What the ketogenic diet actually is
Keto is a very low carbohydrate, high fat way of eating. The whole point is to flip your body's main fuel from glucose, which comes from carbs, to fat. When you keep carbs low enough for long enough, your liver starts turning fat into molecules called ketones, and your brain and muscles burn those for energy instead. That metabolic state is ketosis, and it is the thing every keto rule is built to produce and protect.
In practice that means carbs drop to roughly 20 to 50g of net carbs a day, protein stays moderate, and fat fills the rest of your plate. Net carbs are total carbohydrate minus fiber, since fiber is not digested into glucose. Most beginners start at 20g net carbs because it is the most reliable way to reach ketosis quickly. You can test a higher limit later once you know how your own body responds.
How to begin in one week
You do not need a perfect plan to start. You need a carb limit, a short list of foods you actually like, and a way to handle the first few days. Pick your daily net carb target, clear the obvious carbs out of the kitchen, and decide on three or four simple meals you can repeat. Eggs and bacon, a big salad with chicken and olive oil, ground beef with cheese and avocado: a boring first week is a successful first week.
The fastest way to get your numbers right is to let a calculator do the math. The Keto Macro Calculator turns your height, weight, and goal into exact daily grams of fat, protein, and net carbs, so you are not guessing. When you are unsure whether a specific food fits, the keto food database gives you the net carbs per serving and a plain keto verdict for hundreds of everyday foods.
What to eat and what to avoid
The foods that work on keto are the ones that are naturally low in carbohydrate. Build your plate from fatty meats and fish, eggs, butter, olive and avocado oil, full-fat cheese, and low-carb vegetables that grow above ground, like spinach, broccoli, zucchini, and cauliflower. Avocados, nuts, and seeds round things out. A half avocado has about 2g of net carbs and a useful dose of potassium, which matters more than beginners expect.
The avoid list is shorter than it looks because it is mostly one category: starch and sugar. Bread, pasta, rice, potatoes, and cereal are out. So are sugary drinks, most fruit, beans, and anything built on flour or sugar. Two traps catch nearly everyone early on. The first is fruit, which feels healthy but is concentrated sugar, with a banana running around 24g of carbs. The second is hidden sugar in sauces, dressings, and drinks: ketchup, barbecue sauce, sweet coffee, and most bottled marinades can quietly use up a whole day's carbs.
What the first week feels like
Two things tend to happen in week one. The scale drops fast, and you might feel a little rough. Both have the same cause. Your body stores carbohydrate as glycogen, and every gram of glycogen is bound to three to four grams of water. When you cut carbs, you burn through that glycogen and release the water with it, which is why people often lose three to seven pounds in the first several days. That early drop is mostly water, not fat, and it is normal.
The same shift can leave you tired, headachy, foggy, or crampy, a cluster nicknamed the keto flu. It is not an illness. As insulin falls, your kidneys flush out sodium and water, and the resulting mineral imbalance is what makes you feel off. The fix is electrolytes: more salt, potassium-rich foods, and often a magnesium supplement. This matters enough that it has its own guide. Read Electrolytes and Keto Flu 101 before you start, and you can sidestep most of the misery entirely.
Common early mistakes
Most beginner struggles come down to a handful of fixable errors. Not eating enough salt is the big one, and it is the direct cause of most keto flu. Being scared of fat is the next: keto is a high fat diet by design, and trying to run it low fat and low carb at the same time leaves you hungry, drained, and miserable. Eating too much protein in place of fat can also blunt ketosis for some people, so aim for a moderate amount rather than piling it on.
The rest are about expectations. Snacking all day keeps insulin up and slows the switch into ketosis, so let real meals do the work. Trusting net carbs blindly on packaged keto treats backfires when sugar alcohols and starches sneak in, so read labels. And do not weigh yourself in a panic on day three, when water weight makes the number jump around. If you want to confirm you are actually in ketosis rather than guessing, the next step is to read How to Test for Ketosis and pick a method that fits your budget. Give it two to three weeks before you judge how keto is working for you. The first few days are noise; the trend after that is the signal.