In essence, it is a diet that causes the body to release ketones into the bloodstream. Most cells prefer to use blood sugar, which comes from carbohydrates, as the body’s main source of energy. In the absence of circulating blood sugar from food, we start breaking down stored fat into molecules called ketone bodies (the process is called ketosis). Once you reach ketosis, most cells will use ketone bodies to generate energy until we start eating carbohydrates again. The shift, from using circulating glucose to breaking down stored fat as a source of energy, usually happens over two to four days of eating fewer than 20 to 50 grams of carbohydrates per day. Keep in mind that this is a highly individualized process, and some people need a more restricted diet to start producing enough ketones.
Josh Axe, a doctor of natural medicine and clinical nutritionist, estimates that about 25% of people who try a ketogenic diet experience these symptoms, with fatigue being the most common. “That happens because your body runs out of sugar to burn for energy, and it has to start using fat,” he says. “That transition alone is enough to make your body feel tired for a few days.”
One of the most common side effects of starting the ketogenic diet is the “keto flu.” This term describes the often unpleasant, fatigue-inducing symptoms that occur as the body adjusts from a high-carbohydrate to a low-carbohydrate diet. During the keto flu, the body’s stored glucose begins depleting, and the body starts adapting to producing and utilizing ketones as energy. (2)
As for how it feels to follow the diet, St. Pierre says it will vary from person to person; as with most diet-related things, whether or not a way of eating works is in the eye of the beholder. He’s seen some people feel fine, but that “some clients have found it causes brain fog and lethargy, decreases their performance, disrupts their regular bowel movements, and makes them feel generally crappy,” he says. Some keto adherents also report having pretty atrocious breath, which might be caused by the presence of acetone, a ketone and byproduct of fat metabolism, in exhaled breath.

If you want to slam a protein shake post-workout, that's probably fine as long as you've got room for it in your macros. But shoot for one that is very low—like, zero—in carbohydrates. Pure isolates, such as Signature 100% Whey Isolate, are extremely low in carbohydrate. If you struggle to fit fat in during the day, toss a tablespoon of olive oil in with your shake. You won't taste it, and it gives a quick 13-14 grams of fat.
You want to keep your cheats to none. Be prepared, make sure you’re eating what you need to be satiated (“full”), and make sure you’re satisfied with what you’re eating. If you have to force yourself to eat something, it will never work out in the end. This is just a guideline on how you can eat on a ketogenic diet, so you’re very welcome to change up what kind of foods you eat!
While following keto, you’re encouraged to eat as much fat as you want, no matter the source (e.g., butter, olive oil, meats, cheeses), until you feel full. Fat can have a strong satiety effect, but following this recommendation may not be comfortable for everyone. According to recent research, fatty foods may increase feelings of nausea and bloating for some people. Beyond potential physical discomfort, it also might not be emotionally comfortable for everyone—after all, keto is still a restrictive diet, which typically is not a good choice for anyone with a disordered relationship to food and eating. And as with any restrictive dietary plan, following a ketogenic diet can present challenges when it comes to social occasions, celebrations, office parties, meals out, etc. It also might be necessary to limit or avoid culturally relevant foods on the diet in order to stay in ketosis. For still other people it will be tough to sustain because carbs aren’t only delicious, they’re omnipresent. Avoiding them day to day means fastidious meal prep and planning, and planning for social events that include food or eating.

This lighter soup by I Breathe I’m Hungry offers plenty of fat with coconut milk, chopped pistachios, and coconut flakes. Spices like curry powder and onion flakes make it flavorful. It will even please the non-keto dieters in your family as it’s low in calories, only 117 per serving, and is dairy-free and Whole30 compliant, too. One serving has only 6 grams of net carbs.
Diarrhea can also be due to a lack of fiber in the diet, says Kizer, which can happen when someone cuts way back on carbs (like whole-grain bread and pasta) and doesn’t supplement with other fiber-rich foods, like vegetables. It can also be caused by an intolerance to dairy or artificial sweeteners—things you might be eating more of since switching to a high-fat, low-carb lifestyle.
Bulk buy and cook. If you’re someone who doesn’t like to spend a lot of time in the kitchen, this is the best of both worlds. Buying your food at bulk (specifically from wholesalers) can reduce the cost per pound tremendously. Plus, you can make ahead food (bulk cook chicken thighs for pre-made meat, or cook entire meals) that are used as leftovers, so you spend less time cooking.
When you eat foods high in carbohydrates and fat, your body naturally produces glucose. Carbohydrates are the easiest thing for the body to process, and therefore it will use them first – resulting in the excess fats to be stored immediately. In turn, this causes weight gain and health problems that are associated with high fat, high carbohydrate diets (NOT keto).

If you’re a newbie planning your weekly keto diet plan, make the meals as easy as possible. A keto breakfast, for example, can take advantage of many classic breakfast foods, including eggs, bacon, sausage, and ham. Eggs are real winners in the keto world. They’re extremely versatile, easy to cook, and have just half a gram of carbs but 6 g of protein and 5 g of fat.
In essence, it is a diet that causes the body to release ketones into the bloodstream. Most cells prefer to use blood sugar, which comes from carbohydrates, as the body’s main source of energy. In the absence of circulating blood sugar from food, we start breaking down stored fat into molecules called ketone bodies (the process is called ketosis). Once you reach ketosis, most cells will use ketone bodies to generate energy until we start eating carbohydrates again. The shift, from using circulating glucose to breaking down stored fat as a source of energy, usually happens over two to four days of eating fewer than 20 to 50 grams of carbohydrates per day. Keep in mind that this is a highly individualized process, and some people need a more restricted diet to start producing enough ketones.

When you’re eating the foods that get you there (more on that in a minute), your body can enter a state of ketosis in one to three days, she adds. During the diet, the majority of calories you consume come from fat, with a little protein and very little carbohydrates. Ketosis also happens if you eat a very low-calorie diet — think doctor-supervised, only when medically recommended diets of 600 to 800 total calories.
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