Glycemic Load, the GI's Complex Brother, Turns Out to Be A Good Predictor of Postprandial GLP-1 and GIP Response

What does the glycemic load tell you about these foods? Right, an apple a day keeps the doctor away, ice-cream makes a healthy sweet treat and apple juice is one of those purportedly healthy drinks that are not on iota better than coke. It may be partly due to the correlation of low GL and high micronutrient content that going by the glycemic load is not the worst way to pick quality foods.
After taking one week off, the SuppVersity Science Round Up is back today. As usual the show will kick off at 1PM EST (update: download podcast) and I cannot complain about not having enough stuff to talk about. So here is a sneak peak on the news line-up for today:
  • insulin in the brain - how it's keeping you lean
  • breast cancer - protective & useless micronutrients
  • prenatal caffeine - negative effects on male offspring
  • mouth rinsing - caffeine & carbs, both work
  • ashwaganda - Lance's new dope?
  • betaine - the ergogenic w/ anti-NASH effect
There is more - as usual, so don't worry, there is more than enough left for the "Seconds", tomorrow ;-)

But now for the sugary news of the day...

The myth of the healthy low-GI diet is crumbling like stale white bread. Still, if you look around at diets that work (both for losing weight, as well as for weight maintenance and overall health), you will realize that they are mostly moderate in GI and, what's way more important, almost always have a low GL. It would therefore be negligent to abandon the whole notion of the importance of the glycemic response to certain foods. After all, it's practical relevance has been established in various contexts:
  • performance fuel for athletes (O'Reilly. 2010)
  • blood sugar reductions in type II diabetics (Fabricatore. 2011)
  • various types of cancer (Meyerhardt. 2012; Hu. 2013)
  • other chronic diseases (Barclay. 2008)
That said a recently published study by Shauna S. Runchey and her colleagues from Europe and the US sheds some light on one of the potential underlying mechanisms by which low glycemic load diets can promote health / ward off disease and could help you improve your exercise performance and physique.

Low GL foods are mostly healthy micronutrient-rich foods

What's the Glycemic Load (GL)? The glycemic load (GL) basically is an extension that to the GI concept that renders it practically relevant by weighing the GI with the amount of carbohydrates you ingest. Thus a high GI food with a low carbohydrate content (e.g. many fruits and certain vegetables) will have a medium, in some cases even low glycemic load. As the name already implies the GL does therefore give you an idea of the degree of blood glucose elevation a certain food will actually produce, thus making a statement about both nutrient quantity and quality.
You can find a  very comprehensive list of GLvalues for all sorts of foods and even whole meals here.
Before we do actually take a closer look at the study design and the results, the researchers present in the latest installment of the medical journal Metabolism, I just want to remind you of the fact that many of the natural low GL foods are laden with anti-oxidants, polyphenols, flavenols, fiber and all sorts of other healthy stuff so that many of the aforementioned benefits are probably a result of the perfect synergy of these (and other) micronutrients and cannot be attributed to the low glycemic load, alone.

The latter is obviously also the case in the trial at hand. And this goes despite the fact that the macronutrient composition, 15% energy from protein, 30% energy from fat and 55% energy from carbohydrate, of the high- (GL 250) and low glycemic load (GL125) diets the 89 overweight–obese (BMI 28.0–39.9kg/m²) and lean (BMI 18.5–25.0kg/m²) healthy adults had to follow for 28-days each was identical  (the study used a randomized controlled cross-over design, meaning that all subjects participated in two trials consuming both the high and low GL diet for 28 days each; the wash-out period in between was likewise 28 days long).

In that the 2x higher fiber intake in the low GL group may be only one example of potentially meaningful differences, which could partly explain the differential postprandial incretin (GIP and GLP-1) response you see in figure 1 (Weickert. 2005; Ma. 2012).
Figure 1: Glucose, Insulin, Insulin / Glucose ratio (marker of insulin sensitivity / glucose clearance), GLP-1 and GIP area under the curve after the ingestion of a standardized breakfast containing ~575kcal at a protein / carb / fat ratio of 11/32/80 (high GL) and 114/27/94 (low GL); data expressed relative to intergroup means (based on Runchey. 2013)
In that, it is interesting to remark that the fasting levels of glucose, insulin, etc. did not differ significantly between the high and low GL groups. They were however across the board more favorable in the low GL group, so that you could argue that the difference may have reached statistical significance after another months or so on the respective diets.

