Rapid weight loss diet is a type of diet in which you lose more than 2 pounds (1 kilogram, kg) a week over several weeks. To lose weight this quickly you eat very few calories.
How It Works
These diets are most often chosen by people with obesity who want to lose weight quickly. These diets are less commonly recommended by health care providers. People on these diets should be followed closely by a provider. Rapid weight loss may not be safe for some people to do on their own.
These diets are only to be used for a short time and are usually not recommended for more than several weeks. The types of rapid weight loss diets are described below.
People who lose weight very quickly are much more likely to regain the weight over time than people who lose weight slowly through less drastic diet changes and physical activity. The weight loss is a bigger stress for the body, and the hormonal response to the weight loss is much stronger. The hormonal response is one of the reasons that weight loss slows down over time and also why weight gain occurs when the diet is stopped or relaxed.
Very Low-Calorie Diet (VLCD)
On a VLCD, you may have as few as 800 calories a day and may lose up to 3 to 5 pounds (1.5 to 2 kg) week. Most VLCDs use meal replacements, such as formulas, soups, shakes, and bars instead of regular meals. This helps ensure that you get all of the nutrients you need each day.
A VLCD is only recommended for adults who have obesity and need to lose weight for health reasons. These diets are often used before weight-loss surgery. You should only use a VLCD with the help of your provider. Most experts do not recommend using a VLCD for more than 12 weeks.
Low-Calorie Diet (LCD)
These diets usually allow about 1,000 to 1,200 calories a day for women and 1,200 to 1,600 calories a day for men. An LCD is a better choice than a VLCD for most people who want to lose weight quickly. But you should still be supervised by a provider. You will not lose weight as fast with an LCD, but you can lose just as much weight with a VLCD.
An LCD may use a mix of meal replacements and regular food. This makes it easier to follow than a VLCD.
This diet strategy is becoming more popular. It is often compared to fasting, but the two strategies are slightly different. Time-restricted eating limits the number of hours per day that you can eat. A popular strategy is the 16:8. For this diet, you have to eat all of your meals during an 8 hour period, for example, 10 am to 6 pm. The rest of the time you cannot eat anything. There are some studies that this method can cause rapid weight loss, but there is little information so far about whether the weight loss is sustained.
Fasting is an ancient form of caloric restriction. It has become more popular recently. This is partly because some animal and human studies have shown benefits to fasting for people with diabetes and obesity. There are many different fasting regimens and it is unclear which may be the best. One of the most popular is the 5:2 system. This involves 2 days a week of fasting or VLCD and 5 days a week of eating your normal diet. Diets that incorporate fasting can cause rapid weight loss.
Some fad diets also severely limit calories to achieve rapid weight loss. In some cases, these diets are not safe. In most cases, these diets are not sustainable for long enough to cause long-term weight loss. Once you stop the diet, you are at risk for regaining the weight if you return to your old eating habits. For most people, it is safest to choose a diet in which you lose a 1/2 pound to 1 pound (225 grams to 500 grams) a week.
The Role of Exercise
Rapid weight loss is more about cutting calories than exercising. Talk with your provider about what type of exercise you should do while you are on this type of diet. Your provider may suggest waiting until you are on a more long-term diet to start exercising.
Rapid weight loss diet is usually for people who have health problems because of obesity. For these people, losing a lot of weight quickly can help improve:
- High cholesterol
- High blood pressure
Possible Health Concerns
You should only follow one of these diets with the help of your provider. Losing more than 1 or 2 pounds (0.5 to 1 kg) a week is not safe for most people. It can cause you to lose muscle, water, and bone density. Rapid weight loss can also cause some side effects including:
People who lose weight quickly are also more likely to gain back the weight quickly. This can lead to other health problems.
In general, a rapid weight loss diet is not safe for children. It may also not be safe for teens, pregnant women or older adults unless a provider recommends it.
If you have a health condition, it is a good idea to talk with your provider before starting this or any diet plan to lose weight.
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Academy of Nutrition and Dietetics website. 4 ways low-calorie diets can sabotage your health. www.eatright.org/health/wellness/weight-and-body-positivity/4-ways-low-calorie-diets-can-sabotage-your-health. Updated January 24, 2022. Accessed January 11, 2023.
Academy of Nutrition and Dietetics website. Staying away from fad diets. www.eatright.org/health/wellness/diet-trends/staying-away-from-fad-diets. Updated April 13, 2021. Accessed January 11, 2023.
Flier EM. Obesity. In: Melmed S, Auchus RJ, Goldfine AB, Koenig RJ, Rosen CJ, eds. Williams Textbook of Endocrinology. 14th ed. Philadelphia, PA: Elsevier; 2020:chap 40.
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Review Date 4/29/2022
Updated by: Sandeep K. Dhaliwal, MD, board-certified in Diabetes, Endocrinology, and Metabolism, Springfield, VA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.