Marks Daily Apple
Serving up health and fitness insights (daily, of course) with a side of irreverence.
7 Jun

Dear Mark: The Low Carb Flu

Conquering carbs offers a whole constellation of rewards, not the least of which is a steady, brisk energy unlike most people have known before (well, maybe since the whirling age of 10 or so…). People tell me constantly that they can finally make it through the day without being down for the count every midafternoon. They enjoy enough vigor and vitality to weather a whole day’s worth of activity. The busyness of life becomes easier to handle: the energy demands of daily work or business travel, the mayhem and constant commotion of kids, a weekend’s worth of chores and errands, etc. A skipped meal doesn’t suddenly change the agenda to including procuring a bagel or other false pick-me-up. Nonetheless, for some folks, there’s a common, temporary but still bothersome bump in the road on the way to that Primal prize. Though it varies, it often means a couple weeks of mental fuzziness, fog and fatigue. Although your body might be off to the races, your brain can lag behind like a little brother in a stuffed snowsuit. It’s a game of “hey, wait up!”  while the body’s mechanisms and metabolism align themselves. They call it “low carb flu,” and rest assured it’s just as temporary.

Dear Mark,

I just want to know if anyone who has been Primal for some time had any trouble with cognition in the first few weeks. I can hardly think straight, especially after eating, and I am also low on energy. Will this pass??? Besides that, my body feels great!”

Thanks to Jessica for her question in response to Matt Garland’s excellent guest post last week. It’s a common subject of emails I receive.

First off, I should mention that some folks experience the low carb flu, and others don’t. Overall, those who have been lower carb for some time seem to have fewer problems with the transition taking their carb intake down a notch. As rational as it sounds, this trend still isn’t a hard and fast rule. I know a number of Primal adherents who fit this profile and then went on to experience the symptoms Jessica describes. Others I know went from sky high sugars to low carb cold turkey and felt great from day one. Go figure. It might be due to the “sliding  range of genetic variations” I’ll be discussing in an upcoming post.

Although the low carb flu isn’t universal, that doesn’t mean it’s abnormal. For many people, it takes about two to three weeks to move beyond the temporary fog and fatigue. Studies following the physical performance of low carbers showed that initial disadvantages were erased after this window of time. If your body is used to employing easy glucose carbs and now must create glucose from fats and protein (a slightly more complex but entirely natural mode of operation), it can take some time to get up to speed. Rest assured that our bodies can and are doing the job. It simply takes time to work efficiently. The transition actually shifts metabolic related gene expression, increasing fat oxidation pathways and decreasing fat storage pathways. (That’s nothing to shake a stick at!) Within a few weeks, the body should be fairly efficient at converting protein and fat for the liver’s glycogen stores, which provide all the glucose we need for the brain, red blood cells, muscles, etc. under regular circumstances.

The small amount of specifically targeted research on low carb diets and cognition are small and shoddy at best. A recent study, for example, showed minor memory delay during the first week of strict carbohydrate elimination (as opposed to reduction), but further measures were not taken until carbs were reintroduced. By the same token, those on a traditional American Dietetic Association diet exhibited more confusion during the study tests. I don’t think this study offers any legitimate, useful assessment on either diet.

Nonetheless, there’s the sense that we’re stressing the brain and causing damage by not bathing it in a steady supply of glucose. In fact, we may be doing our brains the ultimate favor. Recent research has pegged Alzheimer’s as a kind of “type 3” diabetes, a disease related to insulin resistance. Truth be told, our brains can function on ketones, fat-metabolism byproducts and key energy sources in low to very low carb diets, which some say may be even healthier yet for our noggins. Check out Jimmy Moore’s interview with Dr. Larry McCleary, a respected neurosurgeon and author of The Brain Trust Program: A Scientifically Based Three-Part Plan to Improve Memory, Elevate Mood, Enhance Attention, Alleviate Migraine and Menopausal Symptoms, and Boost Mental Energy.

