A ketogenic or keto diet consists of high fat, moderate protein, and very low carbs. People adhering to a strict keto diet eat fewer than 50 grams (g) of carbs per day, forcing the body to start using fat-derived ketones for energy instead of glucose (sugar) derived from carbs. This state is called ketosis. Getting into ketosis takes anywhere from two to four days, on average. To enter ketosis, roughly 70–80% of daily calories should come from fat, less than 10% from protein, and only about 5% from carbohydrates or "carbs." Intermittent fasting before starting a keto diet can help speed up the process of getting into ketosis.
Read on to learn what you need to know about getting into ketosis, the timeline of ketone effects, what you can and cannot eat on a keto diet, signs you're in ketosis, ways to get into ketosis faster, and potential risks of staying on a strict keto diet for extended periods.
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Getting Into Ketosis: Timeline of Ketone Effects
The human body's metabolism uses glucose for energy (glycolysis) before it uses fat. Therefore, sugar molecules from carbohydrates that are stored in the body as glycogen must be depleted before ketosis starts. Without stored glycogen (sugar) to burn as fuel, your body enters ketosis as it gradually switches from using glucose for energy to using fat as fuel.
There isn't a one-size-fits-all formula for determining exactly how many daily carbs make the difference between entering ketosis or not. That said, eating fewer than 50 grams of carbs a day is generally considered a ketone-producing diet. Full keto-adaptation, in which your body makes fat its preferred source of fuel during physical activity, can take a few weeks or even months.
The ketosis timeline can vary greatly depending on various factors, such as:
- The amount of glycogen stored in reserve when someone starts a keto diet
- Age (younger people get into ketosis faster)
- Body weight
- Exercise habits
- Basal metabolic rate
Certain illnesses that affect the thyroid (hypothyroidism) can slow metabolism and influence how long it takes to get into ketosis. Ketogenic diets may induce a shift in how the thyroid functions.
Older adults and people with health issues that slow their metabolic rate typically take longer to get into ketosis. Always speak to a healthcare provider before starting a ketogenic diet, especially if you have underlying medical conditions.
The precise timeline for fat loss on a keto diet isn't set in stone and varies from person to person. During the first 24-hour window and the next few days on a keto diet, most weight loss is from water, not fat. Hypothetically, ketosis-induced fat loss kicks in when the body starts burning fat deposits for energy. There is mixed evidence and ongoing debate about the fat-burning superiority of keto diets compared to other weight loss plans.
Getting into ketosis and losing weight on a ketogenic diet is a complex metabolic process; more research is needed to fully understand how it works.
Seeing a Nutritionist
If you're thinking about starting a regimented keto diet at home, talk with a healthcare provider and consult with a registered dietitian (RD) or registered dietitian nutritionist (RDN) before going on a dayslong fast or making radical changes to the amount of carbs, fats, or protein you're eating every day.
Signs You're in Ketosis
Signs of ketosis range from flu-like symptoms to "keto breath." The easiest science-based way to know if you're in ketosis is a simple home test using a ketone urine strip. Any color (from pink to purple) on the ketone test strip indicates ketosis.
Symptoms of carbohydrate withdrawal that may be signs you're entering ketosis include:
- Accelerated weight loss
- Bad breath (halitosis)
- Changes in appetite
- Constipation or diarrhea
- Dehydration from body water loss
- Dizziness
- Fatigue
- Headache
- Irritability
- Gastrointestinal distress (stomach issues)
- Ketones in the bloodstream and urine
- Muscle cramps
- Muscle weakness
- Sluggishness
- Subpar exercise performance in the first few days/weeks
Is There a Way to Get Into Ketosis Faster?
Fasting for 12 hours is the fastest way to get into ketosis. When you're initiating a very low-carbohydrate ketogenic diet, the fewer carbs you eat in the first 24 hours and the next few days, the more quickly your glycogen reserves will get depleted and the faster you'll get into ketosis.
Medium-chain triglyceride (MCT) oil supplements may help you get into ketosis faster. In 2018, a study found that keto-induction was quicker with MCT supplementation. Within the first 24–72 hours of initiating a keto diet, the researchers found that when study participants got 80% of their calories from fat, 13-17% from protein, and 3-6% from carbohydrates—and also took MCT oil—they'd get into ketosis more rapidly than people on the same keto diet who didn't take supplemental MCT oil.
Ketosis and Diet: What's Safe and Possibly Dangerous?
It's difficult to answer the question, "Is ketosis dangerous?" with scientific certainty. Most studies on what's safe and possibly dangerous regarding ketosis and keto diets are of low quality and don't have large sample sizes.
Due to a lack of sufficient high-quality clinical evidence that shows keto diets to be safe and without potential danger, there are ongoing debates about whether the benefits of very low-carbohydrate ketogenic diets that are extremely high in unhealthy fats outweigh their risks.
In general, current research shows that being on a strict keto diet and in ketosis for about 8-12 weeks is safe. After 12 weeks, potential risks and possible dangers tend to increase. Although this topic remains controversial, accumulating evidence suggests that for most people, the long-term health risks of very low-carbohydrate ketogenic diets outweigh the advantages.
Long-term adverse effects of being in ketosis for extended periods may include:
- Micronutrient deficiencies (insufficient vitamins and minerals)
- Electrolyte abnormalities
- Poor cholesterol profiles (too much “bad” LDL cholesterol)
- Increased risk of heart disease
- Increased risk of chronic kidney disease
- Kidney stones (renal calculi)
- Hypoproteinemia (not enough protein in the blood)
- Decreased bone mineral density (osteoporosis)
- Copper-deficiency anemia
- Iron-deficiency anemia
- Ketoacidosis (acidic blood from too many ketones)
- Selenium-deficiency cardiomyopathy
- Hepatic steatosis (non-alcoholic fatty liver disease)
Research suggests that eating a low-carbohydrate ketogenic diet during pregnancy increases the risk of neural tube defects in newborns, even when taking folic acid supplements.
Ketogenic diets are usually high in red meat, processed meats, and saturated fats, which are associated with an increased risk of chronic kidney disease, heart disease, atherosclerosis (plaque buildup in arteries), cancer, diabetes, Alzheimer’s disease, and other illnesses. Additionally, keto diets are low in health-protective foods such as fruits, vegetables, legumes, and whole grains.
Summary
Ketosis is a metabolic state marked by your body using fat for fuel. When someone's not on a ketogenic diet, the sugar molecules (glucose) derived from carbohydrates are the body's preferred energy source. On a stringent low-carbohydrate keto diet, the body is starved of carbs, which forces it to turn fat into ketones, which are used as fuel instead of glucose.
Getting into ketosis takes about two to four days, on average, because that's how long it takes for glycogen reserves to be depleted after someone starts eating a keto diet with under 50 grams of carbs per day. Fasting, limiting carbs, and MCT oil supplements can speed up the ketosis timeline.
