Keto Diet for Beginners: How It Works and What to Eat

The keto (ketogenic) diet is a very-low-carbohydrate, high-fat way of eating. When carbohydrate drops to roughly 20 to 50 grams a day, the liver turns more fat into ketones, and the body shifts toward using fat and ketones for fuel, a state called nutritional ketosis. In trials, keto produces weight loss, but after a year or more the results are similar to, or only slightly better than, other reduced-calorie diets.

Key Takeaways

Ketogenic diets used for weight research usually limit carbohydrate to 20–50 g a day, with 60–75% of calories from fat and moderate protein.
Ketosis typically begins after a few days of eating less than about 50 g of carbohydrate a day.
A meta-analysis of 13 trials lasting a year or more found about 0.9 kg (2 lb) more weight loss on very-low-carb ketogenic diets than on low-fat diets. A 2022 Cochrane review found little to no difference versus balanced-carbohydrate weight-loss diets.
“Keto flu” symptoms such as headache, tiredness, nausea and dizziness are commonly reported in the first weeks.
Keto can raise LDL cholesterol in some people, is usually low in fiber and several vitamins and minerals, and has been linked to kidney stones.
Talk to a clinician first if you take medicine for blood sugar or blood pressure, are pregnant or planning a pregnancy, or have kidney or liver problems.

Evidence grade: Moderate. Dozens of randomized trials and two meta-analyses compare low-carbohydrate and ketogenic diets with other weight-loss diets in people; results consistently show small or no long-term differences, but certainty is rated moderate and few trials run longer than two years.

How Ketosis Works

On a typical diet, glucose from carbohydrates is the body’s main fuel, and the brain relies on it heavily. A 2013 review in the European Journal of Clinical Nutrition explains that after a few days of fasting, or of eating less than about 50 g of carbohydrate a day, glucose stores run low. After roughly 3 to 4 days the liver starts producing more ketone bodies (beta-hydroxybutyrate, acetoacetate and acetone) from fat, and the brain and other tissues use them for energy.

On a normal diet, blood ketones are very low, under 0.3 mmol/L. The same review notes that the brain begins to use ketones as a meaningful fuel when they reach about 4 mmol/L.

The diet itself is not new. According to StatPearls, a clinical reference hosted on the NIH’s NCBI Bookshelf, Dr. Russell Wilder of the Mayo Clinic coined the term “ketogenic diet” in 1921, when he used it as a medical diet for epilepsy. Interest in using it for weight came much later.

What Ketone Levels Look Like on Keto

People sometimes confuse nutritional ketosis with ketoacidosis, but they are different states. In diet-induced ketosis, blood ketones can rise to a maximum of about 7–8 mmol/L while blood pH stays normal. In uncontrolled diabetic ketoacidosis, ketones can exceed 20 mmol/L and the blood becomes acidic. The 2013 review describes this acidosis risk as virtually nonexistent in people whose insulin works normally. Ketoacidosis is a medical emergency that occurs mainly when the body has too little insulin, which is why people who use insulin need medical guidance before trying keto.

Blood measure	Typical mixed diet	Ketogenic diet	Uncontrolled ketoacidosis
Glucose (mg/dL)	80–120	65–80	Above 300
Ketone bodies (mmol/L)	About 0.1	Up to about 7–8	Above 25
Blood pH	7.4	7.4	Below 7.3

Macronutrient Ranges Used in Studies

“Keto” covers several plans. StatPearls describes three main versions, and research trials label diets in different ways, so it helps to know what was actually tested.

Version	Fat	Protein	Carbohydrate	Where it is used
Classic ketogenic diet (4:1 ratio)	About 90% of calories	About 6%	About 4%	Medically supervised epilepsy care; foods are weighed
Modified Atkins diet	About 60–70%	About 25–30%	About 5–10% (usually under 20 g a day)	Less strict medical use in teens and adults
Very-low-carbohydrate ketogenic diet	About 60–75%	Moderate	20–50 g a day	Most weight and metabolic research

Many “low-carb” trials are not ketogenic. In the 2022 Cochrane review, most trials tested diets with more than 50 g and up to 150 g of carbohydrate a day; only 14 of 61 trials used 50 g a day or less. Keep that in mind when you see headlines about low-carb results.

