The ketogenic diet — keto for short — is a way of eating that gets most of its calories from fat, very few from carbs, and a moderate amount of protein. People try it for weight loss, blood sugar control, or because their doctor prescribed it for a condition like epilepsy.

This guide explains, in plain English, what keto actually does inside your body, what you eat, and what to expect in your first few weeks.

How keto works: the switch to ketosis

Your body usually runs on glucose — a sugar that comes from carbs like bread, rice, pasta, and fruit. When you cut carbs down to about 20–50 grams per day, your body runs out of stored glucose in about 2–4 days.

At that point, your liver starts breaking down fat into small molecules called ketone bodies. Your brain and muscles can use these ketones for energy instead of glucose. Living in this state — using ketones as your main fuel — is called ketosis.

Three ketones do most of the work: beta-hydroxybutyrate, acetoacetate, and acetone. Beta-hydroxybutyrate is the main one. Acetone is what gives some people the "keto breath" — a slightly fruity smell — in the first weeks.

You can briefly enter mild ketosis during overnight fasting or hard exercise. Keto just keeps you in that state most of the time.

The macros: what your plate looks like

The standard keto diet aims for these calorie shares (Harvard Nutrition Source):

  • Fat: 70–80%
  • Protein: 10–20%
  • Carbs: 5–10%

On a 2,000-calorie day, that's about 165 g of fat, 75 g of protein, and 40 g of carbs.

For comparison, a typical Western diet is about 30% fat, 15% protein, and 55% carbs. Keto is a big shift — most of the change happens at your dinner plate.

The number to remember is 20–50 grams of carbs per day. Most people stay in ketosis under 50 g; strict versions aim for 20 g.

What you eat — and what you don't

Eat freely

  • Fatty meats, eggs, oily fish (salmon, sardines, tuna)
  • Hard cheeses, butter, cream
  • Avocados, olives, plant oils (olive, coconut)
  • Nuts and seeds (walnuts, pecans, chia, flax)
  • Non-starchy vegetables (leafy greens, broccoli, cauliflower, peppers, zucchini, cucumber)
  • Small amounts of berries — the lowest-carb fruit

Avoid

  • Grains and flour: bread, pasta, rice, oats, cereals
  • Sugar in all forms: candy, soda, juice, most desserts
  • Starchy vegetables: potatoes, corn, peas, winter squash
  • Most fruits — they're high in natural sugar
  • Beans, lentils, peanuts
  • Beer and sweet drinks

A useful rule: if it isn't on the avoid list and grows in the ground or comes from an animal, it's probably fine.

The first weeks: what to expect

When you first cut carbs sharply, you may feel rough for a few days to a couple of weeks. This is often called the keto flu. Symptoms include headache, tiredness, brain fog, hunger, irritability, constipation, and trouble sleeping (Harvard).

Why does this happen? When you cut carbs, your body loses its glycogen stores — and glycogen holds water. So you lose water fast (that's why most people drop several pounds in the first week — up to 10 pounds in the first two weeks). Along with the water, you lose minerals like sodium, potassium, and magnesium. That's the real source of the headache, fatigue, and cramps.

How to handle it:

  • Drink more water than usual
  • Add salt to your food (sea salt, broth, pickles)
  • Eat potassium-rich keto foods: avocado, spinach, salmon
  • Consider a magnesium supplement if cramps are bad
  • Ease in — you can lower carbs over a week instead of overnight

Most people feel normal again within one to two weeks.

Who keto helps — and who should avoid it

Solid evidence for keto:

Talk to a doctor before keto if you:

  • Have type 1 diabetes — risk of dangerous ketoacidosis
  • Have kidney disease, liver disease, or pancreatitis
  • Are pregnant or breastfeeding
  • Have a history of an eating disorder
  • Take SGLT2 inhibitors for diabetes — they raise the risk of a serious form of ketoacidosis (StatPearls)
  • Have certain inherited fat-metabolism disorders (rare, but worth checking)

For children, keto should only be used under medical supervision — almost always for epilepsy.

A few honest notes before you start

The type of fat matters. Harvard researchers stress that focusing on mostly unsaturated fats — olive oil, avocado, nuts, fatty fish — is healthier than building keto around butter and processed meats.

Long-term data is limited. Most keto studies last 12 weeks or less. Adherence drops too — one review found about 38% of people are still on it after 3 years.

Get your labs checked. If you stay on keto, ask your doctor for yearly checks on cholesterol, kidney function, and electrolytes.

Keto isn't magic and it isn't for everyone. But for the right person, done thoughtfully, it can be a useful tool for weight, blood sugar, or specific medical conditions. The next step is figuring out whether it fits you — and that's worth a real conversation with someone who knows your full health picture.

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