Will 800 calories a day put you in ketosis?

October 11, 2025 •

4 min read

According to scientific literature, consuming fewer than 800 calories per day, especially with low carbohydrate intake, can indeed lead to a state of ketosis. This metabolic state is characterized by the body using fat stores for energy instead of glucose. However, a very low-calorie diet of 800 calories a day is not a safe or sustainable approach for most people and must only be undertaken under strict medical supervision.

Quick Summary

A very low-calorie diet of 800 calories or less will likely induce ketosis by restricting carbohydrates and depleting glucose reserves. This approach is not recommended for general use due to numerous health risks, including nutrient deficiencies, muscle loss, and metabolic slowdown. Ketosis can be achieved more safely and sustainably through moderate carbohydrate restriction alongside a balanced intake of protein and healthy fats.

Key Points

Ketosis can be induced by 800 calories a day: A very low-calorie diet creates a severe caloric deficit that forces the body into a state of ketosis by burning fat for fuel.
The 800-calorie approach is a VLCD, not a standard keto diet: VLCDs are based on extreme calorie restriction, whereas standard ketogenic diets focus on carbohydrate restriction with adequate fat intake.
Significant health risks are associated with VLCDs: Dangers include nutrient deficiencies, metabolic damage, loss of muscle mass, electrolyte imbalances, and the formation of gallstones.
Medical supervision is mandatory for a VLCD: An 800-calorie diet should only be followed under the strict guidance of a healthcare professional, especially for those with severe obesity or pre-surgery needs.
Weight regain is common after VLCDs: The rapid weight loss often reverses once the diet is stopped, as the body's metabolism has been negatively impacted.
Safer and more sustainable methods exist to achieve ketosis: A standard ketogenic diet, intermittent fasting, and increased exercise are healthier alternatives for entering ketosis.
Prioritize balanced nutrition over extreme calorie cuts: A focus on moderate carbohydrate restriction and balanced macronutrients is a safer and more sustainable path to achieving and maintaining ketosis.

Understanding How Ketosis Is Achieved

Ketosis is a metabolic state where the body burns fat for fuel, producing molecules called ketones. This happens when glucose, its primary energy source, is limited due to a low-carbohydrate intake. The liver breaks down fat into ketones, which can then be used by the brain, heart, and muscles for energy.

There are two primary pathways to initiate ketosis: severe carbohydrate restriction, as seen in a standard ketogenic diet (typically limiting carbs to 20–50 grams per day), and severe caloric restriction, such as in a very low-calorie diet (VLCD). Since an 800-calorie-a-day diet is a form of VLCD, it will force the body to deplete its glycogen stores and begin producing ketones to power bodily functions. However, the approach and long-term implications of using a VLCD versus a standard keto diet are vastly different.

The Science Behind VLCD-Induced Ketosis

A very low-calorie diet (VLCD), by definition, provides fewer than 800 kcal per day. The severe energy deficit created by this intake is functionally similar to fasting. When the body receives inadequate calories, it quickly uses up its stored glycogen. With these carbohydrate reserves depleted, the liver escalates fat breakdown to produce ketones for energy, initiating ketosis. This process is different from a typical ketogenic diet, which achieves ketosis primarily by limiting carbohydrates while maintaining an adequate—often high—intake of healthy fats.

Potential Dangers of an 800-Calorie Diet

While an 800-calorie diet can put you in ketosis, it is fraught with risks and should not be attempted without medical oversight.

Gallstone Formation: Rapid weight loss, a hallmark of VLCDs, significantly increases the risk of developing gallstones.
Nutrient Deficiencies: Severe calorie restriction makes it extremely difficult to obtain all necessary vitamins, minerals, and essential fatty acids, potentially leading to malnutrition.
Metabolic Slowdown: In response to perceived starvation, the body slows its metabolism to conserve energy. This can make future weight management more difficult and lead to rapid weight regain.
Loss of Muscle Mass: The body may break down lean muscle tissue for energy in addition to fat, especially if protein intake is insufficient.
Electrolyte Imbalances: Ketone body excretion in urine can cause a loss of electrolytes like sodium and potassium, leading to potentially fatal heart rhythm abnormalities.
Fatigue and Dizziness: The low energy intake can cause severe fatigue, headaches, and light-headedness.

This is why such diets are typically reserved for patients with severe obesity or those preparing for bariatric surgery, where the benefits of rapid weight loss under strict medical observation may outweigh the risks.

Safer Alternatives to Achieve Ketosis

For those seeking the benefits of ketosis without the severe risks of an 800-calorie diet, safer and more sustainable alternatives exist.

