Everything You Need to Know About the Keto Diet

Including the not-so-pretty side effects that can come along with the trendy diet plan.

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Whether you're feeling sluggish and looking for a dietary reboot or want to shed a few pounds, the keto diet has surely come up in your search for the most popular diets of 2020.

The low-carb, high-protein eating plan gained mainstream popularity back in 2018 thanks to celeb endorsements (ahem, Kourtney Kardashian) and viral transformation photos — and chances are you have a friend or family member who still won't stop talking about their keto weight-loss success or favorite keto 'fat bomb' recipe. (If this means nothing to you, keep reading.)

Thinking of giving it a try for yourself? Ahead, nutritionists weigh in on the pros and cons of the keto diet, as well as some of the side-effects you might experience (and uh, yes, the 'keto flu' is definitely a thing).

What is the keto diet?

The keto diet (short for ketogenic) is a high-fat, moderate protein, low-carb diet, according to Charles Passler, D.C., a New York-based nutritionist and the founder of Pure Change. On this diet, up to 70 percent of your calories per day should come from fat, 20 to 25 percent from protein, and five to 10 percent from net carbs (which is carbs minus fiber), Passler says.

The goal of the keto diet is for your body to switch from relying primarily on glucose (from carbohydrates) to fueling itself on ketones (from fat), explains Ryan P. Lowery, Ph.D., co-author of The Ketogenic Bible.

“By limiting the sugar and carbohydrates in your diet, your body starts to burn fat at a higher rate, thereby producing ketones that your body starts to utilize,” he says.

When this shift happens, it puts your body into a metabolic state referred to as ketosis. (This usually takes a few days, FYI).

So... what foods can't I eat on keto?

As you've probably guessed, this means you're going to be cutting out pizza, bagels, and other beloved carbs.

As a general rule of thumb, you'll want to keep your net carb intake to no more than 15 to 20 grams a day, explains Sydney Greene, R.D., a registered dietitian at Middleberg Nutrition in New York City. (For reference, one small apple is about 15 grams of carbohydrates.)

Greene says you’ll want to avoid the following foods:

  • Starchy veggies — sweet potatoes, butternut squash, spaghetti squash and beets, as well as other root vegetables, like carrots and parsnips
  • Fruit — Most varieties, including tropical fruits like bananas, mango, pineapple and papaya
  • Processed carbohydrates — Bread, crackers, and junk food
  • Other carbohydrates — Grains, legumes, and corn

"The majority of carbs must be removed from the diet, such as sweets, desserts, and most items with any added sugar," Passler explains. "Eliminating grains, root vegetables, legumes, and fruit may also be necessary for the majority of people." Sadly, due to their carb content, many alcoholic beverages are also off the table.

And, unfortunately, unlike other popular high-protein diets, like Atkins or Whole30, keto doesn't have a follow-up phase or a period where you begin to add in small amounts of foods that were once off-limits, like whole grains or fruit.

What can you eat on keto?

OK, so by now you know that the diet is low in carbohydrates and high in fat. So what does that really mean in terms of what you can eat?

Greene says a general outline of the foods that can be eaten, includes the following:

  • Fruits — minimal, but can include raspberries, blackberries and blueberries
  • Veggies — spinach, kale, fennel celery, cucumber, cauliflower, broccoli, bell peppers, and zucchini
  • Healthy fats — nuts, olive oil, avocados, ghee (like one of Fourth & Heart’s many grass-fed varieties), nut butter and seeds
  • Protein — Poultry, fish, beef, bone broth, full-fat cheeses, plain Greek yogurts and eggs from organic and/or grass-fed sources

Here's a sample keto menu:

Here are some examples of the foods you might enjoy throughout the day while on the keto diet, according to Passler:

  • Breakfast: An average breakfast might consist of two to three eggs, a side of cottage cheese (or a few slices of hard cheese), a half an avocado, and a small helping of nuts, like raw almonds.
  • Lunch: Fill your plate with a palm-sized helping of grilled chicken, half an avocado, non-starchy vegetables, and maybe even nuts if you need a little extra fat. Other popular lunch choices include grilled steak and fresh arugula with avocado (keto devotee Jenna Jameson's favorite).
  • Dinner: A keto-approved dinner could feature lean protein, like grilled salmon paired with steamed or roasted broccoli, and other non-starchy veggies, as well as avocado. (Yes, there are a lot of avocados on this diet.)

