What Is the Keto Diet? How It Works, Pros and Cons

The keto, or ketogenic, diet is a very low-carbohydrate, high-fat way of eating that typically limits carbs to about 20 to 50 grams a day. With so little carbohydrate available, your body shifts toward burning fat and making compounds called ketones for fuel, a state known as ketosis. Keto can lead to weight loss, and it has a long history as a medically supervised therapy for some types of epilepsy, but it’s restrictive, hard to sustain and not safe for everyone.

How ketosis works

Normally, your body runs largely on glucose from carbohydrates. When carb intake stays very low for a few days, your stored glucose, called glycogen, runs down. Your liver then starts converting fat into ketones, which your brain and muscles can use as an alternative fuel.

Nutritional ketosis is different from diabetic ketoacidosis, a dangerous condition in which ketones and blood sugar climb to very high levels, most often in people with type 1 diabetes. That’s one reason people with diabetes should only try keto with close medical guidance.

What people eat on keto

Keto plans often aim for roughly 70% to 80% of calories from fat, 10% to 20% from protein and only about 5% to 10% from carbohydrates.

How quickly a carb budget fills up

This is what makes keto so challenging. Here’s the approximate total carbohydrate in some everyday foods:

A single bowl of rice can use up an entire day’s allowance on a strict plan. That’s why many keto followers count net carbs, which subtract fiber; our guide to net carbs vs total carbs shows how that works.

Possible benefits

• Weight loss. Many people lose weight quickly at first, partly from water as glycogen stores empty, and keto may curb appetite for some. Over the longer term, studies generally find weight loss similar to other calorie-reduced diets.
• Blood sugar. Some people with type 2 diabetes use lower-carb eating patterns under medical supervision to help manage blood sugar, sometimes with medication changes made by their doctor.
• Epilepsy. Ketogenic diets have been used since the 1920s, under close medical care, to reduce seizures in some people whose epilepsy doesn’t respond well to medication.

Downsides and risks

• “Keto flu.” Headaches, fatigue, irritability, nausea and dizziness are common in the first days or weeks. Shifts in fluid and minerals are part of the reason; see our guide to electrolytes.
• Low fiber. Cutting grains, beans and most fruit can lead to constipation and less food for gut bacteria.
• Nutrient gaps. Restricting whole food groups makes it harder to get enough of some vitamins and minerals.
• Cholesterol changes. In some people, especially those eating lots of saturated fat, LDL cholesterol rises.
• Kidney stones and other problems have been reported in some people on long-term ketogenic diets.
• Hard to sustain. Many people find keto tough to maintain socially and culturally, and weight often returns when they stop.

Please talk with your doctor before trying keto if you’re pregnant or breastfeeding; have diabetes, especially if you take insulin or medications such as SGLT2 inhibitors; have kidney, liver, pancreatic or gallbladder disease; or have a history of disordered eating.

A gentler middle ground

If you’re curious about eating fewer carbs, you don’t have to go to keto’s extremes. A moderate approach might mean swapping refined grains and sugary foods for vegetables, beans, nuts and some whole grains, while keeping most of your fat unsaturated. If you do try keto, make olive oil, avocados, nuts, seeds and fish your main fat sources, eat plenty of non-starchy vegetables, and keep processed meats to a minimum. Our guide to healthy fats can help you choose.

The bottom line

The keto diet cuts carbohydrates to about 20 to 50 grams a day so your body runs mainly on fat and ketones. It can lead to short-term weight loss and has genuine medical uses, but it’s restrictive, can cause side effects and isn’t right for everyone. If you’re considering it, check with your doctor first and build it around unsaturated fats and plenty of non-starchy vegetables.