A low carbohydrate-protein supplement improves endurance performance in female athletes

Journal Title (Medline/Pubmed accepted abbreviation): J. Strength Cond. Res.
Year: 2011
Volume: 25
Page numbers: 879-888

Summary of Background and Research Design

Background: It has been shown that a beverage with protein (PRO) and low carbohydrate (CHO; 3% CHO + 0.75% CHO or 4.5% CHO + 1.2% PRO) is just as effective at prolonging endurance as a 6% CHO beverage. It has also been shown that a blend of carbohydrates (i.e. glucose, maltodextrin, and fructose) can be oxidized for energy more rapidly than any single source of carbohydrates alone

Hypothesis: A beverage formulated with 1.2% protein and 3% carbohydrate, as a mixture of glucose, maltodextrin, and fructose, would improve endurance compared to a traditional 6% CHO beverage in female athletes.

Subjects: Fourteen well-trained female cyclists and triathletes, age 30.4 ± 1.6 yrs. old

Experimental design: randomized, double-blind, repeated measures design

Treatment: CHO: 6% dextrose (also known as glucose), 24 kcal/100 mL; CHO+PRO: 1% dextrose; 1% fructose, 1% maltodextrin, 1.2% whey protein isolate; 16.9 kcal/100 mL. (CHO+PRO had 33% less calories than CHO.)

Protocol: Initially, the participants’ VO2max and ventilatory threshold (VT) were determined on a stationary bicycle. The VT is the point at which breathing rate increases at a rate greater the need for oxygen when exercise intensity is being increased at a linear rate. On the day of the trial, the subjects cycled for 3 hrs at intensities alternating between 45 and 70% of their VO2max They consumed 275 mL of a beverage before the trial and then every 20 min while cycling. At the end of the 3 hrs, they cycled to exhaustion at an intensity near their VT. Exhaustion was determined when subjects could not keep up a cycling rate of 60 rpm. It is the time that the cycled here that was used as a metric for endurance. Throughout the ride, 5 respiratory gas samples were collected and 4 blood samples. The gas samples were used to calculate oxidation rates of carbohydrate and fat. Blood samples were analyzed for glucose, insulin, lactate, and myoglobin (marker for muscle damage) concentrations. One week later, the subjects repeated the protocol with the other treatment.

Summary of research findings
  • The CHO+PRO beverage increased endurance compared to the CHO beverage. With CHO+PRO, subjects cycled 49.94 ± 7.01 min and with CHO they cycled 42.36 ± 6.21 min (p < 0.05). The most probable explanation for this phenomenon is that the CHO+PRO beverage allowed the body to efficiently burn the ingested energy and spare endogenous CHO (i.e. glycogen).
  • Blood glucose levels were significantly greater during exercise with CHO compared to CHO+PRO (mean ± SD= 4.47 ± 0.12 mM for CHO and 4.07 ± 0.12 for CHO+PRO).
  • There were no significant differences among treatments for insulin, lactate, or myoglobin concentrations throughout the ride.
  • There were no significant differences in substrate use for energy (carbohydrate or fat) during the first 3 hrs of the ride. Substrate use was not calculated during the ride to exhaustion. This finding is interesting because the CHO+PRO beverage had 50% less calories than the CHO beverage.
  • Heart rate was significantly lower while the subjects consumed CHO+PRO (130 ± 3.13 beats/min) compared to CHO (132.80 ± 2.92 beats/min). This suggests that the heart was more efficient during this trial.

Interpretation of findings/Key practice applications

The CHO+PRO beverage increased performance 15.2% compared to the CHO beverage, despite providing less overall energy. This is a potentially significant finding. However, no data on baseline muscle glycogen levels or pre-exercise dietary intake were presented in this paper. In addition, there is considerable debate regarding the methods used to evaluate exercise performance. Some argue that the time to exhaustion test has greater variability and is less representative of an actual sporting event when compared with a time trial. Others counter that the time to exhaustion endpoint does have relevance for some types of physical activity in which a person does exercise to the point of exhaustion (e.g., tactical exercise like militarymissions or firefighting).
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