Low-carb diets can affect dieters’ cognition skills

Tufts study compared women’s cognition on low-carb and reduced-calorie diets

MEDFORD/SOMERVILLE, Mass. — A new study from the psychology department at Tufts University shows that when dieters eliminate carbohydrates from their meals, they performed more poorly on memory-based tasks than when they reduce calories, but maintain carbohydrates. When carbohydrates were reintroduced, cognition skills returned to normal.

“This study demonstrates that the food you eat can have an immediate impact on cognitive behavior,” explains Holly A. Taylor, professor of psychology at Tufts and corresponding author of the study. “The popular low-carb, no-carb diets have the strongest potential for negative impact on thinking and cognition.”

Taylor collaborated with Professor Robin Kanarek, former undergraduate Kara Watts and research associate Kristen D’Anci. The study, “Low-carbohydrate weight-loss diets. Effects on cognition and mood,” appears in the February 2009 edition of the journal “Appetite.”

While the brain uses glucose as its primary fuel, it has no way of storing it. Rather, the body breaks down carbohydrates into glucose, which is carried to the brain through the blood stream and used immediately by nerve cells for energy. Reduced carbohydrate intake
should thus reduce the brain’s source of energy. Therefore, researchers hypothesized that diets low in carbohydrates would affect cognitive skills.

Study participants included 19 women ages 22 to 55 who were allowed to select the diet plan they preferred — either a low-carbohydrate diet or a low-calorie, macronutrient balanced diet recommended by the American Dietetic Association. Nine women chose a low-carbohydrate diet and 10 selected the low-calorie diet.

“Although the study had a modest sample size, the results showed a clear difference in cognitive performance as a function of diet,” says Taylor.

The 19 dieters completed five testing sessions that assessed cognitive skills, including attention, long-term and short-term memory, and visual attention, and spatial memory. The first session was held before participants began their diets, the next two sessions occurred during the first week of the diet, which corresponded to the week when low-carb dieters eliminated carbohydrates. The final two sessions occurred in week two and week three of the diets, after carbohydrates had been reintroduced for those on the low-carb diet.

“The data suggest that after a week of severe carbohydrate restriction, memory performance, particularly on difficult tasks, is impaired,” Taylor explains.

Low-carb dieters showed a gradual decrease on the memory-related tasks compared with the low-calorie dieters. Reaction time for those on the low-carb diet was slower and their visuospatial memory was not as good as those on the low-calorie diet. However, low-carb dieters actually responded better than low-calorie dieters during the attention vigilance
task. Researchers note that past studies have shown that diets high in protein or fat can improve a person’s attention in the short-term, which is consistent with the results in this study.

Participants were also asked about their hunger levels and mood during each session. The hunger-rating did not vary between participants on a low-carb diet and those on a low-calorie diet. The only mood difference between dieters was confusion, which was higher for low-calorie dieters during the middle of the study.

“Although this study only tracked dieting participants for three weeks, the data suggest that diets can affect more than just weight,” says Taylor. “The brain needs glucose for energy and diets low in carbohydrates can be detrimental to learning, memory and thinking.”

Another study where a faulty hypothesis is “confirmed.” What does the study actually say?

The LC participants was told to eat 0 carbohydrates during week 1, and that’s when the “cognitive impairment” was recorded. Why 0 carbs? I know of no lowcarb diet that recommends such a low carb intake, even during an induction phase.  And hadn’t they heard about “induction flu?” The test was done at a time when the body was switching over from burning glucose to burning fat and many people feel slighlty sick during this period.

I am convinced that if the test had been done at a later stage, no “impairment” would have been found. And, actually, it wasn’t. The test was repeated at week 2 and 3 after the LC group had “added back carbs.” So how much carbohydrate did they add back? 5-8 grams in week 2, and another 5-8 in week3. They were still below Atkins’ induction levels! And that very small amount of carbs eliminated the “cognitive impairment” recorded in week 1.

Perhaps they should have tested in week 2 and 3 without adding any carbs back. Or even better, started out with a higher level of carbs as recommended by any lowcarb diet. What would the results have been then? Would any “impairment” have been detected?

But of course, any impartial evaluation was not the purpose of the study. They just wanted to show that LC diets are bad for you.

For instance, they conclude:

Performance on the reverse digit span was significantly worse for participants in the low-carbohydrate condition after 1 week on the diets relative to the ADA diet (mean number correct + SEM). Performance at other time points did not vary as a function of diet.

“Performance at other time points did not vary as a function of diet?” That’s not exactly true. The LC dieters showed a better performance than the ADA dieters in week 3, at the time limited to 10-16 grams of carbs per day.


……..suggests that the two diet groups reported similar confusion rates for the 48-h and 3-week test sessions, but that ADA dieters reported higher confusion for the 1-week and 2-week sessions

The majority of the tests showed no difference between the groups.

And this “study” proves that low carb diets cause “‘cognitive impairment?” I question the cognitive ability of the researchers!

So does Kimorexia: Benefits of cigarette smoking

If you enjoyed this post, please consider to leave a comment or subscribe to the feed and get future articles delivered to your feed reader.


[…] Original post by mariasols […]

thanks for getting the word out about that flawed logic of the press release Tuffs released. I hope the editors of the science department of the Feb mag this study is coming out in check the data carefully.

did you notice the pre test scores and the week 3 scores for the low carbewrs eating just 10-16 grams of carbs would confirm that Atkins induction level carbs do not impair cognative skills.

Great post. You know, I think I’m just going to stop calling it a study. :-)

Leave a comment