The ketogenic diet—also known as “keto”—has become the latest big thing in weight-loss plans, touted recently by celebs like Jenna Jameson, Mama June, and Halle Berry. The diet involves cutting way back on carbohydrates, to 50 grams a day or less, to help the body achieve a state of ketosis, in which it has to burn fat (rather than sugar) for energy.

Doctors say that keto can be helpful in treating epilepsy; it’s unclear exactly why, but something about a ketogenic state seems to reduce the frequency of seizures. Animal studies have also suggested that the diet may have anti-aging, anti-inflammatory, and cancer-fighting benefits, as well.

But as a general weight-loss plan, keto is more controversial. Some health experts warn against it entirely, citing unpleasant side effects, health risks, and the diet’s unsustainable nature. Even many keto proponents admit that, if the diet’s not done “the right way,” it can be the opposite of healthy.

Here are a few things you should know about the ketogenic diet before you try it as a way to lose weight. Yes, you might drop pounds, but you should also watch out for the following side effects or complications.

RELATED: 4 Things You Need to Know Before Trying the Keto Diet

The “keto flu”

“Some people report that when they start ketosis, they just feel sick,” says Kristen Kizer, RD, a nutritionist at Houston Methodist Medical Center. “There can sometimes be vomit, gastrointestinal distress, a lot of fatigue, and lethargy.” This so-called keto flu usually passes after a few days, she adds.

Josh Axe, a doctor of natural medicine and clinical nutritionist, estimates that about 25% of people who try a ketogenic diet experience these symptoms, with fatigue being the most common. “That happens because your body runs out of sugar to burn for energy, and it has to start using fat,” he says. “That transition alone is enough to make your body feel tired for a few days.”

You may be able to minimize the effects of keto flu by drinking plenty of water and getting plenty of sleep. Axe, who sells keto-related supplements on his website, also recommends incorporating natural energy sources to battle fatigue, like matcha green tea, organic coffee, or adaptogenic herbs.

Diarrhea

If you find yourself running to the bathroom more often while on a ketogenic diet, a quick internet search will show you that you’re not alone. (Yes, people are tweeting about keto diarrhea.) This may be due to the gallbladder—the organ that produces bile to help break down fat in the diet—feeling “overwhelmed,” says Axe.

Diarrhea can also be due to a lack of fiber in the diet, says Kizer, which can happen when someone cuts way back on carbs (like whole-grain bread and pasta) and doesn’t supplement with other fiber-rich foods, like vegetables. It can also be caused by an intolerance to dairy or artificial sweeteners—things you might be eating more of since switching to a high-fat, low-carb lifestyle.

RELATED: 10 Things That Can Cause Diarrhea

Reduced athletic performance

Some athletes swear by the ketogenic diet, not just for weight loss but for improved performance in their sport, as well. But Edward Weiss, PhD, associate professor of nutrition and dietetics at Saint Louis University, doesn’t buy it. “I hear cyclists say all the time that they’re faster and better now that they’re on keto, and my first question is, ‘Well, how much weight did you lose?’” he says.

In a recent study in the Journal of Sports Medicine and Physical Fitness, Weiss and his colleagues found that participants performed worse on high-intensity cycling and running tasks after four days on a ketogenic diet, compared to those who’d spent four days on a high-carb diet. Weiss says that the body is in a more acidic state when it’s in ketosis, which may limit its ability to perform at peak levels.

“Just losing a few pounds is enough to give you a huge advantage on the bike, but I’m very concerned that people are attributing the benefits of weight loss to something specific in the ketogenic diet,” Weiss continues. “In reality, the benefits of weight loss could be at least partially canceled out by reductions in performance.”

Ketoacidosis

If you have type 1 or type 2 diabetes, you shouldn’t follow the keto diet unless you have your doctor’s permission and close supervision, says Kizer. “Ketosis can actually be helpful for people who have hyperglycemia issues, but you have to be very mindful of your blood sugar and check your glucose levels several times a day,” she says.

That’s because, for people with diabetes, ketosis can trigger a dangerous condition called ketoacidosis. This occurs when the body stores up too many ketones—acids produced as a byproduct of burning fat—and the blood becomes too acidic, which can damage the liver, kidneys, and brain. Left untreated, it can be fatal.

Ketoacidosis has also been reported in people without diabetes who were following low-carb diets, although this complication is quite rare. Symptoms of ketoacidosis include a dry mouth, frequent urination, nausea, bad breath, and breathing difficulties; if you experience these while following the keto diet, check in with a doctor right away.

RELATED: Keto Diet Constipation Is a Legit Issue—Here’s How to Deal

Weight regain

Because the keto diet is so restrictive, health experts say it’s not an appropriate diet to follow long-term. (Even Axe says it’s best done for 30 to 90 days, followed by a more sustainable diet plan.) But the problem with that, says Kizer, is that most people will regain a lot of the weight they lost as soon as they go back on carbs.

“It’s an issue with any fad diet, but it seems to be extra common with ketosis,” says Kizer. “When people tell me they want to try it because their friends lost weight, I always tell them, ‘Just watch, I almost guarantee that they’ll gain it all back.’”

