A keto diet is a high-fat, very-low-carbohydrate eating pattern that shifts your body’s primary fuel source from glucose to fat. The typical breakdown is 70 to 80 percent of daily calories from fat, 10 to 20 percent from protein, and just 5 to 10 percent from carbohydrates. For most people, that means eating fewer than 20 to 50 grams of carbs per day, which is roughly the amount in a single bagel.
How Ketosis Works
Your body normally runs on glucose, the sugar that comes from carbohydrates. When you drastically cut carbs, your glucose stores run out within a day or two, and your liver starts breaking down fat into molecules called ketones. These ketones become your brain’s and muscles’ alternative fuel source, a metabolic state called ketosis. You can confirm you’re in ketosis with a simple blood meter; the typical threshold is a reading of 0.5 mmol/L or higher.
The concept isn’t new. In 1921, a physician at the Mayo Clinic named Russell Wilder coined the term “ketogenic diet” as a treatment for epilepsy. The idea was to mimic the metabolic effects of fasting, which had already shown promise in reducing seizures, but in a way patients could sustain long-term. It remained a clinical tool for decades before gaining mainstream popularity as a weight-loss strategy.
Why People Lose Weight on Keto
Several things happen when you enter ketosis that can accelerate fat loss. First, you drop water weight quickly because your body releases stored water as it burns through its glucose reserves. That accounts for the dramatic early results many people see in the first week or two.
The more interesting effect is what happens to hunger. Normally, when you lose weight through dieting, your body ramps up production of ghrelin, the hormone that drives hunger. This is a major reason why diets fail: the more weight you lose, the hungrier you get, and the harder it becomes to stick with a calorie deficit. Ketogenic diets appear to suppress this rebound. Research published in Nutrition Reviews found that ketosis prevents the typical spike in ghrelin that accompanies weight loss, and in some cases actually reduces feelings of hunger below baseline levels. This makes it easier for many people to eat less without feeling deprived.
Fat and protein are also more satiating than carbohydrates on a per-calorie basis. A breakfast of eggs cooked in butter keeps most people full far longer than a bowl of cereal with the same number of calories.
What You Eat (and Avoid)
The bulk of your plate comes from fat-rich whole foods: meat, fish, eggs, cheese, nuts, avocados, butter, and olive oil. Non-starchy vegetables like leafy greens, broccoli, cauliflower, and zucchini fill in the rest. Most people on keto eat liberally from these categories and find that tracking carbs matters more than counting total calories, at least initially.
The foods you cut are anything starchy or sugary: bread, pasta, rice, potatoes, most fruits, beans, candy, soda, and juice. Even foods that seem healthy, like oatmeal, bananas, and sweet potatoes, contain enough carbohydrate to kick you out of ketosis in a single serving.
Counting Net Carbs
Many keto followers track “net carbs” rather than total carbs. The calculation is straightforward: subtract grams of fiber from total carbohydrates, since fiber passes through your body without raising blood sugar. For products made with sugar alcohols (common in keto-friendly snack bars and candy), the UCSF Diabetes Teaching Center recommends subtracting half the grams of sugar alcohol from total carbs, not the full amount. So a protein bar listing 29 grams of total carbs and 18 grams of sugar alcohol would count as 20 net carbs (29 minus 9).
The Keto Flu
Somewhere between day two and day seven, many people hit a rough patch known as the keto flu. Symptoms include headaches, fatigue, brain fog, irritability, nausea, and muscle cramps. It’s not actually the flu. It’s your body adjusting to a radically different fuel source while also flushing out electrolytes along with water.
The fix is simple but important: drink more water than you think you need, and make sure you’re getting enough sodium, potassium, and magnesium. Salting your food generously, eating potassium-rich greens like spinach and avocado, and supplementing magnesium can shorten or eliminate symptoms entirely. Most people feel normal again within a week, and many report sharper mental clarity and more stable energy once fully adapted.
Variations of the Keto Diet
The version described above is called the Standard Ketogenic Diet, or SKD. It’s what most people mean when they say “keto.” But two other variations exist for people with specific athletic needs.
The Targeted Ketogenic Diet (TKD) follows the standard approach most of the time but adds a small dose of carbohydrates 30 to 60 minutes before a workout. This gives muscles a quick burst of glucose for high-intensity exercise without disrupting ketosis for the rest of the day. It’s popular with people who do CrossFit, sprinting, or heavy weightlifting and find that pure keto leaves them underpowered during peak efforts.
The Cyclical Ketogenic Diet (CKD) alternates between five or six days of strict keto and one or two days of higher-carb eating. The purpose is to fully replenish muscle glycogen stores, which matters most for competitive athletes training at high volumes. This isn’t the same as “cheating” on the weekend; it’s a structured refeed designed to support performance while maintaining the metabolic benefits of ketosis throughout the week.
Potential Risks and Side Effects
Beyond the keto flu, there are longer-term considerations worth knowing about. Some people see a significant rise in LDL cholesterol on keto, particularly those who are genetically predisposed. This appears to be driven by the high saturated fat intake. Triglycerides, on the other hand, tend to drop, and HDL (“good”) cholesterol often improves. The overall cardiovascular picture is mixed and likely varies from person to person, so getting blood work done before starting and a few months in gives you actual data rather than guesswork.
Nutrient deficiencies can develop if the diet is too restrictive. Cutting out fruits, whole grains, and legumes removes major sources of certain vitamins, minerals, and fiber. Eating a wide variety of vegetables and considering a multivitamin helps offset this.
Constipation is common, especially early on, because fiber intake drops when you eliminate grains. Loading up on low-carb vegetables and staying well-hydrated usually resolves it.
Who Should Avoid Keto
For most healthy adults, a ketogenic diet is safe to try. But certain medical conditions make it genuinely dangerous. Absolute contraindications include rare inherited metabolic disorders that affect how the body processes fat or produces energy from it, as well as a condition called porphyria. People with a history of acute pancreatitis, advanced liver disease, advanced kidney disease, or familial hypercholesterolemia face elevated risk and would need close medical supervision at minimum.
Anyone taking medication for diabetes or high blood pressure should be aware that keto can lower blood sugar and blood pressure rapidly, which may require adjusting dosages to avoid dangerous lows. Pregnant and breastfeeding women are generally advised against strict ketogenic diets due to the high nutrient demands of those periods.
What to Expect in the First Month
The first few days feel the hardest. Cravings for bread, sugar, and starchy comfort foods are real, and the keto flu can make you question the whole project. By the end of week one, most people have lost several pounds (mostly water) and the worst of the transition symptoms are fading.
Weeks two through four are where the diet starts to feel sustainable. Appetite drops noticeably for most people, and energy levels stabilize. Fat loss begins to show up on the scale at a steadier, more moderate pace, typically one to two pounds per week. Many people notice reduced bloating, fewer energy crashes after meals, and improved mental focus. These subjective benefits are a big part of why keto has such a devoted following, even among people who eventually transition to a less restrictive low-carb approach.