The ketogenic diet is a high-fat, very-low-carbohydrate eating pattern used primarily for weight loss, blood sugar control, and managing drug-resistant epilepsy. It works by forcing your body to burn fat instead of carbohydrates for energy. While it started as a medical therapy in the 1920s for children with seizures, it has since gained widespread popularity for metabolic health conditions and general weight management.
How the Diet Works
When you eat fewer than about 20 to 50 grams of carbohydrates per day, your insulin levels drop and your body begins breaking down stored fat into molecules called ketone bodies. These ketone bodies, produced in the liver, replace glucose as your primary fuel source. Your brain, muscles, and other tissues can all run on them, and ketone metabolism can cover up to 80% of the brain’s energy needs.
The standard macronutrient breakdown calls for 70 to 80% of your daily calories from fat, 10 to 20% from protein, and just 5 to 10% from carbohydrates. For context, 5 to 10% of a 2,000-calorie diet is roughly 25 to 50 grams of carbs, which is less than what’s in a single bagel. This severe carbohydrate restriction is what distinguishes keto from other low-carb approaches and what triggers the metabolic shift into ketosis.
Weight Loss
Weight loss is the most common reason people try keto. When insulin levels fall, your body stops prioritizing fat storage and starts breaking down fat reserves for energy. This shift in fuel source, combined with the appetite-suppressing effect many people experience on high-fat diets, tends to create a calorie deficit without deliberate calorie counting.
Much of the early weight loss on keto is water. Carbohydrates cause your body to hold onto water, so cutting them leads to a rapid initial drop on the scale. Fat loss follows over the subsequent weeks, though long-term weight loss results vary and depend heavily on whether someone can sustain the diet’s restrictions.
Type 2 Diabetes and Blood Sugar Control
Keto has shown some of the strongest results for people with type 2 diabetes. Because the diet sharply reduces carbohydrate intake, blood sugar levels drop and the body needs less insulin to manage them. Clinical studies have documented significant reductions in HbA1c (a marker of average blood sugar over three months), with improvements ranging from 0.6% after just three weeks to sustained reductions from 7.5% down to 5.9% after 15 months.
The impact on medication use is striking. In one year-long study, insulin therapy was reduced or stopped entirely in 94% of participants using it, and all participants on certain oral diabetes medications were able to discontinue them. Another study found that 60% of participants could stop their diabetes drugs after adopting a ketogenic diet, compared with none in the control group. These medication changes happened under medical supervision, which is critical since reducing blood sugar medications without adjusting for the diet’s effects can cause dangerous drops in blood sugar.
Epilepsy
The ketogenic diet was originally developed as a treatment for epilepsy, and it remains a recognized therapy for children and adults whose seizures don’t respond to medication. It has been used in this role for over a century. The exact mechanism isn’t fully understood, but the presence of ketone bodies in the brain appears to have a stabilizing effect on neural activity. For people with drug-resistant epilepsy, keto is typically administered under close medical supervision with carefully controlled formulations to ensure nutritional adequacy.
PCOS and Hormonal Health
Polycystic ovary syndrome (PCOS) is closely linked to insulin resistance, which makes the insulin-lowering effects of keto particularly relevant. In a pilot study of overweight women with PCOS who followed a ketogenic diet for six months, fasting insulin dropped by 54%, free testosterone fell by 22%, and a key hormonal ratio linked to the condition improved by 36%. Participants also lost an average of 12% of their body weight. Two women in the study became pregnant despite previous infertility problems, and there were trends toward improvement in hair growth, menstrual regularity, and other PCOS symptoms.
The likely explanation is that lower insulin levels reduce the signal that drives the ovaries to overproduce androgens (male hormones like testosterone). This chain reaction, from fewer carbs to lower insulin to lower androgens, is what makes keto a plausible dietary approach for PCOS, though the research base is still small.
Brain Health and Neurological Conditions
Beyond epilepsy, researchers are investigating keto’s effects on neurodegenerative diseases like Alzheimer’s and Parkinson’s. Both conditions involve disrupted glucose metabolism in the brain, and ketone bodies offer an alternative energy source that damaged neurons may still be able to use. In Alzheimer’s patients, the presence of ketone bodies appears to improve blood flow in the brain. In Parkinson’s patients, early research suggests ketones may help reduce muscle tremor and stiffness while improving cognitive function.
This area of research is still in its early stages, with most evidence coming from small studies and animal models. But the underlying logic is sound: if the brain can’t use glucose efficiently due to disease, providing it with a backup fuel could slow the damage.
Short-Term Side Effects
Most people experience what’s commonly called “keto flu” during the first week or two. Symptoms include headaches, achiness, nausea, fatigue, muscle cramps, and constipation. These are largely caused by the sudden drop in carbohydrate intake and the loss of water and electrolytes that follows. When your body dumps water in response to falling insulin levels, it flushes sodium, potassium, and magnesium along with it.
Adding extra salt to food and replenishing electrolytes through supplementation or electrolyte-rich drinks can ease these symptoms significantly. The discomfort is temporary for most people and typically resolves within one to two weeks as the body adapts to burning fat for fuel.
Long-Term Risks and Concerns
The long-term safety profile of keto is less clear. Studies have reported that prolonged adherence can lead to elevated LDL cholesterol, high triglycerides, and bone mineral loss. The American Heart Association has noted that the ketogenic diet falls outside the acceptable macronutrient ranges it recommends and excludes most food sources of carbohydrates, which makes it difficult to meet certain nutrient needs over time.
The diet is also strictly off-limits for people with certain rare metabolic conditions. These include primary carnitine deficiency, several fatty acid oxidation disorders, pyruvate carboxylase deficiency, and porphyria. These conditions impair the body’s ability to process fat for energy, which means forcing the body into ketosis can be dangerous or even life-threatening.
For most people without these conditions, the practical challenge is sustainability. The extreme restriction of carbohydrates makes the diet difficult to maintain long term, and many of its benefits, particularly for blood sugar and weight, depend on continued adherence. Regaining weight after stopping keto is common, and the metabolic improvements in conditions like type 2 diabetes tend to appear quickly (often within the first 70 days) but require ongoing dietary commitment to maintain.