What are meal replacement diets? They are a relatively new way of losing weight, and they involve substituting the meals you already eat with prepackaged food.

The replacement meals are carefully designed to be portion controlled, low-fat, low-calorie and nutritionally balanced. They are easy to use, since you don't need to count calories, or worry about what foods to eat – this is all done for you. Usually, the replacement meals provide about 800 to 1000 calories a day.

When you go on a meal replacement diet, the meals you use will come in a packet which you then prepare, and can be soups, stews, chili, food bars, puddings and shakes.

The idea of a meal replacement diet is that you are forced into a 'calorie deficit' – your body burns its fat for the energy you need, and you get thinner. Your muscle mass stays the same.

The replacement meals and snacks are designed so that you don't feel the need to constantly nibble. This hunger reduction avoids the common drawback in other types of diets in which you feel weak and hungry. Replacement meals tend to be small, but taken more often during the day than 'normal' meals – often you will eat every 2-3 hours.

In addition, the most sophisticated meal replacement diets incorporate many kinds of snacks, like crackers, crisps and soup, and drinks like cappuccino, latte and cocoa.

Meal replacement diet products often avoid the use of sugar entirely, and use other sweeteners. The meals are are usually designed to be different for men and women, as the nutritional requirements for each sex are slightly different, and the best products have different meal options for very active people who do a lot of physical activity or sports. Usually there are vegetarian and diabetic options too.

So, how effective are meal replacement diets? In some cases, people can lose five pounds a week, though you are more likely to lose about two pounds a week.

One of the advantages of meal replacement diets is that, though you pay for the replacement meals, the cost of your food is often about the same as the cost of food you would be buying anyway. This is unlike medication or herbal based diets, where the costs are in addition to your normal food. Also, you don’t need to take extra vitamins or minerals while on a meal replacement diet, as your nutritional requirements are covered.

When you start on a meal replacement diet is often the most critical decision you can make. Much of your social life may revolve around meals or dining out, so you have to choose a time to begin your meal replacement diet when you are free of work-related or family events which involve food. Obviously, you cannot continue on the diet if you can’t prepare the replacement meals, so going on vacation to a hotel soon after you you start the diet should be avoided.

Most meal replacement systems recommend that, if you don't already exercise, you help your body by starting on light exercise a few weeks into the program. This can be as simple as walking a set distance once a day. Cycling and swimming are also good choices.

When you end the diet should be based on when you have reached your target weight, or when your weekly weight loss has tapered off.ep