The keto diet promises big weight loss, but is it safe?

If you cut carbs and increase fat, will you really lose weight, elevate your mood, and boost energy levels? That’s the promise of the keto (short for “ketogenic”) diet. But is keto safe, and does it really work? Here’s what you need to know.

What is the keto diet?

In short, it’s a low-carb eating plan with a strong emphasis on fat that sets tight limits on the macronutrients you eat. Macronutrients are the three main types of nutrients that people normally eat a fair amount of—carbohydrates, fat, and protein. People on the keto diet aim to get about 80 percent of their daily calories from fat, 15 percent from protein, and just 5 percent from carbohydrates. The high fat content sets this diet apart from other low-carb diets, such as Atkins.

The American Heart Association advises most healthy adults in the United States to limit their saturated fat intake to 6 percent of their daily calories. For a person who eats a 2,000-calorie diet, that’s about 13 grams of saturated fat daily. The AHA also recommends that most people eat about 3 servings of fiber-rich whole grains, 2 cups of fruit, and 2 1/2 cups of vegetables (mostly leafy greens) each day as part of an overall healthy eating pattern—parameters that are not compatible with the rules of keto.

In everyday terms, a typical ketogenic diet limits carbs to between 20 and 50 grams per day. One large banana, for reference, has about 31 grams of carbs.

Why so much fat and so few carbs?

There are two rationales. For one, dietary fats are naturally satisfying, so by eating lots of them, you’ll fill up faster and feel full longer. Meanwhile, by drastically cutting carbs and replacing them with megadoses of fat, you can change your body’s fuel-burning process.

The body primarily burns carbohydrates for energy. But when you deprive yourself of that typical fuel source, your body shifts to a state called ketosis. During ketosis, the body converts fat into compounds called ketones that can be used for fuel.

What do people eat and what do they cut out?

The keto diet emphasizes the following foods:

• Animal protein, like fatty fish, grass-fed meat, dark meat poultry, bacon, and eggs
• Full-fat dairy, like cheese, butter, and cream
• Healthy oils, like avocado, coconut, and olive oil
• Plant sources of healthy fats, like avocados, walnuts, almonds, and flaxseeds
• Leafy greens and limited non-starchy veggies, like asparagus, and celery
• Fluids to stay hydrated, like water, plain tea, and bone broth

Meanwhile, the keto diet removes these foods from the menu:

• Starchy veggies, like potatoes and corn
• Most fruits except berries
• Legumes such as beans, lentils, and peas
• Grains, including whole grains and anything made from grains
• Sweets, sweeteners, or anything with sugar, like honey, ketchup, soda, or fruit juice
• Low-fat and sweetened dairy, like milk and ice cream

Weight loss and more energy are among the touted benefits

As your body burns off your reserves of carbohydrate, which is stored in your muscles as glycogen, you’ll lose water, so you may see weight loss right away. Many adherents also say they experience better mood, mental performance, and energy levels.

Some research in overweight patients suggests that ketosis can suppress appetite and feelings of hunger. Other research suggests that a ketogenic diet may help some obese patients lose weight and lower their levels of triglycerides and cholesterol.

A 2020 meta-analysis of 14 studies published in the journal Nutrients that compared ketogenic diets to traditional low-fat diets in overweight people found that the ketogenic diets resulted in weight reduction and better cholesterol profiles among those patients who were diabetic.

Another 2020 meta-analysis published in Nutrition & Diabetes also detected improved blood sugar as well as better cholesterol profiles in diabetic patients. Other evidence supports lower blood pressure with the ketogenic diet.

On the other hand, an analysis of studies published in 2021 in Nutrients compared the keto diet to low-calorie or low-fat diets in obese people. The analysis did not detect changes in cholesterol or even body mass index with the keto diet.

Meta-analyses sometimes come to different conclusions depending on which studies they choose and how they examine the data, and it’s possible that certain types of people are more likely to benefit from keto. In any cases, most studies of keto look at small numbers of people for short periods of time, which can only give us limited insight.

