Effects of a High Fat, High Carbohydrate, and High Protein Diet on Endurance Performance in Trained Cyclists and Untrained Controls

Abstract

LEARNING OUTCOME: To describe the effects of a high fat, high carbohydrate, and high protein diet on submaximal endurance times to fatigue in male endurance athletes and controls. A low fat, high carbohydrate (CHO) diet is currently recommended for endurance athletes. But recent studies indicate that a moderate fat diet may be more advantageous. To test this, six trained male cyclists (mean ± SD age 23±3.5, VO 2 max 66±5 ml/kg/min, body mass index (BMI) 23±1.7) and five male controls (age 25±1.5, VO 2 max 44±5 ml/kg/min, BMI 27±4.3) were fed the following diets, in random order, for one week: a high CHO (15% fat, 15% protein, 70% CHO by calories), high fat (35% fat, 15% protein, 50% CHO), high protein (30% fat, 30% protein, 40% CHO), and normal diet (subjects usual diet; 21% fat, 15% protein, 63% CHO). Each diet week was followed by a one week period where subjects consumed their usual diets. Exercise time to fatigue was measured at the completion of each diet following a light liquid meal (20% of total caloric intake) of equal composition as the proceeding diet week. Tests were completed at a consistent power output corresponding to 70 and 75% of the control and trained subjects’ VO 2 max, respectively. Mean ± SD exercise times to fatigue (in minutes) were as follows: Control Subjects (n=5) Trained Subjects (n=6) Normal diet 24±16 37±9 High fat diet 30±28 48±15 High CHO diet 22±11 45±16 High protein diet 22±15 51±18 One-way ANOVA p=0.88 p=0.39 The main limitation of this study was its small sample size, which likely contributed to the lack of significance found in these data. While not statistically significant, trained subjects had increased endurance times by six minutes after the high protein diet, and three minutes after the high fat diet compared to the high CHO diet. Thus, a diet with less emphasis on carbohydrates than traditionally recommended may enhance performance in the endurance athlete and deserves further study.