High-Carbohydrate Vs. High-Fat Diets In Trained Cyclists - Brief Article - Statistical Data Included
R. Brown

Previous studies have examined weight and body composition changes in obese individuals when the proportions of dietary fat and carbohydrate have been altered. Very few studies have examined the effects of macronutrient composition of the diet on body composition in lean subjects engaged in high volumes of physical exercise. A method to determine body fat composition, more accurate then skin-fold measurements and anthropometry, is dual X-ray absorptiometry (DXA). This technique measures both bone mineral and soft tissue composition, and can distinguish fat located in the hips and thighs from that deposited in the trunk. A New Zealand study recently examined the effect of high-fat or high-carbohydrate diets on body composition in athletes using DXA.

DXA was used to examine body composition in cyclists after a 12-week period of either a high-fat or high-carbohydrate dietary regimen to determine whether fat mass and distribution differed between the two diet groups. Thirty male and two female cyclists volunteered to participate; all had participated in competitive cycling for at least two years. Subjects (mean age 26 [+ or -] 9 years), were randomly assigned to receive either high-carbohydrate (HC) or high-fat (HF) dietary advice for a period of 12 weeks. In HF, approximately 45-50% of total energy was derived from fat and 35-40% from carbohydrate. In HC, the proportions were 14% from fat and 65-70% from carbohydrate. Protein intakes were similar for both groups. Each group received a weekly food ration appropriate to their dietary instruction. Each participant received individualized nutritional instruction throughout the study. Five-day food intake was recorded at baseline and every fourth week during the study.

Body weight and total body fat did not change significantly from baseline to week 12 both within and between each diet group. Regional fat distribution changed significantly within each diet group from baseline to week 12. Body fat in the legs decreased significantly in HC and truncal fat increased significantly in HF and HC groups. On examination of individual data, it was clear that the results from two individuals who were siblings skewed the results. This indicates that there may be a genetic predisposition for regional fat distribution with high-fat dietary change. Studies with larger samples are needed to verify this observation.

There were no significant differences in lean tissue mass during the study period. Total bone mineral density (BMD) increased significantly within HF from baseline to week 12. There were significant changes in regional bone density within both groups from baseline to week 12. In HC, BMD increased significantly in the trunk and lumbar regions; in HF, BMD increased in the legs and pelvis. Researchers attributed this to bone mineral accrual of young athletes. Calcium intake tended to be higher in the HF group. Training volume during the study ranged from 115 to 826 km per week (6-25 hours), plus a 40-120 km weekly road race. After 12 weeks of training, maximal oxygen uptake remained at the same high levels as the start of the study.

The results of the study have considerable relevance to the endurance of young athletes. During periods of endurance training, when energy requirements are high, it seems reasonable to suggest that athletes might choose to increase dietary fat intake, even when consuming a significant proportion of fat calories as saturated fats (46%), without the risk of weight gain and/or increased adiposity. It appears that endurance athletes can adapt to high-fat diets without any detrimental effect on physical capacity.

R. Brown, C. Cox, A. Goulding, High-carbohydrate versus high-fat diets: effect on body composition in trained cyclists, Med Sci Sports Exerc 32(3): 690-694 (2000) [Correspondence: Rachel Brown, Dept. of Human Nutrition, University of Otago, PO Box 56, Dunedin, New Zealand. E-mail: rachel.brown@stonebow.otago.ac.nz.]

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