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

Cutting carbs and ramping up dietary fat comes with some major drawbacks.

Medically reviewed in March 2018

Updated on February 1, 2021

Are you willing to cut carbs and load up on fat to lose weight, elevate your mood and boost energy levels? That’s the promise of the keto (short for “ketogenic”) diet. But is keto safe? If it sounds too good to be true, you might be onto something. And if you feel like you’ve heard this song before, you’re not imagining things. 

We looked into the details of this diet craze to break down the pros and cons for you. 

What is the keto diet? 
Simply put, the ketogenic diet is a high-fat, low-carb eating plan that sets tight parameters on the macronutrients you eat. Dieters shoot to get about 80 percent of their daily calories from fat, 15 percent from protein and a skimpy 5 percent from carbohydrates.  

By contrast, the Institute of Medicine of the National Academies advises that American adults get between 45 and 65 percent of their daily calories from carbs, 10 to 35 percent from protein and 20 to 35 percent from fat, with as few calories as possible coming from saturated and trans fats. 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?  

The notion is two-fold. For one, 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 mega-doses 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. 

So, what can I eat, and what do I 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) 
  • 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 diet kicks these off the menu: 

  • Starchy veggies (like potatoes and corn) 
  • Fruits, pretty much all of them, except for 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 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 tout enhanced 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.  

Diets that sharply cut carbs may also lower blood sugar and insulin levels. A 2013 meta-analysis comparing low-carb ketogenic diets to traditional low-fat diets found that the ketogenic diets led to greater long-term weight loss. And not surprisingly, other research suggests that a keto diet that incorporates calorie restriction can help overweight people lose weight. 

‘Keto flu’ and other drawbacks 
Many keto dieters report a constellation of side effects in the early going dubbed “keto flu”: headaches, brain fog and nausea. The sharp drop in carbs can also lead to irritability or bad moods, and some report “keto breath,” an oddly fruity or foul aroma that’s the result of exhaling chemical byproducts of ketosis. 

Cutting beans, whole grains, fruits and many types of veggies from your diet also means you may miss out on key nutrients, including dietary fiber, which is valuable for heart health and healthy digestion. To combat constipation, a possible side effect of the diet, experts recommend drinking extra water to flush your system, and some advise taking fiber supplements. For those with heart concerns, heavy doses of saturated fat likely won’t do you any favors, either. 

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 with uncontrolled diabetes, but could theoretically occur when a low-carb diet is taken to extremes. 

In terms of weight loss, the research is compelling, though not clear-cut. One small study comparing ketogenic low-carb diets with low-carb diets that did not induce ketosis found that the ketogenic diet was no more effective for weight loss, but brought with it drops in mood and energy. More likely, 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. 

“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,” she says. “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 with a heavier emphasis on fat). And just as Atkins has fallen out of favor over the years, it’s unclear yet whether the keto diet will stand the test of time. 

That said, forms of the ketogenic diet actually 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 simply 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. 

So, 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 weight to lose, remember that keto is not your only route. 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, it’s important to 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. 

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