Lots of people these days are interested in the ketogenic diet: It’s one of the most Googled weight-loss terms, and celebs like Kourtney Kardashian have touted its supposed benefits. But the high-fat, low-carbohydrate eating plan is not without its drawbacks.

Experts warn that the diet is extremely restrictive and not sustainable, and that it can lead to nutritional deficiencies, high cholesterol, or a serious condition called ketoacidosis. But even before long-term problems set in, many people who try the diet report other unpleasant side effects. These side effects even have a name in the weight-loss world: keto flu.

Keto flu is an unofficial way to describe how many people feel shortly after starting a ketogenic diet, and it can include both physical and emotional symptom—like nausea, cramping, lack of energy, and irritability, to name a few. It’s what happens when the body and the brain are forced to adjust to a sudden carbohydrate deficiency, says Abbey Sharp, RD, a Toronto-based nutritionist and blogger at Abbey’s Kitchen.

Proponents of the keto diet say that these flu-like symptoms are only temporary, and that certain remedies can help reduce or eliminate them altogether. But is it really worth subjecting yourself to, even if just for a short time? Here’s what our experts say.

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Keto flu symptoms

There’s no scientific definition of keto flu, but it’s often described as flu-like symptoms that start soon after a person cuts carbohydrates largely out of their diet. (To enable “ketosis,” a sort of starvation mode in which the body burns fat rather than glucose, the ketogenic diet allows for only 2% to 5% of a person’s daily calories to come from carbohydrates.) 

“Very often, people don’t feel well when they’re on the ketogenic diet, and it tends to be worse in the early period,” says Edward Weiss, PhD, associate professor of nutrition and dietetics at Saint Louis University. “This is something largely anecdotal—we don’t have studies on this—but it’s probably very real.”

A sudden drop in carbs can lead to a drop in energy levels, with some dieters reporting unusual fatigue, confusion, or brain fog. “The symptoms are from your brain needing to adjust to the new source of energy, while also trying to deal with a drop in electrolyte levels as you lose weight,” says Sharp. It can also cause nausea, stomach pain, cramping, and constipation, as well, due to the diet’s high-fat and low-fiber makeup.

Keto dieters also sometimes report bad breath or foul-smelling sweat and urine. “The smelly factor comes from the fact that acetone, a byproduct of ketone metabolism, seeps out of your body,” says Sharp. (Ketones, a type of acid, are byproducts of fat breaking down in the body.)

Cynthia Sass, MPH, RD, Health’s contributing nutrition editor, says that many of her clients who have tried the ketogenic diet have also reported irritability and changes in mood. And while people don’t necessarily feel hungry on the keto diet—thanks to its high allowance for fat and moderate amounts of protein—some do report serious sugar cravings.

Adopting a ketogenic diet may also hamper athletic performance, says Weiss, even though many athletes try it, thinking it will have the opposite effect. In a recent study in the Journal of Sports Medicine and Physical Fitness, Weiss and his colleagues found that after four days on a keto diet, participants performed worse on anaerobic exercise tasks—which involve short bursts of intense activity—than those who’d recently gone on a high-carb diet.

“Our participants were right in that period of feeling terrible,” says Weiss. “They were tired, hungry, lethargic.” But he and his colleagues suspect there’s a biological reason they performed worse, as well: They had higher levels of acid in their blood, a result of their bodies burning ketones.

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How long does the keto flu last?

“Most people find that you will feel better in a few days, or up to a week, once your body adjusts,” Sharp says. “Whether or not you want to put your body through that is a personal choice.”

As for athletic performance, the participants in Weiss’s study weren’t followed long enough to see if theirs improved after more than just four days on a ketogenic diet. But other research suggests that acid levels in the body tend to normalize after a few weeks, he says, while performance remains compromised. 