‘Keto flu’ and other drawbacks

Short-term improvements are one thing. But the keto diet may come with side effects. Is it worth those downsides, and should people maintain it over the long term?

Many keto dieters report an early side effect called “keto flu”: headaches, brain fog, and nausea. The sharp drop in carbs can also lead to irritability or bad moods, and some people report “keto breath,” an oddly fruity or foul aroma that’s the result of exhaling chemical byproducts of ketosis.

In the 2021 Nutrients analysis, the researchers reported side effects that included, headache, muscle weakness, high levels of uric acid in the blood (which has been linked to gout), nausea, and tired legs. Constipation also affected many participants for months. To address constipation while on the keto diet, experts recommend drinking extra water to flush your system, and some of them advise taking fiber supplements.

There are longer-term effects, as well. Importantly, eating too much saturated fat, like that found in meat and dairy products, is strongly linked to a greater risk of heart disease over time. (Some versions of keto emphasize foods low in saturated fats.) What's more, cutting beans, whole grains, fruits, and many types of veggies from your diet means you may miss out on key nutrients, including dietary fiber, which is valuable for heart health and healthy digestion.

There’s also the remote risk of ketoacidosis, which occurs when a buildup in the body of too many ketones causes the blood to become acidic. Ketoacidosis is typically seen in people who have uncontrolled diabetes, but it has been reported in people following a very low-carb diet for a long time.

As far as maintenance goes, keto has not proven more effective at the one-year mark than conventional weight-loss diets when it comes to weight loss, blood pressure, and cholesterol.

So does it really help with weight loss?

In some people, yes. But as with most restrictive diets, the keto diet may simply be too difficult to maintain over time (hello, carb cravings). And long-term health outcomes for people on the diet are still largely unknown. Many questions remain about health risks like osteoporosis, safety in people with kidney and liver disease or during pregnancy, and potential effects from high fat consumption.

“I’d love us all to learn from this diet to cut back on carbs, pump up fats, enjoy moderate amounts of protein, and eat plenty of veggies. Those are all great things,” Dawn Jackson Blatner, RDN, author of The Flexitarian Diet and The Superfood Swap, told Sharecare via email.

“But there’s no need to take it to such an extreme. At the end of the day, the universal diet will always win. That’s mostly veggies, plus a little of everything else: animal and plant protein, whole grains, and healthy fats.”

Yes, you’re having déjà vu

If the notion of an ultra-low-carb, super-high-fat diet sounds familiar, it is. The general dimensions of the ketogenic diet are similar to the early phases of the Atkins diet (although keto allows less meat than Atkins and places a heavier emphasis on fat). Atkins has fallen out of favor over the years, and it’s unclear yet whether the keto diet will stand the test of time.

That said, forms of the ketogenic diet date back to the 1920s, when it was originally used as a method for reducing seizures in people with epilepsy. Though anticonvulsant drugs have largely replaced the diet for this purpose—and despite the fact that the ketogenic diet is hard to stick with over the long term—it’s still sometimes used to ease seizures in children with epilepsy who do not respond to medication.

Should you try it?

“People struggle to follow these extreme diets. But in the end our ‘SAD’ diet—the standard American diet—is not working either,” Blatner notes. She acknowledges that any diet that asks you to scrutinize the way you eat can be beneficial. “Trying something new can help you become more mindful and conscious of what you eat. And preparing your own food more often, reading food labels, and cutting out processed crap is never a bad thing.”

If you have excess weight to lose, remember that keto is not your only option. A healthy diet, particularly one that’s sustainable over time, combined with regular exercise is still effective. “I think the answer actually is to eat a ton of vegetables and keep the rest—carbs, protein, fat—all moderate, not extreme,” says Blatner.

Either way, before leaping into a diet that asks you to substantially restructure your nutrient intake, consult a healthcare provider to make sure the drops in carbs and increases in fat will suit your health profile—particularly if you have diabetes, heart disease, or a history of disordered eating.