Low Carb Diets Affect Short Term Memory

ConfusionA soon-to-be published study in Appetite [2009 Feb;52(1):96-103] (but apparently already published online earlier this summer) done by a group at Tufts seems to have “proven” that when you remove carbohydrates for three weeks from the diet of people who have depended on them for decades, you get some short-term memory loss, fuzzy thinking and/or mood swings. In what appears to me to be yet another colossal waste of time and money, the Tufts researchers concluded that “the brain needs glucose for energy, and diets low in carbohydrates can be detrimental to learning, memory and thinking.” What? They got grant money for this? Most of my Primal Blueprint 30-day challengers could have told you that for free! If you understand the power of gene expression and the ability of the human body to acclimate, this study only “proves” what we’ve known for over 100 years.

Here’s the gist: the researchers gave memory, mood and “cognition” tests weekly to two groups of women who were dieting over a period of three weeks. Nine women were on a “low-carb” program where they could eat all they wanted of fat and protein-based foods (we say “ad libitum”), but zero carbs the first week and less than 16 grams/day for the remainder. The other 10 women were on a reduced calorie, balanced diet recommended by the American Dietetic Association with plenty of carbs. The results: women on the low carb diet “performed worse on memory-based tasks than ADA dieters… These impairments were ameliorated after reintroduction of carbohydrates.” You can view the complete abstract here.

And based on this they suggest that low-carbs diets aren’t all they’re cracked up to be. Another Tufts/ADA anti-low-carb conspiracy?

There are so many problems with this study I don’t know where to begin. I guess I’ll start with the most glaring issue, since it renders moot almost all the others. The length of the study was only three weeks. If you are a reader of MDA, if you have read Michael Eades, Modern Forager, TheIFLife, Jimmy Moore and any number of “low-carb” blogs, you’ll know that three weeks is about the average acclimation period to any low-carb diet. And at less than 20 grams a day, this was a true low-carb/ketogenic diet. During that three week period, when you cut carbs so dramatically from previously levels, your memory and cognition will likely get fuzzy, your mood will swing and your energy levels will drop. This is normal and to be expected when cells that have been programmed for decades to burn primarily glucose are now being deprived of this main fuel. It’s especially true of the brain. But as we discuss here so often, life is about gene expression and the signals your send those genes. When you deprive a sugar-burning human of his/her carbs and replace those calories with fat and protein, your genes get the signal to start up-regulating fat metabolism and ketone production.  After three weeks, the acclimation (we call it “keto-adaptation”) is usually complete and brain cells start to thrive on a mix of ketones and the small amount of glucose produced in the liver via gluconeogenesis in the complete absence of carbohydrates. In fact, much of the brain prefers ketones to glucose when you are in ketosis. It’s a beautiful thing. Energy overall increases, mood stabilizes, you think more clearly, that regular three-hour hunger for carbs disappears, you burn of fat stores, you can skip meals with no noticeable effects, etc. Had the researchers followed their subjects for a few more weeks they would surely have seen all this. In fact, Lt. Frederick Schwatka wrote about it in the New York Herald in 1880 when describing an expedition with the Inuit: “When first thrown wholly upon a diet of reindeer meat, it seems inadequate to properly nourish the system, and there is an apparent weakness and inability to perform severe exertive fatiguing journeys. But this soon passes away in the course of two or three weeks.” He thrived for two years on that diet. So did Stefansson in 1929.

Other issues I have with the study are the small sample size (less than 20 subjects), the fact that subjects “self-selected” their diets and the fact that little was made about the four-pound average weight loss among both groups, especially since the low-carb group could eat all they wanted of non-carb foods. To me that’s huge…but the researchers had an agenda: “the results suggest that weight-loss diet regimens differentially impact cognitive behavior.” Whatever.

Now, having said all this, remember that the Primal Blueprint “maintenance” level of carbs is between 100 and 150 a day (depending on your age, size, gender, etc). That’s not a ketogenic diet – it simply lowers insulin and forces the body to look for extra energy from fat stores so you don’t gain more fat. Even the PB “sweet spot of weight loss” allows 50-100 grams a day, barely dancing in a range of ketosis and nothing like the <16 grams in this study. But for those of you who wish to maximize fat loss for a while and go “full-ketosis” just keep in mind this all-important three-week window of keto-adaptation, where you may not want to take your SATs or go on Jeopardy for the first three weeks. But after that? Who wants to be a millionaire!

