Keto in plain words: what it is and who it suits
4 November 2026
The diet explained without the jargon, with its limits.
A ketogenic diet, keto for short, is very low in carbohydrate and high in fat, with a moderate amount of protein. With little sugar and starch coming in, the body turns to fat for most of its fuel and makes ketones, which is where the name comes from.
How low is low
Harvard's School of Public Health describes keto as less than 50 grams of carbohydrate a day, and sometimes as little as 20. For comparison, a single cup of cooked rice has about 45. One sample day from Harvard Health, at 2,000 calories, is 165 grams of fat, 75 grams of protein and 40 grams of carbohydrate.
Where it came from
Keto was not invented for weight loss. In 1921 Dr Russell Wilder of the Mayo Clinic used it to treat epilepsy and gave it its name. It reduced seizures in some patients, and epilepsy is still the one use that medical guidelines fully accept.
What it does for weight
People often lose weight quickly at first. Over the longer term the picture is more modest: a Harvard review reports that after six months keto does not lead to clearly more weight loss than other approaches.
Side effects
In the first days many people get what is called keto flu: headache, tiredness, dizziness, nausea and constipation. It usually passes within days to weeks. Over the long term, doctors watch for kidney stones, a lack of some vitamins and minerals, and changes in the liver. Because many keto plates lean on saturated fat, heart health is a concern too.
Who should be careful
People who are pregnant or may become pregnant are advised against keto. People who take insulin or tablets for diabetes can have dangerously low blood sugar if their medicine is not adjusted first. Anyone with liver, kidney or pancreas disease should not start without their doctor. Harvard Health's advice is to speak to a doctor and a dietitian before trying it.
This article is general information, not medical advice.
Sources
Harvard T.H. Chan School of Public Health, The Nutrition Source. Diet Review: Ketogenic Diet for Weight Loss. https://www.hsph.harvard.edu/nutritionsource/healthy-weight/diet-reviews/ketogenic-diet/
Harvard Health Publishing. Should you try the keto diet? https://www.health.harvard.edu/staying-healthy/should-you-try-the-keto-diet
Masood W and others. Ketogenic Diet. StatPearls, National Library of Medicine. https://www.ncbi.nlm.nih.gov/books/NBK499830/
Courtney Y. Keto: it's probably not right for you. Harvard, Science in the News, 2023. https://sites.harvard.edu/sitn/2023/05/10/keto-its-probably-not-right-for-you