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

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

GI glucose glycemic index high GI low GI performance preworkout time to exhaustion TTE

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!

References:

• 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).

Weight Loss, 'Metabolic Damage' and the Magic of Carbs? Human Study Probes Effects of Carbohydrate Content, GL & GI on Diet-Induced Suppression of Resting Metabolic Rate

Weight Loss, 'Metabolic Damage' and the Magic of Carbs? Human Study Probes Effects of Carbohydrate Content, GL & GI on Diet-Induced Suppression of Resting Metabolic Rate

dieting GI glycemic index glycemic load high carb high carbohydrate lose fat low carb low carbohydrate metabolic damage resting metablic rate RMR weight loss

Will slimming down from a 120 cm to a 60 cm waist always ruin your metabolic rate and set you up for weight regain or can high GI protect you from yoyoing?

Broscience tells us: "Carb up to preserve your resting metabolic rate." And in fact, there is some scientific evidence that suggests a link between high(er) carbohydrate intakes and increased thyroid function. The same amount of T3 will trigger a sign. higher stimulation of lipolysis and fat oxidation, for example, on high vs. low carb diets (Mariash. 1980). Low carb diets, on the other hand, lead to significant reductions of the active thyroid hormone and increases in the 'thyroid receptor inhibitor' rT3 - even in healthy individuals and if the energy intake is standardizes (Serog. 1982; Ullrich. 1985). So, is broscience right? Well, overfeeding studies show a similar increase in T3 in response to protein, fat and carbohydrates (Danforth Jr. 1979). So refeeds should work, irrespective of their carbohydrate content...

# Women appear to be particularly prone to # metabolic damage - more on # female fat loss:

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As you can see, it is hardly possible to confirm or reject the "carb up to prevent metabolic damage" (=prevent the diet induced over-proportional reduction in resting energy expenditure) hypothesis based on the existing evidence. A recent study by J. Philip Karl and colleagues who tried to determine "the effects of diets varying in carbohydrate and glycemic index (GI) on changes in body composition, resting metabolic rate (RMR), and metabolic adaptation during and after weight
loss" (Karl. 2015), however, may yet take us one step further towards rejecting or confirming this commonly heard of idea.

Figure 1: Overview of the key parameters of the study design and dietary composition (Karl. 2015).

In said study, Karl et al. randomly assigned adults with obesity (n = 91) to one of four diet groups for 17 weeks. As you can see in Figure 1, the diets all subjects were provided with differed in percentage energy from carbohydrate (55% or 70% | Figure 1, top-right) and GI (low or high, Figure 1, bottom-right) but were matched for protein, fiber, and energy. The study design itself comprised 5 phases:

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"Phase 1 was a 5-week weight maintenance phase in which weight maintenance energy needs were determined by adjusting provided energy intake to maintain stable weight. Mean Phase 1 energy intake was 12.2 MJ/day with 48% energy provided as carbohydrate, 16% as protein, and 36% as fat. Following Phase 1, participants were randomized by the study statistician to their Phase 2 dietary assignment using computer-generated randomization. The four diets differed in carbohydrate content (55%, ModCarb or 70%, HighCarb of total energy) and dietary GI (less than 60, LowGI or 80, HighGI), and were provided for 12 weeks at 67% of the weight maintenance energy intake determined in Phase 1. 

Participants were allowed to increase their energy intake during Phase 2 by requesting additional, randomization-appropriate foods from the metabolic kitchen if too hungry to be adherent. Phase 3 was a 5-week weight maintenance phase during which food was provided according to randomization. Energy intake during Phase 3 was prescribed to support weight maintenance at the new, lower body weight, and was predicted from body weight and energy intake measured at the end of Phase 2, with adjustment for self-reported physical activity. Phase 4 was a 12- month follow-up period during which participants selected and pre pared their own meals after being provided with instructions on fol lowing the diet to which they were randomized" (Karl. 2015)

To assess the effects of this sequence of induction (weight maintenance), and weight stabilization phases, the body weight, body composition, RMR, and metabolic adaptation (measured RMR vs. predicted resting metabolic rate = RMR) of the middle aged study participants (49-64 years) were measured before and after all phases of the study.

Figure 2: (A) Weight loss and (B) percentage of total weight loss attributable to fat mass and fat free mass while consuming provided-food diets differing in glycemic index (GI) and percent energy from carbohydrate (55%, ModCarb and 70%, HighCarb) for 17 weeks (n = 79). Values are mean 6 SEM. Weight loss analyzed by repeated measures ANCOVA, body composition by two-factor ANOVA. a,bMain effect of time; asignificant decrease from baseline (P < 0.001), bsignificant difference from Phase 2 end (P < 0.001). No diet effects (main effects or interactions) for any comparisons. GI, glycemic index; HighCarb, 70% energy from carbohydrate; ModCarb, 55% energy from carbohydrate (Karl. 2015).