Keep in mind also that people in low carb transition can genuinely shortchange their physical needs when they aren’t eating enough fat or when they’re pursuing high intensity exercise routines lasting over an hour, which send the body’s glycogen stores into a nosedive to the ground. I hear this from CrossFitters and P90Xers who insist on maintaining their daily high intensity while transitioning to Primal eating. This isn’t to say that you can’t exercise more intensely for longer on a Primal Blueprint diet (although there’s no real need to for perfectly solid fitness), but more than an hour’s time for most folks requires some extra carbs when glycogen is depleted. If you’re bothered by the mental fog even without intensive exercise, ease up on your carb restriction by adding 25 grams or so to see if it makes a difference. Listen to your body and gauge your sensitivity. Rome wasn’t built in a day, and you’re not going to compromise your overall benefit by taking it slowly.

As any Primal buff would attest, this temporary setback pales in comparison to the benefits once the transition has passed. Without the persistent carb-induced peaks and crashes, you should enjoy a measurable boost in energy – steady and stable – in no time. Hang tight, and know you’ll be on the other side of the hill soon! In the meantime, take good care of yourself with a little extra shuteye and maybe some well-timed green tea for a modest boost when you really need it.

Avid Grokkers: what words of experience do you have for Jessica and other new folks at the Primal table? Be sure to share your thoughts, stories and words of wisdom. Thanks for reading and keep your questions coming!

You want comments? We got comments:

Imagine you’re George Clooney. Take a moment to admire your grooming and wit. Okay, now imagine someone walks up to you and asks, “What’s your name?” You say, “I’m George Clooney.” Or maybe you say, “I’m the Clooninator!” You don’t say “I’m George of George Clooney Sells Movies Blog” and you certainly don’t say, “I’m Clooney Weight Loss Plan”. So while spam is technically meat, it ain’t anywhere near Primal. Please nickname yourself something your friends would call you.

  1. For some people the symptoms could be a result of a candida (yeast) “die-off”. By cutting carbs and lowering blood glucose you’re restricting the candida of it’s food supply. On top of that, many of the PB foods are anti-fungal, killing more and intensifying symptoms.

    Mike wrote on June 7th, 2010
    • I was browsing through all the comments to see if anyone would make the link. This explains most of the so-called ‘low carb flu’ that affects many people new to a primal regime. Yes, the enzymes responsible for liberating ATP from fat need a while to increase their function, and yes, the nervous system does take several days to begin using ketones effectively but these two issues do not explain the majority of symptoms that people notice when they cut out most of their carbs.

      They are, however, starving yeast and IF they have a yeast issue (many do) they they are likely to suffer from a wave of fungal toxins such as acetaldehyde that come with such a die-off reaction. Doing it slower and adding insoluble fibre can help.

      Marek London wrote on June 8th, 2010
    • This makes sense to me. Yeast overgrowth can be very tenacious. Kombucha and lacto-fermented veggies can be very helpful, much more effective than probiotic capsules, in my experience.

      Jeanmarie wrote on June 10th, 2010
      • I will try some Kombucha right now and kick up my intake of Olive leaf extract and silver. That may help me pull out of it.

        Jennifer wrote on April 11th, 2012
  2. I had a little trouble kicking the caffeine when I first started a little over a year ago. A couple of days with a mild headache, then I was good to go. However, I haven’t had the big changes that many others have had. I haven’t lost more than a couple of pounds, and just can’t get rid of the fat over my abs. I’m a bit frustrated and am not sure what I’m doing wrong. Could be that my carbs are still a bit too high. Can’t seem to kick the craving for dark chocolate that still hits every afternoon!

    Jenny wrote on June 7th, 2010
    • Greetings,

      I realize this post is old but I felt compelled to reply. I had intense chocolate, nuts and butter cravings early on my primal journey.

      I suggest taking a magnesium glycinate supplement as that could certainly be the cause of your cravings as both as very high in that essential mineral. I take a good 400-500mg sometimes upwards of 750mg and even 1g on high workout days.

      Perhaps you should try it and see how you feel. Fact: most of us are severely deficient in magnesium.

      Cheers and best of luck!

      DavidC wrote on November 27th, 2013
  3. I had bad withdrawal symptoms when I started the Primal lifestyle, but I was a total sugar addict. I didn’t really have mental fog though, but I did have trouble in the gym for about a week, which has passed.

