How long do you think you could survive without eating at all? You can fast about a month for every 20 pounds of stored body fat. An average, normal weight adult can fast for about 60 days before they have used up all their fat stores and they starve to death. In the 1970s, under the care of his doctor, a man that initially weighed 455 pounds fasted for 382 days and lost 277 pounds.
During prolonged fasting and starvation, your brain is able to gradually change from eating only glucose and begin using another energy source called ketone bodies. Your liver makes ketone bodies out of fatty acids. This change in your brain from using glucose to using ketone bodies for energy is gradual over many days. By the time you have fasted for a week, your brain can get about 7 out of 10 calories that it needs from ketone bodies, but still needs about 3 out of 10 calories from glucose. After 2 to 4 weeks, your brain has fully adapted to starvation and gets about 9 out of 10 of its calories from ketone bodies. But because your brain still requires about 1 out of 10 of its calories from glucose, your body will still break down and eat some of your muscles to feed your brain.
An extreme form of dieting is called the protein-sparing very low calorie diet. It is sometimes also called a protein-sparing modified fast. People on this diet eat only about 500 calories of food per day. If you are fasting, your body will break down your own muscles to make glucose for your brain. If you are trying to lose weight, it is important to preserve your muscles, because muscles efficiently burn fat and if you have more muscle mass you will have a higher resting metabolic rate. To preserve your muscles, you need about 500 calories per day of carbohydrate or protein to supply your brain with the glucose it needs and prevent the breakdown of your own muscle tissue.
People on this diet also take vitamin and mineral supplements – including extra salt and potassium - every day. They are also told to drink about 2 liters per day of water. It is important if you decide to undertake an extreme diet like this that you have medical supervision.
People who strictly follow this diet, eating only 500 calories of protein per day, lose about 5 to 10 pounds (2-5 kg) in the first week, partly due to loss of water when they use up stored glycogen. Then they lose about 2 to 4 pounds per week (1-2 kg) for women and 3 to 6 pounds (1.5-2.5 kg) per week for men. Heavier people lose weight faster because of their higher resting metabolic rate. The usual amount of weight lost is about 15 to 20 pounds (7-10 kg) in one month, 45 pounds (20 kg) in 3 months, and 70 to 90 pounds (31-41 kg) after 6 months.
This is some very impressive weight loss. It takes a very motivated person to undertake an extreme diet like this. But as you can see, if you eat very few calories, you will definitely lose weight. So why do we not recommend a diet like this more often? The answer is that it doesn’t work well in the long term. When the diet ends, people return to their old habits and eating patterns. When we go back to our prior way of eating, we go back to our previous body weight.
Within about six months of completing a very low calorie diet, more than half of the patients have regained more than half of the weight they had lost. Within a year, people are still usually at a lower weight than they would have been if they had never dieted, but they are not at a lower weight than people who followed a more normal, less extreme diet plan.
A better goal for a low calorie diet is to eat between about 800 and 1500 calories per day. On average Americans eat about 2,700 calories per day. According to the Dietary Guidelines for Americans, most moderately active women who are not trying to lose weight need about 2000 calories per day, and men need about 2600 calories per day. On a low calorie diet, women aim for a goal of eating 1000 to 1200 calories per day. For men the goal is 1200 to 1500 calories per day. This is about the same number of calories per day as the average person living in poor areas of central Africa eats. If you are able to eat this few calories every day, you will lose weight.
During prolonged fasting and starvation, your brain is able to gradually change from eating only glucose and begin using another energy source called ketone bodies. Your liver makes ketone bodies out of fatty acids. This change in your brain from using glucose to using ketone bodies for energy is gradual over many days. By the time you have fasted for a week, your brain can get about 7 out of 10 calories that it needs from ketone bodies, but still needs about 3 out of 10 calories from glucose. After 2 to 4 weeks, your brain has fully adapted to starvation and gets about 9 out of 10 of its calories from ketone bodies. But because your brain still requires about 1 out of 10 of its calories from glucose, your body will still break down and eat some of your muscles to feed your brain.
An extreme form of dieting is called the protein-sparing very low calorie diet. It is sometimes also called a protein-sparing modified fast. People on this diet eat only about 500 calories of food per day. If you are fasting, your body will break down your own muscles to make glucose for your brain. If you are trying to lose weight, it is important to preserve your muscles, because muscles efficiently burn fat and if you have more muscle mass you will have a higher resting metabolic rate. To preserve your muscles, you need about 500 calories per day of carbohydrate or protein to supply your brain with the glucose it needs and prevent the breakdown of your own muscle tissue.
People on this diet also take vitamin and mineral supplements – including extra salt and potassium - every day. They are also told to drink about 2 liters per day of water. It is important if you decide to undertake an extreme diet like this that you have medical supervision.
People who strictly follow this diet, eating only 500 calories of protein per day, lose about 5 to 10 pounds (2-5 kg) in the first week, partly due to loss of water when they use up stored glycogen. Then they lose about 2 to 4 pounds per week (1-2 kg) for women and 3 to 6 pounds (1.5-2.5 kg) per week for men. Heavier people lose weight faster because of their higher resting metabolic rate. The usual amount of weight lost is about 15 to 20 pounds (7-10 kg) in one month, 45 pounds (20 kg) in 3 months, and 70 to 90 pounds (31-41 kg) after 6 months.
This is some very impressive weight loss. It takes a very motivated person to undertake an extreme diet like this. But as you can see, if you eat very few calories, you will definitely lose weight. So why do we not recommend a diet like this more often? The answer is that it doesn’t work well in the long term. When the diet ends, people return to their old habits and eating patterns. When we go back to our prior way of eating, we go back to our previous body weight.
Within about six months of completing a very low calorie diet, more than half of the patients have regained more than half of the weight they had lost. Within a year, people are still usually at a lower weight than they would have been if they had never dieted, but they are not at a lower weight than people who followed a more normal, less extreme diet plan.
A better goal for a low calorie diet is to eat between about 800 and 1500 calories per day. On average Americans eat about 2,700 calories per day. According to the Dietary Guidelines for Americans, most moderately active women who are not trying to lose weight need about 2000 calories per day, and men need about 2600 calories per day. On a low calorie diet, women aim for a goal of eating 1000 to 1200 calories per day. For men the goal is 1200 to 1500 calories per day. This is about the same number of calories per day as the average person living in poor areas of central Africa eats. If you are able to eat this few calories every day, you will lose weight.