Keep in mind that snacks are also allowed if they meet the keto meal requirements. Keto favorites include celery with almond butter or cream cheese, or a helping of fresh cucumbers, chopped cauliflower or broccoli, or sliced bell peppers dipped in guacamole.

And if you need a more detailed explanation of what is considered a high-fat, low-carb food, there are a ton of books on Amazon that break down the diet and offer recipes and simplified food plans.

What are the benefits of the keto diet?

Interestingly, the keto diet wasn't originally created as a means for weight loss, says Greene. In fact, the diet was originally introduced by doctors in the early 1900s as a means for treating epilepsy and other neurologic illnesses.

It wasn't until the last few years that the keto diet became known as a method for dropping weight quickly thanks to celebs like Kardashian and Vanessa Hudgens, as well as the influx of keto progress photos on social media.

The keto diet can lead to weight-loss for a variety of reasons: It ups your protein take (which can help manage hunger), it can help reduce your overall calorie intake, and it increases the amount of fat your body burns at rest and while exercising. And, if your stomach is your target area, low-carb diets can be very effective at reducing abdominal fat.

Other benefits of keto include increased energy and improved mental clarity, Passler says. By switching your fuel source from carbs to fats, this can give you more sustained energy throughout the day, rather than the energy spike you're used to from using carbs as your fuel source.

Additionally, studies show that the keto diet can help improve blood lipid profiles (cholesterol levels and triglycerides) and improve blood pressure. Neurological disorders, like Alzheimer’s and Parkinson’s diseases, have also shown to benefit from a keto lifestyle. If any of these health issues are an area of concern for you personally, talk to your doctor to see if keto may be a fit.

But can keto really help with long-term weight-loss?

Mainly, Greene says keto is most often used for short-term, quick weight loss. Many keto diet followers have seen significant weight loss in a fairly short amount of time (take, for example, Sherri Shepherd’s 25-pound transformation).

But, Greene cautions, if the proportions of protein, carbs and fat are not maintained, then “weight loss will plateau or even come back.”

Lowery adds that he sees the keto diet as a way for people to get control over their appetite. “If you can put appetite back in your control and stop riding what we call the ‘carb/insulin rollercoaster’ of periodic ups and downs throughout the entire day, then you will undoubtedly look and feel better.”

He added: “There are a million different ways you can lose weight, but ultimately that isn’t what people want. People want to improve their body composition — meaning lose fat and maintain or gain muscle. In research and in a real-world application, a well-formulated ketogenic diet has been shown to do this over and over again.”

What are the downsides of the keto diet?

It's important to know that not all health experts are on board with the keto diet. Ahead, a few main side-effects and drawbacks of the trendy diet plan.

It takes serious commitment.

The keto diet is restrictive and takes serious commitment, Passler says. "The keto diet is difficult to stick to without proper planning and without giving your system time to adapt," he explains. "Having the proper foods available at home and knowing what to order out is key."

It isn't always healthy.

There are some people who have put their own spin on the diet, following a variation, like "dirty keto," which involves sticking to the diet's macronutrient breakdown, but not necessarily eating quality foods. Popular dirty keto meals, for instance, incorporate processed foods and tons of butter and bacon. There are also tons of packaged, foods out there (like shakes and bars) that, while technically allowed on keto, aren't the best for your overall health.

It can cause the "keto flu".

The list of keto diet side effects is lengthy. One side effect is constipation, since you're cutting out complex carbohydrates, and therefore fiber. You might even experience the dreaded "keto flu," which includes a host of symptoms, like muscle aches, dizziness, nausea, and diarrhea, that mimic the flu.

“The best way to understand this is that your body has been primarily running on glucose and carbohydrates for 30 or 40 or 50 or 60 years of your life,” Lowery says. “It’s going to take more than 72 hours for your body to switch its fuel source from primarily glucose to primarily ketones.”

It can be dangerous in the long-term.

The keto diet was not intended to be maintained long term and, in fact, it lacks adequate research that supports the effects of doing so. Plus, the keto diet can cause vitamin and mineral deficiencies if you aren't supplementing when needed, Greene says.

The bottom line on the keto diet:

Like any diet, the keto diet's menu and lifestyle might not be up your alley. "The best program for any individual is the one that works for them," Passler notes. "It's best if it is based on foods that they enjoy and believe in eating — a plant-based vegan is not a candidate for a program that involves animal products — and can feel good while staying on track long term."

Although the diet may be trendy, it's important to chat with your doctor first to see if it's a safe fit for you. Greene adds: “I highly recommend working with a healthcare professional if you are going to embark on this diet.”

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