Further Reading:

How to Guide: Making the Switch to Primal Living in 6 Easy Steps

The Context of Calories

Have you Decided to Healthy?

TAGS:  mental health

About the Author

Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.

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43 thoughts on “Low Carb Diets Affect Short Term Memory”

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  1. I was just thinking about whether or not I experienced fuzziness or memory loss when I first started going PB. But I can’t remember 😉

    Way to attack this one pro-actively Mark!

    The SoG

  2. I so appreciate your honest and upfrontness Mark. I’m with SoG, I don’t remember any memory loss or fuzziness. And while SoG meant it as a pun, I’ll say in all seriousness, it clearly wasn’t significant enough to be noted.

  3. I wouldn’t think ZERO carbs for the first week could be considered healthy. Even Atkins lets you have like 20 or something.

  4. I experienced some mood swings for about a week. My wife (and I) just figured that was because I was being a pain about not eating dessert anymore.

  5. On the positive side, I think that this study helps support the idea of making small changes slowly over time. Many people will feel slow if they drop carbs to nearly zero over night and I don’t recommend it. I drive on the same roads as these people :).

  6. I believe I saw this one before, and if I recall correctly the low-carb group experienced a larger “drop” in cognition, but still scored higher overall than the low-cal group.

  7. Before people can discuss this intelligently their first needs to be consensus on what “low carb” means.

    I have a text book in front of me that refers to the Zone as a low carb diet (the Zone recommends 40% of calories come from carbs, the “food pyramid” recommends 45-60%). While I hardly consider the zone low carb, it does serve to illustrate how the definitions of the term are all over the map.

    So is low carb 40% of total calories, under 100g a day, under 16, etc..?

    I think it should be painfully obvious that super low carb diets are not healthy in the long run since you would have to curtail fruits and vegetables so heavily.

  8. Chris,

    “low carb” is a relative term. Ask an Atkins adherent and they’d tell you that 0-20 is low. I wouldn’t say 40% is low at all, but it is low relative to the suggested government diet.

    I don’t know why super low carb diets are unhealthy for any reason. Fruit and vegetables don’t seem to be required, perhaps just useful. Check out Hyperlipid (https://high-fat-nutrition.blogspot.com/) to see what I mean. There doesn’t seem to be any benefit to staying in deep Ketosis (unless you’re fighting off cancer, maybe) but some people thrive on it.

    That being said, the weight training element of the Primal Blueprint would be challenging without carbohydrate in the form of fruits and veg to provide *some* glycogen for fuel. While some people do well on ketogenic diets, I haven’t met anyone who successfully did weight training long-term on a ketogenic diet. It doesn’t mean they don’t exist, it just hasn’t been my experience.

    Personally, I aim for 100-120g of CHO per day (with ~40g coming from fiber) when I eat about 2500 cal/day on average. It’s usually about 15% calories from carbs, 35% from protein, and about half from fat. My powerlifting and martial arts training demands that I have some carb fuel.

    On the other hand, when I can’t work out for a week (on vacation or maybe just taking a week off) I do very, very well riding the ketogenic boundary (40-60g CHO/day).

    Others have had similar experience, but your mileage may vary.

  9. Keenan, well said. And I might add that once you are able to play in the 100-150 range every day, dropping to 40-60 or 60-80 on an IF day or a keto day is a no-brainer. It’s when people go from 250 or 350 grams a day to 50 that they experience the transition blues. And dig that 300 grams carbs a day is still on the low end among those who eat 2500 calories and get 60% of it from carbs as our USDA would suggest. (60% of 2500 is 1500 calories and at 4/gram, that’s 375!)

  10. Jane’s remark about it being unhealthy to go without carbs is a common thing I hear but I’m becoming less and less convinced that it’s true. We are *able* to eat carb foods to some extent, but it isn’t strictly necessary and we don’t even make all the enzymes needed to process that stuff. We do make some amylase (starch-digesting enzyme), but not a lot of it.

