While the buzz seems to be heating up around the ketogenic diet, the eating philosophy isn't new. In fact, it’s been used as a treatment for epilepsy since the 1920s and came back into the spotlight in the ‘90s when Dateline ran a segment highlighting it as a treatment option. But how did it go from epilepsy treatment to weight-loss regimen? Well, for starters, more and more celebs — including Vanessa Hudgens and Halle Berry — have recently praised its weight-loss results publicly.

So what is the ketogenic diet? It’s a high-fat, low-carb diet that sends your body into a state of ketosis, during which it uses stored fat as energy. When you eat this way — typically we’re talking less than 50 grams of carbohydrates a day — “you’ll consume an enormous amount of fat,” says Shahzadi Devje, RD, CDE, a dietitian in Toronto, Canada. In fact, by following the keto diet, about 60 to 80 percent of your daily calories will come from fat. When you restrict carbs this way and eat more fat, your body takes about 24 to 48 hours to begin producing ketones, which are created when your body metabolizes fat for energy, says Pegah Jalali, MS, RD, a clinical coordinator at the New York University Comprehensive Epilepsy Center, where she counsels patients on the ketogenic diet. Normally, your body would metabolize carbohydrates as a primary source of energy.

By following the keto diet, about 60 to 80 percent of your daily calories will come from fat.

When most people hear "diet" they think calories, but on the keto diet specific caloric intake isn't the focus. “Mostly, I encourage my patients to eat according to their appetites,” says Jalali. “Your body can be in a ketogenic state at a range of calorie levels, and individual needs fluctuate day to day depending on activity and other factors.” But you won’t want to add a latte to your morning routine or even an extra piece of fruit to your afternoon snack, if those weren’t on your meal plan, says Jalali. “If you eat a lot more, then you risk coming out of ketosis — and if you do not eat enough fat, you also risk coming out of ketosis.” Bottom line: When you follow the keto diet, you must do so to a T for it to be effective. If you introduce cheat meals or cheat days, you’d be eating a high-fat diet and bringing your body out of that fat-burning ketosis state, which could have negative health consequences.

What does a day on the diet look like?

As an example of what a day in the life of the ketogenic diet may look like, here is a sample meal plan that Jalali shares with her patients.

Breakfast

2 eggs, cooked in 1 tablespoon of butter or ghee

½ cup cooked spinach, in 1 tablespoon coconut oil

1 cup coffee, blended with 1 tablespoon butter and a dash of cinnamon

Mid-morning snack

6 macadamia nuts

6 raspberries

Lunch

Tuna salad (4-5 ounces canned light tuna, mixed with 2 tablespoons mayonnaise, ¼ cup chopped celery, ¼ cup chopped green apple, and salt and black pepper to taste), served over 1 cup Romaine lettuce

Salad (½ cup steamed green beans and 8-10 olives), topped with a mixture of 1 tablespoon olive oil and ½ lemon, juiced

Afternoon snack

½ avocado, sprinkled with 1 tablespoon hemp seeds or 1 teaspoon nutritional yeast

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Dinner

8-12 ounces cooked steak, in 1 tablespoon butter

1 cup cauliflower, roasted in 1 tablespoon hazelnut oil

Dessert

1 ounce 90 percent dark chocolate

1 tablespoon crunchy salted almond butter, sprinkled with a dash of cinnamon

The pros of the Keto Diet

A main benefit of the diet, and why many of its followers praise the eating plan, is weight loss. Multiple studies show promising results: In a study in The American Journal of Clinical Nutrition, obese men dropped about 14 pounds after following the diet for a month. And in a longer-term study published in Clinical Cardiology, obese adults adhering to a ketogenic diet for about six months noticed significant weight loss — on average, 32 pounds — as well as reductions in total cholesterol and increases in beneficial HDL cholesterol. A review study in the European Journal of Clinical Nutrition also found that the weight loss seen within the first three to six months of following the keto diet was greater than the loss from following a regular balanced eating style.

Weight loss often occurs on the keto diet because your body is utilizing fat stores for energy — and you may also notice a decreased appetite that often results from the diet. “We don’t know exactly why ketones suppress appetite,” says Jalali. “But my theory is that ketosis is the state we are in during starvation and prolonged fasting, so it might be an evolutionary response to help humans cope.” Plus, your body digests fat a lot more slowly than carbs. “Fat has a longer intestinal transit time compared with carbs,” she says. “So it will prolong that feeling of fullness, especially in between meals.”