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Keto flu remedies

Keto blogs and weight-loss websites recommend taking precautions—like making sure you’re staying hydrated, getting plenty of sleep, and finding ways to manage stress—to reduce the unpleasant effects of the ketogenic diet on your body. Some also recommend electrolytes, ketone supplements, or bone broth (which is high in sodium and other minerals) to replace some of what the body is missing in the early stage of the diet.

Certainly, prioritizing sleep, hydration, and overall healthy habits can keep you from feeling even worse, whether you’re on the keto diet or not. But the experts we spoke with agreed that avoiding ketosis altogether is a smarter way to feel good while you’re trying to lose weight, rather than putting a Band-Aid on something that could have more serious consequences down the road. 

“I don’t really recommend the keto diet in anything other than clinical disease management settings, because it is incredibly restrictive,” says Sharp. (The diet was originally used as a treatment for epilepsy, and scientists are also looking into its potential benefits for people with diabetes or insulin resistance.)

Research on low-carb diets has also shown that while people do tend to lose weight faster in the beginning, there is no long-term difference when compared with other diets of equal caloric intake. “In other words, if you enjoy carbs, a balanced diet that includes them can lead to just as much weight loss,” Sharp says.

RELATED: 6 ‘Bad’ Carbs That Are Actually Good For You

How to avoid keto flu

It is possible to cut back on sugar and carbs—and, yes, lose weight—without experiencing these nasty symptoms, says Sharp. “While you cannot achieve ketosis without a significantly reduced-carb diet, you can absolutely reap a lot of the potential glycemic benefits of a low-carb diet—without some of the downfalls, like keto flu—simply by choosing the right carbs,” she says.

The key is making sure the carbs you do keep in your diet are rich in fiber, like whole grains, vegetables, and fruits. And when you do eat those carbs, Sharp adds, pair them with some fat or protein; this slows their glycemic impact even more, preventing the blood-sugar spikes (and subsequent crashes) that lead to cravings and crappy feelings.

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Sass agrees: In a 2017 column, she recalled one man who became irritable and had trouble sleeping after adopting an ultra low-carb diet. “Those side effects subsided after he added back fruit, pulses, whole grains, and starchy vegetables to his diet,” she wrote.

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Your friend has cut out sugar and feels amazing as a result. Another friend, on the other hand, is on what sometimes appears to be a strict all-candy diet and still stays perfectly healthy and trim. And you have tried both of these dietary tactics and have seen no real changes in your own body.

The same could be said for dairy or carbs — whatever the nutrient may be, you likely know from experience that just because some eating habit works for somebody else, that doesn’t necessarily mean that it’ll work for you. Everyone’s body is different, of course, which means everyone’s body responds to food a little differently. And this, some scientists across the globe are now arguing, points to the potential future of healthy eating. The key to fighting the increasing threat posed by diabetes and obesity may be personalized diets — that is, eating plans tailored specifically for each individual — instead of the generalized nutrition guidelines we have now.

The answer is in your gut — more specifically, the trillions of bacteria currently residing there. Two of the scientists currently studying the interaction between the gut microbiome and diet are Eran Segal and Eran Elinav, the biologists behind the Personalized Nutrition Project in Tel Aviv. Preliminary results from their research, presented earlier this summer at the Human Microbiome conference in Germany, suggest that a computer algorithm can predict how individuals’ bodies will respond to certain foods, thus creating a tailored meal plan for each according to his or her own unique bacterial profile.

This project began more than two years ago and has so far included more than 1,000 people. Segal and Elinav first instructed their participants to wear glucose-monitoring devices, which measured and recorded their levels of blood sugar every five minutes for a week; they also used a mobile app to record what and when they ate that week. Altogether, they collected data on more than 50,000 meals and snacks, plus how each person’s blood-glucose levels responded to each food.

