If you feel lousy on a keto diet (or any variation of a low-carb diet, such as Paleo, carnivore, or even intermittent fasting), you might be experiencing what is often referred as the “keto flu” — a constellation of symptoms that often hammers folks transitioning from a carb-fueled diet to a fat-fueled diet. I’ve been there too; keto flu symptoms made me question the low-carb diet when I first started. It’s hard to sustain your program and achieve your health goals when you’re experiencing fatigue, headaches, brain fog, and muscle cramps. But while these symptoms may be common for some low-carb dieters, especially in the beginning, they’re not inevitable.
The most common causes of keto flu include sodium deficiency, dehydration or overhydration, carb withdrawal, and reduced brain fuel. Symptoms can be similar for each of these (for example, all four can cause headaches and low energy), which can make identifying the cause of your specific symptoms difficult.
If you’re needing immediate relief, you can address these four keto flu causes all at once:
- Consume more sodium to compensate for the deficiency
- Drink electrolyte water to thirst to combat both dehydration and overhydration
- Ease off carbs more slowly to tamp down carb withdrawal symptoms
- Try MCT oil
- Give your brain and body time to adapt to using fat as fuel rather than carbs
If you want to hone in on your keto flu’s specific causes, learn the science behind why these symptoms pop up, or get some tips to dial in the remedies, then that’s what we’re going to dive into today. You could also test out each remedy individually to determine what’s causing your specific symptoms.
First up: Let’s explore why keto flu happens.
Why Does Keto Flu Happen?
Keto flu is the catch-all term for the symptoms triggered by the metabolic changes that happen when you reduce your carb intake to a point your body needs to rely on fat for fuel, rather than carbs (more on this below). A common carb goal for keto dieters is below ~100g of carbs a day. Symptoms can cover quite a range, from muscular to cognitive, and tend to occur within the first few days of eating a keto diet. But this isn’t a hard rule. Some people never get any symptoms, while others may start to feel lousy months into the program. It depends on various factors.
Before we unpack the causes, let’s explain these metabolic changes. Typically our bodies use glucose molecules, AKA broken down carbohydrates, for energy. You eat a banana, your blood sugar rises with the influx of glucose, and your pancreas releases the blood sugar regulation hormone insulin. Insulin then shuttles the glucose into your cells to use for cellular energy.
When you minimize carbs, you minimize those blood sugar levels and insulin. But your cells still need a source of energy, so your body begins to break down dietary fat (which is usually higher on a keto diet) and body fat to produce molecules called ketones for energy instead. So now rather than primarily using carbs and glucose to fuel your body, you’re primarily using fat and ketones. This state is called ketosis.
While these metabolic changes can bring many benefits — fat loss, mental clarity, more stable energy, etc. (you can read more on that here) — they also can have side effects. Let’s get into these side effects and their causes now, the symptoms they provoke, and how to mitigate them.
Keto Flu Causes and Remedies
Cause #1: Sodium deficiency
Low-carb diets minimize the hormone insulin, and one of insulin’s lesser-known jobs is telling your kidneys to retain sodium. That’s why people with diabetes are sensitive to salt. Their high insulin levels keep them from peeing out sodium, impairing their ability to regulate blood pressure.
Low-carb folks have the opposite issue. With insulin suppressed, they excrete too much sodium through urine. Combine that with less dietary sodium (because keto eliminates salty processed foods) and you have a recipe for sodium deficiency.
Sodium deficiency symptoms are eerily similar to keto flu symptoms. That’s not a coincidence. Sodium is an all-star mineral — it facilitates nerve impulses, regulates fluid balance, and influences many hormones. Without enough sodium, you won’t feel or perform your best.
If you’re experiencing headaches, weakness, low energy, fatigue, muscle cramps, brain fog, insomnia, salt cravings, or irritability during a low-carb diet, try consuming more sodium. Salt your food twice as much. Hydrate with LMNT Drink Mix or LMNT Sparkling. Shoot for 4–6 grams of sodium as a baseline, and increase if necessary to replace sweat losses. Experiment and find what works for you — you’ll feel the difference when you get it right.
