Abstract
Introduction Although the acute effects of exercise on blood lipid profiles have been extensively investigated, the data reported are conflicted (Pronk, 1994). It has been recently shown (El-Sayed and Rattu, 1996) that vigorous exercise performed in the morning in highly trained cyclists increases high-density lipoprotein cholesterol (HDL-Chol), but not total cholesterol (T-Chol) or triglyceride (TG). However, the possible effect of different times of day on blood lipid profiles at rest and in response to exercise is not known. The purpose of this study was to assess the effects of two different times of day (09:00 and 17:00 hours) on HDL-Chol, T-Chol and TG at rest and in response to exercise. Methods Ten normal healthy subjects volunteered to participate in the study. Following the determination of VO2max, subjects performed two separate randomised sub-maximal exercise (60% VO2max for 45 min) trials at 09:00 and 17:00 hours. Subjects consumed the same diet 3-h prior to each trial and this was verbally verified and confirmed before the commencement of testing. Venous blood samples were obtained, with no stasis, at rest, post exercise and also following 30-min recovery. Whole blood was mixed gently with EDTA and a small portion of this was used for the measurement of T-Chol and TG concentrations. Small aliquots of EDTA blood were assessed for lactate, haematocrite, and haemoglobin. Changes in plasma volume were calculated using the method of Dill and Costill (1974). The remainder of the blood was then centrifuged and plasma was separated and used for the measurement of HDL-Chol. All lipid measurements were carried out using the reagents and equipment of Boehringer Mannheim, Germany. Results The mean resting values of the blood lipid variables did not vary significantly between the morning and the evening test trials. The concentration of T-Chol, TG and HDL-C increased significantly (P<0.05) following both exercise tests, with no difference being observed between the morning (T-Chol; 4.980.29, TG; 1.030.07, HDL-Chol; 1.270.07) and the evening ((T-Chol; 4.570.18, TG; 1.080.07, HDL-Chol; 1.20.11) trials. Following 30-min recovery, T-Chol, TG and HDL-C returned back to the pre-exercise levels. Post- exercise plasma volume decreased significantly (P<0.05) after both the morning (-12.55%) and the evening (-12.02%) test trials, but was largely restored to the pre-exercise level at the end of recovery. When post-exercise data were adjusted for plasma volume loss, the increase in T-Chol, TG and HDL-C was no longer apparent, and this was similar during both the morning and the evening exercise test trials. Conclusion It is concluded therefore that acute exercise induces significant changes in blood lipid variables mainly due to haemoconcentration and these changes do not appear be influenced significantly by the two different times of day chosen in this study. References Pronk, P.N. (1994). Short term effects of exercise on plasma lipids and lipoproteins in humans. Sports Medicine, 16, 431-448. El-Sayed, M.S (1996). Changes in lipid profile variables in response to sub-maximal exercise and maximal exercise in trained cyclists. European journal of Applied Physiology, 73, 88-92. Dill, D.B., and Costill, D.L. (1974). Calculation of percentage change in volumes of blood, plasma, and red blood cells in hydration. Journal of Applied Physiology, 37, 247-284.