While some say following the diet claim it is limiting at food-related social occasions, Lara Clevenger, MSH, RDN, a dietitian in Edgewater, Florida, says you can easily plan ahead. “I currently follow a ketogenic diet, and eating out is a lot easier than people think,” she says. “You can ask for a burger minus the bun and fries, plus a side of veggies. Or order a chef salad and ask for olive oil as your dressing, minus the croutons. The options are plentiful!” Clevenger says she first started the keto diet for its anti-inflammatory benefits, due to a family history of obesity, hypertension, breast cancer, diabetes and dementia. “I will stay on a ketogenic diet for the foreseeable future,” she says. “I now have two to three times more energy, I no longer have trouble falling asleep and staying asleep, I am better in tune with my hunger and satiety, and I am no longer addicted to sugar. The diet has had more of an impact than I could have imagined!”

In addition to weight loss, studies show that the ketogenic diet may also be helpful in treating many other conditions, such as type 2 diabetes, polycystic ovary syndrome (PCOS), acne and more.

The cons of the Keto Diet

“As with any diet, there are always pros and cons,” says Devje. If you follow the ketogenic diet, you may notice side effects such as bad breath, headaches, nausea and fatigue. The negative symptoms some experience when first starting the diet is often referred to as the “keto flu.” This feeling of tiredness is due to a drop in blood sugar that can cause lethargy, which usually passes in 24 to 48 hours, notes Jalali. You might also feel bloated or constipated — and you may feel the need to urinate more frequently, since ketosis acts as a diuretic, says Jalali.

Because the ketogenic diet is limited in terms of food groups, you’ll be at risk for nutrient deficiencies. “The diet requires a robust supplement regimen, since you can develop certain deficiencies on a ketogenic diet that can inhibit your ability to stay in ketosis,” says Jalali. “You can also develop micronutrient deficiencies that can cause hair loss, and a lot of people become constipated on a ketogenic diet.” Jalali recommends discussing the diet with both a physician and a ketogenic dietitian to work to prevent these side effects.

And if you’re following the diet for weight loss, it's important to know that you may gain weight back when and if you go back to your normal eating habits. “A ketogenic diet is fantastic for some people, but it’s not a lifelong diet for everyone,” says Jalali, who says she sees benefits for people with chronic conditions such as epilepsy, type 2 diabetes and PCOS — but sometimes negative effects for people who go on the diet for weight loss. “A lot of patients find that when they come off the diet, they regain weight very easily,” she says. “The theory is that by being on the ketogenic diet for a prolonged period of time, their bodies become very sensitive and efficient at metabolizing carbohydrates.” Jalali notes that these patients may be able to maintain the weight loss by sticking with a low-carbohydrate diet. While more research on weight-loss maintenance after following the keto diet is needed, research shows that cycles of brief ketogenic Mediterranean diet periods, separated by longer periods of the Mediterranean diet, over one year helped obese adults maintain weight loss.

There are some groups who shouldn’t take the keto diet for a test drive. It can be dangerous for people with type 1 diabetes, as it can increase the risk for life-threatening diabetic ketoacidosis (DKA). Even people with type 2 diabetes following a ketogenic diet should regularly test their blood sugar and ketone levels to prevent DKA. If you have kidney disease, you’d want to skip this diet because you’d need to limit your protein intake. And if you have a history of disordered eating, the diet severely limits carbs and takes a lot of control to follow — which may increase the risk of bingeing or compulsive overeating. Consult with your doctor to determine if this is the right diet for you.

What’s the bottom line?

If you're planning to follow the keto diet, consider consulting with a registered dietitian to determine the most balanced diet possible, as well as any supplements you may need to take. “Without monitoring by a licensed healthcare professional, challenges like poor bone health, gastrointestinal reflux, and changes in cholesterol levels can go unchecked,” says Julie Stefanski, RD, a registered dietitian in York, Pennsylvania. Getting a balance of nutrients on the ketogenic diet can be challenging, which is “no surprise because you’re essentially cutting out healthy whole grains, fruits and some vegetables,” says Devje.

And daily carbohydrate allowances on the keto diet will vary from person to person. “Some adults will have to consume 15 grams of net carbohydrates to stay in ketosis, whereas some professional athletes may be able to consume more than 120 grams of net carbs and be in a ketogenic state,” says Jalali. “This is another reason why it is best to work with a professional to design the diet.”

Because the keto diet is high in fat, you’ll want to make sure to incorporate more healthy fats into your eating plan. These include omega-3s, such as those found in salmon and sardines, as well as monounsaturated fats present in avocado, walnuts, almonds and sunflower seeds.

We don’t have a clear picture on the long-term benefits of the keto diet for weight loss. “Although there seems to be a short-term advantage of the ketogenic diet for weight loss, the advantage seems to shrink at the one-year mark,” says Devje. “The long-term safety of the ketogenic diet is yet to be determined.”

NEXT: Why losing weight too quickly may backfire in the long run

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