    Carla wrote on June 7th, 2010
  4. I was in a brain fog for years with high carb diet. Somewhere between week 5 and week 6 of eating paelo the fog lifted. I can think again!!

    Teena wrote on June 7th, 2010
  5. I’ve noticed several things in the few weeks I’ve been at this seriously. One is that I’m more sensitive to mineral intake. There’s way more salt in the animal stuff I’ve been eating and I have to be sure to get my potassium. Same with high calcium — have to make sure to get my magnesium. Another is that I can’t slack on protein without paying a price, either. All of those help brain fog and I can see that balancing will be a learning process.

    Another aspect of this shift is that I won’t have the high carb diet cravings, highs and crashes, but if I’m unusually active, I’ll bonk! Happened the other day and the only reason I recognized it as such is that I used to (when much younger and fitter) do distance running and cycling. When you bonk, you don’t feel hungry. You just suddenly run out of gas, mentally and physically. As soon as I recognized it, I ate something and was fine.

    All that said, and fear of fat and other newbie anxieties taken into account, I don’t think I’ll ever willingly go back to high carb eating just because it is so damn great not to feel so bloated and strung out all the time.

    I want to go read up on high fat diets as epilepsy treatment. Want to see what they say about your brain on high fat/low carb in those studies.

    slacker wrote on June 7th, 2010
    • The guest blog comments a week or so back talks about this. In some small studies in children over here (UK) there was a big decrease in seizures on a low carb high fat regime.

      Kelda wrote on June 8th, 2010
  6. i made the switch very gradually, over YEARS, so no low carb flu. but i’ll tell ya, cutting out my BAD (5 cups a day) coffee habit last month was BRUTAL withdrawal. the headaches and brain fog!it’s nice to feel calmer now…though i also liked being sorta amped on caffeine and i don’t think it was such a bad habit. something about the pain of quitting makes me want to not start up again. our bodies get used to a certain chemical balance and adjustment certainly can be tricky.

    DThalman wrote on June 7th, 2010
  7. I’ve been playing with low carb for several years. The first time, I went strict Atkins induction all the way with the hope it would allow me to lose weight without too much suffering. Before that, I was in the habit of drinking ‘healthy’ fruit juice all day so the second through fourth days of induction where accompanied by very mild fatigue but also a truly unholy intense constant craving for fruit juice. One day at work, I caught myself just before my hand actually reached out and stole a coworker’s cranberry juice and chugged it! At about day 5, the cravings started to subside and then at day 6, I felt suddenly wonderfully great and energetic with minimal cravings. I think for some, it may be easier on the body to go slowly, but for me, I think the best was that I got through those cravings as fast as possible as they were torture! From there, it was fairly easy sailing and I was strict and loyal for 8 months or so, and then after that, I only occasionally cheated. I found that small amounts of fruits did not trigger any cravings and that the most important was I ate low carb at dinner. Then later, I lost my job and was forced to eat many of my meals at my parents house (super carby). I was depressed and gained a lot of weight. Later, I got another job and then read the book called Good Calories Bad Calories, which scared me straight! Perhaps since I had even when cheating tried to stay more away from carbs, the transition this time was quick with barely any cravings and no fatigue. Although interestingly, this time, perhaps due to also doing weightlifting, I actually gained a few pounds right away on low carb, and then lost it again plus more. Measurement around the waist showed I was slowly losing fat there the whole time though. I am now mostly primal but am not yet convinced that salt is a big deal to worry about (delving into the research just has not given me reason to fear salt in any way)and I still consume some diary, especially cream and butter. I try to stay away from lowcarb ‘fake’ foods, but once in a while, I will enjoy some lowcarb icecream. I am very observant of any carb cravings and I now know if I experience any carb cravings, then I have eaten too many carbs. I also try to keep most if not all carbs eaten to be healthy carbs like fruit and veggies, not antinutrient grains or other potential poisons. This seems to work for me. I have heard from many other females who say at some point after a long time on lowcarb, they began to feel tired and rundown. I can only guess why, and it does seem there are more women than men on this boat, but I think for some people, carefykkt uping the intake of HEALTHY carbs may be needed at some point. For myself, I will cross that bridge if I get to it, but so far, I sleep much better and need less sleep, I am stronger and more energetic, and I really enjoy my meals. I have also converted 2 others who have their own very encouraging lowcarb stories.