    Glucose source: Carbs are not needed for this. As you’ve stated even in this post, we can make glucose from other things. I read someplace that about half the protein we eat above our body’s requirements can be turned into glucose, and about ten percent of fat. And only cells with very few to no mitochondria in the human body are incapable of using ketones for energy.

    Fiber: If you eat enough fat it greases the skids, so to speak. Or that has been my experience. If you don’t use it all for fuel, don’t need it all for hormones and cell-building, and you don’t have your insulin up to pack it away into storage, where else is it going to go?

    Phytonutrients: The number one reason “experts” say we need these is to prevent cancer. If we aren’t eating like crap to begin with and we’re not exposed to massive amounts of carcinogens, our chances of getting cancer are very, very slim to begin with, so what’s to prevent? Most types of cancer, from what I understand, live off of glucose. Don’t give them tons and tons of glucose to live off of and they won’t get a foothold in the first place. To add insult to injury, I’ve learned that researchers suspect that these cells eat up glucose in the first place to protect themselves from–you’ll never guess!–free radicals. Apparently cancer is more susceptible to them than healthy cells are! So it occurred to me to wonder whether antioxidant chemicals might not serve a similar purpose once you’ve got cancer–might they not protect the cancer from dying? I doubt anyone’s going to think to look at that for several more decades and I am not a scientist, alas.

    Are plant foods flavorful? Sometimes. Are they healthy? Sometimes. It helps to know how to prepare them properly since every part of a plant except the fruit has a reason to defend itself chemically in some way. But can we live without them? Certainly. We would have had to, back in the days that we spent at least four months out of the year snowed in somewhere with no farms, no domesticated food animals and no grocery stores. And the Inuit did it up until very recently and the Maasai sometimes do it now.

    On top of all that I’ve also learned that if you don’t eat a lot of glucose-forming foods, it has a vitamin C sparing effect because both glucose and C get into cells by the same receptors. This is one way the Inuit traditionally avoided scurvy!

    My two cents, your mileage may vary. I haven’t abandoned eating plants, but I’m not running around paranoid that I’m going to die if I don’t eat truckloads of them, either.

  11. “And the Inuit did it up until very recently and the Maasai sometimes do it now.”

    Go without plant food, I mean. Obviously the Maasai are not snowed in anywhere. 🙂 🙂 🙂

  12. I am on Kimkins and I never had headaches from cutting carbs. Must not have noticed it.

  13. “Fiber: If you eat enough fat it greases the skids, so to speak. Or that has been my experience. If you don’t use it all for fuel, don’t need it all for hormones and cell-building, and you don’t have your insulin up to pack it away into storage, where else is it going to go?”
    I have had the very same experience, although I get more than enough fiber from vegetables. As for phytonutrients I think they should not be avoided if they give us so much benefit. We certainly do not need plants, but they sure do add a lot.

    One person who I always refer back to for health is Jack LaLanne. He stays buff and healthy at 92 with several raw veggies and fruit, tons of vitamins with an emphasis on desiccated liver tablets.

  14. I’d fund a study that stated “Low carb diets get you jacked abs!” Is there anything like that coming out? Hahaha, but yes Mark I agree, the three week period is borderline conspiracy and 100% negligent for those people out there looking for real answers. Well done sir, keep fighting the good fight.

    All the Best,

    Andrew R

  15. Mark,

    Thank you for writing this.

    I started Atkins after being a frustrated near-vegan (I ate seafood, but no dairy, and no other meat) for a year. Before that, I ate meat, but no dairy, and many years ago, I was a complete vegetarian for a couple of years (but did eat dairy).

    OK, my point: The first day on Atkins I felt great (the momentum, I guess). The 2nd and 3rd day, not so great. I couldn’t sleep because I kept getting up to pee 20 bazillion times a night. Cutting carbs makes you shed a ton of water weight. Which the pro-carb lobby makes a big deal out of: “Oh, the weight loss is only water weight.” As if water retention is GOOD FOR YOU and alleviating it is BAD. But I digress.

    After the 3rd day, I felt just fine, and continued to feel better and better. And I saw results, and don’t miss carbs at all.

    Again, thanks for your research and dedication. It’s good to speak truth to power.

