What is the Keto Diet
The ketogenic (keto) diet is a very low-carbohydrate, high-fat eating approach that shifts the body into a metabolic state called ketosis, where fat becomes the primary fuel source instead of glucose from carbohydrates.
The ketogenic diet (commonly called keto) is a dietary approach characterized by very low carbohydrate intake (typically under 20–50 grams per day), high fat intake (60–75% of calories), and moderate protein (15–30%). By severely restricting carbohydrates, the body depletes its glycogen (stored glucose) reserves and shifts into a metabolic state called ketosis — in which the liver converts fat into ketone bodies (acetoacetate, beta-hydroxybutyrate, acetone), which become the primary fuel source for the brain and muscles.
The keto diet was originally developed in the 1920s as a treatment for drug-resistant epilepsy in children, where it remains clinically used today. Its popularity as a weight loss and metabolic health intervention exploded in the 2010s and has remained extraordinarily high.
Ketosis: How It Works
Under normal conditions, the body preferentially burns glucose (from carbohydrates) for energy. Glucose is readily available, easily converted to ATP, and tightly regulated by insulin. When carbohydrate intake drops below the threshold needed to sustain normal glucose levels (~20–50g/day depending on the individual), the body:
- Depletes liver glycogen stores (within 24–48 hours of carb restriction)
- Increases fat mobilization from adipose tissue
- The liver converts fatty acids into ketone bodies
- The brain and muscles adapt to using ketones as the primary fuel
The transition into ketosis typically takes 2–4 days of strict carbohydrate restriction. Blood ketone levels can be measured via breath meters, urine strips (less accurate), or blood ketone meters.
Standard Keto Macronutrient Breakdown
For a 2,000-calorie diet:
| Macronutrient | % of Calories | Grams |
|---|---|---|
| Fat | 70–75% | 155–167g |
| Protein | 20–25% | 100–125g |
| Carbohydrates | 5% | 25g |
Net carbs = total carbs minus fiber. Because fiber isn't absorbed, most keto practitioners track net carbs rather than total carbs, which allows for more vegetables and high-fiber foods.
What You Eat on Keto
Foods allowed:
- Meats, poultry, and fish
- Eggs
- High-fat dairy (cheese, butter, heavy cream)
- Non-starchy vegetables (leafy greens, broccoli, cauliflower, zucchini)
- Avocados, olive oil, coconut oil
- Nuts and seeds (in moderation)
- Berries (in small amounts)
Foods avoided:
- Bread, pasta, rice, and all grains
- Sugar and sugary beverages
- Most fruit (high in sugar)
- Starchy vegetables (potatoes, corn, beans)
- Processed high-carb foods
Evidence for Keto
Weight loss: The keto diet consistently produces short-term weight loss comparable to or slightly better than low-fat diets in head-to-head research. Much of the initial rapid weight loss (1–5 lbs in week one) is water weight from glycogen depletion — each gram of glycogen is stored with ~3g of water. Long-term adherence rates are similar to other restrictive diets.
Metabolic health: Beneficial effects on blood glucose, type 2 diabetes management, and triglyceride levels are well-documented. Some studies show reductions in HbA1c comparable to medication in type 2 diabetics.
Epilepsy: Robust clinical evidence for reduction in seizure frequency in drug-resistant epilepsy, particularly in children.
Athletic performance: Mixed evidence. Useful for endurance athletes who adapt over time; may impair performance in high-intensity activities that rely on glycolytic (carbohydrate-fueled) energy systems.
The Keto Flu
During the transition to ketosis (days 1–7), many people experience a cluster of symptoms called the keto flu:
- Fatigue and brain fog
- Headache
- Irritability
- Nausea
- Muscle cramps
These symptoms result primarily from electrolyte and fluid shifts as the kidneys excrete more sodium during glycogen depletion. Increasing salt intake, supplementing with electrolytes (sodium, potassium, magnesium), and staying well hydrated significantly reduces keto flu severity.
Potential Downsides and Risks
- Sustainability: Very restrictive — eating out, social meals, and food variety are challenging long-term
- Cholesterol: Some individuals (particularly LDL "hyperresponders") experience significant LDL cholesterol increases; lipid monitoring is advisable
- Kidney stones: Increased risk with prolonged ketosis, especially if uric acid levels rise
- Not appropriate for: Those with certain metabolic disorders, pancreatitis history, liver conditions, or who are pregnant without medical supervision
- Nutrient gaps: Limiting fruit, whole grains, and legumes can create deficiencies in B vitamins, fiber, and certain micronutrients
Keto vs. Other Low-Carb Approaches
| Diet | Carbs (daily) | Key Difference |
|---|---|---|
| Standard American diet | 200–300g | No restriction |
| Moderate low-carb | 100–150g | Reduced but not ketogenic |
| Low-carb (Atkins-style) | 50–100g | May partially support ketosis |
| Ketogenic | <20–50g | Sustained ketosis |
| Targeted/cyclical keto | Varies | Strategic carb refeeds around workouts |
For most people, the most sustainable diet for long-term health is the one they can consistently follow. With a caloric deficit being the primary driver of weight loss, keto is one effective tool — but not uniquely superior to other diets when protein intake and adherence are controlled.