Keto Diet Side Effects

The ketogenic diet, commonly referred to as “keto,” is a low-carb diet that encourages the body to burn fat as its main source of fuel in place of glucose. Although it dates back to around 500 BC, in contemporary medicine, keto was introduced in the 1920s to treat drug resistant epilepsy, particularly in children.

Recently, the diet has also been hailed for its medical role in treating some cases of neurological conditions such as Alzheimer’s Disease, Parkinson’s Disease, stroke, brain trauma and migraines, among others.

In the mainstream diet scene, keto caught on as weight loss tool in the 1970s, propelled into the spotlight alongside Dr. Robert Atkins’ take on a low-carb, high-fat, high-protein eating plan.

“When the body uses fat as its primary energy source, it causes a metabolic state known as ‘ketosis,’ which occurs from the use of ketone bodies derived from fat for energy, rather than glucose (carbs),” explains Kim Bowman, F45 Training Sports Nutritionist.

Bowman notes that the aim of the ketogenic diet is to keep the body in a state of ketosis for the duration of the diet period to boost fat loss.

Based mainly around meals comprising high amounts of fats—including fatty fish, nuts, seeds, avocados, butter and healthy oils—keto restricts your daily carb intake to 5% or less of your daily calories, with 75% derived from fat, and the remainder from protein.

The diet’s weight loss effects are in part due to water weight loss, alongside a reduction in calories with the avoidance of carbs.

In addition, according to Bowman, once the body is in a state of ketosis, an individual’s metabolism becomes efficient at burning fat for energy, which can lead to fat loss. As the body shifts to using fat as its main source, research notes that improvements in blood sugar regulation and insulin sensitivity are very possible.

Keto Diet Limitations

As with any extreme alteration to your diet, Bowman cautions: “Always consult with your primary care physician before undergoing a ketogenic diet.” She also notes that even if approved by your physician, the keto diet may not a sustainable approach to eating long-term.

Given this restriction, following the keto diet may not be suitable for anyone with the following health conditions.

Kidney or Liver Problems

If you suffer from any chronic or acute issue with your kidneys or liver, this diet should be avoided. “There are increasing metabolic demands on both organs due to the increased daily percentage of protein and fat,” explains Bowman.

Dietary Restrictions

“Anyone with dietary restrictions should avoid the keto diet as it is already highly restrictive, given the extreme low carb consumption,” says Bowman.

For example, if you follow a vegan diet, you might struggle to consume calories from a variety of fat and protein sources, depriving you of multiple groups and access to key specific nutrients and vitamins.


Although research on following keto while pregnant is lacking, the general recommendation is to eat a wide variety of healthy carbs, proteins and fats while pregnant. This provides both you and the baby with essential nutrients and vitamins (such as folic acid, calcium and iron).

As such, cutting back severely on carbs may restrict important food sources during pregnancy. What’s more, you should not be losing weight during pregnancy, rather gaining as the baby grows. Given that many opt for a keto diet due to its weight loss benefits, it might not be the most suitable diet during pregnancy.

Side Effects of the Keto Diet

Keto Flu

A widely discussed side effect of “going keto” is the keto flu, which mimics some symptoms of the common flu.

“There are a few known symptoms that have been reported initially including headache, weakness, constipation, nausea and vomiting,” outlines Bowman. “These may occur as a result of the body adapting initially to a low-carb state with change of primary energy source from carbs to fat.”

However, as the body adjusts, these symptoms should diminish with time.

Nutrient Deficiency

As outlined by the WHO Global Action Plan for the Prevention and Control of Noncommunicable Diseases, healthy diets are generally whole-rounded and higher in fruits, vegetables, nuts and seeds, whole grains, legumes, and contain less animal-based foods. With keto, many of these food sources are off limits.

This can result in a drop of the essential nutrients and vitamins received by the body. As Bowman explains: “Following a keto diet can lead to micronutrient deficiencies such as magnesium and phosphorus, as well as macronutrients of fiber-critical compounds for the health of our gut microbiome.”

Depriving the body of key vitamin and mineral groups can lead to a drop in energy levels, focus and even muscle cramps, among other symptoms.

Fluid-Electrolyte Imbalance, Dehydration, or Kidney Problems

“During a state of ketosis, the kidneys are working overtime which increases urination and a loss of electrolytes, which can result in dehydration, lightheadedness, and may also lead to kidney stones or acute kidney injury,” Bowman cautions.

Brain Fog and Mood Swings

A diet comprising low-carbs can lead to cravings due to an imbalance of macronutrients. And although this can level out over time, it may bring about pesky side effects—brain fog being one. “As the brain requires glucose from carbohydrates to function optimally, extreme low carb diets may impair cognitive focus and lead to irritability,” explains Bowman.

Bad Breath

The process of ketosis may cause you to develop bad breath, as the byproduct is released through urinating and exhaling, alongside protein metabolism, in which the body produces ammonia during the breakdown.

As a high measure of water is required to excrete ammonia, a lack of proper hydration may cause a build up in the body, requiring sufficient hydration to balance. Without this hydration, there may be a lingering smell on the breath.


Diets rich in soluble and insoluble fiber from a variety of whole grains, vegetables, fruits, legumes, among other foods groups, are known to regulate bowel movements and reduce the likelihood of constipation. Low-carb diets are therefore a culprit of the bowels becoming backed-up. Among the keto-approved remedies to counteract this are increased hydration, consumption of non-starchy vegetables, and cutting back on dairy. People on keto diet are also encouraged to eat high fiber fats, such as flax and chia seed and avocado.

Post-Diet Side Effects

As the body re-adjusts to breaking down a medley of foods post keto, you may experience side effects—some less pleasant than others.

First, if you suddenly fall back into your old ways without a tapering period, you may find weight quickly piling back on. Whether that’s due to fluid retention, falling into unhealthy eating habits, or hunger pangs, how your body stores food will change as it once again utilizes glucose for energy. To minimize the effects, a monitored transition from keto to a more balanced form of eating should be followed.

In addition, according to Bowman, a poor transition off the ketogenic diet which immediately reintroduces large amounts of refined sugar and processed carbohydrates can spark spikes in blood sugar and cravings for sugary food.

“These blood sugar changes are often accompanied by feelings of irritability and fatigue,” she says. As such, it’s recommended to slowly wean off the diet in stages to avoid sugar highs.

Remember, keto is an extreme form of dieting, and the body requires an adjustment period at both the beginning and end. Rather than the “all or nothing” approach, you should slowly bring specific carbs back into the diet to ward off any unwanted side effects.


The diet that best suits your body will provide you with sufficient fuel and the nutrients you need to sustain you. This may be a ketogenic diet, if following this eating pattern generates an energized, active and healthy body. However, this may not be suitable in the long-term, depending on your motivation, nutritional needs, goals, and budget.

Although a ketogenic diet can promote potential benefits to a number of health markers, it may not be suitable for every individual, especially those with specific conditions. Make sure you seek advice and guidance from a health care professional before starting a ketogenic diet.

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