This is for my coursework in health and fitness. How much weight can be lost in one week when eating 100 calories or less And how much weight can be lost in a week on a diet of 1000 calories?

An average person, at rest, will need about 60 calories per hour to stay alive (about 1,500 calories per day). A moderately active person will burn another 500 calories a day in his activities of daily living. So, on average, a person needs 2,000 calories a day.

Now, there is about 4,000 calories (1 gram of fat =9 calories, times 454 grams in a pound) to a pound of fat. For every 10 grams of fat lost, there is about a gram of water that it is lost with it. Thus, you really need to burn about 3,600 calories to lose one pound.

If you are eating only 100 calories a day, you would be losing 1,900 calories in a day, or 13,300 calories in a week, which is about 3 and 1/2 pounds of weight loss.

There is one more wrinkle to this, though. When you initially begin to lose weight, the body will first lose the stores of glycogen in the liver. The body stores about 2 pounds of glycogen. Glycogen loves water, and for every pound of glycogen, there is between 1-2 pounds (really 1/2 liter) of water with the glycogen. So, for every gram of glycogen lost, there is about 1 centiliter of water lost. Unlike fat, glycogen produces only 4 calories per gram. So, 2 pounds of glycogen produce only 3,600 calories (454 grams in 1 lb X 2 lb X 4 calories per gram). So, within 2 days, an ordinary person, having lost 3,600 calories, will lose 2 lb of glycogen, and 2 lb of water! Therefore, within the first 2 days, the person will have lost 4 lb of weight!

When you consider the above, then, on a 100 calorie a day diet, an ordinary person will lose about 6.5 lb of weight in the first week. Thereafter, he will lose about 3.5 lb per week.

Note that I have ignored other mechanisms such as the body converting metabolic systems into a starvation mode. Thus, for example, basal metabolic rate(BMR) decreases as the body enters a starvation mode, so that fewer calories are burned at rest when someone starves himself. As well, on 100 calories a day, a person will not get enough protein, forcing the body to break down muscle in order to sustain itself. Less muscle means lower BMR which again diminishes the calories lost.

Hope that helps.