The effect of low-carbohydrate vs. low-calorie diets on exercise metabolism and performance

Statement of the Problem The purposes of the study were (1) to determine if a difference in performance levels exists between subjects on low-carbohydrate diets compared to those on low-calorie diets, (2) to determine if greater changes in body composition (total fat and weight) occur between subjects on low-carbohydrate compared to those on low-calorie diets, (3) to determine the type substrate metabolized at rest and during exercise of subjects on low-carbohydrate diets compared to those on low-calorie diets, (4) to determine the concentration of blood lactate at rest and after exercise in subjects on low-carbohydrate and low-calorie diets and note any differences that may exist between the groups.

Procedure of the Study Fifteen college women, five of whom acted as control and five of whom were in each diet group, were subjects for the study. All were required to meet basal conditions prior to their weekly test. All subjects were given a pretest before the two diet groups began their treatment. The treatments consisted of a low-calorie diet for one group and a low-carbohydrate for another. Four weekly tests were given to all subjects after the treatments were begun. All tests were conducted between 7:00 and 1:00 a.m. and followed a definite routine.

Subjects rested quietly for 30 minutes, the last five of which an expired air sample was collected and the resting heart rate determined. Then, the subject was weighed, was measured for skinfold thicknesses in six sites, and gave a finger-tip blood sample. The exercise portion the test consisted of a graded treadmill test and another finger-tip blood sample. During the treadmill test, the subject walked until she reached 90% of HR_max. Every three minutes, during the last minute of the stage, an expired air sample was obtained and heart rate recorded. Contents of the air and determination of the heart rate were determined by standard procedures. Four minutes after termination of the test, a finger-tip blood sample was taken. The lactate content was determined by an enzymatic technique. The two diet groups kept daily records of their caloric and/or carbohydrate intake.

An analysis of covariance was used to determine if differences existed among the three groups in all variables of the weekly tests. If a difference existed, a Newman-Keuls Post Hoc test was performed on the groups to determine which were different from the others.

Summary of Findings In summarizing the results of the study, the following conclusions seem justified.

1. No difference in performance levels were seen between either diet group and furthermore, none existed between the diet groups and the control group.

2. Body composition (fat and weight) did not change significantly between the two diet groups and only at the first week of treatment did the diet groups experience a greater reduction than the control group.

3. No differences occurred in the resting R values between any of the groups. Differences occurred in the exercise R values during

both low-calorie and control. No significant differences existed between the diet groups or between the diet groups and the control group in either the resting or exercise lactate levels.