What Is Low Carb Flu, or Keto Flu? And Ways to Beat It

low carb keto fluOver the first few days (up to two weeks) of eating low-carb, you may run into some frustration. Where is all of this energy I’m supposed to have? Why do I want to mow through that bag of chips right now? Am I coming down with a cold? For some people, the transition from burning glucose to burning fat comes with unwanted symptoms that range from slightly uncomfortable to miserable. This transition period is known as keto flu, or low-carb flu. It’s real, and it can be pretty terrible.

But, it’s temporary.

What is Low-Carb Flu?

Low-carb flu, or keto flu, is a set of symptoms that you may feel over the first few days of limiting carbohydrates. Low-carb flu isn’t a flu or infection at all, and it’s not a medical term. It got its name because some of the symptoms of carb restriction can feel like you’re sick with the flu.

Low-carb flu has dissuaded millions of people from pursuing and sticking to a healthy diet. You can laugh now that you’re fat-adapted and humming along on stored body fat, but you’ve forgotten just how terrible the transition from sugar-burning to fat-burning can be.

Symptoms of Keto Flu, or Low-Carb Flu

It shows up differently for everyone. Some people, likely the ones who are metabolically flexible to a degree before even starting, won’t notice much trouble. That’s somewhat rare. More often, people new to carb restriction will experience some degree of:

  • Headaches
  • Brain fog
  • Malaise, fatigue, listlessness, and other synonyms for “exhaustion”
  • Feeling lightheaded or dizzy
  • Irritability
  • Mood changes
  • Constipation
  • Diarrhea
  • Nausea
  • Vomiting
  • Muscle aches
  • Lack of motivation
  • Feelings of anxiousness
  • Cravings

At some point, you’ll just have to accept the reality of the situation: you’re shifting from a sugar-burning metabolism to a fat-burning metabolism. You’re building the metabolic machinery necessary to burn fat. You’re updating your body’s firmware, and it’s a big update. That takes time.

How Long Does Keto Flu Last?

Generally, you can expect keto flu to last 4-7 days.

Most commonly, people who have symptoms with low-carb will experience symptoms If the results of one study are representative, it takes about five days on a low-carb, high-fat diet to increase AMPK and start building new fat-burning mitochondria.12 And sure enough, most people report that the low-carb flu lasts from four to seven days—right on target.

But that doesn’t mean we have to like it. So, what can you do to speed up the transition and reduce the pain and suffering?

Here are a few strategies to help you cross the rocky terrain of keto flu more quickly.

11 Keto Flu Remedies to Make Low Carb Easier

  1. Eat fatty fish or take fish oil
  2. Support your stress systems
  3. Don’t skimp on salt
  4. Eat enough potassium
  5. Take magnesium
  6. Stay hydrated
  7. Eat more fat
  8. Include medium-chain triglycerides (MCTs)
  9. Consider ketone supplements
  10. Move around at a slow pace
  11. Reduce carbs gradually

1. Eat fatty fish or take fish oil

One theory is that low-carb flu is caused by the release of stored arachidonic acid from adipose tissue. Since AA is the precursor to inflammatory molecules implicated in headaches,3 a sudden rush of AA into the blood—as happens in obese and overweight people during initial weight-loss—could be responsible.4 If this is true, taking extra fish oil or eating fatty fish like sardines or salmon should counter the omega-6-induced inflammatory response triggering the headaches. If this isn’t true, eating fish is still a good idea.

2. Support your stress systems

There’s a good chance you have been fueled by glucose for most of your life. So, when glucose suddenly isn’t available, your body might think you’re in danger – that you’re in a time of scarcity or famine. That triggers your stress response, and your adrenal glands release cortisol, which makes you store body fat.

An easy way to combat this is with adaptogens – supplements that act directly on your body’s stress mechanisms. Adaptogens help to modulate the stress response so that the physical effects of stress are less pronounced.

3. Don’t skimp on salt

Going low-carb increases salt requirements on multiple levels. First, when your body dumps glycogen, it doesn’t just dump the water that accompanies it. You’re also losing tons of sodium. Second, a byproduct of low insulin is reduced sodium retention,5 so you’re both losing and failing to hold on to it. Third, going on a Primal eating plan inevitably entails eating more fresh food and less unprocessed food. Unprocessed food is usually low-salt; processed food often comes with added salt. Nothing a little extra salt can’t fix.

Add salt to taste. Drink salty bone broth (Peter Attia likes bouillon dissolved in hot water, but I prefer the real stuff). Sprinkle a little salt in your water.

