The Ketogenic Diet: What is Keto, and Does it Work?

Finally: a complete beginner’s guide to understanding the keto diet fad.
Published September 25, 2019

Imagine being able to eat all the fat you ever wanted and still lose weight. Butter, burgers, bacon, steak, cheese—no holds barred. Except maybe one. In exchange, you’d have to forgo nearly all carbs like bread, bagels, pasta, pizza, and even most fruit. In reality, that’s the trade-off required for keto-diet success.

Although it may sound new, the ketogenic diet (or “keto” for short) was developed nearly a century ago by Mayo Clinic physician Russel Wilder, MD, to treat epilepsy. Now it’s one of the hottest weight-loss crazes. 

If you’re considering jumping on board, here’s what you need to know. 

What is a ketogenic diet? Keto for beginners

Lots of people assume keto is a high-protein diet, but it’s actually much higher in fat. On the flip side, it’s extremely low in carbs, with just 5% to 10% of calories coming from carbohydrate food sources. Considering most of us consume roughly half of our calories from carbs, adopting a keto lifestyle could mean a lot of changes. 

The goal of the keto diet is to force the body into ketosis, a metabolic condition in which the body burns fat in place of its preferred fuel source, carbohydrates. “Ketosis is a back-up system for the body to use when it doesn’t have access to food, such as in times of starvation,” says Toby Amidor, MS, RD, author of The Healthy Meal Prep Cookbook: Easy and Wholesome Meals to Cook, Prep, Grab and Go. “It is not meant as a long-term way to provide the body with energy.” 

What happens when your body doesn’t get the carbs it needs for fuel? First, it burns available sugar stores known as glycogen, releasing water—and water weight—in the process. (It’s why many people experience rapid rates of weight losses in the first few days and weeks on a keto diet—and why the reintroduction of carbs, which replenishes glycogen and water stores, reverses water weight loss.)

Next, the body breaks down protein to produce the simple sugar glucose, the body’s favorite source of energy. This neat trick is called gluconeogenesis. Trouble is, only a limited number of amino acids can be converted to glucose, so this process only covers a fraction of the body’s energy needs.

Following the use of glycogen and water losses, and a brief spell of gluconeogenesis, the body eventually turns to its last possible fuel option: fat. However, because fat can’t cross the blood brain barrier to power the brain, it’s converted it into a form the brain can readily use—namely an alternative form of energy called ketones, a fat byproduct produced by the liver. When your body uses ketones for energy instead of its usual glucose, you’re effectively in ketosis.

​How does consuming more fat than carbs on keto lead to weight loss?

Considering fat has more than twice as many calories per gram as carbohydrates, you might be wondering how eating more of it can help you lose weight. “The focus on fat and proteins appears to promote satiety, making it easier to restrict calories,” says Jeffrey Volek, PhD, RD, a professor of human sciences at The Ohio State University.

But there’s another incredibly basic reason why the keto diet works, at least in the short term. When an entire food group (carbs) comes off the table, you’re left with fewer foods to choose from, making it easier to eat less. And just like on any other diet, when you cut calories, you lose weight.

Despite dramatic claims surrounding the keto diet, it’s unclear how long the results last. In the first couple of weeks, you can expect rapid and substantial weight loss, sometimes as much as 10 pounds. Yet that’s mostly from the diet’s diuretic effect: Tapping into your body’s sugar stores leads the release of fluids. But as soon as you bend the rules by eating, say, a slice of pizza, you replenish your sugar—and fluid—stores, and the water weight returns.

Even though some studies show the keto diet has a small edge over a low-fat diet plan in the short term, many keto followers start to regain some of their losses in as little as five months.

​Different types of keto diets

Keto might sound like a single diet, but just like the Paleo diet, there are several variations:

1. ​Standard ketogenic diet (SKD)

True to its keto roots, the SKD is a super-low-carb, high-fat diet with moderate protein. It typically contains 70% to 80% of calories from fat, about 10% to 20% protein, and only 5% to 10% carbohydrates. 

2. ​Cyclical ketogenic diet

Popular among athletes and bodybuilders, the cyclical ketogenic diet works in cycles of five low-carb ketogenic days, followed by two higher-carbohydrate re-feeding days. Not all keto experts are fans, though. “This probably isn’t a good idea for most people, especially those using the ketogenic diet for weight loss or to improve insulin resistance,” Volek says. 

3. Targeted ketogenic diet

Like the cyclical ketogenic diet, the targeted keto diet is another favorite of athletes, who follow the low-carb approach to eating, then boost carb intake around intense workouts to replenish glycogen stores. 

​4. High-protein ketogenic diet

This higher-protein version of the keto diet increases protein to 35% of calories, while reducing fat to 60%. Not only is it still high in fat, but it may not even work. “In most keto diet plans, high protein intake is not encouraged because it may inhibit ketosis by encouraging gluconeogenesis,” says Patti Urbanski, MEd, RD, LD, CDE, a clinical dietitian and diabetes educator at St. Luke's Hospital in Duluth, Minnesota.