    Eva wrote on June 7th, 2010
    • Yep, read Taubes if you really want to understand why you are going Primal and why you should stay Primal, forever.

      I won’t EVER forget reading that book.

      Kelda wrote on June 8th, 2010
  8. Wow – what a great response.

    As for the 2 week time frame, yeah – it pretty much was that for me. However, I did do the crash and burn method – no sugars for 2 weeks…. since we started off with south beach. Which phase one use to be pretty much primal / low carb. I’ve noticed that has changed, in his newer books.

    Anyway – after that – we found MDA and we added some more variety in veggies which I think helped a lot. And, honestly cooking/eating primal is a LOT easier than SB/carb counting.

    Mostly it takes time – and like everyone else has said – you need to listen to your body.

    Also, I think it depends on how you approach things. I’m a all or nothing type of person. While it may not be the best approach – it works for me. I noticed some people like to count, others like to go slowly into primal. The thing is to figure out how you do things normally — and then make primal work for you with your normal style of doing things.

    good luck… hopefully it will work out as well for you as it has for us.


    Ame wrote on June 7th, 2010
  9. Hi Mark,

    In the German Ketokenic Diet booklet aimed at cancer patients on how to use diet as a treatment, it is mentioned that it may take some days for the brain to effectively swap from using glucose into using ketone bodies for energy.

    In that sense it is not recommended to swap straight into a very low-carb regime, but to instead aim at daily 50g carb intake for a week or two before going lower.

    The whole booklet can be found here in English:


    Sami Paju wrote on June 8th, 2010
  10. I have been primal for 4 months. 51 male went from 23 PBF to under 14. Lost 20 lbs, over 4 inches in waist. Awesome health improvements. When I began, I had a low grade headache for 2 weeks. If I excercised, it got worse. After two weeks, fog cleared and began working out. Now Im in the best shape in my life and I was a marathon guy and moderate athalete all my life. Hang in there… your body will learn how to use good food just as it was designed!!

    Joe Seebald wrote on June 8th, 2010
  11. Just finished my first week of Primal. My main “flu” symptom has been headaches, all day long for about three days, now off-and-on. They aren’t debilitating, just annoying.I haven’t noticed any big change to my energy level, but cravings and roller-coaster blood sugar subsided almost immediately. I’ve been pretty much at 50-80 carbs a day. Next goal is to try to stay under 50. In my first week I lost 5 lb., probably mostly water but encouraging nonetheless. I also feel less puffy, but it’s more of a subjective feeling, not a measurable thing.

    jennybug wrote on June 8th, 2010
  12. I would like to add another possible cause of low carb flu. The new Atkins book says that there is evidence that switching to a lower carb lifestyle can have a diuretic effect. This is part of the reason people can lose so much weight the first week. They are now suggesting having a cup of bouillon to help with the symptoms.

    September wrote on June 8th, 2010
    • I would think salt water is better for you than boullion cubes if thats what you meant. They have MSG (and who knows what else) in them, and if you are sensitive to MSG, that can cause migraines. I had an adrenal problem for years which caused water to just run through me without hydrating me, and my doctor recommended salt water with unprocessed sea salt or pink Himalayan salt. You should pay about 10 bucks or the like for a bag or else you aren’t getting the real stuff

      cindyk wrote on July 8th, 2012
      • only really really CHEAP bullion cubes have msg in them these days, read labels. And bullion tastes better heh.

        Marie wrote on February 4th, 2014
  13. Could these few people who suffer with the “low carb flu” just have a metabolic-type of a high carb eater?

    Oliver wrote on June 8th, 2010
    • I too wonder about this. I know Sisson doesn’t subscribe to the different metabolic type theories, but from my own personal experiences, I’m starting to. Some people handle carbs just fine, gives them energy, and doesn’t make them gain weight. For them, weight loss just comes from portion control. Me on the other hand, I get fat from carbs. Too many makes me feel like crap, etc.

