What is keto, does it work, and who is it for?
Keto means restricting carbohydrate to roughly 20 to 50 grams a day until the body switches to ketones from fat as its main fuel. The fast first-week loss is mostly water held with glycogen, not fat. With calories and protein matched, research does not find keto burning more fat than other diets — its real advantage is that it makes many people full enough to eat less on their own.
Keto produces both people who do extremely well on it and people who cannot last a month, and that split is itself the clue to how it works.
What is keto, and how much carbohydrate a day?
Generally, restricting carbohydrate to about 20 to 50 grams a day — low enough that the liver begins producing ketones from fat to use instead of glucose. A common split is 70 to 75% of energy from fat, 20 to 25% from protein, and the remainder carbohydrate.
Worth knowing: the threshold differs between people. Someone training hard may eat more carbohydrate and still remain in ketosis.
Why is the first week so dramatic?
Because it is water, not fat. Glycogen stored in muscle and liver carries water with it, roughly 3 grams per gram of glycogen. Cut carbohydrate and glycogen depletes, taking the water with it — which can be 1 to 3 kilograms in the first week.
This is why keto feels like it works instantly, and it is the same reason the weight bounces straight back the moment carbohydrate returns. Neither has anything to do with fat.
Does keto burn more fat than other diets?
With the variables controlled, no. The cleanest test is Hall and colleagues (2016), who kept 17 men in a metabolic ward where everything was measured, then switched them from high-carbohydrate to ketogenic with calories and protein held constant. Daily energy expenditure rose slightly, by about 57 calories — but the rate of body fat loss actually slowed during the ketogenic phase.
In free-living studies where people choose their own intake, keto often wins slightly in the short term because it makes them eat less. The difference tends to disappear by twelve months.
So who does it work for?
Keto's real advantage is fullness and simplicity. Removing an entire category of food means not deciding meal by meal, which many people find easier than counting. And high-fat, high-protein meals do keep people full longer at the same calories.
Beyond weight, it has a clear medical use in drug-resistant epilepsy in children, where it has been used since the 1920s. In type 2 diabetes there is evidence of good short-term glucose control and medication reduction in some people — but that has to be done under supervision, because the medication has to change with it.
What are the common side effects?
Keto flu in the first week — headache, fatigue, irritability, poor concentration — mostly from water and electrolytes leaving together. Drinking enough and getting enough salt helps considerably.
Constipation, from the fibre lost when grains and fruit go. Rising LDL in some people, sometimes substantially, particularly in lean and very active individuals — which is why a lipid panel is worth doing if you intend to stay on it. And falling adherence, which shows up in nearly every long-term study.
Who should not do it?
Anyone with type 1 diabetes without medical supervision. Anyone taking an SGLT2 inhibitor, because of the risk of euglycaemic ketoacidosis, which is dangerous and hard to recognise. Pregnant and breastfeeding women. Anyone with pancreatic disease, certain liver conditions, or an inherited disorder of fat metabolism. And anyone who has had their gallbladder removed, who will find large amounts of fat harder to digest.
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