Foods to eat on a keto diet

While you’ll eat almost unlimited fat and more than enough protein on this plan, there’s little room for healthful carbohydrate-containing foods like bread, pasta, and fruit. 

Here’s a breakdown of the typical foods you’ll eat on keto: 

70% to 80% of calories
Cream cheese
Oils (e.g., olive, canola, peanut, and coconut)
Animal fats (e.g., duck fat, lard, and tallow)
Pork rinds
Nuts and seeds
Organ meats
Full-fat Greek yogurt
Full-fat cottage cheese
Deli meat
Fish and seafood
Leafy green vegetables (e.g., spinach, kale, arugula, and romaine)
Low-carb vegetables (like broccoli, cauliflower, cabbage, zucchini, asparagus, and mushrooms)
Small servings of berries and melon

For a typical 2,000-calorie diet, this translates to about:

  • 155 to 175 grams of fat

    Note: The World Health Organization recommends no more than 30% of calories from fat, which equates to 60 grams in a 2,000-calorie diet.
  • 50 to 100 grams of protein
  • 25 to 50 grams of carbs

Foods to avoid on a keto diet

For the keto diet to work, you’ll need to reduce carbs—a lot! Sure, you’ll nix nutritionally empty desserts, candy, soda, and sweetened drinks. But you’ll also have to say goodbye to healthful whole grains, potatoes, beans, most fruits, and even many veggies. “With so many foods being eliminated or eaten in small amounts, there’s no viable way to take in adequate nutrients your body needs to stay healthy,” Amidor says. “Since fruits, grains, and carb-containing vegetables are limited, nutrients like fiber, antioxidants, potassium, magnesium, and vitamins A and C are especially of concern as this may lead to multiple nutrient deficiencies.” 

Food categories you’ll eliminate on the keto diet

1. Grains and starches

Whole grains like brown rice, quinoa, oatmeal, whole-wheat pasta, and whole-grain bread work wonders for healthy digestion. But you won’t find them on a keto diet. Filling your plate with fibrous veggies like broccoli, asparagus, cauliflower, brussels sprouts, kale, collards, and chard can help to some degree, but constipation may still be an issue. 

Without grains, you may also have difficulty obtaining the magnesium your body requires (310 mg for women, 400 for men) for more than 300 different chemical reactions, such as maintaining healthy nerves and muscles, building protein, helping your body use glucose efficiently, and regulating blood pressure. Magnesium can be found in keto-approved almonds, spinach, cashews, and avocados. 

2. Starchy vegetables

While keto gives the green light to lots of vegetables, higher-carb potatoes, parsnips, corn, carrots, and beets are off the menu. That can make getting the potassium needed for healthy blood pressure a challenge. It can also rob you of disease-preventing antioxidants.

3. Most fruit, except small portions of berries and melon

Like vegetables, fruit is loaded with antioxidants, not to mention immune-supporting vitamins A and C. And yet? The keto diet outlaws most options. A few exceptions include small servings of lower-carbohydrate strawberries, blackberries, and cantaloupe. To keep carbs below seven grams per serving, limit portions to half a cup.

​4. Beans, chickpeas, lentils, peas, and edamame

Most people think of beans and legumes as high-protein foods, but they’re also loaded with healthy complex carbs. Go bean-free and you’ll miss out on one of nature’s top sources of potassium, antioxidants, and fiber.

​5. Some condiments and sauces

A little ketchup, barbecue sauce, or teriyaki sauce is a genius way to infuse foods with flavor for relatively few calories. But on keto, their added sugars makes them a no-go. Instead, dieters stick with lower-sugar mustard, soy sauce, and red-wine vinegar.

​Potential health benefits of a keto diet

Even though keto can be super restrictive, it could offer some health benefits. A small number of studies show it may reduce symptoms of Alzheimer’s and Parkinson’s diseases and improve hormone profiles in women with polycystic ovary syndrome. Right now, experts agree more long-term research is needed.

There is, however, stronger evidence that lower- and low-carb diets may help lower blood sugar in people with type 2 diabetes. “Several studies have reported that the keto diet can significantly reduce [a long-term marker of blood sugar control known as] HbA1c in individuals with diabetes when compared to a traditional low-fat, moderate carbohydrate eating plan,” Urbanski says. In fact, a recent Diabetes Therapy study found that when people with diabetes combined a ketogenic diet with other treatment methods for one year, 94% of them were able to lower or eliminate their insulin requirements and reduce the need for most other medications by 48%. Again, more research is needed to uncover the optimal approach for people living with diabetes.

Potential​ ​side effects and health risks of a ketogenic diet

For most people, the side effects and health risks of the keto diet may far outweigh any potential benefits.

