Obesity, weight loss and very low-calorie diets (VLCDs)

Traditional weight loss methods include low- calorie diets that allow between 800 to 1,500 kcal a day and encourage regular exercise. However, an alternative method that moderately to severely obese people may consider for significant, short-term weight loss is a very low-calorie diet (VLCD).

Many very low-calorie diets are commercially prepared formulas of 800 kcal or less that replace all usual food intake. Others, such as the grapefruit diet (also called the Hollywood Diet), rely on eating a lot of the same low-calorie foods. Very low-calorie diets are not the same as over-the-counter meal replacements, which are substituted for one or two meals a day.

How effective are very low-calorie diets?

A very low-calorie diet may allow a severely to moderately obese person to lose about 1.4 to 2.3 kilograms (three to five pounds) per week, for an average total weight loss of 20 kilograms (44 pounds - more than three stone) over 12 weeks. Such a weight loss can improve obesity-related medical conditions, including diabetes, high blood pressure, and high cholesterol.

However, very low-calorie diets are no more effective than more modest dietary restrictions in the long-term maintenance of reduced weight. Health experts in the UK offer these guidelines: 

Consider a very low-calorie diet only after the failure of conventional diets.

Ensure they provide a minimum of 450 kcal/day for women, 500 kcal/day for men.

Very low-calorie diets may be suitable to follow in hospital before an operation, but are no more successful over the longer term.

Drawbacks of very low-calorie diets

To be healthy, we need a balance of foods from different food groups. It's quite difficult to get good nutrition in as few as 800 kcal (even a dietician would have trouble doing this), especially if one eats the same foods day after day.

Potential adverse effects of a very low calorie diet may include hair loss, fatigue and constipation, as well as some potentially more serious effects such as irregular heart rhythms and osteoporosis.

Also, once you go off the diet, you will be likely to put the weight back on unless you change your lifestyle and commit to healthy eating and regular physical activity.

By sticking to a long-term commitment, you can prevent your weight from drifting back up the scale.