine, cucumbers were among those vegetables most commonly part of the diet. The changes in lipid levels (mmol/L) and body weight are shown in Table 2. In males, the differences between before and after exercise were found to be -0.24 +/- 0.04, -0.18 +/- 0.06, +0.17 +/- 0.05, -0.09 +/- 0.03, -0.07 +/- 0.02 mmol/L of triglycerides, cholesterol, HDL-cholesterol, LDL-cholesterol and VLDL-cholesterol, respectively. The differences in triglycerides, HDL-cholesterol, LDL-cholesterol and VLDL-cholesterol were significant (p < 0.05), while the differences of total cholesterol were found to be insignificant. In females, differences between before and after the exercise were found to be -0.07 +/- 0.03, -0.13 +/- 0.06, +0.05 +/- 0.03, -0.03 +/- 0.02, -0.01 +/- 0.02 mmol/L of triglycerides, cholesterol, HDL-cholesterol, LDL-cholesterol and VLDL-cholesterol, respectively. All of these differences were found to be insignificant. Resting supine vital signs from aerobic exercises before and after exercise training of 12 weeks are given in Table 3.

In both males and females, after exercise training, resting heart rate reduced significantly (p < 0.05), while resting mean systolic and diastolic blood pressures remained the same. After 12 weeks of exercise training, in response to aerobic exercise diastolic blood pressures reduced significantly (p < 0.01), while the mean heart rate and systolic blood pressure remained the same before and after exercise training. Reduction in the mean body weight was found to be -1.1 +/- 0.3 kg (p > 0.05) and 0.9 +/- 0.4 kg (p > 0.05), in males and females, respectively. After exercise training, the marked increase in maximal O2 uptakes indicates improvement in the subjects' physical fitness.

Discussion

It is important to evaluate the effects of life style of risk factors among young people. It seems logical to consider that establishment of proper habits of physical activity during childhood and adolescence is useful for prevention of cardiovascular diseases in later life. In the present study as indicated by Vo2max, exercise training increased physical fitness in both men and women. This is a very useful way to avoid CHD, because studies have established that lack of physical activity or low Vo2max is suspected to be associated with an increased incidence of CHD [5-8].

Several studies of lipid change have been done [1, 5, 6]. In male subjects, a significant decrease of triglycerides, LDL-cholesterol and VLDL-cholesterol levels was found in association with regimens of 12-week exercise. Among females, there were no significant changes. The earlier literatures of changes of male's serum triglyceride levels in association with various types of sports or athletic activities is particularly conflicting. In vigorous endurance training studies, plasma triglyceride levels generally were found to decrease [2,4,12] with a few exceptions [1,7,8]. LDL-cholesterol levels have not been frequently reported to decrease [2,3], as several studies found no significant change [12]. Bonetti et al. [7] reported a rise in the serum triglycerides level of volleyball players after a match and attributed this to catecholamine release due to exertion. However, Streja and Mymin [8] did not recognize any significant changes in the serum triglycerides level in 32 persons with ischemic heart disease placed on a jogging program requiring 70 to 80% of the maximum oxygen uptake 3 times a week for 13 weeks. Low-intensity endurance exercise training has also been frequently reported to induce favorable changes in plasma lipids. Yamasaki et al. [5] observed significant deceases in the triglycerides levels of young adults given a light-to-moderate exercise program which required 40% and 50% oxygen at a maximum uptake for 20 and 30 minutes, respectively, 3 times a week for 6 weeks. Much less information is available for females, but they appear to differ from the male changes in triglycerides. Like the present study, one previous study has shown no significant changes in triglycerides levels of females after exercise training [1].