What to Eat and What to Limit

Keto meals are built around foods with little or no carbohydrate. Net carbs are total carbohydrate minus fiber, and many people count net carbs so they can keep eating fibrous vegetables.

Eat often	Eat in measured amounts	Usually limited
Non-starchy vegetables (leafy greens, broccoli, cauliflower, zucchini), eggs, fish, poultry, meat, olive oil, avocado	Nuts and seeds, cheese, plain Greek yogurt, berries	Bread, rice, pasta and other grains; potatoes and other starchy vegetables; beans and lentils; most fruit; sugar, sweets and sweet drinks

Food choices matter for diet quality. A 2021 review in Frontiers in Nutrition notes that very-low-carbohydrate diets tend to cut fruits, vegetables, whole grains and legumes and are often low in thiamin, folate, vitamins A, E and B6, calcium, magnesium, iron and potassium. They are also typically high in saturated fat. Leaning on olive oil, nuts, seeds, avocado and fish, and filling half the plate with non-starchy vegetables, helps close some of those gaps. For background on fat sources, see our guide to dietary fats, and if you are curious about MCT oil, read whether MCT oil is the same as coconut oil.

Your First Weeks: Keto Flu, Electrolytes and Fiber

Keto flu

“Keto flu” is not a medical diagnosis. It is the name people use for symptoms in the first weeks. A 2020 study analyzed 448 posts from 300 users of online forums. Of the 101 users who described their own experience, the most common symptoms were flu-like feelings, headache, fatigue, nausea, dizziness, “brain fog,” stomach discomfort, low energy, feeling faint and changes in heartbeat. Reports peaked in the first week and faded after about four weeks. These were self-reports, not a controlled trial.

Water and electrolytes

Some of the fast drop on the scale in the first week or two is not fat. The 2021 review notes that rapid early weight loss on ketogenic diets comes largely from fat-free mass such as body water and stored glycogen. Dehydration, low sodium and low magnesium are among the adverse effects reported on strict ketogenic diets.

Drinking enough fluid and eating mineral-rich, low-carb foods is a practical start. The NIH Office of Dietary Supplements (ODS) lists green leafy vegetables such as spinach, nuts and seeds as magnesium sources, and spinach, salmon, chicken, beef and broccoli as potassium sources. Adults are advised to get 3,400 mg (men) or 2,600 mg (women) of potassium a day, and 400–420 mg (men) or 310–320 mg (women) of magnesium. Whether you should add salt depends on your blood pressure and medications, so ask your clinician.

If you prefer a drink mix, OPA ION+ Select Electrolyte is a caffeine-free powder made without added sugars. It provides magnesium (as di-magnesium malate), potassium (as potassium chloride), sodium from Himalayan pink salt and vitamin B12. It is meant to support everyday hydration; it is not designed to change ketone levels or fat loss.

Fiber

Ketogenic diets are typically low in fiber, and constipation is one of the most common complaints. Non-starchy vegetables, chia and flax seeds, nuts and avocado add fiber without many digestible carbs. Our article on high-fiber diets and digestive wellness explains why fiber matters.

What Reviews Show About Weight and Adherence

Keto helps some people eat less, which is how most diets work. Trials do not show a large advantage over other approaches.

Versus low-fat diets. A 2013 meta-analysis of 13 randomized trials with at least 12 months of follow-up (about 1,400 participants) found that very-low-carbohydrate ketogenic diets (50 g or less a day) led to 0.91 kg more weight loss than low-fat diets. The keto groups also had lower triglycerides, slightly lower diastolic blood pressure, higher HDL cholesterol and higher LDL cholesterol.
Versus balanced-carbohydrate diets. The 2022 Cochrane review pooled 61 trials with 6,925 participants. Low-carbohydrate weight-loss diets probably made little to no difference compared with diets getting 45–65% of calories from carbohydrate: about 1.1 kg more at 3 to 8.5 months and about 0.9 kg more at one to two years (moderate-certainty evidence).
Staying on the diet. In the 12-month DIETFITS trial of 609 adults, a healthy low-carb diet and a healthy low-fat diet produced similar weight loss (6.0 kg vs 5.3 kg). The low-carb group was coached toward the lowest carbohydrate intake they could maintain, yet at 12 months they were eating about 30% of calories from carbohydrate on average, far above keto levels. Strict restriction is hard to keep up.
Appetite. In a 2021 NIH inpatient study, 20 adults ate as much as they wanted of an animal-based ketogenic diet and a plant-based low-fat diet for two weeks each. They ate about 689 fewer calories a day on the low-fat diet. The study was short and did not test long-term weight.