      Sam wrote on June 8th, 2010
      • it would be interesting to have all those who suffer badly from the switch to low carb on a test for their MT, maybe they would all (or mostly) show they were primarily carb eaters. That would be some decent proof.

        I don’t know either way, but it is at least possible.

        Oliver wrote on June 8th, 2010
        • obviously there is genetic variance depending on the climate our genes evolved in. The obvious example of this is the Eskimo who eat almost exclusively fat and protein. Conversely, if you lived tropical, you’d probably be eating pineapple and bananas all day long. I would imagine that there is a wide variance among modern humans on carb amounts that our bodies can handle depending on what predominant genes got passed along.

          Sam wrote on June 8th, 2010
  14. Two interesting things that can influence the low-carb flu that have been mentioned at coolingInflamation and Hyperlipid both actually have to do with your gut flora.

    1:FIAF ( at Hyperlipid, which in a nutshell is your gut flora if carb adjusted will scream at your brain when you change your diet and can make you feel pretty bad while adjusting.


    2: Gut biofilms ( covered at coolinginflamation. Where bad gut flora or carb adjusted flora can form films that hide themselves from your immune system and tie up metal cations (usually mg and calcium but in autism cases heavy/toxic metals too) and mess with nutrient absorption (because its literally a barrier between you intestine wall and nutrients and it tries to bind things like mg and calcium to itself instead of you getting to use it).

    when switching to a diet like paleo/primal during the adjustment period it can suddenly start breaking down the biofilms and adjust the ratio of your gut flora, which can mean you get the shock of whatever is making up that biofilm, and the bad bacteria die off all at once, in addition to whatever signals the bacteria send to your brain (intresting discussions linked here at paleo hacks

    Funnily enough a bunch of old school cure alls and some of the new school medicines as well as a paleo diet are all things that attack biofilms and bad gut flora ratios/carb loving bacteriums. (AVC, Pre and Probiotics, Fermented foods, antibiotics(only attack the bacteria outside the biofilms so temporary relive of symptoms), pectin/fruit fibers, spices like tumeric, starving carb reliant bacteria with a high protein/fat diet)

    Robert wrote on June 8th, 2010
  15. After going primal (in February), I suffered low carb flu for about a week. But then for a couple months I’d wake up some days feeling great, some days feeling sluggish and achy. I finally connected the sluggish days with high dairy intake the day before.

    I’ve never had an issues with dairy, so this has surprised me. My thought is that in the past, my body was so out-of-whack due to high carb intake that the affects of dairy was lost in the noise.

    I am now experimenting with types/quantity of dairy to see if I can add back some cheese, which I miss. But thought I’d mention it as a possible affect people might want to be aware of.

    john wrote on June 8th, 2010
  16. Yerba Mate tea with mint seems to have helped me through this.

    Martin wrote on June 8th, 2010
  17. I havent really suffered from the ‘carb flu” mentally or physically. Apart from my first long bike ride shortly after starting, I seem to have plenty of energy. I’ve only changed to a primal lifestyle recently and think that rather than “carb flu” I suffer from “carb psychosis”. I can’t stop thinking about bread, sandwiches, pasta, rice etc. I think this will pass with time. In an effort to take the focus away from this psychosis, I started my own blog,, so that I could document my journey and particularly to refocus on creating healthy meals and alternatives in line with the primal blueprint while fooling my brain to think I am eating these things my subconscious can’t seem to let go of.

    Steve wrote on June 9th, 2010
  18. I can confirm that it goes away after 2-3 weeks. Now after a few months I don´t have it anymore even if I eat an almost zero carb meal with lots of protein and fat.

    I had a relatively long “binge” where I eat a lot of sugar in 2-3 weeks after being primal for 3 months. After the binge the first few days were again harder but just a few days and not a few weeks. So 3 months primal, then 3 weeks high-carb didn´t reset my metabolism completely to high-carb but I felt a change when I started being primal again.

    ramon wrote on June 9th, 2010
  19. The post by Robert says alot about the causes of this carb flu. Another common issue along this line might be what is known as the “herxheimer reaction”. This happens anytime you have yeast overgrowth and you start starving the yeast by cutting out sugars/carbs. There are 79 known toxins released from yeast when it starts to die. This may make you feel very foggy,tired and achy. This could be some explanation for what people might think is some sort of carb w/d when it’s actually the release of toxins by the overgrowing candida in the gut and colon.