These types of back-and-forth weight fluctuations can contribute to disordered eating, Kizer says, or can worsen an already unhealthy relationship with food. “I think this diet appeals to people who have issues with portion control and with binge eating,” she says. “And in many cases, what they really need is a lifestyle coach or a professional counselor to help them get to the bottom of those issues.”

Less muscle mass, decreased metabolism

Another consequence of keto-related weight changes can be a loss of muscle mass, says Kizer—especially if you’re eating much more fat than protein. “You’ll lose weight, but it might actually be a lot of muscle,” she says, “and because muscle burns more calories than fat, that will affect your metabolism.”

When a person goes off the ketogenic diet and regains much of their original weight, it’s often not in the same proportions, says Kizer: Instead of regaining lean muscle, you’re likely to regain fat. “Now you’re back to your starting weight, but you no longer have the muscle mass to burn the calories that you did before,” she says. “That can have lasting effects on your resting metabolic rate, and on your weight long-term.”

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Increased risk of heart disease and diabetes

Axe says that, when done right, the ketogenic diet includes lots of vegetables and lean sources of animal protein. In other words, it’s not an excuse to eat butter and bacon—although some people may try to do just that.

That’s why many health experts are concerned about people on the keto diet, especially those who try it without the guidance of a doctor or nutritionist. Doctors say that high-fat diets like this one may raise cholesterol levels, and some studies suggest that they increase the risk of diabetes. Some have even called it a “cardiologist’s nightmare.”

Just this week, a 25,000-person study presented at the European Society of Cardiology Congress in Munich suggested that people on the lowest-carb diets had the highest risk of dying from cancer, cardiovascular conditions, and all other causes. Another study, published this month in the Lancet, also found that people who followed diets that were low in carbs and high in animal proteins had a higher risk of early death compared to those who consumed carbs in moderation. (The opposite was true, however, for low-carb dieters who opted for plant-based proteins over meat and dairy.)

“Whether you’re in the paleo camp or the keto camp or the vegan camp, everyone agrees that we want to have a nutrient-rich diet,” Axe says: “Lots of vegetables, herbs, spices, and plant-based sources of fat and protein, too.”

“If you’re not doing that, you’re promoting disease in the body—it’s that simple,” Axe says. (And yes, that’s true even if you still lose weight in the beginning.) “If you’re just going to eat butter and bacon,” he adds, “I’d rather you not do keto at all.”

“Granola has a heavier weight than cereal because it is baked with added honey, sugar, and oil,” says Flipse. Though it can contain whole grains, normally a 1/4 cup serving contains between 130–150 calories.

Instead, Flipse recommends using granola like a condiment, sprinkling it on top of yogurt or mixing it with lower calorie cereal.

When selecting a granola, inspect the ingredient list and look for those made with whole grains. If it includes three or more types of sugar (honey, molasses, and brown sugar, for example), put the bag back on the shelf. If these sugars were added together, they would be the first ingredient on the list.

You’ve probably heard that when you lose weight fast, it’s usually water weight. Or maybe you point fingers at water weight after stepping on the scale when you’re feeling totally bloated. But what exactly is water weight–and how do you get rid of it?

Water weight is when fluid collects in your tissues, causing them to swell–and it can make you feel pretty miserable. “Water weight is where the body retains fluid that normally would go to the kidneys,” explains Lynn Mack, MD, associate professor of internal medicine at the University of Nebraska Medical Center. Instead of peeing out that extra fluid, your body stores it between your organs and skin, she says.

That can be uncomfortable–but it’s usually temporary, and it doesn’t mean you’ve gained actual weight.

Of course, simply knowing that water weight often goes away on its own doesn’t make bloating or swelling feel any better. So here are some tips on how to both prevent water weight and lose it if it’s already there. 

RELATED: 5 Signs Your Bloating Could Be Something Serious

Causes of water weight

Salt and carbs. One of the most common causes of water weight is too much salt in your diet. Sodium binds with water and keeps it trapped in the body. “The higher the sodium in the diet, the more fluid retention a person will have,” says Dr. Mack.

Carbs can also have an impact on fluid retention, specifically if you start adding them back after a period of restricting them.

“The carbohydrates we don’t use right away for energy we store as glycogen,” explains Joanna Sheill DiCicco, a registered dietitian at Henry Ford Center for Health Promotion in Detroit. “Glycogen pulls in water, so the more glycogen we are storing, the more water we are taking in.”

“When we are on restrictive diets and at first lose weight quickly, that really is just water weight from the loss of stored glycogen from our muscles,” says DiCicco.

RELATED: 20 Little Ways to Drop the Pounds and Keep Them Off

Menstruation. Many women retain water weight the week before their period due to fluctuating hormones. Fluid retention may reach its peak the first day of your actual period, before subsiding for that cycle.

“With this type of fluid retention, the breasts can get really tender and some women get belly fullness,” says Dr. Mack.

You might also notice swelling in your face, legs, arms, and pubic area in the days leading up to your period.

