When starting a ketogenic diet, some people experience initial side effects from carbohydrate restriction known as ‘keto flu.’
These symptoms can have some mild and potentially severe effects on the body.
While the condition is popularly known as keto flu, people also commonly refer to it as induction flu, low carb flu, and Atkins flu.
This article will explain what it is, why it happens, and the best strategies for avoiding or beating it.
What is the Keto Flu?
Firstly, it is not the real flu.
It just shares the name because it has several of the same symptoms.
Coming from a high carbohydrate diet, the body is well-adapted to using glucose for fuel.
However, when restricting carbohydrate, the supply of glucose falls before the body has adapted to burning fat for fuel.
In other words, your body is in ketosis but not fully keto-adapted.
If you are curious about this, you can find out your level of ketosis by using ketone strips.
The liver and gall-bladder need time to upregulate the number of fat-burning enzymes to burn larger amounts of fat efficiently.
Severely restricting carbohydrate is a massive change to the way the body works and your body needs time to adjust to the metabolic changes.
When Does it Start?
There is no exact timeframe, but symptoms may appear as quickly as 10-12 hours after starting to restrict carbohydrate.
For some people, it might be slightly earlier or later.
Of course, there are also people who won’t experience the dreaded keto flu at all.
How Long Does it Last?
Based on anecdotes, this induction flu lasts somewhere between two days and about two weeks.
The worst symptoms appear in the first few days and then taper off.
Regarding the intensity of the symptoms, this likely depends on the previous diet, hormonal state, and prior carbohydrate intake.
For people who are following extremely low carbohydrate diets (especially zero carb diets), these symptoms may be more intense.
Fortunately, there are several ways to reduce the side effects or even completely remove them.
So, what kind of things should you be looking out for?
Some symptoms may be mild, and in extreme cases, others can be more serious.
As previously mentioned, the intensity of the effects will depend on the hormonal state and metabolism of each person.
- Difficulty sleeping: Some people have problems getting to sleep and waking up in the middle of the night is not uncommon.
- Digestive issues/upset stomach: Mild stomach discomfort can be a problem for some.
- Fatigue: As the supply of glucose falls with nothing (initially) to replace it, a lack of energy is a common symptom.
- Headaches: A headache is a frequent side effect of going low carb in the first day or two.
- Irritability: Making large-scale dietary changes affects hormones and may make people irritable.
- Lack of focus/lethargy: As energy levels drop, a lack of focus and lethargy is a natural result of cutting the carbs.
- Mental Fog: A lack of mental clarity, cloudy memory, and a spacey feeling can accompany a sudden reduction in carbohydrate.
- Nausea: Occasionally, people report nausea during the first several days.
- Sleepiness: Again, this feeling is due to a lack of energy from your body continually trying to burn (a falling supply of) glucose.
- Sugar and carbohydrate cravings: refined carbs and especially sugar can feel very addictive, and giving them up can lead to withdrawal symptoms.
- Arrhythmia: Otherwise known as heart palpitations, some people experience these in the first few days of a very low carb diet. This symptom tends to be due to mineral deficiencies.
- Constipation: Changes in bowel movements are a common complaint. In this case, they are usually caused by dehydration from the body flushing stored water as glycogen drops.
- Cramps: Also a side effect of dehydration and mineral deficiencies, some people experience cramps — particularly in the legs and feet. These events may occur more frequently during the night.
- Dizziness: This is only in more extreme cases, but dizziness can occur. It happens as a result of low blood sugar and sodium deficiency.
- Drowsiness: While similar to a lack of energy, feeling drowsy can be dangerous if an individual is driving or in public.
- High (or low) blood pressure: In severe cases, high or low blood pressure may be a symptom.
Before we look at how to solve these problems, it’s important to know why they occur and we’ll examine this in the next section.
Why Do These Keto Flu Symptoms Happen?
One simple reason why these side effects occur is that the body is trying to burn glucose, but being fed fat.
Keto-adaptation takes time.
Over a potential lifelong diet of high-carbohydrate meals, the body has developed a wealth of enzymes to burn carbohydrate, but it is ill-prepared for burning significant amounts of fat.
As an analogy, it is like trying to fuel a diesel car with gasoline/petrol.
Once someone switches to a very low-carb, high-fat diet, the body will begin to upregulate the production of fat-burning enzymes.
However, this may occur over a period of a few days or it may take a few weeks, and this adaptation phase can be a painful process.
All the ‘keto flu’ symptoms disappear once the body starts burning fat (ketones) instead of carbs (glucose).
How Can We Make the Adaptation Phase Easier?
Nobody wants to feel like they have the flu for days on end, and all it takes is a bit of research to minimize these side effects.
For me, there are five important considerations during keto-adaptation;
- Hydration (water consumption)
- Fat consumption
If we carefully consider each of these points, then we can seriously reduce (or completely remove) the symptoms of keto flu.
1. Water: Stay Hydrated
The first thing to remember is that when we are consuming a high carbohydrate diet, our body stores lots of glycogen.
Glycogen is a form of sugar that is easy to use for the body, and it is stored in our muscles and liver.
Upon starting a ketogenic diet, these glycogen stores progressively deplete and at the same time flush the no-longer-needed stored water. As we lose this water, we also lose electrolytes such as sodium, potassium, and magnesium.