4. Eat enough potassium

You also lose potassium when you go low-carb and dump all that water weight.6 To replenish your stores, Use Lite-Salt (a potassium salt) along with your regular salt, and eat lots of non-starchy green vegetation, like spinach. Other great potassium sources include avocados and yogurt (if you get real yogurt, the bacteria have consumed most of the sugar).

5. Take magnesium

Notice a theme here? Electrolytes matter when you’re going keto.

Although losing water doesn’t really flush out magnesium like it does other electrolytes, we do need extra magnesium to regulate sodium and potassium levels in the body.7 Leafy greens like spinach (again) are great sources of magnesium, as are most nuts and seeds. Even though low-carb and sweeteners don’t usually mix, I’d say the huge amount of magnesium in blackstrap molasses makes a tablespoon worth adding.

You may have to dip into the supplement bin for this one. Any magnesium ending in “-ate” will do: glycinate, citrate, malate, etc. And once again, it’s one of those cases where almost everyone can probably use extra magnesium regardless of their current diet. It’s simply a good nutrient to have.

6. Stay hydrated

People tend to focus on the electrolytes you lose with water loss, but there’s also the water. If you’ve ever been dehydrated, you know the symptoms—dizziness, fatigue, mental confusion—match those of the low-carb flu. Pay attention to your thirst and get yourself a good source of mineral water with a TDS of at least 500 mg/L (or make your own using mineral drops), like Gerolsteiner, to boost your intake of minerals that may be lost to water shedding. Don’t drink healthy-sounding things like reverse osmosis water without remineralizing it.

7. Eat more fat

The study I cited earlier in which a low-carb, high-fat diet increased AMPK had another experimental group who also experienced AMPK upregulation: lean adults given a bunch of fat to eat. It turns out that both carbohydrate restriction and fat feeding can increase AMPK activity. In both instances, the amount of fat available for burning increases. By supplementing your endogenous fatty acids (the stuff coming off your body fat) with exogenous fatty acids (dietary fat, or the fat you eat), you can maximize the AMPK activation and, hopefully, get to a state of metabolic flexibility faster. You may not lose as much body fat this way, but you’ll be happier, less fatigued, and more likely to stick with the diet.

8. Include some medium chain triglycerides (MCTs) in that fat

MCTs metabolize differently than other fats. Rather than store them in the body fat or use them in cell membranes, the body sends them directly to the liver for burning or conversion into ketones. So a good percentage of the MCTs we eat become ketones, which provide some additional fuel to glucose-deprived bodies that haven’t quite adapted to a fat-based metabolism. Coconut fat is the natural source of MCTs, though only about 14-15% of the fatty acids in coconut oil are MCTs. If that’s not giving you the boost you need, MCT oil is an isolated source of the ketogenic fatty acids. You can go even further and get just caprylic acid-based MCT oil, which isolates the specific fatty acid with the most ketogenic potential.

9. Consider ketone supplements

Part of the low-carb flu comes down to poor energy availability: when you take away the energy source you’ve been relying on all your life, it takes a while to feel normal. Similar to MCTs but more so, ketone esters “force” ketone availability. And while I’m skeptical of taking large amounts of supplemental ketones on top of a high-carb diet, I can imagine them helping the newly low-carb speed up the adaptation process and overcome the low-carb flu.

10. Move around a lot at a slow pace

If you haven’t read Primal Endurance, consider grabbing a copy. It really fleshes all this out. But long story short? Hold off on the extended sugar-burning training—long CrossFit WODs, long hard endurance efforts, 30 minute interval workouts, P90X—until you’re fat-adapted. Do some intense stuff, but keep it really intense and brief. Short 2-5 rep sets of full body lifts, brief 5-10 second sprints (with plenty of rest in between), things like that. The bulk of your training should consist of easy movement keeping your heart rate in the fat-burning zone (180 minus your age) until you’re adapted and the low-carb flu has abated. Hikes, walks, light jogs, cycling, swimming are all great depending on your level of fitness, and they’ll jumpstart the creation of new fat-burning mitochondria to speed that process up.

11. Reduce carbs gradually

The vocal ones, the people who post on message boards and leave comments and submit success stories, are generally going to be more extreme. They’re going from 400 grams of carbs a day to 20 grams. They’re going all in. They’re going cold turkey (literally: they’re eating entire meals consisting entirely of cold turkey to avoid carbs). That doesn’t work for everyone.

Another option, and one that might work even better for most people, is to gradually reduce carbs. By reducing carbs more gradually you reduce the shock to your system and give your body the chance to find its sustainable sweet spot. You might do best on 150 grams a day (that’s about where I am, in fact). You might like 120, or 130, or 70. The point is going gradually allows you to take a journey through all the possible permutations of carb/fat/protein intake. It’s quite possible that 140 grams a day works best for you, but because you immediately launched into a very low-carb 20g/day diet and failed miserably, you’re turned off from the idea altogether.

