Low-carbohydrate diets

Low-carbohydrate diets can help some people lose weight or manage blood sugar, but they are not for everyone.

Low-carbohydrate diets cut back on carbohydrate-rich foods, such as bread, pasta, rice, potatoes, sugar and sweet drinks, and replace them with more protein, fat and non-starchy vegetables. They are popular for weight loss and are sometimes used to help manage type 2 diabetes. Here is what they involve, what the evidence shows, and who should talk to a doctor before starting one.

What counts as low carb

There is no single definition, but the American Diabetes Association and many researchers use these bands:

  • Low carbohydrate: fewer than about 130 grams of carbohydrate a day, or less than about a quarter of daily calories.
  • Very low carbohydrate, or ketogenic: about 20 to 50 grams a day, low enough that the body switches to burning mostly fat and produces ketones.

For comparison, the standard recommended range in the US is 45 to 65% of calories from carbohydrate, which on a 2,000-calorie day is roughly 225 to 325 grams.

Low carb, slow carb and the glycemic index

These terms often get mixed up. A low-carb diet reduces the amount of carbohydrate you eat. A "slow-carb" or low-glycemic approach is different: it keeps carbohydrates but favors the kinds that raise blood sugar more gradually.

The glycemic index (GI) measures that speed. It ranks carbohydrate foods by how much they raise blood sugar after eating, compared with pure glucose, which scores 100. A score of 55 or less is low, as for most beans and lentils, oats and many fruits; 70 or more is high, as for white bread and many breakfast cereals. GI does not tell you how much carbohydrate a food contains. The glycemic load does, by combining a food's GI with the amount of carbohydrate in a normal serving, which makes it the more practical measure. A food's GI also changes with ripeness, processing and cooking, and with what else is eaten in the same meal.

What the evidence shows

For weight loss, low-carb diets work about as well as other approaches that cut calories. In the DIETFITS trial, published in 2018, 609 adults followed either a healthy low-carb or a healthy low-fat diet for a year. Both groups lost a similar amount, about 13 pounds (6 kg) on low carb and 12 pounds (5.3 kg) on low fat, and the difference was not significant. Some people lose weight faster in the first weeks of a low-carb diet, partly because the body sheds water along with its stored carbohydrate. Over the longer term, the diet that works best is the one a person can stick with.

For people with type 2 diabetes, eating fewer carbohydrates can lower blood sugar, and the American Diabetes Association recognizes low-carb eating as one of several reasonable options. It needs to be planned with the care team, because medicines may have to be adjusted.

Building a healthy low-carb plate

  • Vegetables first. Fill half the plate with non-starchy vegetables: leafy greens, broccoli, cauliflower, zucchini, peppers, tomatoes, mushrooms and green beans.
  • Protein in normal portions. Fish, poultry, eggs, tofu, lean meat, and beans and lentils if your carbohydrate allowance includes them. Protein is filling, but it still has calories, and more is not automatically better.
  • Choose fats for quality. Olive oil, avocado, nuts, seeds and oily fish are better choices than butter, cream, fatty meat and processed meats such as sausage and bacon. Diets heavy in saturated fat can raise LDL ("bad") cholesterol, and processed meat is linked to a higher risk of colorectal cancer.
  • Keep the fiber. Cutting grains, fruit and beans can leave a diet short of fiber. Vegetables, nuts, seeds, berries and, where they fit, beans and lentils help make up for it.
  • Cut refined carbohydrates first. The easiest gains come from sugary drinks, sweets, white bread and pastries, rather than from whole fruit or legumes.

Check with your doctor first if

  • you have diabetes and take insulin or a sulfonylurea: cutting carbohydrates can make blood sugar fall too low unless the dose is adjusted
  • you take an SGLT2 inhibitor, such as empagliflozin, dapagliflozin or canagliflozin: combining these drugs with a very low-carb or ketogenic diet has been linked to ketoacidosis, a dangerous condition that can occur even when blood sugar isn't very high
  • you have kidney disease
  • you are pregnant or breastfeeding
  • you have, or have had, an eating disorder
  • you have high LDL cholesterol or heart disease
  • the diet is meant for a child or teenager

Learn more in our pages on carbohydrates, sugar management, the MASLD diet and weight loss.

Sources

Last reviewed November 2026. This article is general information, not medical advice.