Nutritional Ketosis with a Standard Keto Diet: Limiting carbohydrate intake to under 50 grams per day while consuming adequate fat and protein is the most common way to achieve and maintain ketosis safely.
Intermittent Fasting: Periods of fasting can help deplete glycogen stores and push the body toward ketosis more quickly when combined with a low-carb eating plan.
Increase Healthy Fat Intake: Consuming more healthy fats, such as MCT oil, can aid in ketone production and help you feel more satiated, making a lower-carb diet easier to follow.
Regular Exercise: Moderate to high-intensity exercise helps burn through glycogen stores, encouraging the body to shift to fat for fuel and accelerate the onset of ketosis.

Comparison of VLCD (800 Calorie Diet) and Standard Ketogenic Diet


Feature	800-Calorie (VLCD) Ketogenic Diet	Standard Ketogenic Diet
Caloric Intake	Extremely low, under 800 kcal/day.	Normocaloric or moderately hypocaloric, tailored to individual needs.
Primary Driver of Ketosis	Severe caloric deficit and low carbs.	Strict carbohydrate restriction, usually under 50g daily.
Medical Supervision	Required due to high health risks.	Not typically required for healthy individuals, but medical advice is recommended.
Duration	Short-term (e.g., 2–12 weeks), not sustainable long-term.	Can be followed long-term with proper planning and monitoring.
Risks	High: gallstones, metabolic damage, muscle loss, nutrient deficiency, electrolyte imbalance.	Moderate (especially initially): keto flu, constipation. Risks can be mitigated.
Sustainability	Very low due to severe restriction and side effects.	Good, especially with a focus on nutrient-dense foods.
Effectiveness for Weight Loss	Rapid initial weight loss, but often followed by significant regain.	Effective and sustainable weight loss when followed correctly.

What the Experts Say

Medical and nutritional experts are unanimous in their caution regarding VLCDs. As highlighted by ScienceDirect and other health resources, VLCDs are not recommended for routine weight loss and should only be undertaken with intensive medical supervision for specific populations, such as those with severe obesity or prior to bariatric surgery. Dietitians emphasize that the weight lost is often regained rapidly and that a slower, more sustainable approach is more effective for long-term success. A standard, well-formulated ketogenic diet, by contrast, focuses on macronutrient balance rather than extreme caloric deprivation, offering a safer and more durable path to nutritional ketosis.

Conclusion: Is 800 Calories the Right Path to Ketosis?

Yes, consuming 800 calories a day can technically put you in ketosis, but it is a dangerous and unsustainable method. This severe calorie restriction, part of a Very Low-Calorie Diet (VLCD), forces the body into a starvation-like state that triggers ketone production. However, the numerous health risks—including muscle loss, metabolic damage, and severe nutrient deficiencies—far outweigh the benefits for most people. For safe and long-term results, a standard ketogenic diet that focuses on balanced macronutrient intake is the recommended approach. Always consult a healthcare professional before starting any extreme diet plan. For more in-depth information on safely entering and maintaining ketosis, consult authoritative sources like the National Institutes of Health.

Frequently Asked Questions

No, it is not safe for most people to follow an 800-calorie diet without medical supervision. This is a very low-calorie diet (VLCD) that can lead to severe health problems, including nutrient deficiencies, metabolic slowdown, and heart problems.

Due to the drastic caloric restriction, an 800-calorie diet can force the body into ketosis relatively quickly, often within a few days, by rapidly depleting glucose stores.

A VLCD, like an 800-calorie diet, primarily relies on extreme caloric restriction to induce ketosis. A standard ketogenic diet, by contrast, relies on severe carbohydrate restriction while providing adequate calories from fat and protein.

While rapid weight loss occurs on an 800-calorie diet, a significant amount of this loss is likely to be lean muscle mass and water, not just fat. A key aspect of VLCDs is the risk of muscle atrophy.

Common side effects of a VLCD include fatigue, headaches, constipation, nausea, dizziness, and gallstone formation. More serious risks, such as electrolyte imbalance and heart issues, can also arise.

No, VLCDs are generally not effective for long-term weight loss. While they produce rapid initial results, studies show that most people regain a significant portion of the weight lost, often due to a slowed metabolism.

A safer way to enter ketosis is by following a standard ketogenic diet that restricts carbohydrates to under 50 grams per day while maintaining a balanced intake of fats and proteins. Incorporating exercise and intermittent fasting can also accelerate the process safely.