Your gut converts the food you eat into sugars, which are subsequently released into the bloodstream; from there, these sugars are either converted to energy or stored away as fat. Certain foods cause too muchsugar to flow into the bloodstream, and this too-high level of glucose in the blood is what can lead to things like diabetes and obesity. But what foods do this? This is part of the point of nutrition guidelines, to recommend the foods least likely to cause this potentially dangerous spike in blood glucose

But from their data, Segal and Elinav could see that the people in their study were responding to similar foods in wildly different ways. “Already, we could see at a very large scale that, indeed, for any food we looked at, we could see a huge variability in the response,” Segal said. “Some people, you give them sugar and they have a very faint response — even to pure sugar. Whereas others, they have a huge response. And this holds for basically every food that we examined.” And there were more surprises. “Some individuals, they eat whole-wheat rice and their blood-sugar levels remain low, and when they eat ice cream they spike,” Segal said. But for others the results showed just the opposite.

Theirs is not the first study to find an individualized response to similar foods. Studies of twins, for instance, have shown that even people with identical DNA sometimes respond differently to the same diet. And so these results, the researchers argue, suggest “that a universal diet, or universal guidelines, they could never work for everybody, because people are different,” Segal said. “General guidelines are going to have limitations, and they might actually be bad for some people.”

But why might this be the case? Segal and Elinav thought that if they could understand the underlying mechanism that might explain these differences in reactions, they could possibly learn to predict them. Their investigation takes a turn here into the gross: They took stool samples from 800 of their participants, sequencing the genes in each person’s DNA, and used this to complete profiles of the bacterial composition of each individual’s gut. (Basically, they knew that a particular gene is present in a particular type of bacteria, so if they found that gene, it means that bacteria is present, too.) They combined this with the records on their glucose responses to certain foods and used the two data sets to create a computer algorithm, which would create a list of foods that would not trigger that spike in blood-glucose levels.

To investigate the algorithm’s accuracy the researchers started the study that would later be presented at the Human Microbiome conference. They used the algorithm to tailor diets for 25 individuals, all of whom had high enough blood-sugar levels to be considered prediabetic. Some of the foods included on the “approved” list were not exactly the foods you might expect. “For some people it included chocolate, ice cream, pizza — things a dietitian would not prescribe,” Segal said. (Plenty of others didn’t, of course, and stuck to things like whole grains or veggies.) For one week they ate according to their personalized food plan; the following week they ate a diet that was similar in total calories consumed and was in line with more typical dietary guidelines for prediabetics. After the week following their personalized diet, fewer individuals experienced those spikes in blood glucose when compared to their week on the standard diet; some of them even saw their blood-sugar levels dip back down to healthy levels.

It’s an intriguing finding, though very much still a preliminary one. More research needs to be done involving many thousands more people, who are followed for a longer period of time, before anything becomes definitive or clear. And it’s also worth noting that other scientists working on the link between the gut microbiome and diet are skeptical of the notion that this research will eventually lead to eating plans tailored for an individual person. Jens Nielsen, a biochemical engineer at Chalmers University of Technology, believes that it’s more likely that this research will eventually lead to groupings of people, categories of individuals who respond to particular foods in similar ways.

Nielsen is co-author of a study published last month in the journal Cell, which found that people with more diverse populations of gut bacteria are healthier even if they are overweight, when compared to people with less diverse bacterial profiles. Within a decade or so, Nielsen expects that his work may be applicable to weight loss. He’s currently working on the inverse of this problem, investigating the microbiomes of children in developing nations who simply cannot put on weight, even when eating foods expressly designed to help them do so.

And Segal and Elinav expect their work, too, will one day be made available to a wider group of people interested in a personalized meal plan, though, again, the practicalities here are undeniably unpleasant. (Musing on future applications of their work, Segal and Elinav could see a world in which it becomes mainstream to mail stool samples into the lab to get diet advice.) We’re still many years away from that, but the more these researchers look into it, the more individual differences they find, each discovery undermining the idea of blanket nutrition guidelines a little further. “The entire nutritional paradigm we all base our decisions on in our study is proven to be at least partially wrong,” Elinav said. “So we are shifting the paradigm to individuals.”

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This article originally appeared on nymag.com

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