RELATED: Here’s How PMS Can Change in Your 20s, 30s, and 40s

Pregnancy. Pregnancy can cause you to gain water weight, especially as you get closer to your due date. You may see swelling in your hands, feet, or ankles. Hormones are partly to blame, but your growing baby also puts strain on your blood vessels.

“With pregnancy, you have a big belly so the [pressure causes] the fluid to go out into the tissues, and it has trouble getting back into the vessels,” says Jennifer Wu, MD, an ob-gyn at Lenox Hill Hospital in New York City.

If your only symptom is swelling, it’s probably normal (although the weight may not all come off the minute you deliver your baby). If you have sudden swelling that hurts, you may have developed a blood clot (especially if the problem is only in one leg) or a spike in blood pressure. Either way, if you have these symptoms, get to a doctor right away.

RELATED: 17 Things No One Tells You About Recovering From Childbirth

Hormonal birth control. Just like there’s a connection between pregnancy and menstruation and water retention, hormonal birth control can also sometimes cause water weight.

Both the estrogen and progestin in birth control pills can be culprits, says Dr. Mack. Usually the water weight isn’t major and doesn’t last long, Dr. Wu adds, but you may want to talk to your ob-gyn about other birth control options.

RELATED: The Most Effective Birth Control, Ranked

Cortisol. Cortisol is best known as a “stress hormone,” although it’s actually much more than that. It’s involved in keeping blood sugar levels stable, balancing metabolism, reducing inflammation, and even forming memories.

Water retention as a result of elevated cortisol levels isn’t common, but it can happen.

“You’d have to have a pathophysiologic release of cortisol for that,” says Dr. Mack. In other words, there would have to be a lot of cortisol. “Just being stressed won’t do that.” (Phew.)

Cushing syndrome, for instance, might cause water retention. This is when tumors on the pituitary or adrenal glands release too much cortisol into the blood. People with low levels of thyroid hormone (hypothyroidism) can develop swelling around their eyes, says Dr. Mack.

Travel. Sitting for long periods of time on cross-country flights or lengthy road trips can cause water retention.

“Your muscles contract literally from sitting for too long,” says Dr. Mack, and your feet and legs may swell in response as the fluid pools there.

RELATED: 13 Reasons Your Feet Are Swollen

Medications. Certain meds can cause you to retain fluid. These include drugs for high blood pressure like calcium channel blockers, corticosteroids, and nonsteroidal anti-inflammatory drugs (NSAIDs). Some diabetes drugs, called thiazolidinediones, also do this.

Your doctor or pharmacist will be able to tell you if fluid retention is a side effect of any medications you’re taking and if there are alternatives that might not cause water weight.

Poor circulation. Our circulatory systems become weaker as we get older, or sometimes as the result of a more serious condition like heart failure (which is also more common as we age), says Elizabeth Kavaler, MD, a urologist with Lenox Hill Hospital.

The valves in the veins of our legs, which are supposed to keep blood flowing upward to the heart, collapse a little, so the blood pools in the lower extremities and causes fluid retention.

“It’s physics,” says Dr. Mack. “You just get more pressure pushing down on those legs.”

How to lose water weight (or at least control it)

Avoid super-salty foods. It’s not so much the saltshaker on your table you have to worry about, it’s processed foods. These contribute about 75% of our salt intake, according to the FDA, since many use salt as a preservative.

“All of your processed, packaged foods are going to have more sodium simply because [the manufacturers] want them to stay on the shelf longer,” says DiCicco.

Cook from scratch when you can, using non-processed items like fresh fruits and vegetables. If you do need a packaged item, read the label and compare sodium content across similar products.

RELATED: 25 Surprisingly Salty Processed Foods

Drink MORE water. You might think that putting more water into your body just adds more water weight. In fact, the opposite is true. If your body feels starved for water, it will hold on to whatever water it has.

If you’re retaining water, make sure you’re getting plenty of H2O, especially if you’re also eating salty foods.

It might also help to limit tea, coffee, and alcohol, all of which can be dehydrating. Cranberry juice, on the other hand, has a slight diuretic effect and may help flush out some excess water.

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Exercise regularly. Physical activity is key to losing water weight. Not only will you be sweating out some fluid, but you’ll also get thirsty and want to drink more water, says DiCicco.

Moving around can also help decrease puffiness. If you’re driving long distances, stop the car at regular intervals so you can get out and stretch your legs. Walk around when you can on planes, buses, or trains, and do simple exercises with your feet and legs while seated.

Regular exercise is also important if you’re pregnant (although resting with your feet up is smart as well).

RELATED: Tracy Anderson’s Favorite Exercises to Do While Traveling

Eat hydrating foods. This is almost as good as drinking water. “Foods that have a high water content help with increasing one’s overall hydration,” says DiCicco. Taking in more fluids–even in the form of hydrating foods–will ultimately help your body excrete water, she says.

Watermelon, spinach, strawberries, and cantaloupe, among other fruits and veggies, all have a lot of water.

Eating potassium-rich foods like tomatoes and sweet potatoes (and most fruits and vegetables) can also help you get rid of excess salt, says DiCicco.