Tuesday, May 3, 2016

Endurance Athletes May Benefit From High-GI Meals Before Competition: 18% Increased Endurance During Time Trial

Hillariously sweet, super cheap, and surprisingly effective: glucose!
While most of you will probably already have discarded the notion that you can only lose weight if you eat a diet with an extremely low GI (on a side note: Taubes' own study falsified his "insulin theory of obesity", recently), the notion that the ingestion of high glycemic index foods before exercise could ruin your performance, because your glucose levels, after an initial spike, will plummet and you will crash, still looms large.

With that being said, athletes around the world have, somewhat paradoxically, stuck to complex carbs before competition only to then fuel their performance with extremely sugary intraworkout bars, gels and drinks.
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Now, a recent study from the University of Florida, shows that "a fiber-free, glucose-derived meal may be superior to a low glycemic fiber-free whole meal in supporting moderately high intensity exercise in some highly motivated recreationally trained endurance athletes" (my emphasis in Waggener. 2016) - not exactly what you'd expect, right?

No reason to feel embarrassed, though: Even the authors of said study expected that their likewise fiber-free whole milk supplement with a low glycemic (LGM | GI = 41), would "equal or surpass the effects of an iso-kilocaloric, high glycemic (HGM) pre-exercise meal of glucose" (GI = 100).
Timing + intensity (and fiber) may matter, here! While there are surprisingly few studies that actually compare high vs. low GI carbohydrate sources pre-workout, a 2001 study by Kirwan using totally different CHO-sources (2 different breakfast cereals: rolled oats (moderate GI, 61; MOD-GI) or puffed rice (high GI, 82; HI-GI), combined with 300 mL of water) suggests that high-fiber low GI meals are advantageous if (a) the time-trial begins after 45 minutes of rest (instead of 2h of medium-intensity exercise) and you are (b) training / competing at a low intensity of only 60% VO2peak and thus for 2h+, which is obviously not exactly representative of endurance sports competitions or resistance training workouts w/ higher intensities.
Furthermore, the scientists expected that both would kick the ass of an artificially flavored placebo. when either of the three treatments was consumed 30 minutes before a 2-hr submaximal steady state ride at 55% of VO2 max that was followed by brief rest, and then by a final time-trial to exhaustion on the cycle ergometer (TTE) at 80% of VO2 max - the test was repeated on three separate days.
"The test drinks in this study consisted of whole cow’s milk (837.2 kJ / 320 ml fluid volume: 16 g carbohydrate, 10.7 g fat, and 10.7 g protein; Publix, Inc.) and two contrast drinks, one consisting of glucose polymer drink iso-kilocaloric to the whole milk supplement (837.2 kJ / 300 ml fluid volume: 50 g carbohydrate, 0 g fat, and 0 g protein; Cardinal Health, McGraw Park, IL) and a placebo/control drink (water). The placebo was an artificially flavored, unsweetened beverage (2 g of NutraSweet in 300 ml bottled water; [...])" (Waggener. 2016).
To avoid that the subjects' baseline diet would mess with the results, the diet in the days before the test was standardized (high in carbs 60.5% = typical end. athlete diet) and controlled via food logs.
Figure 1: Stamina (length in minutes) during the time trial after 1h of low-medium intensity exercise (Waggener. 2016).
Much in contrast to what the scientists expected, the LGM (whole milk) did not exert a greater ergogenic effect in this study when compared to the isokilocaloric HGM (glucose).

In fact, there was a non-significant (+18% time to exhaustion, blue bar in Figure 1 | not significant due to the large inter-individual differences) advantage for the HGM meal, which outperformed both, the LGM and the placebo on the most important test: the time to exhaustion trial - and that in spite of the fact that it was performed after a 2h! medium intensity workout, during which the subject's serum glucose and insulin levels should have performed the initially hinted at, purportedly ergolytic (=the opposite of performance-enhancing) roller-coaster ride.
Ah, and before you ask: The carbs - high or low GI - did not reduce the fat oxidation, the respiratory quotient was the same. Intra-workout fat oxidation doesn't matters much (or not at all) for fat loss, but alas... (Waggener. 2016).
Bottom line: As surprising as it may seem, the scientists' conclusion that a glucose-derived meal "may be superior to a low glycemic fiber-free whole meal in supporting moderately high intensity exercise in some highly motivated recreationally trained endurance athletes" (Waggener. 2016), is accurate, even if the time between the ingestion of the glucose solution and the actual high intensity work is so long that the "high GI carb bashing) would have you expect the athletes to fall into a hypoglycemic coma ;-)

What? Oh, yes! It would certainly be nice to see this being repeated with different types of exercises or athletes, but with the recreationally trained cyclists with a rather wide range of relative VO2 max values, the subjects in the study at hand are more representative of the average fitness enthusiast than pro-athletes or noobs | Comment on FB!
  • Kirwan, John P., et al. "Effects of moderate and high glycemic index meals on metabolism and exercise performance." Metabolism 50.7 (2001): 849-855.
  • Waggener, Green T., et al. "The Effect of a Low Glycemic vs. High Glycemic Pre-Exercise Meal in Recreationally Trained Endurance Cyclists." (2016).