Short post today on a new study suggesting that restricting carbohydrates in women with gestational diabetes mellitus (GDM) does not reduce their chances of needing insulin to control blood sugar during pregnancy. Spanish researchers randomly assigned 152 women with GDM to a control diet or a "low carbohydrate" diet. Each of the diets contained at least 1800 calories. The control diet contained 55% of calories from carbohydrate. And the "low carbohydrate" diet? It specified 40% of calories as carbs, or at least 180 grams. Definitely not low-carb, and significantly more than most of these women need, given the increased insulin resistance that is the hallmark of GDM. In addition, compliance was measured with three-day food records kept by the participants in their own homes rather than in a metabolic ward or other controlled setting where the amount of carbs actually consumed could be accurately assessed. The women's food records indicated they consumed between 155-191 grams of carbs daily; however, it's generally accepted that most people tend to underreport the amount of food they eat. The researchers concluded that a low-carbohydrate diet does not reduce insulin needs, but this is misleading given that this diet contained considerably more carbs than the women should have been advised to eat.
I've read other studies looking at similar "low carbohydrate" diets that actually weren't low enough to be therapeutic. Part of the problem is that there's no universally accepted definition of that term, but in my opinion, up to 190 grams of carbohydrate daily really seems like a stretch. Had these women been prescribed a diet containing less than 100 grams instead, I'm fairly confident the number requiring insulin would have been much lower than those following the control diet. I suspect they would have also had an easier time maintaining a healthy rate of weight gain during their pregnancies.
Unfortunately, given that the media often only report the "conclusions" in research abstracts, the take-home message will be that carb restriction doesn't decrease the need for insulin in GDM, so there's no reason to try it. And that's a shame, because we know that decreasing carbohydrates does lower insulin requirements, provided that the amount consumed is low enough to keep blood sugar at a healthy level. 40% of caloric intake clearly exceeds that limit.
1. Morena-Castilla C, et al. Low-carbohydrate diet for the treatment of gestational diabetes: a randomized controlled trial. Diabetes Care. e-published ahead of print April 5, 2013 do1:30.2337/dc12-2714
Franziska Spritzler, RD, CDE