The fatter you are the more you benefit

Suggested read: "Burn Fat Like on Crack W/ GLP-1" (read more)
The probability that a longer study period would have yielded statistically different inter-group differences is by the way much higher in the participants with higher body fat percentage (women >30%, men >25%; measured by DEXA), where the differences in incretin (GLP-1 and GIP) baseline levels had a 2x higher p-value, i.e. were much closer to being statistically significant, than they were in the lean(er) study participants.

Against that background it is also questionable how significant the switch from a higher to a lower GL diet would be for you, who will hopefully have made it below the 30%, respectively 25% body fat mark for men and women already.

"Tell me about those glycemic loads"

If you take a peak at GL table, you will still realize that going by the glycemic load, can lead you to health-foods you may not even have considered eating or drinking before. What about the anti-catabolic tomato juice, for example? It has a GL of 4 compared to the highly juicy version of coke, called "apple juice" with a GL of 12 (even in its unsweetened, natural incarnation; cola has "only" 15, see table 1 for more).
Table 1: The glycemic loads of a couple of selected drinks and foods (picked from various websources)
As the example of the openly hailed apple juice, but also the unrighteously dreaded ice-cream goes to show you, GL based food choices can make sense... after all, you'd be way better off drinking tomato juice during a workout and having some ice-cream on your cheat day than drinking the some "healthy apple juice" and having a handful of dried fruits, like raisins as a sweet treat.

But isn't the GL just another fad? Not really. The GI is in fact mainstream and practically useless. Low glycemic load diets on the other hand have actually busted the even more mainstream-ish "just cut your fats" diets that are still hailed as the go-to standards by way too many nutritionists in various studies - e.g. 5x more weight and 2x more fat loss from a low GL vs. low fat diet in Ebbeling, 2007.
Bottom line: Just as with all those quantitative markers of "food quality", a term that has the word "quality", not quantity, in its name already, you better not base your dietary choices one concept alone. That the glycemic load of your diet probably is one of the better (mainstream) indicators of dietary quality.

That this is also due to the fact that a low GL does usually correlate with a higher micro-nutrient is yet a different story, though. Furthermore, it should be obvious, even if there may be certain timepoints, e.g. post-workout, and purposes, e.g. glycogen replenishment, when you want to prefer meals with higher glycemic loads (e.g. instant oats vs. quaker oats, bananas vs. apples and rice vs. salad).

  • Barclay AW, Petocz P, McMillan-Price J, Flood VM, Prvan T, Mitchell P, Brand-Miller JC. Glycemic index, glycemic load, and chronic disease risk--a meta-analysis of observational studies. Am J Clin Nutr. 2008 Mar;87(3):627-37.
  • Fabricatore AN, Wadden TA, Ebbeling CB, Thomas JG, Stallings VA, Schwartz S, Ludwig DS. Targeting dietary fat or glycemic load in the treatment of obesity and type 2 diabetes: a randomized controlled trial. Diabetes Res Clin Pract. 2011 Apr;92(1):37-45.
  • Hu J, La Vecchia C, Augustin LS, Negri E, de Groh M, Morrison H, Mery L; the Canadian Cancer Registries Epidemiology Research Group. Glycemic index, glycemic load and cancer risk. Ann Oncol. 2013 Jan;24(1):245-251.
  • Ma J, Pilichiewicz AN, Feinle-Bisset C, et al. Effects of variations in duodenal glucose load on glycaemic, insulin, and incretin responses in type 2 diabetes. Diabet Med 2012;29(5):604–8.
  • Meyerhardt JA, Sato K, Niedzwiecki D, Ye C, Saltz LB, Mayer RJ, Mowat RB, Whittom R, Hantel A, Benson A, Wigler DS, Venook A, Fuchs CS. Dietary glycemic load and cancer recurrence and survival in patients with stage III colon cancer: findings from CALGB 89803. J Natl Cancer Inst. 2012 Nov 21;104(22):1702-11.
  • O'Reilly J, Wong SH, Chen Y. Glycaemic index, glycaemic load and exercise performance. Sports Med. 2010 Jan 1;40(1):27-39. 
  • Runchey SS, Valsta LM, Schwarz Y, Wang C, Song X, Lampe JW, Neuhouser ML. Effect of low- and high-glycemic load on circulating incretins in a randomized clinical trial. Metabolism. 2013 Feb;62(2):188-95.
  • Weickert MO, Mohlig M, Koebnick C, et al. Impact of cereal fibre on glucose-regulating factors. Diabetologia 2005;48(11): 2343–53.
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