I’ve always been struck by the imprecision of how the terms “low-glycemic-index diet” and “low-glycemic-load diet” are used. In theory, these are diets that reduce post-meal blood glucose and their benefits derive from this property. In practice, they often differ from typical diets in multiple ways. If a “low-glycemic diet” is also higher in fiber and protein and/or lower in palatability, calorie density, and carbohydrate, can we really attribute its beneficial effects to its impact on blood glucose per se? To gain a better understanding of the depth and pervasiveness of this problem, I did a scientific literature search for randomized controlled trials of low-glycemic-index/load diets, selected the first ten I found, examined the details of the assigned diets, and evaluated the findings.
I performed a search in Google Scholar using the search terms “randomized trial low glycemic”. I scanned down the list until I had identified the first ten studies that met the following criteria:
- Randomized controlled trial
- Compares a low-glycemic-index or low-glycemic-load diet to a control diet
- Title describes the low-glycemic diet only according to its glycemic index/load, not other diet characteristics (e.g., high protein, legumes)
- Any length (single meal to long-term diet trial)
I extracted the following information from each study:
- First author
- Year published
- Summary of subject characteristics
- Length of intervention
- Diet summary
- Detailed diet description
- Was the between-diet comparison well controlled for glycemic index/load? Yes = differences in glycemic index/load were achieved without substantial differences in the types of foods consumed. Moderate = the types of foods differed substantially between groups, but major nutritional characteristics like protein and fiber intake were well controlled. No = major nutritional characteristics like protein and fiber intake differed between groups.
- Outcome of the intervention
My primary outcome was how well each diet was controlled for dietary variables other than glycemic index/load.
I compiled a table of the findings here. Trials were published between 2004 and 2011, and were all highly cited (105 to 413 citations according to Google Scholar). They lasted between 4 weeks and 18 months, and 6 of 10 involved subjects with diabetes.
Among the ten trials, five were clearly not controlled for glycemic index/load (“no”), four were moderately well controlled (“moderately”), none were well controlled (“yes”), and one didn’t provide enough information to judge.
Overall, the results of the trials suggest that low-glycemic diets can be helpful for blood glucose control in diabetes, but their effects are modest in people without diabetes. They are not very effective for weight or fat loss (relative to control diets, which varied) and they seem to have little impact on birth outcomes in the context of gestational diabetes.
In this limited but unbiased sample of highly cited papers on low-glycemic diets, I found that none isolated glycemic index/load as a variable, 40 percent were moderately well controlled for glycemic index/load, and 50 percent were poorly controlled for glycemic index/load. Another way of stating this is that all of the diet comparisons differed in ways other than glycemic index/load, and five of them differed in major ways that are likely to be confounding.
This is only a problem if we attribute the outcomes of these studies to glycemic index/load per se. If we all understand that “low-glycemic diet” often means “beans, nuts, fresh fruit, and slow-digesting whole grains that may improve health via multiple mechanisms relative to typical diets”, there’s no problem. This is the interpretation I believe is most accurate, but it’s not what I see in most discussions of these studies.
The findings of this brief literature survey suggest that we should be cautious about attributing the effects of low-glycemic diets to their impact on glycemia per se.