Additionally, we also lose water due to a reduction in insulin levels.
As carbohydrate intake reduces, our blood glucose levels drop significantly. This fall has the knock-on effect of reducing our circulating insulin levels.
Higher insulin levels encourage our kidneys to retain sodium, so when our insulin levels drop, this stored sodium is excreted.
In other words, we lose yet more water as we excrete this sodium through urination.
As we are losing a significant amount of water during the first few days of carbohydrate restriction, it’s important to stay hydrated.
Dehydration can easily occur and helps explain many of the symptoms such as headaches and cramps.
Thirst is a great cue which we should listen to. While I don’t believe in an arbitrary water recommendation that fits everyone, it’s worth considerably upping water intake during the keto-adaptation phase.
If you experience headaches or any cramping, then there’s a good chance you’re just not drinking enough.
Sodium is one of the most important nutrients to human health.
As previously mentioned, our body flushes large amounts of this electrolyte away during the adaptation stages of keto.
Quickly losing large amounts of sodium in this way can lead to many deficiency symptoms such as headaches, thirst, brain fog, and lethargy.
This explains a significant part of what keto flu is — symptoms of electrolyte deficiencies and inadequate hydration.
On the negative side, critically low sodium levels may also lead to hypotension (low blood pressure). This condition results in feelings of dizziness, lightheadedness, and in severe cases, even passing out.
It is, therefore, critical to get extra sodium when starting on a ketogenic diet.
How Much Sodium is Enough?
Starting on a ketogenic diet without upping water and salt intake is likely going to cause keto flu symptoms.
Phinney and Volek, respected low-carb research scientists, recommend 3-5 grams of sodium per day (4).
This amount works out to around 1.5 to 2 teaspoons of salt.
Here are some ways to ensure sufficient sodium intake;
- Focusing on naturally sodium-rich foods like eggs, meat, and fish
- Making bone broth and salty soups
- Including sea vegetables such as kelp and kombu
- Liberally salting each meal
And don’t worry too much about high salt consumption.
In fact, studies show that—for most people—salt consumption plays a minor role in hypertension (high blood pressure) compared to sugar and refined carbohydrate intake (5).
Also, in the presence of lower insulin levels, the body does not store salt in the same way.
Sodium and potassium need a delicate balance in the body, and this ratio is essential for controlling the fluid balance in every cell.
As sodium levels fall when starting a ketogenic diet, so too does potassium as the body excretes it through urine.
Not surprisingly, symptoms of potassium deficiency also mimic those of the keto induction flu (6);
- Heart palpitations
There are several ways to keep potassium levels high;
- Make yourself aware of low carb foods that are high in the mineral.
- Due to the link between sodium and potassium, keeping sodium intake high helps to preserve potassium levels.
- Emphasize leafy greens — spinach and seaweed are especially high in the mineral.
- All meat and fish contain decent amounts of potassium, but these foods are generally part of a ketogenic diet anyway.
In my opinion, magnesium is the single most important mineral.
The fact that our magnesium levels drop when starting a keto diet is an important consideration.
For instance, sufficient magnesium intake also helps regulate potassium and sodium levels (9).
Short-term magnesium deficiency is associated with;
- An increase in food cravings (10).
- Symptoms such as nausea, fatigue, and cramps (11).
Here are some strategies to maintain sufficient magnesium levels;
- Many delicious low carb foods are high in magnesium, so try to emphasize them.
- If you’re not getting enough dietary magnesium, it’s a good idea to consider using supplements. Magnesium citrate is one of the best, or you can use a magnesium complex like the above.
Ensuring a proper dietary intake of magnesium, potassium, and sodium will assist in providing sufficient levels of electrolytes.
Through this and drinking enough fluids, you can usually neutralize or dramatically reduce most of the keto flu symptoms.
5. Fat Consumption – Get Enough
When riding a bike for the first time, how do you learn? By riding more!
Likewise, eating a sufficient amount of dietary fat helps your body to acclimatize to burning fat for fuel quickly.
Fearing fat is one of the biggest mistakes that so many people make. For instance, limiting dietary carbohydrate, but then eating low-fat chicken breasts and trimming the fat from red meat.
If carbohydrate is low, then fat needs to be high. Without a sufficient supply of energy, we will feel terrible.
Prioritize sources of fat in the diet, from foods such as;
- Fatty meats (pork belly, bacon, ribeye steak, roasted chicken with skin).
- Oily fish (salmon, mackerel, trout, herring, sardines).
- Healthy fruit sources of fat such as olives and avocado.
- Eat a handful of nuts each day – macadamia nuts are one of the tastiest!
- Use healthy sources of fat liberally – butter, ghee, coconut oil, olive oil, avocado oil, and animal fats all make good choices.
Temporarily eating more fat can help hasten the keto-adaptation phase, and then you can cut down once fully adapted.
Ketogenic diets can be healthy and they have a lot of benefits if you eat right, including weight loss and better health markers.
However, it is important to fully research them before adopting the diet for the first time.
Some of these possible symptoms can be scary, and all it takes is a bit of knowledge to avoid them.
If you do experience these symptoms, then do be prepared and make sure you get enough water and electrolytes.
Lastly; remember that although these keto flu symptoms can be a pain, they don’t last forever.
For more on keto diets, see this guide to ketogenic diets and sports performance.