The short answer - not much when you look for differences between diets that are high or low glycemic index.
But y'all know I'm not about the short answer already, so how about we take a look at a newly published study, No effect of a diet with a reduced glycaemic index on satiety, energy intake and body weight in overweight and obese women, that has some very interesting findings that were notable but ignored in the paper.
Make a note of the paper title, it's important later!
Let's start with some background. For this study, researchers recruited otherwise healthy, obese women who had hyperinsulinemia. They were free of underlying conditions like diabetes, were not taking medications for cholesterol, hypertension or other medical conditions, were not pregnant or breastfeeding, and had not recently dieted to lose weight, nor recently lost or gained weight, thus deemed "weight stable" to participate.
The researchers then determined their habitual food intake and calories consumed each day, noting the subjects consumed an average of 1859-calories/day - 48.8% carbohydrate, 16.7% protein and 34.4% fat [227g carb, 77.6g protein, 71g fat].
The study was then underway, with the women consuming one of two diets for 12-weeks in a cross-over design, so the study lasted a total of 24-weeks. The objective of the study was to "investigate whether a diet with a reduced glycaemic index (GI) has effects on appetite, energy intake, body weight and composition in overweight and obese female subjects," so the researchers provided cereals, breads, rice, pasta and potatoes with different glycemic index for the women to consume while following either the low or the high glycemic diet.
The women were told to consume their foods as they did in their habitual diet and continue eating how they usually ate before starting the study.
In the paper, the researchers noted that "[t]here were no differences in energy intake, body weight or body composition between treatments. On laboratory investigation days, there were no differences in subjective ratings of hunger or fullness, or in energy intake at the snack or lunch meal."
They concluded, "This study provides no evidence to support an effect of a reduced GI diet on satiety, energy intake or body weight in overweight/obese women. Claims that the GI of the diet per se may have specific effects on body weight may therefore be misleading."
Yesterday, Sandy Szwarc at Junkfood Science blog penned her article Carbs humbug? — Are carbs really fattening? about the study, with her perspective being that this is one more piece of evidence that carbohydrates are not fattening.
She wrote, "Despite oft-repeated fears that refined “bad” carbs send our glucose and insulin levels soaring, these researchers found “no differences in glucose, insulin and non-esterfied fatty acid responses to the lower versus higher GI breakfasts, with no differences in either total area under the curve or single measurements at any time point.” Concerns that carbohydrates in the diet stimulate insulin production and are responsible for obesity and illness were recently examined here."
I point to the article at the Junkfood Science blog because it's a good example of seeing the forest while missing the trees.
Let's look at the data in the study now, comparing the dietary changes with their baseline starting values...with the baseline number first, followed by the measured effect of each diet (values are rounded up or down where appropriate).
Low-Glycemic Index Diet, 12-weeks
Baseline - Low-GI Diet
Weight: 87.5kg - 89.1kg
Waist: 103cm - 105cm
Fat Mass: 42.3kg - 44.54kg
Calories: 1859/day - 1928/day
Carbohydrate: 227g/day - 248g/day
Protein: 78g/day - 82g/day
Fat: 71g/day - 69g/day
High-Glycemic Index Diet, 12-weeks
Baseline - High-GI Diet
Weight: 87.5kg - 89.2kg
Waist: 103cm - 106cm
Fat Mass: 42.3kg - 42.9kg
Calories: 1859/day - 1874/day
Carbohydrate: 227g/day - 223g/day
Protein: 78g/day - 83g/day
Fat: 71g/day - 71g/day
The researchers didn't spend much time in the paper discussing the weight gain, fat mass increase, or change in waist circumference. In fact, they noted "[t]here were no differences in body weight, waist circumference or fatness between intervention periods. Weight increased during both intervention periods, although weight gain did not differ between treatments."
They didn't discuss the differences from baseline, nor seek to understand how it is that hyperinsulinemic women, who are obese, gained both weight and body fat while consuming diets that were not that much higher in calories than baseline - certainly not high enough to theoretically gain as much as they did!
But more importantly - look at the increase in waist circumference - more than one inch (3cm) is not something to sneeze at in just 3-months, nor is the scary gain of 4.9-pounds of body fat while consuming the low-GI diet for 12-weeks!
Why did that happen?
The researchers didn't discuss the possibility that high insulin played a role or that blood glucose from carbohydrates consumed had anything to do with it.
Perhaps it happened because insulin and blood glucose didn't budge?
The paper does not include baseline values of insulin and glucose in the table to compare it to each diet, but does provide a telling look at how dietary glycemic index, in the context of a high carbohydrate diet, when compared with each other had no effect on either:
And in case you missed it, what this showed, quite nicely I'll add is that 12-weeks of a high carb diet - whether the perceived junky high-GI or "healthy" low-GI carbs - can make you gain weight, body fat and see your waist grow bigger too...while doing absolutely nothing to resolve a state of hyperinsulinemia!
But yeah, the brilliant conclusion was that there was no difference between low-GI and high-GI...let's not confuse ourselves with the more important findings here!