What happens when we completely restrict calories to lose weight for a day or two?
 |
If we completely fast for a day or two, weight loss would certainly be rapid and this fact is encouraging for “crash dieters.” However, the composition of the weight loss may not be as expected. As much as 60 to 70 percent of that weight loss might be attributable to water loss. Meanwhile, much of the remaining weight loss would be carbohydrate, and to a lesser degree, fat and protein. Keep in mind that glycogen stores bind water. As mentioned earlier, scientists estimate that every gram of glycogen sponges about three grams of water. So during that fasting period when liver glycogen is broken down for energy, water will move out of liver cells into our blood, circulate to our kidneys, and be urinated out. This process makes the scale go down rapidly as the loss of a ½ of glycogen would lead to about 2 pounds of total weight loss. |
What happens if we continue to fast for longer periods?
As the fast continues beyond a day or two, liver glycogen is no longer a major energy storage resource. Body fat breakdown is in high gear and becomes the major fuel source. Keep in mind that because all cells in the body have at least a minimal need for glucose at all times, our liver will need to generate some glucose. Amino acids become the major resource for this process. Most of the amino acids will be derived from skeletal muscle protein at first. Thus, with severe energy restriction you can certainly count on burning body fat, but you will also lose body protein (i.e., muscle mass). This is usually not what we want!
How much body protein would we lose during fasting?
Even though your body would be fueled mostly by fat during prolonged fasting, protein would still make a remarkable contribution to your weight loss. The reason lies in the energy density differences between fat and protein. Consider this example: if a man has been fasting for five days, on the fifth day he might be deriving about 75 percent of his energy from body fat and the remainder from protein. If he expended 2,000 calories that day, then 1,500 calories would have come from fat and 500 calories from body protein. If we calculate the mass (weight) of the fat and protein used it would be roughly 165 grams of fat and 125 grams of protein. That’s roughly 1/3 pound of fat and a ¼ pound of protein. Some weight loss from water would be expected due to its association to lost protein.
If starvation were to endure for even longer, less body protein would be broken down on a daily basis and used as energy. This happens for a couple of reasons. First, our brain would require lesser amounts of glucose as it adapts to use more ketone bodies. As we discussed in Carbs/Protein/Fat, ketone bodies are made in our liver during periods of high fat utilization. This is a survival mechanism serving to reduce the rate of loss of body protein. During prolonged starvation, the cause of death is usually related to body protein loss. Amazingly, our brain can replace about half of its glucose requirement with ketone bodies after a week or so of complete starvation. Second, during prolonged energy restriction the thyroid gland may release less and less thyroid hormone. This slows our RMR and in turn decreases the requirement for protein breakdown.
What happens to our body composition during semi-starvation?
During extended periods when our energy intake is mildly to moderately restricted and most of the diet energy is derived from carbohydrate and protein, the composition of the weight loss would be different than during complete starvation. Since glycogen stores would be partially restored in response to meals, this would lead to less reliance upon the breakdown of our body protein. Furthermore, our diet will also provide protein to replace some of the amino acids used for energy.
Insulin would promote the rebuilding of body protein, especially muscle, as well as liver and muscle glycogen. Contrary to the complete fast (zero energy) there would not be the early rapid weight loss that is attributable mostly to water. The weight loss experienced during extended periods of a mild to moderate energy restriction will largely be a mixture of fat, some protein, and a little water. However, the relative fat to protein contribution to energy expenditure would be much more favorable versus complete fasting. In addition, resistance training and eating more protein will also help minimize body protein loss.
Some moderate energy-restricted diet plans (1,000 to 1,200 Calories) include protein levels that well exceed the RDA. This design is believed to help spare body protein during weight loss. The reason is that the diet protein-derived amino acids can be used for glucose production, thus sparing some body protein from breakdown. Furthermore, if the energy restriction is also limited in carbohydrate (as popular today), amino acids can also stimulate the release of insulin, although to a much lesser degree than carbohydrate. Insulin will help move amino acids into skeletal muscle and dampen the protein breakdown processed. Further still, the branched chain amino acids, particularly leucine, plays a direct role in promoting muscle protein manufacturing.
Can we lose only fat during weight loss?
When body weight is reduced, we must expect some obligatory loss in protein, water, and minerals. This only makes sense because these nutrients were important to a person before the weight loss. Even though fat tissue is composed of about 86 percent fat, when fat cells expand more of the other nutrients are needed to support the new size and metabolism of the larger cells and tissue. For instance, cell membranes of fat cells must expand and more enzymes may be needed. Furthermore, new fat cells may have been made during the accumulation of body fat. On the contrary, when fat is mobilized from fat cells, these cells shrink, thereby decreasing the need for the extra supporting nutrients. Also, when the body was heavier, the amount of skeletal muscle and density of the bones may have been a little greater to support and move the larger body. Researchers usually find that heavier people have denser bones. Thus, as body weight decreases, it is only reasonable that these areas will decrease as well. Excessive skin and some connective tissue would be broken down during weight loss as well; both of these tissues are protein rich.
If you incorporate resistance training in your efforts to change your body composition, it certainly is possible to lose primarily body fat. Here, the maintenance of body protein, minerals, and water may be necessary as you enhance your muscle mass. In fact, it is possible that you might not even lose weight as you lose body fat. This might be indicative for people who are slightly overweight compared to those who are obese.
Recent Comments