What You Need to Know About Very Low-Calorie Diets

How they work, who they're for, and safety considerations

very low calorie diet
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A very low-calorie diet (VLCD) is a rapid weight-loss program where calories are severely restricted. VLCDs 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 to 900 a day, very low-calorie diets should only be followed in certain cases and under a doctor's supervision.

How They Work

VLCDs are designed to produce rapid weight loss at the beginning of a weight-loss program. In most cases, liquid protein shakes or meal-replacement bars are taken in place of food for a period of time ranging from several weeks to several months. However, some very low-calorie diet plans include lean proteins, such as fish and chicken.

The bars and shakes are not the same as the "diet" kind that you purchase in the grocery store. Rather, meal replacements are specially formulated to contain adequate vitamins and nutrients so patients' nutritional requirements are met.

Who They're For

Very low-calorie diets are intended for patients whose body mass index (BMI) is 30 or higher. VLC 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.

Very low-calorie 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, and/or medication needs.

Your physician will decide whether or not a very low calorie diet is appropriate for you.

Weight-Loss Expectations

An obese patient can expect to lose about 3 to 5 pounds per week while following a very low-calorie diet. The average weight loss for a 12-week VLCD is about 44 pounds. This amount of weight loss can significantly improve obesity-related medical conditions, such as diabetes, high blood pressure, and high cholesterol.

Within three to six months, a patient may be able to lose about 15 percent to 25 percent of their initial weight if they start with a very low-calorie diet and transition to a healthy lifestyle, a calorie-controlled eating plan, and exercise program.

Research has shown the long-term results of VLC diets vary significantly. Weight regain is common. Combining a VLC diet with behavior therapy, exercise, and follow-up treatment may prevent this.

VLC participants typically maintain a 5-percent weight loss after four years if they adopt a healthy eating and exercise plan.

Side Effects & Concerns

Many patients who follow a very low-calorie diet for four to 16 weeks experience side effects such as fatigue, constipation, nausea, or diarrhea. These symptoms usually improve within a few weeks and rarely prevent patients from completing the program.

The most common serious side effect of a very low-calorie diet is 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.

The 900-Calorie Diet

Some dieters who are not obese or who are slightly overweight may try to lose weight by eating 800 or 900 calories a day. While the plan may work in the short term, it is not likely to work over the long term. These diets are neither healthy, nor sustainable.

Many dieters who go on very low-calorie diet—like the 900-calorie diet—rebound and binge eat when they get too hungry. It is possible to regain any weight you lose and even put on extra weight as a result.

For these reasons, it's generally not a good idea to follow fad diets or trendy weight-loss programs that provide only 900 calories a day or less. You'll see many plans advertised in magazines and online, some with healthy claims attached to them. But without proper nutrition, you are likely to get tired and develop or exacerbate health problems.

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