The practical takeaway is that the best diet is the one you can follow safely for a long time. If keto feels too restrictive, time-based approaches are another option; see our overview of the benefits of intermittent fasting.

Safety and Interactions

Common short-term effects. Headache, fatigue, nausea, dizziness, constipation and stomach discomfort are the most frequently reported, mostly in the first few weeks. Strict ketogenic diets can also cause dehydration and low sodium or magnesium.

Cholesterol. LDL cholesterol rose on average in the 2013 meta-analysis. The 2021 review describes trials in which roughly 3 in 10 participants on very-low-carb diets had a notable LDL increase. If you plan to stay on keto, ask your clinician about checking a lipid panel before you start and a few months in.

Kidney stones and other effects. Kidney stones are one of the main potential risks; they have been noted frequently in children following ketogenic diets for epilepsy. Reports from medically supervised ketogenic diets also include raised uric acid and triglycerides and inflammation of the liver or pancreas.

Talk to a clinician first if you:

take insulin or other glucose-lowering medicine. In ketogenic diet trials, these medicines often had to be reduced, and one participant in the NIH study withdrew because of low blood sugar;
take blood-pressure medicine, diuretics or other medicines that affect fluid, sodium or potassium. ODS notes that ACE inhibitors, angiotensin receptor blockers and potassium-sparing diuretics can reduce potassium excretion, so check with your doctor or pharmacist before using potassium-containing electrolyte products;
are pregnant, breastfeeding or could become pregnant. In a large U.S. observational study described in the 2021 review, women with low carbohydrate intake before conception were about 30% more likely to have a baby with a neural tube defect;
have kidney, liver or pancreas problems, or a history of disordered eating.

Magnesium supplements have a separate upper limit of 350 mg a day for adults; ODS notes that higher amounts can cause diarrhea, nausea and cramping.

Frequently Asked Questions

How long does it take to get into ketosis?

Usually a few days. The 2013 review describes ketone production rising after about 3 to 4 days of very low carbohydrate intake (under about 50 g a day). The exact timing depends on how strictly you limit carbohydrate and how active you are.

How many carbs can you eat on keto?

Most weight-focused ketogenic plans keep carbohydrate between 20 and 50 g a day, according to StatPearls.

How long does keto flu last?

In online reports analyzed in a 2020 study, symptoms peaked in the first week and became uncommon after about four weeks. Extra fluids and mineral-rich foods are sensible first steps. If symptoms are severe, or you feel faint or notice heart palpitations, stop and talk to a healthcare professional.

Does keto change cholesterol levels?

Often, yes. In trial pooling, ketogenic diets lowered triglycerides and raised HDL, but they also raised LDL on average, and some people see large LDL increases. A blood test is the only way to know how you respond.

Are raspberry ketones the same as the ketones made on keto?

No. Raspberry ketone is the main aroma compound in red raspberries and is used as a flavoring. ODS notes there is not enough research to draw firm conclusions about it for weight, and it does not put you into ketosis. The ketones made on keto are beta-hydroxybutyrate, acetoacetate and acetone, produced by your own liver.

Sources

Disclosure: OPA Nutrition sells supplements, including products mentioned in this article. This content is for education only and is not medical advice. Statements about dietary supplements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease. Talk to your healthcare provider before using any supplement, especially if you take medication or have a medical condition.

Written and reviewed by Michael Gonzales, Founder, OPA Nutrition · Last reviewed: October 17, 2026 · How OPA researches and writes

A Beginner's Guide to the Keto Diet: How It Works and What Studies Show

Key Takeaways

How Ketosis Works

What Ketone Levels Look Like on Keto

Macronutrient Ranges Used in Studies

What to Eat and What to Limit