It
has been shown that the protein requirements for athletes may
well exceed that suggested by the (USRDA) .80 g/kg/day. If an
individuals protein requirement increases in response to exercise,
then changes in protein metabolism will become apparent. When
the body is in a homeostatic state, protein synthesis is equal
to protein degradation and the protein requirement of the body
for tissue maintenance is satisfied. The most common way to detect
changes in protein metabolism is to assess nitrogen balance of
the body.
Positive nitrogen balance occurs when the total nitrogen excreted
in the urine, feces and sweat is less than the total nitrogen
ingested. Positive nitrogen balance must exist for new tissue
to be synthesized. When dietary protein intake or total energy
intake is inadequate to maintain tissues total nitrogen balance,
negative nitrogen balance occurs and new tissue is unable to
be synthesized. When the body is in nitrogen balance, protein
and energy intake is sufficient to maintain tissue protein needs
and the amounts of nitrogen entering and exiting the body are
equal.
The results of nitrogen balance studies on endurance athletes
indicates that these athletes have protein requirements that
exceed the USRDA of 0.8 g/kg/day. A study found that endurance
athletes (defined as training for at least 12 hours per week
for at least 5 years) require 1.37 g/kg/day of protein to maintain
nitrogen balance compared to 0.73 g/kg/day for sedentary individuals.
It appears that weight training can also lead to a daily protein
requirement that exceeds the current USRDA. It has been found
that 2.0 to 2.2 g/kg/day of protein was barely sufficient to
maintain nitrogen balance during moderate intensity weight training.
Furthermore, weightlifter's protein requirements increased proportionally
to training intensity. Research has shown that 2.0 to 2.6 g/kg/day
of protein are required for periods of very intense weight training,
whereas protein intakes of 2.0 g/kg/day maintained a positive
nitrogen balance during periods of less intense weight training.
It is clear that athletes need to consume more protein than
the current USRDA for 0.8 g/kg/day in order to maintain nitrogen
balance. Conversely, since the requirements of carbohydrates,
and overall calories also increase with physical activity, the
recommended proportion of calories from protein does not change
significantly. With a calorie sufficient diet, protein requirement
values needed to maintain positive nitrogen balance of both weight
trained and endurance trained athletes constitutes intakes of
12% to 20% of total daily calories.
Paul GL. Dietary protein requirements of physically active
individuals. Sports Med 1989; 8:154-176.
Obese individuals eating a slightly higher protein diet (25%
of calories from protein), lost significantly more weight and
body fat than those eating a slightly lower protein diet (12%
of calories from protein). (Skov, et. al., 1999)
Overweight women consuming a diet with a carbohydrate/protein
ratio of 1.4 (125 g protein/day) lost more weight and body fat
than those eating a ratio of 3.5 (68 g protein/day).
Layman DK, Boileau RA, Erickson DJ, Painter JE, Shiue H,
Sather C, Christou DD. (2003) A reduced ratio of dietary carbohydrate
to protein improves body composition and blood lipid profiles
during weight loss in adult women. J Nutr. 133(2):411-7.
Skov AR, Toubro S, Ronn B, Holm L, Astrup A (1999). Randomized
trial on protein vs carbohydrate in ad libitum fat reduced diet
for the treatment of obesity. Int J Obes Relat Metab Disord.
23(5):528-36.
Also see Macronutrient
Ratio Studies.