    1. Hi Ruth,

      I’ve been reading through all the comments on Mark’s site (and I’d like Mark to chime in as well) and your’s struck me a little odd. As an endurance runner, hydration is at the forefront of my daily routine. It is because proper hydration is key to proper cell function.

      Dehydration is not a healthy or pptimal state to be in. For one, it makes your blood less viscous which adds stress on your heart. But what I wanted to point out is, how do you know what is the right amount of water retention? Or that the water you shed from lack of carbs was above the normal and healthy state of hydrated.

      I think if you’re going to condemn any study or research (which are usually based on measurements and protocols) you should make sure your claims don’t fall into the same category of speculation.

      Observation and anecdotal experience would be slammed by you and Mark if Tufts or any scientific study used that methodology to draw conclusions.

      This is not to indict you on your observation, but just to keep pure and consistent that the arguments on this side don’t end up being the very same that you would condemn on the other side of the issue.

      Mark, can you help me and Ruth in further understandings about hydration, cell function, etc. and it’s importance on health. And maybe clarify what quantifies as Water Retention as opposed to normal and healthy water content in the human body?


  16. I definitely noticed the “fuzziness”. But I’ll add some perspective here. I went low carb (< 20gr) for a healthy lifestyle with weight loss as the most immediate result desired. I lost 125 pounds in 7.5 months. The first three weeks were tough. I had headaches, I was a bit surly, and I had to concentrate a little more to complete my work. I would compare it to having a few mild flu symptoms.

    But look at the upside of working through that! I lost 125 pounds, got off numerous medications, I look much better, my confidence is higher, I’m physically more active (because I can be now),I’ve motivated numerous others to lose weight the same way. To top it all off my memory, clarity of analysis, and overall enjoyment of work has improved quite a bit from before the diet adjustment.

    To me this temporary “memory loss” as they call it was nothing more than a minor bump in the road. I now hover around 70-100 carbs a day and feel great.

  17. Where does red wine fit in all this? Is ethanol considered a carb or a fat? I know that the body metabolizes it like a fat, but????? Most days I consume close to zero carbs (except what’s in a glass of milk and an apple, tomato, red pepper,…. blah blah etc. no bread. no cookies. no pasta. no rice.) but, hey, what about my beautiful half bottle of dry red Australian wine? Do I have to forgo my reward for getting home alive?

    I was thinking recently since I’ve been taking Harp Seal Oil capsules (seem to have a better DHA to EPA ratio than fish oil), that possibly the reason people in the far north didn’t get depressed during the winter when there was no sunlight is because of the omega 3 in the seal blubber they ate. Then these days it’s no surprise the amount of addiction, glue sniffing, gasoline sniffing and suicide that goes on since these poor people are being provided with garbage carbs from ‘civilization’. Bring on the caribou and seal blubber!!! Eating whale meat might actually be a life saving option. PETA go away.

  18. Well, I just started low-carb yesterday, so I’ll know soon if it gives me “fuzzy brain”. My plan is to make this a lifestyle now, not just a short term “diet”. I’d like to lose 10# or so, then just stay away from sugar and grains – how much healthier I’d be! Problem is, I need more good ideas to keep me on track (“what the heck can I eat?!”), so I did this blog carnival, I hope people will share their favorite ideas or recipes!


  19. i experienced lower endurance on atkins on my 7 mile runs initially – this was cured but upping the fat – what may also be happening is that those beginning very low carb do not compensate enough with fat… we still unconsiously cenosr our fat intake – years of anti fat conditioning

    i agree that there is an adapive period – but i suspect people are still not eating enough fat – and possibly too much protien.. we should, on a ketogenic diet at least, be eating more fat than protien


  20. Markus, you are correct. People still shy away from dietary fat when they are trying to lose stored body fat. It’s a tough thing to learn because it’s counterintuitive, especially in light of 30 years of being told fat is bad.

    AS for running on Atkins or PB, I’m not sure you could race well on ketones (since there’s no way to store them) but you can certainly find a “sweet spot” where you run easily and have plenty of fats/ketones to handle that load. I have some athletes (people who, against my better advice, still want to run/cycle/swim hard every day 🙂 who have acclimated to a high fat, low carb diet over a period of months and find they are performing in races quite well. But they DO need to throw in some pure glucose during those longer harder events, since the limiting factor in a race is still muscle glycogen.

