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Energy Restriction: The Effect It Has On The Female Muscular Function.

Today I am going to present a study as it relates to female endurance athletes and the effects of a calorie restricted diet. Can this be a good thing?

Many women who are hoping to lose a few pounds turn to energy restriction as a way to accomplish the caloric deficit needed to lose bodyweight. The normal protocol for this method is restricting calories to about 75% of maintenance level while continuing their normal workout programs. The one concern with this method is that on a calorie restricted diet, substrate utilization in the body often changes, which could cause a change in performance values.

In this article, I'll discuss a study (Belcastro, A., McCargar, L., McKenzie, D. & Parkes, S., 1998) where female endurance athletes were assigned to two groups, one that ate at maintenance, and one at 75% below for 2 weeks while continuing their normal exercise training program and the effects that this caloric restriction created.

The Study

When someone goes on a diet, their body's method of substrate utilization begins to change, which may alter muscle tissue structure and function due to a selective breakdown of functional muscle proteins. Often, this change increases the amount of fat oxidation, which is what one hopes for when they go on a reduced calorie diet.

The Subjects

    The 14 subjects used for this study had the following requirements: eumenorrhea, not currently following a therapeutic diet program, a body fat that was between 16 and 20%, a VO2 max level of at least 42 ml/kg/min, and were running at minimum, 30 km/week as well as participating in other cross training activities for at least 3 hours of the week (Belcastro et al., 1998).


Enter the total distance covered in meters in 12 minutes and then press the 'Calculate' button.

1600 meters = 1 mile
5280 feet = 1 mile
3 feet = 1 meter

Distance meters
Vo2 Max

    Prior to the experiment, the researchers took baseline measurements for various muscle performance measures so they could be compared after the caloric deficit was induced.

The First Four Days

    During the first 4 days of the study, the groups ate a maintenance level diet that had been determined through the Harris-Benedict equation assuming a heavy activity energy factor.

    While they were on this diet, if the women started seeing changes in their weight, their daily caloric allotment was altered to find their individual maintenance level, which was then used to determine the 25% calorie reduced diet.

Fourteen Day Testing Period

    After this period, the subjects were divided into two groups; one continued to eat at this determined maintenance level and the other group assumed the reduced diet.

    During the 14-day testing period, the subject's meals were weighed and measured to ensure accuracy and they were instructed to consume all the food given to them and only supplement the diet with noncaloric beverages.

    Substrate Utilization

      Substrate utilization, the type of fuel the body uses during rest and exercise, was measured by calculating the subjects REE (resting energy expenditure), and then resting and postprandial respiratory quotient through indirect calorimetry.

      This technique monitors the concentration of oxygen and carbon dioxide in the air that the subject breathes out while in an enclosed space.

    Muscular Function

      Muscular function was measured by an isokinetic dynamometer which calculates average speed and torque over a range of motion in both the concentric and eccentric muscle actions. The exercise selection was that of a leg extension and measurements were taken at velocities 30, 90, 120, and 180 degrees/sec for each of the 4 leg extensions at maximum intensity that were performed.

      Click To Enlarge.
      Leg Extensions.

      The values that were recorded were both peak (highest torque obtained throughout the exercise) and average (average torque throughout the range of motion) torque (Belcastro et al., 1998).

The Results

    Before the study was conducted, all subjects had similar measurements of age, weight, height, number of years exercising and VO2 max. During the study, the average time the women spent exercising in the modes of running, cycling and swimming was 100 minutes a day and the average number of miles ran was 38 km per week (Belcastro et al., 1998).

    The results of the study showed that after the 2-week period on the diet, the non-diet group (100% of caloric intake) had the same measurements for bodyweight, BMI and skinfolds, while the dieting group (75% of caloric intake) had significant decreases in these measurements.

Height In Inches:
Body Mass Index

    It should be noted as well that lean body mass did not significantly change throughout the study, thus showing that these women lost predominately fat mass. One interesting finding is that the dieting group's resting RQ was seen to be significantly lower than that of the non-dieting group post study and lower than it had originally been in the pre-study measurement testing.

    This demonstrates the fact that going on a calorie reduced diet will cause your body to burn a greater percentage of its overall daily calories from fat as compared to carbohydrates, thus changing substrate utilization.

    In regard to muscle function, no significant differences were seen in overall muscle function between pre and post diet in either group (Belcastro et al, 1998).

    Eccentric & Concentric

      Eccentric and concentric values did change however for both groups. In the non-dieting group, eccentric values taken at 90 degrees/sec increased whereas in the dieting group, it increased at 180 degrees/sec.

      On the concentric side of things, the 120 degree/sec measurement decreased in the dieting group and concentric peak torque for the non-dieting group increased at the 30 degree/sec measurement (Belcastro et al., 1998). While there were differences seen, as stated above, there were no major noticeable differences that would cause a huge decrease in performance level as a result of going on a short-term calorie reduced diet.


A few factors to keep in mind with regard to this study are the results that showed an increase in torque value could have been due to the training effect in the individual and not whether or not they were following the diet.

Additionally, since the diet only took place over 2 weeks, this may not have been enough time to fully determine the effects of following a reduced calorie diet over a longer period of time that is often required to lose a greater amount of weight.

If the subjects had continued with this diet for a greater duration, or if there had been more of a restriction, these results could have been significantly different.

So What Is The Take Home Message From This Study?

    This study suggests that short-term, moderate dieting appears to have no effect on muscular strength values and that it will enhance your body's ability to burn a greater percentage of calories from fat therefore helping reduce bodyfat levels.

    As long as the diet is properly balanced, this type of restriction appears to be a safe and effective method without risking athletic performance.


  1. Belcastro, A., McCargar, L., McKenzie, D., & Parkes, S. (1998). Effect of Energy Restriction on Muscle Function and Calcium Stimulated Protease Activity In Recreational Active Women. Canadian Society for Exercise Physiology, 23(3)