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Here's what you need to know about the high-fat, low-carb diet everyone's talking about.

December 16, 2016

Google has released the top search terms of 2016, and when it comes to weight loss, it turns out folks were especially drawn to the ketogenic diet. It was one of the 10 most-searched diets this year, landing halfway down the list (just a few notches below the taco cleanse!). But if you weren't among the keto-curious in the last 12 months, you're probably wondering now, Is this something I should try? (And what does ketogenic mean again?) Read on for a quick primer on the plan, and my bottom-line advice.

What is the ketogenic diet?

In a nutshell, it's a high-fat, low- to moderate-protein, low-carb eating plan. On a ketogenic diet, roughly 75% to 90% of daily calories come from fat; 6% to 20% come from protein; and 2% to 5% come from carbohydrates.

It was originally devised as a tool for controlling epileptic seizures (though doctors aren't exactly sure how it works) before there were drugs to treat seizures. In the past few decades, it has reemerged as patients and parents seek alternatives to pharmaceuticals.

But the ketogenic diet has also been adopted as a weight loss plan. The goal of the diet is to achieve ketosis, a state in which the body is using fat as its primary fuel, rather than carbs. After three to four days on a ketogenic diet, back-up stores of carbohydrates, called glycogen, become depleted and ketosis kicks in, triggering some weight loss and the appearance of a leaner physique.

But in terms of dropping pounds, the primary advantage of a ketogenic diet is that it doesn't leave you hungry, since it involves eating a good deal of satiating fats, and the state of ketosis has been shown to reduce appetite.

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What does the research say?

A recent Spanish study tracked 20 obese volunteers who followed a low-cal ketogenic diet (about 800 to 1500 calories daily) and a supervised exercise program. Over four months, the subjects lost an average of 40 pounds, including a significant amount of belly fat, while preserving their muscle mass and strength. Another study, published in The American Journal of Clinical Nutrition, found that in obese men, a high-protein ketogenic diet reduced hunger and food intake more than a high-protein, medium-carb non-ketogenic diet did.

A 2013 meta-analysis of 13 studies published in the British Journal of Nutrition looked at the effects of the diet on long-term weight control and cardiovascular health. The research showed that adults on a low-cal ketogenic diet (with no more than 50 grams of carbs per day) lost more weight and experienced better changes in blood pressure, triglycerides, and “good” HDL cholesterol than people who followed a conventional low-fat diet (with less than 30% of calories from fat). 

But a small 2006 study that compared a ketogenic diet to a moderately low-carb non-ketogenic diet (with 40% of calories from carbs) found no differences in weight loss, or hunger. And the study participants on the non-ketogentic diet had better moods, more energy, and lower levels of inflammatory markers. "The use of ketogenic diets for weight loss is not warranted," the study authors concluded.

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What is it really like?

With only 50 grams of carbs to "spend" per day, your food options are limited. Breakfast might be whole eggs with low-carb veggies and avocado, for example. Lunch could be a salad generously dressed with EVOO and balsamic vinegar, and topped with chicken. A typical snack is nuts or seeds. And dinner might be salmon with veggies sautéed in coconut oil.

I’ve had clients eat this way, lose weight quickly, and feel fantastic—at first. But all of my clients who follow a ketogenic plan eventually break down and eat potatoes, fruit, or dessert (or drink several glasses of wine).

I’ve also seen ultra-low-carb diets trigger a serious change in people’s moods. Recently a client told me that he morphed from a happy-go-lucky guy into a total grump, and his family begged him to abandon the diet. Another woman whose husband tried the plan told me that he became intensely irritable and had trouble sleeping; but that those side effects subsided after he added back fruit, pulses, whole grains, and starchy vegetables to his diet.

When I experimented with the ketogenic diet, I felt incredibly cranky as well, and obsessed about foods I wasn't supposed to eat—like black beans, bananas, and sweet potatoes. That never happens when I have them in moderation. I also didn’t feel good about the fact that cutting those foods meant missing out on the vitamins, minerals, antioxidants, fiber, and prebiotics they provide.

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The bottom line

In order for a weight loss approach to work, it has to have "stick-with-it-ness," and make you feel well, physically and emotionally. Otherwise you're miserable and risk gaining back any weight you lose (and possibly more).

In terms of safety, the biggest risk associated with a ketogenic diet is the potential for ketoacidosis, which occurs when ketosis goes too far. When you eat fats (like avocados and olive oil), they're broken down into fatty acids and ketones; if excess ketones build up in the body, your blood becomes acidic. Severe ketoacidosis can lead to coma, or even death; and acidosis in general can trigger bad breath, headaches, dizziness, muscle cramps, and constipation.

You can test your ketone levels using urine strips, to make sure they don't creep too high. But my advice is to only adopt a ketogenic diet under the supervision of a physician or dietitian. If you attempt it on your own, I suggest modifying the diet to allow more carbs—especially the ones you know you can’t live without. In my experience, moderation is generally the key to shedding pounds for good, optimizing health, and living a balanced, enjoyable life.

Cynthia Sass is Health’s contributing nutrition editor, a New York Times best-selling author, and a consultant for the New York Yankees. See her full bio here.