At Verywell, we believe there is no one-size-fits-all approach to a healthy lifestyle. Successful eating plans need to be individualized and take the whole person into consideration. Prior to starting a new diet plan, consult with your healthcare provider or a registered dietitian, especially if you have an underlying health condition.
What Is a Very Low-Calorie Diet?
A very low-calorie diet (VLCD) is a rapid weight-loss program in which calories are severely restricted, often to 800 calories or fewer. A calorie level this low may be considered semistarvation or a crash diet. This diet is dangerous unless you are under medical supervision for reasons that necessitate weight loss, such as surgery. It was developed in the 1970s for patients whose body mass index (BMI) is 30 or higher—people who need to lose weight quickly because of the health consequences of obesity.
Body Mass Index (BMI) is a dated, biased measure that doesn’t account for several factors, such as body composition, ethnicity, race, gender, and age. Despite being a flawed measure, BMI is widely used today in the medical community because it is an inexpensive and quick method for analyzing potential health status and outcomes.
These diets are often used to help obese patients achieve significant, short-term weight loss as part of a comprehensive weight-loss program. Because food intake is so limited and calories are restricted to about 800 a day, very low-calorie diets should only be followed in certain cases and must be supervised by a doctor.
For example, very low-calorie diets are not normally used for patients with a BMI between 27 and 30 unless they have medical conditions related to their weight, such as diabetes or high blood pressure. These diets are not usually prescribed for children or teens.
They are also not usually considered appropriate for older people due to potential side effects, pre-existing medical conditions, or medication needs. Your physician will decide whether a very low-calorie diet is right for you. The very low-calorie diet is not a long-term solution as it is too low in energy and nutrition.
What Experts Say
"A very low-calorie diet is a medically supervised plan eliciting rapid weight loss in those with a high BMI. Because patients eat 800 calories or less per day, it should only be done under a doctor’s supervision and paired with specialty foods to prevent nutrient deficiencies."
—Chrissy Carroll, RD, MPH
Very Low-Calorie Diet: What You Can Eat
In most cases, people following a very low-calorie diet consume liquid protein shakes or meal-replacement bars in place of food for a designated period, ranging from several weeks to several months. However, some very low-calorie diet plans include lean proteins, such as fish and chicken, or limit intake to one type of food (a type of mono diet).
Very Low-Calorie Diet: What You Can't Eat
The very low-calorie is based on limiting calories and not calorie-dense foods. For very low-calorie diets that rely on meal replacements only, you will not eat anything other than those products.
Very Low-Calorie Diet: Preparation & Tips
Your doctor-prescribed very low-calorie diet may suggest when to consume meal replacements and other foods outside of the replacement items (if any; often, you do not eat anything other than prescribed foods).
You may be required to purchase meal replacement products if that is part of your prescribed diet. Bars and shakes used in a very low-calorie diet are not the same as diet products at the grocery store. Instead, meal replacements are specially formulated to contain adequate vitamins and nutrients, so patients' nutritional requirements are met.
Pros of a Very Low-Calorie Diet
A very low-calorie diet can be a good choice for specific people and situations. It is almost always prescribed for medical reasons to help manage obesity and related conditions.
- Efficacy: Doctors often prescribe very low-calorie diets when other eating plans and weight-loss attempts have failed. Studies show these diets work well, at least in the short term. However, they need to be followed carefully and have a behavior component that teaches patients how to change their eating habits for the long term.
- General nutrition: Because the meal replacements in very low-calorie diets are specially prepared for this purpose, they help patients get the essential nutrients they are not otherwise getting from food. This is why medical supervision is key to the safety of a very low-calorie diet.
- Improved body composition: A greater rate of weight loss in a shorter period can result in physical improvements, including fat loss, waist circumference, the relative amount of lean mass compared to body fat, nutritional status, and walking stride.
- Reduced side effects of some conditions: A very low-calorie diet can improve symptoms in patients with diabetes, psoriasis, osteoarthritis, and obstructive sleep apnea. Obesity may cause symptoms and side effects with these conditions, so weight loss helps alleviate them.
- Preparation for surgery: In some cases, preliminary weight loss is important before bariatric surgery. A very low-calorie diet can help people with obesity lose weight to prepare for surgery.
Cons of a Very Low-Calorie Diet
Due to the severe calorie restriction in a very low-calorie diet, there are health risks. Healthcare practitioners will weigh these against the potential benefits of the diet.
- Side effects: Many patients who follow a very low-calorie diet for four to 16 weeks experience side effects such as headaches, weakness, fatigue, nausea and vomiting, constipation, dehydration, low blood sugar, bad breath, diarrhea, and dizziness. These symptoms usually improve within a few weeks. Hair loss is a potential long-term side effect of very low-calorie diet.
- Medical supervision: To safely follow a very low-calorie diet, you must have a prescription and be monitored by a physician, which will require time and money. Typically, health insurance does not cover the cost of meal replacements (of course, you will not be buying any groceries or restaurant meals during the very low-calorie diet).
- Sustainability: This diet is a short-term solution for weight loss, meant to be followed for a limited time. After that, patients should transition to a maintenance plan that includes healthy eating, exercise, and other lifestyle changes.
- Lack of nutrients: The nature of a very low-calorie diet means it can be challenging to obtain enough nutrients. One negative result of the diet is the potential for bone density loss due to low calcium levels.
- Gallstones: Gallstones often develop in people who are obese, especially in women. They are even more common during rapid weight loss. Your healthcare provider may be able to prescribe medication to prevent gallstone formation during rapid weight loss.
Is a Very Low-Calorie Diet Right For You?
Very low-calorie diets are unique in that they are medically supervised and eliminate all foods except for specially formulated meal replacements. Some other diets take aspects of this diet and modify it for general use.
The USDA recommends a diet of about 2000 calories (this can vary based on age, gender, weight, and activity level) for weight maintenance and reducing calories depending on individual health and lifestyle factors for weight loss. The very low-calorie diet reduces intake severely, down to 800 calories or less, and must be overseen by a doctor.
A very low-calorie diet should only be followed if prescribed to you by your doctor. Your doctor will let you know if the benefits of a very low-calorie diet outweigh the potential risks in your specific case. Do not attempt this diet without discussing it with your doctor and committing to expert supervision.
A Word From Verywell
Many people who go on very low-calorie diets rebound and binge eat when they get too hungry. It is possible to regain weight and even put on extra weight, making it a good idea to follow diets or weight-loss programs that provide 900 calories a day or more. You'll see many plans advertised in magazines and online, some with healthy claims attached. Without proper nutrition, you are likely to tire easily and develop or exacerbate health problems.
A doctor-supervised very low-calorie diet can offer nutrition, at least, although it is likely to be tough to stick with. Still, in certain cases, a doctor may recommend a very low-calorie diet as the best way to lose weight in the short term. It should be followed with behavioral changes and a healthy lifestyle.
Remember, this or any diet may not be necessary for you, and many diets out there don’t work, especially long-term. While we do not endorse fad diet trends or unsustainable weight-loss methods, we present the facts so you can make an informed decision that works best for your nutritional needs, genetic blueprint, budget, and goals.
If your goal is weight loss, remember that losing weight isn’t necessarily the same as being your healthiest self, and there are many other ways to pursue health. Exercise, sleep, and other lifestyle factors play a major role in your overall health. The best diet is always the one that is balanced and fits your lifestyle.