Wednesday, October 26, 2005

Weight Loss: Glycemic Index (GI) and Glycemic Load (GL)

If you read the headline today, Reducing GI does not boost effects of low-calorie diet, you might be left with the impression that reducing the glycemic load in your weight loss diet will have no benefit. Think again!

As I read through the article, I wanted to see the data, so I headed over to the American Society for Nutrition to read the paper in their journal, the Journal of Nutrition. The abstract was freely available, but didn't add any more information from the article about the study, so I ponied up the $8.00 for access to the full-text titled, Reduced Glycemic Index and Glycemic Load Diets Do Not Increase the Effects of Energy Restriction on Weight Loss and Insulin Sensitivity in Obese Men and Women.

The reason I wanted to see the full paper is because, to-date, the research that is published shows there are a number of benefits to reducing the glycemic load in your diet and this study seemed to counter the evidence we already have supporting reduced glycemic load.

Specifically the benefits shown in various studies include:
  • Increased insulin sensitivity
  • Greater losses of body fat on a low GI/GL calorie restricted diet
  • Significantly improved measure of triglycerides
  • Increased HDL coupled with lowered LDL cholesterol
  • Higher levels of satiety reported by those following a lower GI/GL diet

Personally, I think the GI/GL is a bit too complicated for practical use by the general public. Too many things confound the usefulness of the GI/GL in an everyday free-living setting - mixed meals simply register all over the place on the GI/GL scale, making it difficult to accurately assess how a meal is really going to affect you.

That said, the research into the why and how of the GI/GL is beneficial to our understanding of how different foods - specifically carbohydrate foods and mixed meals - affect our metabolism. This particular study asserts that low glycemic index and load diets have no differing effect on weight loss or insulin sensitivity when compared with a high glycemic index/load diet - the abstract makes that assertion clear: Weight loss and improved insulin sensitivity scores were independent of diet composition. In summary, lowering the glycemic load and glycemic index of weight reduction diets does not provide any added benefit to energy restriction in promoting weight loss in obese subjects.

So, what did the full-text say?

  • Each diet group lost body weight during the 12-wk feeding phase of the study but the amount lost did not differ among the groups.
  • At 12-weeks the HF and LGI groups tended to maintain their lean body mass (LBM).
  • The calculated HOMA scores were significantly improved at 12 wk compared with baseline in all 3 groups. The improvement in the LGI group was significantly greater than the improvement in the HF group at wk 12.

This was the first phase of the study. Let's look at the results in some real numbers, rather than general statements, shall we?

At 12-weeks, the real pounds lost by each group were calculated as:

  • High-GI lost 9.3kg average, or 20.5-pounds
  • High-Fat lost 8.4kg on average, or 18.5-pounds
  • Low-GI lost 9.95kg on average, or 21.9-pounds

At twelve-weeks, which group had a greater weight loss? The Low-GI group lost 1.4-pounds MORE than the High-GI group. Statistically this is not considered "significant," but if you were the individual trying to lose weight, which approach would you consider better?

So far as lean body mass losses, how did each group do? (Remember, LOSING lean body mass is BAD)

  • High-GI lost 4.8kg on average, or 10.6-pounds of LBM
  • High-Fat lost 2.6kg on average, or 5.7-pounds of LBM
  • Low-GI lost 3.04kg on average, or 6.7-pounds of LBM

Still with me? Good...

When you look at both total weight lost and the loss of lean body mass, here is how each group did for FAT LOSS:

  • High-GI lost a total of 20.5-pounds, with 10.6-pounds of LBM, for a fat loss of 9.9-pounds
  • High-Fat lost a total of 18.7-pounds, with 5.7-pounds LBM, for a fat loss of 12.8-pounds
  • Low-GI lost a total of 21.9-pounds, with 6.7-pounds of LBM, for a fat loss of 15.2-pounds

Now which dietary approach would you want to follow?

Can it even get any better? How did the various groups do with insulin as measured by serum insulin?

  • High-GI at 12-weeks had a reduction of 20.1 pmol/L
  • High-Fat at 12-weeks had a reduction of 6.3 pmol/L
  • Low-GI at 12-weeks had a reduction of 28.5 pmol/L

But, the study was 36-weeks, wasn't it?

What happened in the 24-weeks following the initial 12-weeks?

Well, researchers set the participants out into "free-living" conditions and instructed them to continue to follow their group diet. How did that go? Ummm....all groups lowered their glycemic load by choosing foods that were low glycemic index, thus confounding the final results!

All 3 groups, despite receiving dietary instruction for their specific assignment, consumed diets of relatively low glycemic index and low glycemic load when making their own food selections. The glycemic indices of the diets at 24 wk differed (P = 0.014), with subjects in the LGI group initially consuming a lower glycemic index diet than the other 2 groups; however, at 36 wk, diet glycemic indices did not differ among the 3 groups (P = 0.14). The 5-d food record review showed that subjects in the LGI group tended to choose lower glycemic index foods but members of the other 2 groups simply increased dietary fat.

Basically this tells us that the final 24-weeks of this study are useless to measure if there does exist a benefit for weight loss OR improvements in other health risk markers since everyone in the study was eating a diet that was low glycemic index.

How the researchers had the gall to assert this data shows that the glycemic index/load offers no benefit is beyond me.

The data from the time participants were within controlled conditions - 12-weeks - is very compelling - especially for lean body mass preservation while losing weight.

Yes, the total weight lost between the groups is not "statistically significant," but the data clearly shows when you assess both total weight lost and lean body mass lost, the low glycemic index/load approach was significantly better with more body fat lost and more lean body mass maintained!

The fact that the last 24-weeks of the study were corrupted by all groups eating a lower glycemic index/load and higher fat diet should have been stated as a confounder in the abstract and abstract results with the researchers being open about how and why their final results are confounded and that reaching a conclusion is impossible due to the confounding factors!

If nothing else this study speaks volumes about how important it is to go to the data - the primary source - and see what happened, do the math and make an evaluation for yourself about what the study found. While the headline today is screaming that a low glycemic index/load offers no benefit for weight loss - this data shows the opposite - those on the low GI/GL diet lost more pounds, more body fat and preserved more lean body mass!


  1. Anonymous1:53 PM

    Thanks for looking beyond the headline and providing this analysis. As often happens, the conventional wisdom created by superficial reporting leads us in the wrong direction!

  2. What a fallacy it would be to think that a diet high in glycemic load has no effect on weight loss!
    Foods with a high GL are very quickly broken down in the body into sugar and cause a spike in insulin levels, resulting in a craving for more carbohydrates. The key to long term weight loss is breaking this vicious cycle.Foods with a low GL are broken down more slowly and keep blood glucose and insulin levels from sharp fluctuations.