The keto diet, which is a low-carbohydrate diet focussing on high quality proteins and fats, has risen to popularity due to its impressive weight loss results. By eliminating starchy carbohydrates, many people have been able to realise their weight loss goals.
In addition, the keto diet has been associated with a number of health benefits, including improved insulin sensitivity, reduced hypertension and cognitive improvements[1].
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However, for all the positives, many people who embark on the keto diet experience a range of short-term side effects, commonly known as the “keto flu.”
So what side effects can you expect when initiating the keto diet, and how can you minimise them?
Insomnia
A little known side effect of initiating the keto diet is insomnia.
The exact reason why some people experience difficulty sleeping on commencement of keto is not entirely clear, but it is likely do to with switching the body’s primary metabolism from carbohydrates to fat.
There is some evidence that eating a higher carbohydrate diet leads to increases in blood glucose, which increases the uptake of tryptophan by the brain[2].
This subsequently increases the synthesis of serotonin, which leads to increased melatonin production[2].
As melatonin is known to have a significant impact sleep, this could help to explain why some people experience difficult sleeping when commencing the keto diet[2].
Equally, a meta-analysis of carbohydrate consumption and sleep found that a higher-carbohydrate diet resulted in increased time in REM sleep[3]. (Most of the studies included were short-term interventions)
It should be noted however, that many people following a keto diet actually report significantly improved sleep after the initial transition to keto[4].
In fact, keto leads to a number of health benefits, such as weight loss, which can dramatically improve sleep in overweight individuals, and significantly reduce the impact of sleep apnea[1][4].
In order to minimise sleep disturbances when initiating keto, take a more gradual approach to allow your body enough time to adapt.
Nausea
Nausea, stomach pain accompanied by an urge to vomit, is commonly reported as an early side effect on the keto diet.
In fact, in a consumer report on “keto flu” symptoms, approximately 16% of participants claimed they experienced nausea after initiating the keto diet[5].
This is likely due to the fact that the keto diet consists of 70-75% of calories coming from fat sources, whereas a traditional Western diet contains approximately 35% fat[6].
Fats take longer to digest and are emptied from the stomach much more slowly than carbohydrates. This could lead to more feelings of uncomfortable fullness and nausea, than on other lower fat diets[7].
Nausea is a temporary side-effect that should resolve within a few days to weeks.
This time allows the body to adapt to the new macronutrient composition of your keto diet.
If you are used to eating a low/moderate fat diet, it is advised to slowly increase your fat intake, to avoid potential side-effects.
Fatigue
Muscle fatigue and general lethargy are also commonly reported side effects.
As mentioned in our article on ketosis and ketogenesis, your body preferentially metabolises glucose from carbohydrates for energy.
However, upon initiation of the keto diet, your body has to switch from carbohydrates (the body’s preferred energy source) to ketones from fat.
This change is significant, and results in an adjustment period.
During this period, it is not uncommon to feel physically fatigued and lethargic, from doing everyday tasks that would normally be easy[5].
A study conducted on healthy young women found that upon initiation of a low carb diet, routine daily activities, like taking the stairs or going for a walk, felt more strenuous[8].
This is likely due to the higher presence of ketones that accompany entering into ketosis, as studies have shown a direct relationship between blood ketone concentration and perceived effort by participants[9].
Fatigue is generally short-lived, and is a result of switching from glucose to primary metabolism of ketones for energy.
Taking multi-vitamin supplements can help during this period, and ensure that micronutrient imbalances are not contributing to feelings of fatigue.
Check out our article on micro and macronutrients on the keto diet, for an in-depth guide to micronutrient supplementation on the keto diet.
Irritability and mood changes
Irritability and mood swings can commonly occur after initiation of the keto diet[5].
This is because on a higher carbohydrate diet, your body goes through peaks and troughs of blood glucose, depending on when you ate your last carb-rich meal[10].
After eating a carb-rich meal, blood glucose levels rise, triggering the secretion of insulin. Insulin causes blood sugar to drop, which after reaching low levels, induces cravings for further carbohydrates[10].
After this drop in blood sugar, you would normally eat food containing carbohydrates. However, on the keto diet this is not possible, due to the level of carbohydrate restriction[10].
This means that when transitioning to keto, the drop in blood glucose is not proceeded by further ingestion of carbohydrates, and the blood glucose peak and trough cycle ends.
Given that the brain normally derives almost all of its fuel from glucose, this reduction in blood glucose leads to feelings of irritability, mood swings, and confusion[10].
This is temporary, and lasts for a few weeks until your body adapts to lower blood glucose levels.
It is important to ensure proper hydration and supplementation with electrolytes to try and mitigate some of the irritability and mood swings that accompany commencement of keto.
Getting past this initial stage is challenging but worth the effort.
Once these symptoms settle, following the keto diet can lead to less irritability, a more stable mood, and can even be used to treat mood disorders[11].
Dizziness and headaches
As mentioned, initiation of keto is proceeded by a significant drop in blood glucose[10].
This drop means the brain needs to switch from blood glucose to ketone bodies, as the primary source of energy[10].
This transition can result in headaches or symptoms such as dizziness and light-headedness as your body adjusts to the new diet.
In fact, dizziness or “brain fog” is one of the most commonly reported symptoms associated with the keto flu[5].
The transition to fuelling your body with ketones takes time, during which it is common to experience symptoms such as brain fog.
By ensuring proper hydration and electrolyte balance, through a healthy keto diet and micronutrient supplementation, it is possible to ameliorate some of these symptoms.