Keto flu 

One of the very first side effects of going keto is the “keto flu.” Despite its name, the keto flu isn’t a flu at all. It’s actually a combination of symptoms that strike as the body tries to adjust to ketosis, including headache, nausea, fatigue, difficulty sleeping, and irritability.

Other common keto flu symptoms include:

  • Vomiting
  • Constipation
  • Bad breath
  • Diarrhea
  • Weakness
  • Muscle cramps
  • Dizziness
  • Exercise intolerance
  • Rash

But what causes the keto flu? Because there is very little published, peer-reviewed studies that specifically examine it, there’s no consensus on exactly what triggers it.

Most experts think the symptoms stem from carb withdrawal; others point to dehydration, since burning through the last of your body’s carbohydrate supplies results in fluid loss. While the keto flu generally subsides after a week or two, staying well-hydrated can often alleviate symptoms.

​Other potential​ ​side effects and health risks of a ketogenic diet

Many health experts express concern over other, more serious repercussions of this eating plan. “The side effects, missing nutrients, and the near or total elimination of entire food groups groups really should raise a red flag,” Amidor says. As does the near elimination of carbohydrates. The truth is, recent research finds that people who eat high carbohydrates with a low to moderate intake of fat tend to have the healthiest diets. So unless you’re committed to eating huge quantities of nutrient-rich, low-carb vegetables like spinach, kale, and bok choy on the keto diet, it can be difficult to obtain many of the key vitamins and minerals normally found in carbohydrate-rich plant foods while staying in ketosis. This can translate to nutrient deficiencies and imbalances linked to poor bone health, kidney stones, and potentially gout.

Nutrient deficiencies aside, there are other potential keto dangers, particularly if you have diabetes. “People who have diabetes should absolutely work with their medical care provider if they are interested in trying a keto diet, especially if they take medication,” Urbanski says. Why? When a person living with type 1 or type 2 diabetes loses weight, their blood glucose can begin to drop, too. Normally that’s a good thing. But if this happens too quickly to someone taking diabetes medication, it could lead to dangerously low glucose levels, Urbanski explains.

There have also been rare reports that ketosis can cause a dangerous condition known as ketoacidosis, in which the blood’s pH plummets to potentially lethal levels. However, if you’re healthy, the chances of developing ketoacidosis on keto are slim. While a small number of cases have been reported in healthy, nondiabetic people on the diet, ketoacidosis is generally unlikely, as the level of ketones generated is nowhere near the amount needed to lower blood pH.

Is the keto diet safe?

Perhaps the biggest keto concern is its impact on heart health since it prescribes an unconventionally high dietary fat intake without differentiating between saturated and unsaturated fats. “Some people believe that going on keto is an open invitation to eat bacon, processed meats, and fatty cuts of meat, but that isn't the case,” Amidor says. “These foods are high in saturated fat that’s been linked to heart disease.” While research on keto and heart health has been inconsistent, several studies echo this concern. For instance, one meta-analysis of 11 studies found that going keto resulted in a greater increase in harmful LDL cholesterol than a low-fat plan.

Other disadvantages of a keto diet

Health issues aside, one of the biggest challenges of the diet is that it’s difficult to sustain over the long run, which is why compliance is notoriously low. One meta-analysis of 12 different studies published in Diabetic Medicine found that, despite advice to limit carb consumption to less than 50 grams of carbs a day, most keto dieters usually eat between 132 to 162 grams of carbohydrates. 

There are lots of other reasons to carefully consider going keto such as:

  • Practicality: Eating out on keto can be a real challenge, Amidor says. So can meal prep if you’re cooking for a significant other, spouse, or a family.
  • Unintended consequences for your kids: “If a parent eliminates certain foods on the keto diet, the rest of the family are likely to see and hear about it, which could mean poor modeling of healthful eating behaviors for the kids in that household,” Amidor says. “It can send the wrong message that fruits, grains, and some vegetables are ‘bad.’”
  • The hassle factor: To really know if you’re in ketosis, many experts recommend using ketone strips to test your urine ketone levels daily, which can be inconvenient (and icky).
  • Stress: Forgoing certain foods may work for a while, but eventually, following such a restrictive approach to eating could take its toll.

The upshot: Do keto diets work?

The ketogenic diet may be effective for weight loss in the short term, but many experts agree that it’s extremely difficult to follow for a prolonged period of time, and that research on its long-term effects are limited. What’s more, keto diets can be heavy in red meat and other fatty foods, an approach to eating that’s not generally considered healthful.Given the potential side effects, health risks, and the emotional toll of trying to maintain such a restrictive diet, there are healthier ways to lose weight.

Karen Ansel, MS, RDN is a journalist and author specializing in nutrition, health, and wellness. Her latest book is Healing Superfoods for Anti-Aging: Stay Younger, Live Longer.


This article was reviewed for accuracy in June 2021 by Angela Goscilo, MS, RD, CDN, Nutrition Manager. The WeightWatchers® Science Team is a dedicated group of experts who ensure all our solutions are rooted in the best possible research.

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