If you're thinking about starting a keto diet, chances are you've heard of this pesky thing called keto flu. But what exactly is it and how long do you have to endure it?
While keto flu is not actually a flu or a diagnosis you may hear from your doctor, it is something many people feel they experience when they transition onto a keto diet.
As if cutting carbs wasn’t hard enough, common flu-like symptoms add a unique layer of difficulty for keto goers - enough to discourage even the most dedicated dieters from sticking to their new lifestyle choice.
However, many will claim there is a bright light at the end of the tunnel. If you can grin and bear it, you might come out on the other end feeling more energetic, less hungry, and most importantly, highly efficient at burning fat for long-lasting energy - supporting your fitness and fat loss goals!
What is Keto Flu?
Keto flu, or ketosis flu, describes a set of common flu-like symptoms that occur when switching from a high carb diet to a high fat ketogenic diet. These symptoms can range from low energy levels to brain fog and confusion.
While keto flu is ultimately not dangerous for most people, it can be quite uncomfortable. Luckily, these side effects are rarely long term and there are simple keto flu remedies you can try to make things a bit easier on yourself.
For many, just staying hydrated can do wonders!
What Causes Keto Flu?
The goal of the keto diet is to transition the body into a metabolic state of ketosis through significant carb restriction. In ketosis, the body becomes more efficient at utilizing fatty acids (ketone bodies) as its main fuel source, and, in theory, more efficient at losing weight.
However, teaching your body to be metabolically flexible - switching from carbs to fat as your primary source of energy - can be a big change for most people.
We are biologically built to love and crave carbs (1,2). This is largely because these delicious carbs are quickly broken down into glucose - which is the most accessible energy source we can use. And when we don’t give our body what it wants, it can put up a bit of a fight - resulting in some discomfort along the way.
Additionally, reworking your metabolism doesn’t operate like a light switch - it takes time to adjust. During this adjustment period, you may have changes in water and sodium, hydration, blood sugar levels, mood, appetite, and energy levels to name a few (3).
Who Will Get the Keto Flu?
Some don’t notice much change and report feeling at their best on the keto diet almost immediately - while others feel wretched for weeks. Unfortunately, there isn’t a good way to know how you’ll feel on a very low carbohydrate diet until you try it.
However, we can often assume that your previous diet and lifestyle will have a major impact on how big of a transition this will be.
If your carb intake was really high prior to making the switch, you likely aren’t very metabolically efficient or fat-adapted, and learning to rely on fat for fuel may be a big swing.
On the other hand, if you’ve typically eaten moderate to low carb and have been fairly active in the past, your body may better understand how to tap into this fuel source already, and your transition might not be as difficult.
Similarly, if you’ve ever successfully tried intermittent fasting or typically work out fasted, your body may already have had some experience with increased fat burning for energy.
When Does Keto Flu Start?
Most people report that symptoms of the keto flu often start within 24-48 hours of following a strict keto diet. This window can vary for some, however, it is unlikely to take more than seven days of strict adherence to the diet to start feeling flu-like.
Meaning, if you’re still feeling good by the end of the first week, you’ve likely avoided the keto flu all together!
Side effects typically begin right around the time your body starts to run out of available carb stores - most people can store roughly 500 grams of carbohydrate in their muscles as reserve fuel (glycogen) (8).
At this time, your body is forced to find alternative fuel and starts the process of metabolizing fatty acids into ketones - putting you on your way to ketosis.
Technically you can speed up the process of getting into ketosis by fasting or exercising more, but this doesn’t mean you will necessarily shorten your ketosis flu in the process.
How Long Does Keto Flu Last?
Keto flu can last anywhere from a couple of days to a couple of weeks, and some may find their symptoms last up to a month!
It is almost impossible to tell exactly how long you will have to endure it, but you can rest assured that this miserable feeling is ultimately temporary and will resolve itself in time.
Additionally, as your carb cravings subside and your body learns to normalize blood sugar levels, metabolize fatty acids efficiently, and you adjust to this new style of eating, many find they feel better than before they even started the keto diet.
Of course, this isn’t a guarantee either - if you are having a miserable time and sticking to your meal plan feels damn near impossible, you may want to rethink it.
Do You Have to Get Into Ketosis?
If the thought of suffering through a diet flu scares you, you aren’t alone. But this also may not be a reason to avoid trying keto altogether.
What you can take away from all of this is that diet success - including on keto - is the result of consistency, not perfection.
Like other healthy approaches to dieting, you don’t need to go into this with an all or nothing mindset.
Ketosis is certainly intriguing for many, but the research is still developing around how beneficial it is for weight loss outside of calorie control. Meaning, as far as we know it is not possible to lose weight on keto without also paying attention to portion control.
So keep this in mind as you consider opting for a super low carb plan - if your goal is to lose weight, as long as you control your daily calories, you’ll get results.
Outside of this, its paying attention to how your food choices are making you feel from the inside out.
If sticking with nutritious, keto-friendly foods satisfies your taste buds and your cravings, and ultimately gives you energy throughout your day - this is a good reason to keep going.
Ready to make the switch to a keto meal plan? We might not be able to cure keto flu, but we can take on all the meal planning, shopping, and cooking every week so you can get ready-made keto-approved meals delivered each week, giving you one less thing to stress about.