    Susan wrote on June 10th, 2010
  20. Grains and dairy are very mucus forming and when eating a lot of these foods one will store a lot of mucus in the body. When people stop eating dairy and grains and start eating paleo the body sarts to unload all that extra mucus. Hence, the cold or flu symptoms.

    davolf wrote on June 11th, 2010
    • My son has Aspergers syndrome, and in my research, I found out that in autistic children, or anyone with any form of leaky gut, gluten and casein, (the proteins in wheat and dairy) pass through the gut wall and enter the blood stream. From there, they pass through the blood-brain barrier into the brain and act like Opiates. I’m betting that a lot of us have this problem and don’t even know it. Never tried opium, but I would guess the withdrawl would not be pleasant.

      cindyk wrote on July 8th, 2012
  21. Get those vitamins into your diet…vitamins that are capsules, not “rocks”,(tablets), and quality Omega 3 oils…they do make a difference.

    Cj wrote on June 13th, 2010
  22. I’m sure feeling it. I quit cold turkey because I know myself and if I don’t do this quick, I’m just gonna suffer and stop trying. This is my second day of PB, and it’s only 6:30 in the evening and I’m falling asleep. I also just ate another small square of dark chocolate (had one at 3:30pm) to help kill a sugar craving. However, I’ve only had two meals today and I feel healthy and full. I’m very happy about that where as before I needed to eat every two hours! So, I’ll just keep slugging through this as best I can.

    Lori B. wrote on August 7th, 2010
    • That’s not low carb flu, that’s not eating enough. Read the comments, this is something that hits people after about a week.
      If you don’t like the idea of adding more fat to your cooking then add coconut milk (no not light fer christ’s sake) and buy some almonds to nibble on between meals.
      You might think that I’m trying to sabotage your weight loss but I just want you to be able to live on PB, not just diet on it.

      Morten G wrote on December 7th, 2010
  23. I think I’m one of the people who went from lotso sugar to none and felt great… lucky me! I came right off of a vegan diet (was only vegan for a month), during which I ate tons of grains and sugar, had NO energy and felt close to death (mostly kidding). Anyway, the primal blueprint perked me (and my abs) right up fast! It’s been a month, i’ve lost 8 pounds, and my “sugar blues”/melancholy is G-O-N-E. thank you PB!!

    Lauren wrote on September 8th, 2010
  24. What kind of protein do you carry with you? I always have nuts and cranberries, but sometimes don’t feel like eating them.

    Maxmilliana wrote on December 16th, 2010
  25. In my twenties (I’m now 67) I figured out I was hypoglycemic. Since then I have had to eat every 3 – 4 hours or not function, especially the brain,and I get clumsy. I have eaten mostly low carb, no junk food, since then, but eat and crave bread. Before being on this site, I thought whole grain was healthy. Is this chronic carb flu?

    Maxmilliana wrote on December 16th, 2010
  26. My symptoms were mostly a thick mental fog for about a week. I just stuck with it and increased my fat intake, which helped some, not a lot. I also was scattered mentally, especially my short term memory–I’d be mid sentence and totally forget what I was going to say.

    I had been quite lethargic off and on before and could never pinpoint why. That’s gone now, of course. I have more energy than I know what to do with!

    As of now, my mental function is better than ever now, along with my energy levels and general health. Several other issues that I’d been trying to resolve disappeared and I truly feel a million times better than I ever have.

    Sterling wrote on December 22nd, 2010
    • My brain felt like there was a ‘satellite delay’ between having a thought and the words coming out of my mouth – almost like I was thinking & speaking in slow motion.

      9 months later and brain is all back to top speed and wonderfully so first thing in the morning and even after lunch, although I can lose it slightly if I don’t get enough sleep and I eat too much fruit.

      Day 7-9 I sweated constantly, just like when you have the flu – I was boiling. Guess my fat burning mode kicked in and the body got a bit of a shock!

      Debs wrote on September 21st, 2011

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