You can judge your ketone sweet spot by how you feel after the first week. Or, you can measure your ketones and see what levels make you feel your best.

That’s what I’ve got, folks. Those are the tips that work best for me and mine. Those are the tips that science suggests actually work. What about you? How have you gotten over the low-carb flu?

Thanks for reading, everyone. Take care.

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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  1. Here’s another idea: gradually fat adapt by doing “intermittent fat fasting:” that is establish a carb window outside of which you eat zero net carbs and very low protein in order to keep insulin as low as possible. Gradually shrink the length of your carb window, and the amount of carbs allowed within that window until your carb window is a just few hours in the evening during which you eat 100 or fewer grams of carbs.

    Once this becomes easy, you are fat adapted, and you will have no trouble eating any form of keto that you like. If you want, you can experiment with targeted Keto (TKD) or more extreme intermittent fasting like OMAD (one meal a day), for example.

  2. If I don’t get carb flu does it mean I’m not fat burning or reducing my carbs enough? I’m very new to this but started on 75g of carbs 20%, 60% fat and 20% protein (can’t remember the g’s) I’ve been doing the diet for 4 weeks but did have a big break where I ending up back on a high carb diet for 2 days. So perhaps I’ve started again and am now only 2 weeks in. Any advice would be good to this Keto virgin 🙂

    1. At least from what I’ve seen, most people don’t keto adapt eating 75 grams of carbs. That’s technically a low carb/high fat diet, which might be why you didn’t have the severe keto flu (it is pretty unmistakeable and a lot more severe than just going low carb).
      Now that you’ve spent a few weeks eating low carb, try restricting your carbs more (so under 50 grams) and use an online calculator to figure out where your protein should be (it might already be on target for you). I think Ketogains has a good one, as does ketodietapp .com . At this rate, you’ll be slow rolling into keto, so you have a very good chance of not having a keto flu!
      Also, if you haven’t, read Sisson’s new book Keto Reset Diet. It is an excellent resource for sensible keto eating. Sounds like you’ve already gone through the first phase of his plan, so you’ve got a head start!

      1. Thank you Casey. To be honest I don’t seam to be very good at this diet as I keep falling off the wagon. Usually when I’m out and get over hungry or with friends (this week was KFC with the chips and fizzy drink) very bad I know and probably ruined any head way I made. I have a very social life and eat out a lot with people that do not understand what I am trying to achieve so sticking to very low carbs EVERY day is difficult for me.
        Am I right in saying that this kind of behaviour will not be tolerated by the body and I will have to stat again when ever this happens? I feel I may just end up on a high fat, medium protein, and mostly low carbs but with big binges every now and then diet. which I appreciate won’t get my into ketosis but is it actually bad for me due to the high fat content. I guess what I’m asking is with a Keto diet is it “all or nothing” I’m not morbidly obese, I’m 10st 3lb and 5’6” but just want to shift the 40+ muffin top and live healthier. Any good correct advice is appreciated . Thank you x

        1. HI Lisa, you may want to check out Leanne Vogels keto work, she talks about carb refeeding quite a bit. Her version of keto is more geared towards women who can get hormonal issues from low carb.

  3. Anecdotally, before I started eating VLC (I keep to under 20g carbs per day), I did research and found numerous sources stating that the sure-fire way to avoid “Keto flu” was to dramatically up your intake of electrolytes. I already supplemented with magnesium, but I began adding several shakes of Morton’s Lite Salt (sodium/potassium blend) to every glass of water I drank throughout the day. I’ve cycled on and off this diet 4 or 5 times now, and never experienced even a day of “Keto flu”.

  4. The best course for a lot of people, would be rather than jumping in and cleaning out the pantry, would be to start by replacing sugar in the tea and/or coffee with stevia or similar, and after a couple of weeks start tapering off the grains by way of gradually using up what they’ve got on hand. This way they could transition on to a low carb diet without the unpleasant side effects.

  5. Oof this “flu” is the worst, I’m tired, dizzy, have a bad tase on mouth and don’t want to do anything. I tried the past few dats eating way more fat and a little more meat. But ugg still feel I’ll. Im going to go buy some avocados tomorrow and see if that helps. If not I’m going to have 1 slive of my homemade flax/wheat sourdough bread drenched in butter tomorrow and see if that helps.

  6. excuse my typos! Forgot to mention I fat fasted the first 3 days as bezt I could with what I had (8-10 carbs) I don’t recommend it. Ugg. Today I had a bit more (25) carbs abd still feel sick. Trying to decide if I should go totally 0 carb or try 30…

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