Cause #2: Dehydration or overhydration
You don’t just pee out more sodium on keto, but also more water. This can increase the risk of dehydration (losing too much water) and bring symptoms like headaches, weakness, low energy, fatigue, thirst, muscle cramps, constipation, dark urine, dry skin, decreased urinary volume, and rapid heartbeat.
But don’t start hydrating like a beached manatee. Drinking too much water can dilute blood sodium levels and exacerbate low sodium symptoms — fixing one problem (dehydration) at the expense of another (overhydration). Overhydration symptoms can include headaches, confusion, muscle cramps, low energy, irritability.
The symptoms of dehydration and overhydration sound pretty similar, right? This can make diagnosing the culprit tricky. An efficient way to prevent both is to drink electrolyte water to thirst. Thirst is a brilliant guide to hydration needs. Receptors in your brain constantly monitor fluid balance, triggering thirst if you need more water. Adding electrolytes (especially sodium) to the mix helps prevent overhydration as well as sodium deficiency.
Cause #3: Carb withdrawal
Carbs and sugar activate reward pathways in your brain, so eliminating them can cause carb cravings and other “withdrawal” symptoms (keto flu). I hesitate to use the word “withdrawal” because sugar isn’t an addictive drug. It can, however, trigger similar brain systems (like causing a release of dopamine) as addictive drugs.
“Similar to drugs of abuse,” write the authors of one paper, “glucose and insulin signal to the mesolimbic system to modify [increase] dopamine concentration.”
Translation: Depriving yourself of carbs (which your digestive system breaks into glucose) deprives you of dopamine-driven good feelings. That’s why food companies cram so much sugar into everything. It’s rewarding!
If you’re experiencing hunger, carb cravings, mood fluctuations, low energy, headaches, and fatigue, you can try transitioning to keto more slowly, starting by reducing your carb intake to 50–100 grams daily. See how you feel at that level, wait a week or two for your body to adjust, then continue to ratchet down. This slow but steady approach can allow you to ease off carbs without triggering cravings.
Also, keep in mind that keto doesn’t work for everyone. You might feel your best at 100 or more grams of carbs per day. That’s okay! Just don’t give up on keto without optimizing your sodium and hydration status first (causes 1 and 2). That way, you’ll know it’s a carb problem, not a salt or fluid problem.
Cause #4: Less brain fuel
Like we learned above, when you reduce your carb intake, your body has to use fat and ketones for fuel instead of carb. That means your brain has to use fat as fuel, too.
Some folks can transition to this fat-burning state in mere hours. Athletes, for instance, tend to be fat-burning machines. But some people can take a few days or weeks for their body to get fully up to speed and to feel “locked in” on keto. During the transition, cognition-related keto flu symptoms can arise, including headaches, brain fog, low energy, and fatigue.
Luckily, if you’re minding your keto macros, electrolytes, and hydration, your symptoms shouldn’t last more than a few days as your body gets up to speed. Consuming medium-chain triglyceride (MCT) oil is also a well-documented intervention for raising blood ketone levels, which may speed keto adaptation and relieve keto flu symptoms.
If you try out MCT oil, start with a teaspoon daily and work up to a tablespoon or two. If you get too frisky with this supplement, you may find yourself sprinting to the bathroom, praying desperately for it to be unoccupied.
Beating Keto Flu
Keto flu symptoms aren’t inevitable. Address the main causes — sodium deficiency, hydration issues, carb withdrawal, or low brain fuel — and the symptoms should improve.
I hope these tips prove useful in your quest to beat keto flu. And if nothing seems to work, remember that low-carb diets aren’t mandatory for being a healthy human. You might fare better with a different approach that includes more carbohydrates. Listen to your body and adjust accordingly!
