How a Low-Carb Diet Puts Your Body Into Ketosis

November 5, 2025 •

4 min read

According to the Cleveland Clinic, if you eat between 20 and 50 grams of carbohydrates each day, it will typically take two to four days to enter ketosis. Ketosis is a natural metabolic state where your body uses fat-derived ketones for energy instead of glucose. This profound metabolic shift is triggered primarily by a significant reduction in carbohydrate intake.

Quick Summary

Ketosis is a metabolic state achieved by restricting carbohydrates, which depletes glucose stores and lowers insulin. This forces the body to break down fat for energy, producing ketones. Factors like protein intake, exercise, and metabolism influence the transition time, which typically takes a few days.

Key Points

Carb Restriction is Key: Consuming fewer than 50 grams of carbohydrates daily is the primary trigger that forces the body to switch from glucose to fat as its main fuel source.
Glycogen Depletion: Before ketosis begins, the body must first use up its stored glucose, known as glycogen, which typically takes a few days.
Hormonal Shift: Lowered glucose levels reduce insulin, which signals the body to release fatty acids from fat stores. Glucagon then promotes the liver's production of ketones.
Ketone Production: The liver converts fatty acids into ketone bodies (beta-hydroxybutyrate, acetoacetate, and acetone) that serve as an alternative energy source for the body and brain.
Monitor Protein Intake: Eating too much protein can be converted into glucose, potentially hindering ketosis, so moderate protein intake is important.
Expect Transition Symptoms: Many people experience a temporary 'keto flu' with symptoms like fatigue and headaches as their body adapts to the new fuel source.
Ketosis vs. Ketoacidosis: Ketosis is a normal, non-pathological metabolic state, while ketoacidosis is a dangerous, life-threatening condition, often associated with uncontrolled diabetes.

The Science Behind Entering a Ketogenic State

Ketosis is a metabolic process that occurs when your body's primary fuel source shifts from carbohydrates (glucose) to fat. This change is initiated by restricting carbohydrate intake to below a certain threshold, typically less than 50 grams per day. In the absence of glucose, the body undergoes a series of metabolic adaptations to survive, which involves producing ketones for fuel.

Depleting Glycogen Stores

When you consume carbohydrates, your body breaks them down into glucose, which is then either used for immediate energy or stored in the liver and muscles as glycogen. When you drastically cut your carb intake, your body's glycogen reserves are quickly depleted. This process can take anywhere from a couple of days to over a week, depending on factors like your activity level and previous diet. Regular exercise can accelerate this process by helping to burn through remaining glycogen stores more quickly. As the body exhausts its glycogen, it needs a new source of energy.

Hormonal Changes and Fat Mobilization

Once glucose levels drop, the hormone insulin decreases significantly, while levels of other hormones like glucagon and cortisol rise. Insulin typically signals the body to store fat, so its reduction is a key signal for fat release. This drop in insulin activates hormone-sensitive lipase, an enzyme that breaks down triglycerides in fat cells, releasing fatty acids into the bloodstream. These fatty acids then travel to the liver.

Ketogenesis in the Liver

In the liver, fatty acids are converted into ketone bodies through a process called ketogenesis. The primary ketone bodies produced are beta-hydroxybutyrate, acetoacetate, and acetone. The liver cannot use these ketones for its own energy needs, so it releases them into the bloodstream. From there, these ketones can be used by various tissues throughout the body, including the brain, which adapts to using ketones as a primary fuel source when glucose is scarce. This is the state known as nutritional ketosis.

The Importance of Macronutrient Ratios

To effectively initiate and maintain ketosis, a precise balance of macronutrients is required, not just a simple reduction in carbs. Excessively high protein intake can inhibit ketosis. The body can convert excess protein into glucose via a process called gluconeogenesis, which can prevent the switch to fat-burning. This is why most ketogenic diets emphasize high fat, moderate protein, and very low carbohydrate intake.

High Fat Intake: Provides the necessary fuel for ketone production. Healthy fat sources include avocados, olive oil, nuts, and seeds. Coconut oil, which contains medium-chain triglycerides (MCTs), is particularly effective at boosting ketone levels rapidly.
Moderate Protein Intake: Supplies amino acids for essential functions and for gluconeogenesis to fuel glucose-dependent cells, without providing so much that it interferes with ketosis.
Very Low Carbohydrate Intake: The most crucial factor, forcing the body to seek alternative fuel sources. A typical threshold is 20–50 grams of net carbs per day.

What to Expect During the Transition (The 'Keto Flu')

During the initial phase of transitioning into ketosis, many people experience a collection of temporary symptoms known as the 'keto flu'. This is not a formal medical condition but a common side effect of the body's adaptation. These symptoms, which usually last for a few days to a week, include:

Headaches
Fatigue and lethargy
Brain fog
Irritability
Nausea
Constipation
Muscle cramps
Increased thirst

These symptoms are often linked to electrolyte imbalances and dehydration, which occur as the body loses water during glycogen depletion. Staying hydrated and supplementing with electrolytes like sodium, potassium, and magnesium can help mitigate these effects.

Comparison of Metabolic States


Feature	Glucose-Dependent State	Ketogenic State	Diabetic Ketoacidosis (DKA)
Primary Fuel Source	Glucose from carbohydrates	Ketones from fat	Glucose & high ketones
Carbohydrate Intake	High, variable	Very low (20-50g/day)	Varies (often due to illness)
Insulin Levels	Normal to high	Low and stable	Extremely low or absent
Blood Ketone Levels	Low (<0.5 mmol/L)	Moderate (0.5-3.0 mmol/L)	Very high (>10 mmol/L)
Blood pH	Normal	Normal	Dangerously acidic
Condition	Normal metabolism	Therapeutic metabolism	Medical emergency

Authoritative Reference

For an in-depth, clinical perspective on ketosis and its applications, the NCBI Bookshelf provides a comprehensive article on the biochemistry of ketogenesis.

Conclusion

Achieving ketosis is a deliberate metabolic shift initiated by significantly reducing carbohydrate intake and moderating protein, which forces the body to transition from burning glucose to fat for energy. This process, regulated by hormones like insulin and glucagon, involves depleting glycogen stores and prompting the liver to produce ketones. While the initial transition may involve temporary 'keto flu' symptoms, a balanced high-fat, low-carb diet is the most reliable way to enter this metabolic state. Monitoring ketone levels and staying hydrated can help manage the process effectively. Consulting a healthcare professional before starting is always recommended, especially for individuals with underlying health conditions.

Frequently Asked Questions

The fastest way to get into ketosis is a combination of severe carbohydrate restriction (under 20 grams per day), moderate protein, and increased physical activity to deplete glucose stores more quickly.

For most people, it takes about two to four days to enter ketosis when following a very low-carbohydrate diet, though this can vary based on individual metabolism and prior diet.

Yes, exercise helps accelerate the transition into ketosis by depleting the body's glycogen stores, forcing the body to turn to fat for fuel sooner.

The 'keto flu' is a common side effect during the initial days of ketosis, characterized by fatigue, headaches, and irritability. It's caused by the body adjusting to a new fuel source and electrolyte imbalances due to water loss.

Yes, you can measure ketone levels using urine strips, a blood ketone meter (most accurate), or a breathalyzer that measures acetone.

Excessive protein can be converted into glucose through gluconeogenesis, which can raise blood sugar and inhibit ketone production. Moderate protein provides necessary amino acids without interfering with the metabolic shift.

Insulin and glucagon are key regulators. Low carb intake reduces insulin, which prompts the release of fatty acids. Glucagon then signals the liver to produce ketones from these fatty acids.