Interestingly, the analysis of this data revealed no significant inter-group differences in terms of any of the relevant study outcomes. Yes, you read me right: This means that neither the GI, nor the GL, nor the carbohydrate content of the diet had statistically significant effects on weight loss, body composition, RMR, or the metabolic adaptation aka "metabolic damage" due to weight loss.

Figure 3: Measured resting metabolic rate as a function of predicted metabolic rate (Karl. 2015). Note: If there was no "metabolic damage", the solid line which represents the ideal body-weight dependent decline of energy expenditure and the dashed line which represents the actual ratio of the measured to the predicted RMR should be congruent.

While there were no inter-group differences and neither the amount or the type of carbohydrates had an effect on the reduction of the metabolic rate, there is still one interesting result you can see in the right graph in Figure 3. Said graph depicts the ratio of the measured to the predicted metabolic rate during the 5-week weight maintenance phase. If you look closely, you will realize that it suggests that having a high predicted RMR, i.e. being heavier, being taller and being more muscular, is associated with a non-significant decline of the non-predicted reduction of the energy expenditure (=metabolic damage) and thus a narrowing of the gap between the solid and dashed line.

"Solid and dashed? I don't get it!"

You're asking how I can support this hypothesis? Well, the dashed line that represents the true ratio of the actual to the predicted RMR approaches the theoretical one (the solid line) for higher RMR values. If this was more than a trend, it would suggest that two things: (a) Losing less weight and thus maintaining a higher predicted metabolic rate protects against metabolic damage (that would be useless). And (b) being tall and muscular and thus having a naturally high(er) predicted RMR can protect you from suffering metabolic damage when you lose weight.

Unfortunately, it's not possible to tell which (if any) of the two options is correct. If I had to make an educated guess, though, I would say it's a combination of both: The weight change of an average 5.5 kg did not wary too much and was withing 95% confidence intervals of [-7.1 kg, -4.6 kg]. In conjunction with individual physiological qualities of people with higher baseline RMRs, it could still explain the narrowing of the gap between predicted and true RMR after dieting.

Figure 4: Changes in body composition (absolute value in kg) after 20 weeks and after weight loss phase 2 (Karl. 2015).

Bottom line: As Karl et al. point out, "neither low-GI relative to high-GI diets nor moderate-carbohydrate relative to high-carbohydrate diets showed differences with respect to effects on changes in body composition or resting metabolism during weight loss when confounding dietary factors were tightly controlled in a study providing all food for 22 weeks" (Karl. 2015).

This does not just go against the mainstream assumption that low GI and/or low(er) carbohydrate diets facilitate weight loss, fat loss and weight maintenance (see data in Figure 4 for an overview of these parameters, it also contradicts the initially mentioned broscientific assumption that carbohydrates, in general, and high GI carbs, in particular, have a protective effect against the unexpected diet-induced reduction of basal energy expenditure many people know as "metabolic damage". If there's anything of which the study at hand suggests that it could protect you from such unexpectedly large decrease in RMR, it's not high GI carby, but rather an already high(er) baseline RMR (see Figure 3).

And what does that tell us? Right! Since a high predicted RMR is a function of (a) being male, (b) being tall, and (c) being muscular, all three attributes may protect you from diet-induced "metabolic damage" | Let me know your thoughts and comment on Facebook!

References:

• Danforth Jr, Elliot, et al. "Dietary-induced alterations in thyroid hormone metabolism during overnutrition." Journal of Clinical Investigation 64.5 (1979): 1336.
• Karl, J. Philip, et al. "Effects of carbohydrate quantity and glycemic index on resting metabolic rate and body composition during weight loss." Obesity 23.11 (2015): 2190-2198.
• Mariash, C. N., et al. "Synergism of thyroid hormone and high carbohydrate diet in the induction of lipogenic enzymes in the rat. Mechanisms and implications." Journal of Clinical Investigation 65.5 (1980): 1126.
• Serog, P., et al. "Effects of slimming and composition of diets on VO2 and thyroid hormones in healthy subjects." The American journal of clinical nutrition 35.1 (1982): 24-35.
• Ullrich, Irma H., Philip J. Peters, and M. J. Albrink. "Effect of low-carbohydrate diets high in either fat or protein on thyroid function, plasma insulin, glucose, and triglycerides in healthy young adults." Journal of the American College of Nutrition 4.4 (1985): 451-459.