The study in question is in the February 2009 issue of the scientific journal Appetite. The researchers in the psychology department at Tufts University allowed 19 women to choose between a low-carb diet and a standard ADA diet to follow for 3 weeks. Before the study began, and again at 1, 2, and 3 weeks the subjects tooks a number of tests of mood and various types of cognitive ability.
What did the low-carb diet consist of? This is a critical question, and the answer is going to surprise you. For the first week, the subjects were instructed to eat ZERO carbohydrate. Think what this means: no vegetables, no fruit, no dairy products, no nuts. Not even eggs. The second week they were allowed to add 5 to 8 grams of carbohydrate per day, and the third week they were allowed to go "way up" to 10-16 grams per day. We are not told whether fiber was allowed to be subtracted from the totals. Note that this is way below even the most restrictive of the popular low-carb diets, such as Atkins Induction phase which allows 20 grams of carb per day.
What were the results? Well, it's pretty interesting that even with this severe carb restriction there really wasn't much of an effect past the first week. At the end of week one, there were some differences in a minority of the cognitive tests. This isn't going to surprise anyone who has experinced "carb crash" when the body runs out of glycogen stores in the first week. What does Atkins recommend when this happens? Temporarily increase carbs until the body adjusts! Can you imagine the carb crash these people experienced on zero carbs? By the end of the first week, most people who experience this are over the worst of it, but I'm not surprised that some in the study had a little residual impairment.
Despite the fact that the zero carb group did less well on a couple of the tasks, the low-carb group actually consistently performed better on a task requiring sustained attention. In the discussion, the researchers point out other studies which have shown that "eating meals high in protein or fat in the short term reduces fatigue and improves tasks requiring vigilant attention relative to meals high in carbohydrate."
The study also assessed mood, and found no difference between the groups except that the ADA group did worse on a measure of confusion.
I found it sort of amusing that the researchers seemed to attribute the improvement in cognitive tasks at the second and third week to the very small increase in carbohydrate intake. They seemed to think that this would replete the body's glycogen stores. In fact, this is unlikely since a normal glycogen store is in the vicinity of 400-500 grams of carbohydrate. More likely, the body at least partially replaces some of the glycogen via gluconeogenesis. At the same time, it's converting to primarily using fat for energy instead of carbohydrate. The bottom line is that our bodies adjust to a low-carb diet over a period of time.
The Moral of the Story? Don't believe sensationalistic headlines. Get the facts. And know that many people who respond well to low-carb diets report that after the first week or two they feel the lifting of a mental fog they didn't even realize was there.
Anci, Kristen, Watts, Kara, et al. "Low-carbohydrate weight-loss diets. Effects on cognition and mood ." Appetite. 52/1 (2009)
Image © Vasiliy Yakobchuk