Why You Might Have Flu-Like Symptoms When Beginning The Keto Diet

The first thing to understand about the ketogenic "keto" diet is that it was developed as a medical treatment for epilepsy, one that was to be administered, ideally, under a doctor's care (via Cleveland Clinic). It's still used in treating epilepsy that doesn't respond to mainstream anticonvulsive therapies, according to a 2020 paper published in the academic journal Frontiers. But since the 1960s, the keto diet has increasingly come to be associated with rapid weight loss. 

Initially, keto was used clinically only to help people with obesity to lose weight quickly (via a 2013 study published in the European Journal of Clinical Nutrition). Over the decades, more individuals have come to use a keto diet to lose weight. At the same time, doctors were beginning to prescribe the diet in the hopes of reducing their risk of cardiovascular disease and improving their insulin sensitivity, as insulin resistance is the primary feature of type 2 diabetes. In time, the keto diet would be prescribed for patients with acne, polycystic ovary syndrome, dementia, and other neurological disorders, various cancers, and respiratory issues, although in all cases, based on more preliminary research. 

The point we're trying to make here is that even if you haven't been prescribed keto by your doctor, it's a medical treatment, nonetheless. And like most medical treatments, keto is associated with a number of side effects, including symptoms that mimic the flu. 

The same signs that tell you you're in ketosis are consistent with flu symptoms

There's a lot to love about the keto diet, as you're encouraged to eat all the bacon, butter, and cheese you want via Coleman Natural and Healthline. But the keto diet only works for weight loss if you follow its macronutrient formula (70-75% fat to 150-20% protein to 5-10% carbs) to the letter and eschew glucose. That formula turns your body into an efficient fat-burning machine, according to a 2020 paper published in the academic journal Frontiers. 

Specifically, what happens when you deprive your body of carbs, your body looks for energy in glucose stored in your liver and also begins breaking down muscle in order to release glucose, according to The Nutrition Source put out by the Harvard T.H. Chan School of Public Health. After several days of this, there's no more glucose for the body to use, so, for energy, the body takes to burning fat. In doing so, the liver produces the chemicals known as ketones, which serve as an alternative fuel for your body and for which the diet is named. 

It takes anywhere from a couple of days to a week to induce ketosis. During this so-called "induction phase" is when a number of symptoms begin to kick in, including fatigue, dry mouth, increased thirst, decreased urination, constipation, decreased hunger, headache, nausea, and sleeplessness (via MedicineNet). Sounds like the flu, no? 

Keto flu is not the actual flu, but it can really discourage some keto dieters

If keto is such an effective weight-reduction method, why does medical obesity remain a problem in the U.S.?  The issue is that keto is only effective when you consume the prescribed ratio of macros and eschew glucose, which puts your metabolism into a state of ketosis, in which ketones are released by the liver and your body becomes a fat-burning machine, via The Nutrition Source. Simply put: the keto diet is quite strict and specific. While a ketosis state indicates the body is burning fat, it also means you'll feel pretty poorly for up to two to four weeks, per a 2020 paper published in the academic journal Frontiers. 

The simple truth is that not everyone is willing to tolerate what amount to flu symptoms in the name of weight loss, or even in the name of improving one's health overall. Moreover, the National Library of Medicine's StatPearls, which was updated in June 2022, points out that "the long-term health implications are not well known due to limited literature." We'll have to be patient when it comes to learning about keto's long-term health effects. But in the meantime, we've got some expert advice on how to deal with what's known as "keto flu." 

Tips for getting through the keto diet's difficult first phase

If you made the choice to try keto and stuck with it long enough to experience the symptoms of ketosis, isn't it safe to assume that you were genuinely motivated by a desire to improve your health? If so, Marcelos Campos, MD, writing for Harvard Health, advises that even if your symptoms are tempting you to abandon the diet, you shouldn't give up without a fight. First, it's not uncommon for undesirable symptoms to show up in the first few days of any significant dietary change that you might make, and they almost always disappear in time. Second, you might find yourself regretting it if you abandon the diet because of some admittedly unpleasant symptoms that, again, are almost guaranteed to dissipate as you get more used to the diet.

Before abandoning keto, Dr. Campos suggests eating more frequent meals, making sure to include lots of colorful veggies, taking it easy during the times you're feeling poorly, and drinking enough water to stay hydrated. StatPearls recommends drinking water boosted with added electrolytes to help with feelings of malaise. 

Dr. Campos also suggests skipping supplements, as they're unlikely to help. If you're experiencing fever, with or without chills, you may be suffering from actual flu, and not merely keto flu — which is not associated with fever. In that case, a consult with your healthcare provider might be in order.