December 11, 2008
Brain Performance Lowered On Low Carb Diet

The brain is very sensitive to blood sugar level. The brain also craves carbo and sugars. Not coincidentally, women who go on a low carb diet perform more poorly at some cognitive tasks than women who go on low calorie diets which cut all types of calories.

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.

The more general lower calorie diet has more carbs in it than the low carb diet. The sugars in those carbs probably helps feed the brain and cause higher brain performance.

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.

This was only a 3 week study. Possibly on a longer diet the result would be different. A diet with severe carbo restriction which drives the body to make ketones might performance higher cognitive performance than the low-carb diet used above.

Share |      Randall Parker, 2008 December 11 11:09 PM  Brain Appetite

Fat Man said at December 12, 2008 8:19 AM:

I would tell my wife, but she would kill me.

Lono said at December 12, 2008 10:03 AM:

I can confirm this, in fact, occurs.

Last year around this time, on a bet, I did an extreme low carb diet - droppping about 37.5 pounds in about 6 weeks.

(not enough to win the bet, however, but it cleaned out any excess fat cells around my organs which was most welcome)

The first week - I felt a little dizzy, the remaining 5 weeks thereafter I felt normal - and was able to perform programming tasks sufficently, but I felt like my edge was off, that I lacked the innovatitve, out of the box, thinking that is the halmark of my work style.

Within 48 hours of returning to a reasonable carb intake, my previous creative, imaginative, innovative approach to problem solving had returned in full - with no lasting side effects except for a passionate dislike for Beef Jerky, Eggs, and all forms and flavors of Buffalo Wings.

It was an interesting feeling to have my intellect returned - as I could not realize the significance of my deficit until I had my faculties fully restored.

I, for one, would not recommend anyone undertake such a diet without specifically limiting it to no longer than six weeks.

That is long enough to clean up your system (almost like a registry clean if you'll excuse the analogy) and allow you to return to a healthy and active lifestyle.

It certainly made me appreciate my unique intellectual gifts - and I would prefer to never again lose full access to my reason, at least not voluntarily.

tom bri said at December 12, 2008 10:56 AM:

I have spent long periods, months on end, on a low-carb diet. I liked it. I didn't notice any lowering of mental faculties. In fact I felt a whole lot better. I continue now to have a fairly low carb intake, but I am not a fanatic about diet.

There was very little information in that article. I would be interested to know what sort of low carb diet was being used. Was it high fat or high protein? What did they consider low carb? Zero carb?

momochan said at December 12, 2008 2:07 PM:

The devil really is in the details. For the past 5 years I've been eating a diet completely bereft of grains, fruit, potatoes, and starchy vegetables such as carrots and pumpkins. But I eat at least 1/2 cup cooked beans with every meal -- I become ravenously hungry, not to mentioned constipated, if I miss that component. So my diet is more low on the glycemic index than low in carbs.
Anyway, the most noticeable psychological effect is a lack of mood swings. I haven't noticed any effect on my memory, one way or the other. My immune system is stronger, and I don't gain weight.
Seeing as how this diet has been a definite plus for me, and I would recommend anyone to at least give it a try, I have to wonder about the exact conditions of the study. What does low carb mean to them?

Randall Parker said at December 12, 2008 10:17 PM:

momochan makes an important point about glycemic index. Regardless of whether you are trying to cut your total carbs consumed eat food of lower glycemic index.

If you are trying to eat less carbo then eating low glycemic index food is especially important because what carbo you do eat will be broken down more slowly and therefore the sugar will enter your blood more slowly over a longer period of time. This will keep the brain fed for a larger part of the day.

Lono said at December 15, 2008 10:33 AM:


That is a good point - I find my concentration, and also problem solving skills, to be significantly impaired for a time after eating high glyemic index foods (alone without offsetting protein) for breakfast.

The deficit is far more problematic than that of what I experienced on the near-zero carb diet I was on, but mercifully it usually lasts less than an hour or so.

Removing sodas from my diet completely, for over a year now, has certainly improved my overall concentration as well - although I will have a "go fast" energy drink every once in a while - if I do not have any serious analytical work that needs to be done that day.

Mirco said at December 16, 2008 6:48 AM:

There are problems with this study:
1) Small size of the group selected (19)
2) Age (from 22 to 55)
3) They were able to choose the diet they want follow
4) We don't know how the diets were (how much calories? How much carbos?)
5) We don't know if the reduction in mental abilities is a direct consequence of the diet or a secondary consequence (constipation and related problems or something other).

Brock said at December 19, 2008 7:19 AM:

These women were probably insulin resistant from years on a standard Western diet. When the body switched to ketones the brain would not be able to quickly switch gears due to years of sugar abuse. It takes much longer than three weeks to restore insulin sensitivity in the brain.

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