Chylomicrons and VLDL are considered to be lipoproteins which transport triglyceride. Chylomicrons transport dietary fats, while VLDL transports fat that has been synthesized from fatty acids and carbohydrate, mainly in the liver. When the diet contains more fatty acids than are needed immediately as fuel, they are converted into triacylglycerols in the liver and packed with specific apolipoproteins into VLDL. Activation of lipoprotein lipase by apoC-II causes the release of free fatty acids from cholesteryl esters of the VLDL. The loss of triglycerols converts VLDL to LDL. LDLs carry cholesterol to peripheral tissues, where cholesterol enters cells by receptor mediated endocytosis [27]. Exercise mainly activates lipoprotein lipase (LPL) of adipose and muscular tissues and promotes catabolism of lipoproteins that are rich in triglycerides [9]. It seems quite possible that a decrease in triglyceride level after exercise training may be caused by the longer activation of LPL of adipose and muscular tissues.

This study found that females had lower mean plasma triglycerides values than males. This is in accordance with the finding that the activity of LPL is significantly higher in normal females than in males [10]. It has been reported that the LPL activity of adipose tissue is significantly higher in long distance male runners than control males, while in females the difference is not significant [9]. The difference of the effect of exercise on LPL between males and females accounts for the fact that significant decrease occurred in males while no such change was observed in females.

The effect of exercise on HDL-cholesterol is controversial. In some circumstances, the adoption of a program of vigorous exercise has been followed by modest reduction in cholesterol concentration [11], while other studies have found no such change [1] or a decrease [6]. Interestingly, the study by Syubbe et al. [13], which compared the effects of high-intensity versus low-intensity exercise training, reported a negative correlation between changes in plasma HDL-cholesterol levels and exercise intensity. This study shows an increase in HDL-cholesterol in males and no significant changes in females after exercise. The exact mechanism of exercise-induced elevation of HDL-cholesterol is not clear. A change in daily food composition may be the cause of changes in plasma HDL-cholesterol. However, the subjects of this study were living in the same hostel, and thus taking similar diet from the university mess before and during the exercise training.

The variable most strongly associated with the increase in HDL-cholesterol was the decrease in triglycerides. A negative association had earlier been demonstrated in cross sectional studies between triglycerides and HDL-cholesterol levels [28]. The common denominator in the metabolism of HDL and VLDL has been suggested to be LPL [29], the enzyme responsible for the catabolism of triglyceride-rich lipoprotein in the peripheral tissues. A negative correlation has been reported between serum triglycerides concentration and the activity of LPL in adipose tissue [30] and post-heparin plasma [10]. However a positive relationship is present between HDL-cholesterol and the activity of LPL in adipose tissue in normal subjects [31].

The present study concludes that a short-term exercise program improves lipid patterns more in males than in females. Another study has reported similar findings [14]. However, when Body Mass Index (BMI) values, before and after exercise training were compared, they were similar both in male and female. It is considered that in this study, the reduction of blood lipids resulted from physical activity, because our subjects were living and eating together and nutritional habits and diet was not changed before and during the study period; therefore diet was not a factor in this study.

Paffenbarger and Hyde [32], in studies on San Francisco Longshore wolfers and Harvard University graduates, found that mortality from ischemic heart disease was much lower at any age in individuals normally engaged in physical activities. The present results suggest that routine exercise of moderate intensity may alter the lipid pattern and reduce the risk of CHD. Previous studies have reported that low cholesterol is related with low incidence of CHD, but recent clinical studies showed that a low triglycerides level, rather than a low serum total cholesterol level, is closely related with a low incidence of CHD [15]. In the present study, triglycerides and LDL-C decreased while HDL increased after exercise program, so it seems that all these changes are more beneficial than changes in total cholesterol, which remained the same after exercise training. Although, in the present study the exercise training decreased the lipid profile favorably in male subjects than in female, the resting heart rate decreased in both male and female subjects. Therefore, this study concludes that daily physical exercise may be important for preventing cardiovascular diseases in later life, making it essential to establish a life style with optimum nutrition and physical activity in childhood and adolescence.

Acknowledgments

The present study was supported by "Smile-Spreading Medical Information for better Life through Electronic media". This work was carried out at Shanghai Medical University, People's Republic of China. The author gratefully acknowledges Professor Dr. Mohammad Nawaz, Dr. Zeng Mei, and Dr. Wang Ju for their technical help and suggestions.

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Copyright: 1997, Chinese Medical Association (Taipei)