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Plasma lipid variations in response to diet and exercise McKenzie, Donald Chisholm

Abstract

The purpose of this study was to determine the plasma lipid variations during periods of low calorie diet and low calorie diet plus increased physical activity. Four male graduate students, with above normal activity levels, volunteered for the 10 week study which was divided into five experimental periods. The first, or control condition involved a two week period during which the subjects received a regular diet of normal foods equivalent to approximately 3600 calories per day. During this period 'normal' activity was maintained. The second treatment condition involved a 10 day period of a low calorie diet, with continued 'normal' activity. The low calorie diet was equivalent to approximately 1800 calories daily, of natural foods, plus one multiple vitamin pill. The third experimental period was similar to the control period; a two week period during which the subjects received approximately 4000 calories, per day, of the regular diet. Again, 'normal' activity was maintained. The fourth treatment condition was identical to the second experimental period with the additional requirement of increased daily energy expenditure, per subject, of 500 calories. The final experimental period consisted of a two week period of the regular diet with 'normal' activity. Blood samples were taken, following an overnight fast, twice during each experimental period: once mid-way through the period and again at the end. Plasma triglyceride and free fatty acid concentrations were measured in duplicate in each sample. The results of orthogonal comparisons among treatment means showed a statistically significant increase in the plasma free fatty acid concentration during the low calorie diet and the low calorie diet plus exercise treatment conditions. Increased mobilization of free fatty acids from adipose tissue triglycerides in response to the insufficient dietary supply of substrates for metabolism was cited as the mechanism responsible for the rise in free fatty acid concentration. Neuman-Keuls method was used to examine the effect of the increased physical activity during the low calorie diet periods; the results showed that the increased physical activity had no significant effect on the plasma free fatty acids. Similar statistical procedures applied to the plasma triglyceride values showed a significant decrease in the plasma triglyceride concentration during the low calorie diet and the low calorie diet plus increased physical activity periods. The stress of the low calorie diet on the habitually active subjects was responsible for the decreased levels. The lipid and carbohydrate content of the normal and the low calorie diets, as well as increased peripheral uptake of triglycerides, were suggested as possible explanations for the plasma triglyceride changes. The additional daily output of 500 Calories during the second stress condition was not of sufficient magnitude to elicit a further decrease in plasma triglyceride concentration.

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