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Laura Dolson

Low-Carb Diets Bad for the Brain? (Don't Worry!)

By December 14, 2008

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brainIt's all over the news -- a new study supposedly showing that low-carb diets impair brain function. Titles of news articles go from the relatively restrained "Low-Carb Diets Can Affect Dieter's Cognition Skills" to the ridiculous one in Backpacker Magazine: "Low-Carb Diets Can Make You Stupid". One even says that low-carb diets can "shrink the brain"! So, what is the truth about this research?

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

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December 15, 2008 at 2:43 pm
(1) Richard Feinman says:

The authors are not responsible for mis-representation in the press, but there is substantial the bias within the paper. This includes a couple of statements about the biochemistry that are incorrect.

Authors write: “However, as the body does not have vast glycogen stores, a continual dietary source of carbohydrates must replenish these stores. The body will consume its glycogen stores in a matter of 1–2 days. Low-carbohydrate diets, particularly in the initial introductory phase, contain little or no carbohydrate—restricting intake to below 20 g/day.”

Dietary carbohydrate is not required to replenish glycogen stores. Protein provides glucose through gluconeogenesis and people on low carbohydrate diets typically maintain glycogen at 60 % of starting values. Depletion of glycogen is seen in prolonged exercise but that is different. Also, whereas we think of glycogen as the more the better, this is really only true of athletes. Glycogen is a storage substance like fat and excess will lead to higher blood sugar and may not be generally good.

“the body will begin to metabolize body fat into ketone bodies, which can then be used, albeit less efficiently, by the brain and body as fuel.”

Ketone bodies are MORE efficient fuels than glucose. I went back and did the calculations.
The efficiency of acetoacetate is 0.2 ATP/g or, in terms of calorimeter values, about 34 % (hydroxybutyrate is slightly higher). Glucose efficiency, by comparison, is 0.14 ATP/g or 26 %.

The real problem is that the paper is patently propaganda. It starts out “…to propose that very low carbohydrate diet plans may have long-term effects on cognitive functioning in individuals following such diets in comparison to individuals consuming adequate levels of carbohydrate. “ This would, of course, be Ok, even though it is not a long-term study, and even though we don’t really know what “adequate” carbohydrate means, but the paper ends with the un-attributed quotation:
‘Lose 30 pounds before the holidays,” “drop a jean size in two weeks,” and “7 days to a slimmer you” are phrases designed to lure the prospective dieter into adopting one of the multitude of diet plans over all the others’
and then follows that with
‘Low-carbohydrate diets have gained in popularity because they make promises of rapid weight loss…’
I don’t think those ads are for low carb diets. In general, the author does not even give the semblance of a scientific study.

Most people who are overweight consider it a great psychological burden, not to mention the distress that accompanies diabetes for which carbohydrate restriction always plays a part. There is little to support the idea that the American Dietetic Association diet has had great success in helping these people. That the author of the article, who is in a psychology department, has set out to limit the options of people who are fighting fat and is insensitive to this psychological stress is really sad.

December 21, 2008 at 11:19 am
(2) lowcarbdiets says:

Thanks so much, Dr. Feinman, for this insightful commentary!

December 22, 2008 at 4:21 am
(3) Adrienne Larocque says:

I went low-carb 3 years ago, and I can attest to the lifting of the “mental fog.” I compare it to being stoned on carbs! Not only do my brain and my immune system function much better now, I lost 30 lbs in 3 months and my triglycerides went from 174 to 49! As a scientist (Ph.D. in Geochemistry) I know that not every study that gets published is well-designed. Based on my own experience, I believe that the body needs a little time to adjust to low-carb living, because glucose is so easy to use as fuel. Of course, just because something is easiest does not mean it is best (compare grabbing a burger at McD’s with cooking your own dinner using fresh ingredients). Now, on those occasions when I eat too much carb, I end up with stomach cramps, constipation fatigue and even nausea.

May 9, 2011 at 7:48 am
(4) Lisa in Colorado says:

Thank you for clearing this up!
I saw this on TV and was a little worried about it, I appreicate the clarification!!!

November 27, 2012 at 1:15 pm
(5) LMS says:

I have been low-carbing for several months. I took a lactose breath test this morning for which I had to drink 1 1/2 cups of milk (18 grams of carbs) at one time. I came across this article because my head is in a fog, and I wanted more information on what might be going on. I’ve been very successful in studying for some important exams while I’ve been low-carbing, but now I feel the same way that I’ve felt when trying to study or work after having taken an antihistamine too recently. (I’m having to read this post several times because the words are swimming around in my head, and I’m not sure whether it makes sense.) How about a study on that? It doesn’t exactly fit the brain-shrinking and stupid theories.

September 14, 2013 at 4:04 am
(6) Lee says:

This article is complete BS. As a recovering anorexic, a low carb diet can definitely have serious effects on the brain. I started off on a low carb diet to lose weight. As I got thinner and thinner I developed an eating disorder and hypoglycemia along with that because of my severe restricting. Carbs control blood sugar levels and fuel the brain. When the brain doesn’t get carbs it eats away at muscle and fat stores and when you have no fat stores left it eats away at your brain. The brain is a muscle. I had no fat left on my body. My brain was deprived of glucose – depleting its energy supply and compromising my brain’s power to concentrate, remember, and learn. Mental activity requires a lot of energy. Low carb diets are not healthy.

October 6, 2013 at 6:30 pm
(7) lowcarbdiets says:

Hi, Lee, and sorry you had a bad experience with carbohydrate reduction. We will have to agree to disagree about much of what you’ve said here, starting with “the brain is a muscle”. Best wishes.

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