Influence of low and high dietary fat on physical performance in untrained males.

Med Sci Sports Exerc 1999 Jan;31(1):149-55
Pogliaghi S, Veicsteinas A

MedlineŽ abstract : "PURPOSE: Dietary manipulations have been used in athletes to enhance aerobic performance. We intended to verify whether the quality of energy substrate provided by food (percentage of total calories from fat and carbohydrates) per se has the ability to affect aerobic performance in sedentary humans. METHOD: Fourteen healthy sedentary males were sequentially submitted to 4-wk eucaloric diets (spontaneous diet: 30% of total calories from fat; low-fat, 15% fat; high-fat, 55% fat; and spontaneous, 30% fat). After each diet period, individual body mass, percentage body fat (plicometry), VO2max (incremental bicycle-ergometry) and endurance (pedaling time to exhaustion at 75%VO2max) were measured. VO2, VCO2, VE, R, and heart rate (HR) were measured at rest and during exercise tests. Body composition and performance data (VO2max and endurance) were compared for significant differences by repeated measures ANOVA. RESULTS: Subjects' body weight, percentage body fat, and fitness status (indicated by intercept and slope of the HR/VO2 linear relationship) did not change significantly during the study, thus ruling out the influence of these potential confounders. For a given workload, VO2, VE, and R were unaffected by diet composition. VO2max and endurance time were not significantly modified by the different diets. CONCLUSIONS: Our study showed no impact, positive or negative, of diet's macronutrient composition on physical performance. It appears that the quality of energy substrate provided by food does not have the ability to affect either maximal or submaximal aerobic performance in untrained individuals. "

My Comments:  Diets that prescribe a certain percentage of your calories from fat and high fat diets seem to be the latest fad.  This is only one of several studies to show that as long as you have an adequate intake of protein and carbohydrates, dietary macronutrient (fat, carbohydrates, protein) percentages do not significantly affect athletic performance and body composition.  Research has shown time and time again that it is total caloric intake that is important in determining weight gain or loss and not the macronutrient profile of the diet.  Consuming 30% of your calories from fat does not magically put your body in a "fat burning zone".   Proponents of high fat diets often claim that it takes three weeks for your body to adapt to a high fat diet and that after this adaptation period the body burns fat more effectively.  Researchers measured Respiratory Quotient (R) which is an indication of fuel substrate choice (fat vs carbohydrates) and found that it was not significantly different between diets.  This study, like many others, has shown no benefit from a 30% fat or high fat diet.  Although the author did not measure cholesterol, triglycerides or insulin resistance, these factors are often elevated by high fat diets, especially diets high in saturated fats.