Starting a ketogenic diet means shifting your body’s primary fuel source from carbohydrates to fat by eating 70% to 80% of your daily calories from fat, 10% to 20% from protein, and only 5% to 10% from carbohydrates. In practice, that means keeping your daily carb intake under 50 grams, and many people start closer to 20 grams to enter ketosis faster. Here’s how to actually do it.
What Happens in Your Body on Keto
When you drastically cut carbs, your body runs low on its preferred fuel: glucose. In response, your fat cells release stored fat into the bloodstream as free fatty acids. Your liver picks these up and breaks them down, producing a byproduct called acetyl-CoA. Normally, this molecule feeds into your main energy-production cycle. But without enough carbohydrates, a key ingredient in that cycle (oxaloacetate) gets rerouted to make the small amount of glucose your brain and red blood cells still need.
This creates a bottleneck. Your liver can’t burn all the acetyl-CoA it’s producing, so it converts the excess into ketone bodies, which are released into your bloodstream and used as fuel by your brain, heart, and muscles. This metabolic state is called ketosis, and reaching it is the entire point of the diet. Most people enter ketosis within two to four days of restricting carbs below 50 grams.
What to Eat
The backbone of a keto diet is nutrient-dense fat. Good staples include avocados and avocado oil, olives and cold-pressed olive oil, coconut and unrefined coconut oil, butter, cheese, full-fat Greek yogurt, and whole eggs. For protein that pulls double duty as a fat source, fatty fish like salmon, sardines, tuna, and anchovies are ideal.
Nuts and seeds fill in the gaps nicely. Almonds, walnuts, pecans, pistachios, Brazil nuts, flax seeds, chia seeds, and hemp hearts all offer fat along with fiber and micronutrients. Nut and seed butters work well as dips for low-carb vegetables or stirred into recipes.
For your 5% to 10% carbohydrate allowance, focus on non-starchy vegetables: leafy greens, broccoli, cauliflower, zucchini, bell peppers, asparagus, and mushrooms. These give you fiber, vitamins, and minerals without using up your carb budget. A medium plain bagel alone contains more than 50 grams of carbs, so the shift away from bread, pasta, rice, and potatoes is significant.
Watch for Hidden Carbs
Processed foods are full of sugars disguised under names you might not recognize. The CDC lists common ones to scan for on ingredient labels: cane sugar, turbinado sugar, corn syrup, high-fructose corn syrup, rice syrup, molasses, caramel, honey, agave, and juice concentrates. Any ingredient ending in “-ose” (glucose, fructose, maltose, dextrose, sucrose, lactose) is a sugar. Terms like “glazed,” “candied,” “caramelized,” and “frosted” also signal added sugar during processing.
Condiments are a frequent culprit. Ketchup, barbecue sauce, teriyaki sauce, and many salad dressings can contain several grams of sugar per serving. Even foods marketed as “low-fat” often compensate with added sugars. Reading labels carefully, especially in the first few weeks, keeps you from accidentally knocking yourself out of ketosis.
Getting Through Keto Flu
In the first few days to a week, many people experience what’s commonly called “keto flu”: headaches, fatigue, irritability, brain fog, nausea, and sometimes muscle cramps. This happens because your body is losing water and electrolytes rapidly as it depletes its stored carbohydrates. Each gram of stored glycogen holds onto several grams of water, so the initial weight loss is largely fluid, and that fluid takes sodium, potassium, and magnesium with it.
The fix is straightforward: add more salt to your food, drink electrolyte-rich beverages (look for sugar-free options), and eat magnesium-rich foods like nuts and leafy greens. Staying well hydrated helps too. Keto flu typically resolves within one to two weeks as long as you stay consistent with your carb restriction. If symptoms last longer than ten days or become severe, that’s worth discussing with your doctor.
How to Know You’re in Ketosis
You’re officially in nutritional ketosis when the ketone body beta-hydroxybutyrate reaches at least 0.5 mmol/L in your blood, up from a baseline of roughly 0.1 mmol/L on a standard diet. There are three ways to test this, and they vary considerably in accuracy.
Blood ketone meters are the most reliable option. They measure beta-hydroxybutyrate directly from a finger prick and give you a real-time reading. The downside: the test strips are relatively expensive and the finger prick can be uncomfortable.
Urine strips are cheap, painless, and require no equipment. However, they measure a different ketone (acetoacetate) and reflect an average since your last bathroom visit rather than your current level. They’re also less accurate over time because as your body becomes more efficient at using ketones, fewer spill into your urine. This means the strips may show decreasing readings even as you become more deeply adapted to ketosis.
Breath meters measure acetone in your exhaled breath. Small studies show a positive correlation with blood ketone levels, but no breath ketone device has been cleared by the FDA, so accuracy varies between products.
For most people starting out, urine strips are a reasonable and affordable way to confirm you’ve entered ketosis. If you want ongoing precision, a blood meter is worth the investment.
What to Expect Beyond the First Weeks
Once keto flu passes, many people report increased energy, reduced hunger between meals, and improved mental clarity. The reduced hunger is partly because fat and protein are more satiating than carbohydrates, and partly because stable blood sugar means fewer energy crashes and cravings.
Weight loss on keto tends to be rapid in the first week or two, mostly from water loss, then settles into a steadier pace. The diet’s effectiveness for weight loss is well-documented in shorter-term studies, but longer-term effects are less clear-cut. Animal research published in Science Advances found that while a ketogenic diet protected against weight gain, extended adherence led to elevated cholesterol, fatty liver changes, and impaired blood sugar regulation. These findings are in mice and don’t translate directly to humans, but they do reflect a pattern seen in some human studies where LDL cholesterol rises on keto.
If you plan to follow a keto diet for more than a few months, periodic blood work to check your lipid panel and liver markers is a reasonable precaution, especially if you have a family history of heart disease.
Who Should Avoid Keto
Certain conditions make a ketogenic diet genuinely dangerous. People with rare metabolic disorders affecting fat processing, including carnitine deficiency, carnitine palmitoyltransferase deficiency, and mitochondrial fatty acid oxidation disorders, cannot safely produce ketones and should never attempt this diet.
Pregnant and breastfeeding women face a risk of ketoacidosis on very low-carb diets, so keto is not recommended during these periods. People with a recent heart attack or stroke should avoid it due to increased arrhythmia risk. Those with acute intermittent porphyria can experience dangerous relapses triggered by carbohydrate restriction.
One particularly important interaction: people with type 2 diabetes taking SGLT-2 inhibitors (a common class of blood sugar medication) face a serious risk of a life-threatening condition called euglycemic diabetic ketoacidosis when combining the drug with a ketogenic diet. This combination is strongly discouraged. If you take any diabetes medication, work with your prescriber before starting keto, because your dosages will almost certainly need adjustment as your blood sugar drops.