Headaches can be relieved by over the counter medications such as paracetamol or aspirin, but please consult a doctor before taking any form of medication.
If you are able to make it past the first few days or weeks, it is possible for keto to lead to significant improvements in cognition, including attention[12].
Reduced exercise tolerance
One of the most common complaints on initiation of the keto diet is a reduction in athletic performance and reduced exercise tolerance[5].
This is unsurprising given the usually high dependence on carbohydrates for athletic work output.
Due to the shift of carbohydrate to fat fuel, many people experience depleted energy during exercise, resulting in a lower capacity for work output.
Studies on short-term keto dietary changes have demonstrated a decline in exercise performance[8].
A study in young healthy women found significant decreases in cycling time to fatigue as well as increased feelings of perceived exertion (measured by the Borg scale)[8].
Similarly, a study following athletes on a short-term dietary intervention echoed the above results.
Athletes on a 2-week low-carb diet saw decreased workloads as well as increased perception of exertion during exercise[13].
This isn’t all bad news for athletes however; Once the body has become used to utilising ketones instead of glucose (called keto-adaptation), performance may even be enhanced, especially in endurance athletes[14].
Diarrhea
Gastrointestinal symptoms, such as diarrhea are a frequently reported side effect of the keto diet[5].
In fact, Joe Rogan mentioned, on his JRE podcast, how surprised he was at the level of diarrhea he experienced when staring the carnivore diet (an animal-protein only version of the keto diet).
There may be a number of reasons why this occurs.
Firstly, changing the proportions of carbohydrates and fats in your diet can have significant effects on the microbial composition of your gut[14].
This microbiome change can result in increases in certain types of bacteria which, at least initially, could result in diarrhea[14].
Secondly, with a decrease in carbohydrates, there is less starch and soluble fibre to provide bulk to faeces, and absorb some of the water in the gut[15].
This combined with increased amounts of fat, which is more challenging to digest, can lead to looser bowel movements[15].
This can be exacerbated by over-consuming gut-irritating beverages, such as coffee.
Finally, many individuals tend to increase the amount of sweeteners in their diet when following a keto diet[16].
This serves as a way to address sugar cravings, without the sugar.
However, as mentioned in our articles on artificial sweeteners and sugar alcohols, some sweeteners have laxative effects, which can lead to diarrhea if consumed in high quantities[16].
This diarrhea normally settles after a few weeks, but it is advised to consume moderate amounts of coffee and sweeteners, to minimise initial keto-induced diarrhea.
It is also important to ensure adequate hydration and electrolyte intake, to minimise any potential electrolytes lost through diarrhea.
Constipation
In contrast to diarrhea, some people may experience the opposite problem, and struggle with constipation[5].
As the keto diet eliminates starchy carbohydrates and grains, it can also lead to reductions in fibre intake[15].
In some individuals, this leads to constipation.
It is not exactly clear why a keto diet leads to constipation in some and diarrhea in others, but individual differences are likely a responsible to a certain degree.
Fortunately, fixing constipation is relatively straightforward in most cases.
By incorporating low carbohydrate, fibre-rich vegetables (like leafy greens) into your diet, constipation can usually be avoided.
Take a look at our full guide on food choices on the keto diet.
If you struggle to get the necessary fibre from your keto diet, a fibre supplement can also be a beneficial alternative.
It is also important to ensure adequate hydration, as dehydration can exacerbate constipation and is likely to be a contributing factor.
Carbohydrate cravings
Sugar and carbohydrate cravings are quite common upon initiation of the keto diet, and can persist in the longer-term[5].
Entirely restricting a food group, especially if you are used to consuming it, can lead to both physiological and psychological cravings[17].
By using natural alternatives to sugar (natural sweeteners like stevia), it can be possible to consume sweet-tasting foods while maintaining ketosis.
This is often sufficient to curb sugar cravings while on the keto diet.
It is also important to ensure adequate caloric consumption on the keto diet, including healthy fats, low carbohydrate vegetables and good sources of protein.
Over-restricting calories, even while on a keto diet, can lead to increases in cravings[17].
This can be very detrimental to your diet and there is some evidence that this could lead to binge eating[17].
Ensuring an adequate amount of calories from healthy keto sources, and substituting sugar for natural sweeteners can help curb cravings, and keep your keto diet on track.
Increased thirst and polyuria
A common side-effect of the keto diet that may cause initial concern is an increase in thirst and urination frequency and volume.
Increased urination, known as polyuria, can be a concern as it is often associated with serious health conditions, such as kidney disease.
The main reason for this increase in thirst and urination is to do with a reduction in carbohydrate intake.
Carbohydrate molecules bind to water molecules, in a ratio of 1:3 grams of carbohydrate to water respectively[18].
As a result, a significant amount of water is retained on a high carbohydrate diet. When carbohydrates are restricted however, these water molecules are shed, leading to increased urination[19].
This expulsion of water is thought to be the reason for the increase in thirst, which creates a cycle of higher consumption and expulsion of water[19].
This is more significant when initially transitioning to the keto diet, but tends to persist over the longer term, at a higher level than on higher-carbohydrate diet.
Supplementation with micronutrients can ensure good electrolyte balance, and potentially minimise negative effects.
Key takeaways
Commencement of the keto diet is associated with a number of short-term side effects. As discussed, most of these symptoms (referred to as the keto-flu) are self-limiting, and are not a health concern.
Although the transition from burning glucose to utilising ketones is significant, there are a number of things you can do to ease the change.
By easing into the diet, ensuring good hydration and supplementing where necessary, you can minimise some of the initial side effects, and make your introduction to keto less challenging.
