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Researched
and Composed by
Jacob Wilson, BSc. (Hons), MSc. CSCS
What is
the effect of Carbohydrates and Fats on 24 hour nitrogen balance?
As has been discussed, energy has a tremendous nitrogen sparing
effect (Millward, 2004). However, a related topic concerns the
differential effects of fats and carbohydrates on nitrogen balance. In
this context McCarger (1989) investigated the effect of a high
carbohydrate and high fat diet on nitrogen retention, substrate
utilization, and serum hormone concentrations in six healthy male
subjects. 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. This led Millward (2004) 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.”
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, Kaiser, and Tesch
(1981) investigated the effect of depleting muscle fibers of glycogen on
strength levels. It was found that glycogen depletion in both fiber
types in the vastus lateralis was associated with impaired maximal
muscular strength produced during a single dynamic contraction, as well
as with reduced muscle fatigue patterns. Further it has been well
established that a decrease in intensity, that would be associated with
decreased glycogen stores would cause a significant loss of adaptation
(Wilson and Wilson, 2005; Hickson et al.1981, 1982, and 1985; Sheply et
al.,1992). 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 (2004) 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 is that leucine intake enhances
insulin secretion (Koopman, 2004), and independently increases protein
synthesis (Crozier, 2005; Garlick, 2005). It is generally thought that
insulin enhances protein balance through hindering protein degradation (Biolo,
1999; Wolfe, 2002), which was supported by this study.
However, the role of insulin in stimulating protein synthesis is in
debate (Koopman, 2004). In vitro studies have supported insulin’s role
in regulating protein synthesis, while a number of in vivo studies show
either no stimulation of protein synthesis or various discrepancies
(Hiller et al., 1998; Boirie, 2001).
As an illustration Biolo et al. (1999) 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. (1995) when they found that maintained
amino acid levels in the presence of hyperinsulemia increased protein
synthesis. Further Hiller and colleagues (1998) suggested that
discrepancies seen between in vitro studies and in vivo studies centered
around plasma concentrations of insulin increase. To test this question
Hiller et al. (1998) 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 insulinogenic
response to food, may enhance protein accretion. The efficacy of
combining carbohydrates and protein on insulin secretion was
demonstrated by Ivy et al. (2002) who found that the combined effects of
protein and a high glycemic carbohydrate were greater on stimulating
insulin secretion then their independent effects ( also for a review on
insulin’s role in protein accretion see Knowlden, 2003)
Summary
In summary it appears that both carbohydrates and proteins have similar
nitrogen sparing effects. In this context it is may be advisable to
increase fats when carbohydrates are lowered. However because
carbohydrates are critical to athletic performance 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. This latter effect may be beneficial when manipulated for
protein accretion.
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