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Old 01-14-2009, 05:49 AM   #14
Darryl Shaw
Senior Member
Join Date: Apr 2008
Posts: 681

Originally Posted by Emily Mattes View Post
Is it too low in carbs? I dunno, that seems like an OK level of carbs to me.
Weightlifting and Crossfit type workouts are typically short high intensity affairs which are dependent on aerobic/anaerobic glycolytic energy systems with the principal fuel source being glycogen so the fact that Scott is running out of gas during his workouts combined with the evidence of his fitday log clearly indicates that he isn't eating enough carbs to adequately replenish his glycogen stores between workouts.

The Effect of Carbohydrates and Fats on 24 Hour Nitrogen Balance
As has been discussed, energy has a tremendous nitrogen sparing effect [34]. However, a related topic concerns the differential effects of fats and carbohydrates on nitrogen balance. In this context, McCarger [83] investigated the effects of a high carbohydrate or high fat diet on nitrogen retention, substrate utilization, and serum hormone concentrations in six healthy male participants. The diets were administered at maintenance and at 75% of maintenance calories. Results indicated that the high fat diet produced slightly greater nitrogen retention in the 75% restricted diet than the high carbohydrate diet, while no differences existed between diets at maintenance. Results such as this have led Millward to suggest that "for now energy intakes can be considered independently from the composition of that energy as determinants of NB, thus simplifying the issue [34]." However, these results need to be replicated; particularly, in the context of exercise training.

Carbohydrates and Fats in Resistance training exercise
While carbohydrates and fats may spare nitrogen in a similar manner, it is important to recognize that carbohydrates are critical for high intensity exercise. As an illustration Jacobs et al. [84] investigated the effect of depleting muscle fibers of glycogen on strength levels. It was found that glycogen depletion in both fast and slow fiber types in the vastus lateralis was associated with impaired maximal muscular strength produced during a single dynamic contraction, as well as with increased muscle fatigue patterns. Further, it has been well established that a decrease in intensity can cause a significant loss of adaptation [43,85-88]. These results suggest that a decrease in carbohydrates may indirectly decrease muscle tissue, or impair further adaptations.

Interaction between carbohydrates and protein/amino acid intake
Koopman and colleagues [89] investigated the effects of carbohydrate (0.3 g per kg-per hour) (CHO), carbohydrate and protein (0.2 g per kg-h) (CHO-PRO) and carbohydrates, protein and leucine (0.1 g per kg-h) (CHO-PROL) on net protein balance, and amino acid oxidation rates. Results indicated that net balance was negative in the CHO condition, and positive in the CHO-PRO and CHO-PROL conditions, with the latter attaining the highest values. These results paralleled plasma insulin concentrations, with insulin being highest in the CHO-PROL condition, intermediate in the CHO-PRO condition, and lowest in the CHO condition. The net balance was improved through increased protein synthesis and decreased protein breakdown in the CHO-PROL condition relative to the other two conditions. Further protein oxidation was lowest in the CHO-PROL condition. The rationale may be that leucine intake enhances insulin secretion [89], and independently increases protein synthesis [90,91]. It is generally thought that insulin enhances protein balance through hindering protein degradation [40,92], which was supported by this study.

However, the role of insulin in stimulating protein synthesis is in debate [89]. In vitro studies [93-95] have supported insulin's role in regulating protein synthesis, while a number of in vivo studies have shown discrepancies in protein synthesis [96]. As an illustration Biolo et al. [92] found that insulin infusion increased protein synthesis at rest, but not after resistance training exercise. The authors concluded that it was the decreased amino acid availability which depressed the stimulatory effect of insulin. This was supported by Biolo et al. [97] when they found that maintained amino acid levels in the presence of hyperinsulemia increased protein synthesis. Further, Hiller and colleagues [96] suggested that discrepancies seen between in vitro studies and in vivo studies centered around plasma concentrations of insulin. To test this question, Hiller et al. [96] raised plasma insulin levels to concentrations similar to studies conducted in vitro, while maintaining amino acid concentrations. It was found that hyperinsulemia increased protein synthesis greatly. Therefore, mechanisms which enhance the insulin response to food may enhance protein accretion. The efficacy of combining carbohydrates and protein on insulin secretion was demonstrated by Ivy et al. [98] who found that the combined effects of protein and a high glycemic carbohydrate were greater on stimulating insulin secretion than their independent effects.

Summary of the effect of carbohydrates and fats on protein balance
In summary it appears that both carbohydrates and proteins have similar nitrogen sparing effects [34,83]. In this context it may be advisable to increase fats when carbohydrates are lowered. However because carbohydrates are critical to athletic performance [84] the athlete should be conscious of decreased intensity and performance with decreased carbohydrate intakes. Finally, there appears to be an interaction effect between protein and carbohydrates in stimulating insulin secretion [98-100]. This latter effect may be beneficial when manipulated for protein accretion purposes.


Quantification of the metabolic response aids in ascertaining the nature and extent of the energy requirements imposed by exercise. During high intensity exercise, virtually all of the energy is supplied by the net oxidation of glycogen while fat oxidation plays a more prominent role during lower intensity exercise. Therefore, the lower limit of carbohydrate required above resting needs is equal to the portion of the total energy cost derived from carbohydrate sources. There is no upper limit of additional carbohydrate intake that could be eaten to satisfy the extra caloric requirement since carbohydrate intake will restore any endogenous energy stores that were used during exercise, regardless of the intensity of exercise. The recommendation of a high carbohydrate intake to provide caloric balance in exercising individuals is supported by the observation that exercise performance at high intensity is improved by a high carbohydrate diet, and exercise performance at low intensity is relatively insensitive to the source of the caloric intake. Limited dietary studies are consistent with predictions based on the metabolic response. At exercise intensities below 65% VO2 max, the percent fat and carbohydrate in the diet makes little difference on exercise performance, provided adequate time is allowed to adapt to a high-fat diet. On the other hand, exercise ability during high-intensity exercise is significantly limited by a high-fat diet. A consideration of importance beyond the aspect of energy balance is the anabolic effect of insulin on muscle protein synthesis after exercise. Provision of carbohydrate after exercise is likely to stimulate muscle protein synthesis to a greater extent than a corresponding amount of fat. Dietary fats may offer practical advantages to the athlete but if fats are consumed at the expense of carbohydrate intake, many established benefits of high carbohydrate intake in terms of performance may be sacrified.

(Bold = my emphasis.)
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