The keto diet is a very low carbohydrate diet (under 50 g per day) and high in fat, designed to force the body to produce ketone bodies from fat rather than energy from glucose. Popularized for weight loss over the past decade, it was originally developed in the 1920s to treat drug resistant epilepsy in children. Five effects on metabolism and weight now have solid scientific evidence behind them.
In brief – The keto diet is an eating pattern where fat provides 70 to 80% of calories and carbohydrates less than 10%, which puts the body into ketosis: the liver converts fatty acids into ketone bodies used as fuel by the brain and muscles. Clinical trials show greater initial weight loss than a low fat diet (Bueno et al. 2013), reduced appetite linked to suppressed ghrelin (Sumithran et al. 2013), improved triglycerides and HDL, lower HbA1c in people with type 2 diabetes under medical supervision (Hallberg et al. 2018), and effects that persist at two years in people who stay on the diet. These benefits remain conditional on medical supervision, especially for anyone on diabetes medication.
Definition: what is the keto diet?
It restricts carbohydrates to 20-50 g per day, or about 5 to 10% of calorie intake, compared with 45 to 55% in a typical diet. Fat covers 70 to 80% of calories and protein 15 to 20%. This breakdown deprives the body of its usual fuel, glucose, and forces it to switch to an alternative energy metabolism based on fat.
This diet is not simply a low sugar diet. The carbohydrate restriction must be strict and sustained enough to deplete liver glycogen stores within 24 to 72 hours and trigger ketosis, a metabolic state measurable through blood levels of beta hydroxybutyrate.
How the keto diet affects metabolism
Once liver glycogen is depleted, the liver increases lipolysis and converts free fatty acids into ketone bodies: acetoacetate, beta hydroxybutyrate and acetone. These molecules cross the blood brain barrier and become the brain’s main energy source, since it cannot normally use fatty acids directly.
- Liver and muscle glycogen depleted within 1 to 3 days.
- Increased lipolysis and release of free fatty acids into the blood.
- Hepatic ketogenesis: production of acetoacetate and beta hydroxybutyrate.
- Brain and muscle use of ketone bodies as an alternative fuel.
- Lower insulin secretion, the hormone that normally promotes fat storage.
This sustained drop in circulating insulin explains much of the observed metabolic effects: less fat storage, more mobilization of stored fat, and insulin sensitivity that can improve in people with insulin resistance.
What the science shows: 5 proven effects
Several controlled trials and meta-analyses have measured the effects of the diet on weight and metabolism. Here are the five best documented results, with their sources.
1. Greater weight loss than a low fat diet, in the short and medium term. A meta-analysis of 13 randomized trials in overweight adults found greater weight loss and a larger drop in diastolic blood pressure with the keto diet than with a low fat diet, with follow up of up to two years (Bueno et al., 2013, British Journal of Nutrition). The authors note, however, that the gap, while statistically significant, remains clinically modest.
2. Appetite suppression via ghrelin. In 39 adults losing weight followed for 8 weeks, the rise in ghrelin (the hunger hormone) that normally accompanies weight loss was blunted as long as participants stayed in ketosis, with lower subjective appetite scores than after carbohydrates were reintroduced (Sumithran et al., 2013, European Journal of Clinical Nutrition). This mechanism explains why some people report fewer cravings while in ketosis.
3. Improved triglycerides and HDL. A systematic review of 62 randomized trials calculated an average triglyceride drop of 19.96 mg/dL and an HDL increase of 3.61 mg/dL (Chang et al., 2026, Endocrine Practice). LDL rose by an average of 8.49 mg/dL in the same analysis, a point the authors consider still poorly assessed due to a lack of long term cardiovascular data.
4. Lower HbA1c and reduced medication needs in people with type 2 diabetes. In a controlled trial of 262 participants followed for one year with continuous medical supervision, insulin therapy was reduced or stopped in 94% of users, and sulfonylureas were discontinued entirely in the treatment group (Hallberg et al., 2018, Diabetes Therapy). These treatment adjustments were made by a medical team, not through self-management.
5. Benefits that persist at two years in people who stay on the program. A two year follow up of the same cohort shows that gains in HbA1c and weight persist among participants who remained in the program, with 83% still being followed at one year (Athinarayanan et al., 2019, Frontiers in Endocrinology). The gradual dropout over time illustrates the real difficulty of maintaining such strict carbohydrate restriction long term.
