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The Keto Diet Promises Big Weight Loss, But Is It Safe?

Cutting carbs and ramping up dietary fat has potential drawbacks.

Salmon, nuts, eggs, strawberries, avocados, coconut and blueberries are some of the foods you can eat on a keto diet.

Medically reviewed in March 2022

Updated on March 31, 2022

If you cut carbs and load up on 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 that sets tight limits on the macronutrients you eat, with a strong emphasis on eating fat. 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 Dietary Guidelines for Americans 2020-2025 does not recommend blanket percentages of protein or carbs, but it does advise American adults to limit their saturated fat intake to 10 percent of their daily calories. It also recommends that people eat 6 servings of grains and 2 servings of fruit each day, plus generous amounts of starchy vegetables and legumes each week 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 (fatty fish, grass-fed meat, dark meat poultry, bacon, eggs) 
  • Full-fat dairy (cheese) 
  • Healthy oils (avocado, coconut, and olive) 
  • Plant sources of healthy fats (avocados, walnuts, almonds, flaxseeds) 
  • Not-so-healthy fats (butter, heavy cream) 
  • Leafy greens and limited non-starchy veggies (asparagus, celery) 
  • Fluids to stay hydrated (water, plain tea, bone broth) 

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

  • Starchy veggies (like potatoes and corn) 
  • Most fruits except berries 
  • Beans and legumes 
  • Grains (including whole grains) and anything made from grains 
  • Sweets, sweeteners, or anything with sugar (honey, ketchup, soda, fruit juice) 
  • Low-fat and sweetened dairy (milk, 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.

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. To combat constipation while on the keto diet, experts recommend drinking extra water to flush your system, and some of them advise taking fiber supplements. For those with heart concerns, heavy doses of saturated fat may be risky. (Some versions of keto emphasize foods low in saturated fats.)

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. 

Article sources open article sources

Merck Manual Professional Version. Macronutrients. Accessed March 30, 2022.
Dietary Guidelines for Americans, 2020-2025.
Harvard T.H. Chan School of Public Health. Diet Review: Ketogenic Diet for Weight Loss. Accessed March 29, 2022.
Choi YJ, Jeon SM, Shin S. Impact of a Ketogenic Diet on Metabolic Parameters in Patients with Obesity or Overweight and with or without Type 2 Diabetes: A Meta-Analysis of Randomized Controlled Trials. Nutrients. 2020;12(7):2005. Published 2020 Jul 6.
Yuan X, Wang J, Yang S, et al. Effect of the ketogenic diet on glycemic control, insulin resistance, and lipid metabolism in patients with T2DM: a systematic review and meta-analysis. Nutr Diabetes. 2020;10(1):38. Published 2020 Nov 30.
López-Espinoza MÁ, Chacón-Moscoso S, Sanduvete-Chaves S, Ortega-Maureira MJ, Barrientos-Bravo T. Effect of a Ketogenic Diet on the Nutritional Parameters of Obese Patients: A Systematic Review and Meta-Analysis. Nutrients. 2021;13(9):2946. Published 2021 Aug 25.
Masood W, Annamaraju P, Uppaluri KR. Ketogenic Diet. [Updated 2021 Nov 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-.
Wheless JW. History of the ketogenic diet. Epilepsia. 2008;49 Suppl 8:3-5
Crosby L, Davis B, Joshi S, et al. Ketogenic Diets and Chronic Disease: Weighing the Benefits Against the Risks. Front Nutr. 2021;8:702802. Published 2021 Jul 16.

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