  21. “I am on Kimkins and I never had headaches from cutting carbs. Must not have noticed it.”

    That’s surprising. Is Kimkins still advising low carb, low calorie and low fat all together? I thought that place shut down?

  22. Sue,
    Agreed. i was under the impression that Kimkims was universally deemed insane and unhealthy and kicked to the curb. I don’t know enough about it.. but kimkims was the “diet” where you only ate like 500 calories per day right?

    The SoG

  23. That’s right. Also, if you’re going to do low carb it should be high fat not low fat and low calorie as in Kimkins.

  24. Of course we will also be getting greater amounts of omega 3 on a paleo/primal diet and therefore this should counteract any memory loss from the first few weeks of non-carbs.

  25. I had somewhat of the opposite response, when I started low carbing it was because I had finally discovered I was right on the cusp between diabetes, impaired glucose tolerance and reactive hypoglycemia, and a lot of the mental fuzziness and physical energy drops I’d been experiencing most of my life cleared up. Also my depression symptoms stayed under control on 1/6 of the medication and my ADD symptoms became almost non-existent

    By testing


    I discovered that on the Healthy High Carb Low Fat diet my BG was shunting back and forth several times a day from the 150 – 180 range to 70 or less, and sometimes higher and lower. By limiting the carbs and cutting out the highs the lows looked after themselves.

    There was something of a running in period but once my metabolism switched from glucose to ketones I find my brain works optimally with my BG in the 70 – 90 range.

    As to the “fat makes you fat” myth, just ask any farmer what to feed animals to fatten them. One wonders why human dieticians don’t seem to know this, it was common knowledge pre-Ancel Keys and seems finally to be returning to prominence

  26. Wondering if coconut oil can help with the transition? There’s been research that it produces ketones and has quick results for alzheimer patients. Maybe it can help lift the fog? Plus it’s all fat.

  27. Fuzzy head, mood swings, sleepy, this is part of the transition when you start to come of high carb diet.
    Had them all. 18 months later 75lbs lighter, cant remember the last time I had a cold, Best condition Ive been in for years.
    The study is short term b— crap.
    Keep rocking Mark.

  28. The brain fog does pass. I drank eleuthero tea (siberian ginseng, not the same as regular ginseng). It is a herbal remedy for memory improvement and it really helped.
    My job depends on me doing mental arithmetic all day and I do it fine with low to no carbs, it is just a transition phase you go through.

  29. I would suppose that a short term study of those quitting smoking (or any addiction such as carbs) would have similar results, and likewise conclude that the brain needs nicotine for optimal learning, memory and thinking.

  30. I definitely experienced the “carb flu”, but I also cut my carb intake by a lot initially. I wouldn’t say my mind was fuzzy – it was more having big energy level swings. I would feel very tired at times in the middle of the day, and had some head aches. I experienced that on and off for about 2-3 weeks, but after that my energy kicked into over-drive! I’ve felt great ever since, went from 185 to 170 lbs and still feeling fantastic. The results of that study are only negative because it only lasted for 3 weeks. The best results come AFTER that!

    Grok on!

  31. I used to leave no carb unturned but I always had really low energy and had a foggy feeling quite often. Last week I went low carb, and my energy levels soared and there was no brain fog.I also noticed my energy levels felt smooth and stable, I could go hours without eating and I didn’t feel hungry.

    But every morning I woke up feeling with my arms and legs feeling so shaky and weak. I had all of this energy but my body felt physically sick. I wound up eating a couple of buckwheat pancakes and that made a difference the next day because I didn’t have the shakes. I think I should’ve either upped my fat intake or I should have reduced my carbs more slowly.

  32. I am on week 6. I eat up to 50 grams of carbs and no more everyday, often less. Fat at 60%, Protein at 25%. I had a rough couple of weeks, then I was cruising and lost 5.5 lbs. I upped my energy output lifting weights, and now my cheeks are red and I am tired and I feel like I am comin down with a cold or flu. Is this the start of Ketosis?