In practice: who it suits and in what context
This diet is one of the nutritional approaches linked to the deregulated nutrient sensing pillar, which covers levers that act on glucose and fat metabolism. It mainly suits a short term weight loss goal or a specific medical approach to type 2 diabetes, always under supervision. It is one of the levers studied for insulin resistance, alongside regular readings of fasting blood glucose. Some people use MCT oil to help enter ketosis without cutting carbohydrates as drastically.
The table below compares the keto diet with two other eating patterns commonly recommended for prevention.
| Criterion | Keto diet | Mediterranean diet | Low fat diet |
|---|---|---|---|
| Share of carbohydrates | 5-10% | 40-50% | 50-60% |
| Weight loss at 6 months | Fast, often greater | Gradual | Gradual |
| Lipid profile | Triglycerides down, HDL up, LDL sometimes up | Triglycerides down, LDL down | LDL down, triglycerides little changed |
| Long term adherence | Difficult, high dropout | Good, solid mortality data | Moderate |
| Recommended supervision | Medical, especially if on treatment | Not systematic | Not systematic |
This comparison echoes the findings on the Mediterranean diet, whose decades of longevity data remain stronger today for long term use.
Protocol and precautions
The transition to the keto diet often comes with a few days of fatigue, headaches and irritability, sometimes called the keto flu, linked to initial fluid and sodium loss. Drinking enough water and slightly increasing salt intake usually eases these symptoms.
- Spread fat across varied sources (olive oil, fatty fish, nuts) rather than relying only on saturated fat.
- Monitor kidney function and lipid levels before and during the diet, especially beyond a few weeks.
- Adjust any diabetes or blood pressure medication only with a doctor, never alone.
- Reintroduce carbohydrates gradually at the end of the program to limit weight regain, a phenomenon known as the yoyo effect.
Harvard’s Nutrition Source notes that very low carbohydrate diets such as the keto diet are highly unbalanced in the short term, with a risk of urinary sodium loss, dehydration and vitamin deficiencies when fruit and vegetables are excluded, and it recommends not following them for more than a few weeks without medical advice (Harvard T.H. Chan School of Public Health, The Nutrition Source). It is contraindicated during pregnancy and in cases of pancreatitis, kidney disease or liver disease, and should be avoided by people with type 1 diabetes because of the risk of ketoacidosis.
Frequently asked questions
How long does it take to enter ketosis?
Ketosis generally sets in between 24 and 72 hours after starting a strict carbohydrate restriction, once liver glycogen stores are depleted. The timing varies with physical activity, muscle mass and residual carbohydrate intake. A blood or urine test for beta hydroxybutyrate confirms ketosis more reliably than a subjective feeling.
Does the keto diet burn more fat than a standard diet?
Over periods of several months, comparative trials show slightly greater weight loss with this diet than with a low fat diet, but the gap remains modest once adjusted for total calorie intake. Part of the rapid initial weight loss reflects water bound to glycogen, not fat mass alone.
What are the cholesterol risks of the keto diet?
It generally improves triglycerides and HDL, but raises LDL in some of the people who follow it. Data on the long term cardiovascular consequences of this LDL increase remain limited, which justifies monitoring lipid levels during the diet, especially beyond a few months.
Can you follow a keto diet with type 2 diabetes?
Supervised clinical trials show lower HbA1c and reduced medication doses in people with type 2 diabetes under medical follow up. This approach should never be started alone, since poorly adjusted ongoing treatment (insulin, sulfonylureas) creates a risk of hypoglycemia. Close follow up with the treating physician is essential before any severe carbohydrate restriction.
Why do people regain weight after a keto diet?
Weight regain after stopping this diet is mainly due to the reintroduction of carbohydrates, which restores glycogen stores and the water bound to them, and to the difficulty of maintaining such strict restriction over time. Studies show a gradual dropout of participants over the months, a sign that long term adherence, more than the metabolic mechanism itself, is the main obstacle.
Medical disclaimer. The keto diet is a restrictive eating pattern that profoundly changes metabolism: it should not be started without medical advice, especially in cases of diabetes, ongoing treatment, pregnancy, or kidney, liver or pancreatic disease. The information provided here is for informational purposes only and does not constitute medical advice. It does not replace a consultation. Ask a healthcare professional before changing your diet, taking dietary supplements or starting a new practice, especially if you have a medical condition, are pregnant or are under treatment. Dietary supplements do not replace a balanced diet or medical follow-up.
