The ketogenic diet was originally developed as a medical treatment for epilepsy in the 1920s, and it remains a frontline therapy for drug-resistant seizures today. But over the past decade, it has expanded well beyond neurology. The diet now has meaningful evidence behind it for people managing type 2 diabetes, significant excess weight, fatty liver disease, and polycystic ovary syndrome. It also has clear limitations and real risks for certain populations.
People With Drug-Resistant Epilepsy
This is the oldest and most established use of the ketogenic diet. For people whose seizures don’t respond to two or more medications, keto is a recognized treatment option at any age. More than 70% of patients with pharmacoresistant epilepsy show a positive response to the diet. In children eating very low carbohydrate (around 10 grams daily), 65% achieved at least a 50% reduction in seizures, 35% improved by 90% or more, and some became seizure-free within six months.
Adults benefit too, though the results are more variable. In studies of adults with refractory epilepsy aged 15 to 86, seizure reduction of 50% or more occurred in 20% to 70% of patients, and full seizure freedom was achieved in 7% to 30%. Modified versions of the diet, including lower-glycemic approaches, have also shown strong results: in one group of patients aged 5 to 34, half achieved a 90% or greater reduction in seizures after about 20 weeks.
The ketogenic diet is also considered the gold standard treatment for two rare metabolic conditions: GLUT-1 deficiency syndrome (where the brain can’t properly use glucose for fuel) and pyruvate dehydrogenase deficiency. For these patients, the diet isn’t optional or experimental. It’s the primary therapy.
People With Type 2 Diabetes
The ketogenic diet can produce substantial improvements in blood sugar control. Across multiple studies, HbA1c (a measure of average blood sugar over roughly three months) dropped significantly. In one study, HbA1c fell from 8.9% to 5.6% after 90 days. In longer trials, reductions from 7.6% to 6.3% held at one year, and from 7.5% to 5.9% at 15 months. Most of the improvement tends to appear within the first 70 days.
Perhaps more striking is what happens to medications. In one trial, 60% of participants were able to stop certain diabetes drugs entirely, while none in the comparison group could. A larger study documented that overall diabetes medication prescriptions (excluding metformin) dropped from 57% to 30% at one year. Insulin therapy was reduced or stopped in 94% of users, and all participants on sulfonylureas discontinued them. In another study, 8 out of 11 participants stopped insulin after 15 months. Some participants even achieved remission, defined as an HbA1c below 5.7% with no medications.
When compared head-to-head with conventional low-fat, calorie-restricted diets, the ketogenic approach produced greater HbA1c reductions over 12 months, though some longer comparisons show the gap narrowing over time.
People Looking to Lose Weight
For weight loss, the ketogenic diet works, particularly in the first six months. In a randomized trial comparing a low-carb ketogenic diet to a moderate-carb, calorie-restricted, low-fat diet, the keto group lost more weight at both six months and twelve months. The keto group went from an average of about 100 kg to 92 kg over a year, while the low-fat group went from roughly 97.5 kg to 95.8 kg. A meta-analysis of studies in people with type 2 diabetes found that keto produced about 2.9 kg more weight loss than standard diets at three months and 2.8 kg more at six months. By 12 months, however, the difference in body weight between diets often becomes smaller.
This pattern is worth understanding. Keto tends to produce faster early results, which can be motivating. But the long-term advantage over other well-followed diets is modest. The diet that works best is the one you can sustain, and keto’s restrictiveness makes adherence a genuine challenge for many people.
People With Fatty Liver Disease
Non-alcoholic fatty liver disease affects roughly a quarter of the global population, and the ketogenic diet shows a notably fast effect on liver fat. In a study published in the Proceedings of the National Academy of Sciences, participants with fatty liver saw a 31% reduction in liver fat content in just six days on a ketogenic diet. Body weight dropped only about 3% in that same period, meaning the liver fat reduction wasn’t simply a byproduct of weight loss. Insulin resistance in the liver also fell by 58%. These rapid changes suggest the diet has a direct metabolic effect on how the liver processes and stores fat.
Women With PCOS
Polycystic ovary syndrome is closely tied to insulin resistance, which makes it a logical target for a diet that dramatically lowers insulin levels. A systematic review and meta-analysis found that the ketogenic diet reduced insulin resistance, lowered luteinizing hormone levels (a hormone that’s typically elevated in PCOS and disrupts ovulation), and improved weight in women with a BMI over 25. Compared to other dietary approaches, keto showed superior effects on both insulin sensitivity and reproductive hormone profiles, though its effects on androgen levels and cholesterol were less consistent.
Endurance Athletes
A specific subset of athletes may benefit from keto-adaptation: those in long-duration endurance sports. The core advantage is a dramatically increased ability to burn fat for fuel. Endurance runners adapted to a high-fat, low-carb diet achieved peak fat-burning rates of 1.6 grams per minute, more than double the 0.7 grams per minute seen in equally fit runners eating a traditional high-carb diet. Some keto-adapted athletes burned fat at rates above 1.85 grams per minute, even at high exercise intensities (above 85% of their maximum oxygen capacity).
This matters for ultra-endurance events where glycogen depletion is a limiting factor. By relying more on fat (an essentially unlimited fuel source), these athletes can sustain effort longer without needing as much mid-race fueling. For power and sprint athletes, however, the story is different. High-intensity, short-burst performance depends on carbohydrates, and keto typically impairs it.
Early Evidence in Cognitive Decline
There is growing interest in ketogenic diets for neurodegenerative conditions, but the evidence is far less mature than for epilepsy or diabetes. A systematic review rated the evidence using the American Academy of Neurology’s criteria and found it “probably effective” for cognitive improvement in people with mild cognitive impairment and in people with mild-to-moderate Alzheimer’s disease who don’t carry a specific genetic risk factor (the APOE4 gene variant). For those who do carry that variant, the evidence was rated “unproven.” In Parkinson’s disease, the diet was rated “possibly effective” for non-motor symptoms like mood and cognition, but “unproven” for motor symptoms like tremor and rigidity. These are preliminary findings, not treatment recommendations.
Who Should Not Follow a Keto Diet
Several rare but serious metabolic conditions make the ketogenic diet dangerous. People with carnitine deficiency, fatty acid oxidation disorders, or pyruvate carboxylase deficiency cannot properly produce ketones. On a ketogenic diet, their bodies would attempt to mobilize fat for energy but fail to complete the process, potentially leading to severe low blood sugar, coma, or death. People with acute intermittent porphyria should also avoid the diet, as carbohydrate restriction is a known trigger for flare-ups of that condition.
Beyond these absolute contraindications, the diet raises legitimate concerns for cardiovascular health. A meta-analysis of studies in normal-weight adults found that the ketogenic diet significantly increased LDL cholesterol by about 1.08 mmol/L (roughly 42 mg/dL) and total cholesterol by 1.47 mmol/L (about 57 mg/dL). HDL cholesterol also rose modestly, by 0.35 mmol/L, and triglycerides did not change significantly. The LDL increase is large enough to warrant monitoring, particularly for anyone with existing heart disease risk factors or a family history of cardiovascular problems.
Common Side Effects in the First Weeks
Even for people who are good candidates, the transition period is rough for many. The so-called “keto flu” typically hits in the first one to two weeks and can include headaches, fatigue, nausea, dizziness, and irritability. The most commonly reported remedies among people who’ve experienced it are increasing sodium intake, followed by supplementing magnesium and potassium. The underlying issue is that a sharp drop in carbohydrates causes the kidneys to excrete more water and electrolytes. Proactively salting food generously and eating potassium-rich foods (like avocado and leafy greens) can reduce or prevent these symptoms in most people.