Weight Control 101 - Looking into the Effects of Low Calorie Diet Plans

Low Calorie Diet

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The use of very low calorie diets to lose weight has a checkered history, which includes a long list of adverse health effects that range from losing one's hair to death from unbeatable heart irregularities. In addition, the effect of very low calorie diets on the body has not been completely defined. Some studies have been done on people who were not obese to start with, and many studies have been of short duration. This last point was one of the mistakes of the "Dr. Atkin's Diet Revolution," which claimed increased calorie loss if you were on a carbohydrate-restricted diet. The apparent benefits from a low-calorie, high-protein diet did not persist with diets of longer duration. It was proved that over the long term, the number of calories in the diet was what counted, not whether the calories were from protein or carbohydrate.

One of the continuing concerns about very low calorie diets has been the loss of body protein. That is un-desirable because it decreases active metabolic tissue, such as muscle, and that decreases the number of calories your body uses, even at rest. The end result of that situation is that you need fewer calories to maintain your body after a diet than you needed before you started the diet, which means you have to eat even less to avoid obesity. Worse still, protein can be lost from vital organs, such as the heart muscle.

To clarify what such a very low calorie diet really does in obese middle-aged women, investigators at Ohio State University, Columbus, studied 15 such women for 4-6 months (American Journal of Clinical Nutrition, 45: 1987, 381). They were on a diet of only 420 calories a day and 280 calories (70 grams) were from protein. The women stayed on the diet from 12.9 to 22.4 weeks (average 18.1 weeks). They each lost from 0.8 kg (1.76 pounds) to 1.6 kg (3-52 pounds) of bodyweight a week, an average of 1.1 kg (2.42 pounds) a week.

These were rather heavy women with an average of 46.8% of their bodyweight as fat before they started the diet. After the diet, the average percent of bodyweight as fat was 35.5%.

Body composition studies were surprising in that 83% of the bodyweight lost was fat and only 17% was from body protein. Previous studies have shown that as much as a third of the weight loss on such very low calorie diets is from protein. This difference in studies may be caused by the length of time the subjects were studied. There is a sharp loss of body protein the first two days of fasting. The longer studies, like this one, may result in less protein loss and more bodyfat loss.

Another interesting point was how the fat loss affected the body. The investigators tried to determine which measurements best reflected the loss of bodyfat - various skin-fold measurements or others. They found that measuring the circumference of the trunk was the best indicator of body-fat loss. That should be interesting to overweight women because it means that fat loss will be reflected in a slimmer abdomen, a desirable goal of many. But it must be pointed out that this study was on obese women and men may respond differently. Men tend to develop fat in the abdominal area and a high percentage of it is in-side the abdominal cavity. When worn- en develop a large abdomen, a higher portion is apt to be under the skin rather than inside the abdominal cavity. Differences in the types of bodyfat cells involved determine the location of fat deposits.

This study does not mean that very low calorie diets are safe. Certainly we cannot recommend any such diets that are not under careful and adequate medical supervision. Another aspect of such diets and how they affect body composition is the amount of exercise a person does, which doesn't seem to be considered in this study. Exercise helps to prevent loss of muscle. That is why starting a good walking program and an exercise routine that uses all the different body muscles is an important addition to a diet that severely restricts calories. Such an exercise program can help avoid body protein loss and make the final results of any calorie-restriction program much more successful in the long term.


One of the most disconcerting facts about calorie restriction to lose weight is that the longer a person stays on such a diet, the less effective it is in causing loss of bodyfat. Eventually, the body does such a remarkable job of adjusting to a lower energy intake that bodyfat loss literally stops. That is a great advantage to many people in the world who must subsist on a low calorie intake.

Why does this happen if weight loss is dependent on calories consumed? Because the other side of weight loss is how many calories the body uses. Two factor's have been cited as a cause for the slowing of weight loss while staying on a diet. One is the loss of body protein from muscle. That can significantly decrease the calories the body uses.

The other explanation is a decrease in metabolism caused by changes in the thyroid hormone. The latter is often expressed as the "thyroid shutting down," which is not quite correct. Thyroid hormone produced by the thyroid gland is called T-4. When the T-4 is measured as part of your blood test on a medical examination, that test is done to measure your thyroid function. But the story doesn't end there. T-4 refers to the amount of iodine in the thyroid hormone. Your liver and kidneys act on T-4 to form the active thyroid hormone, called T-3 because it contains less iodine. This is the hormone that really acts on your cells to promote metabolism. The problem is further complicated by the fact that a major portion of thyroid hormone is bound to proteins in the blood. Only the unbound, free T-3 actually acts on your cells. The theory of the change in thyroid hormone function during dieting is that your liver and kidneys stop converting so much T-4 to T-3. Instead, they form an inactive thyroid hormone, called reverse T-3, that will not stimulate your cells. During severe dieting, the level of T-4 may not change, but there is a decrease in T-3 and an increase in reverse T-3. At the same time, there is a decrease in the resting energy requirements (resting metabolism).

The changes in metabolism associated with a very low calorie diet in the 15 women in the study from Ohio State University were also evaluated. One of the important features of this study was the long-term diet, lasting 12.9 to 22.4 weeks. Previous studies of metabolism have been short term.

As with other studies, the T-4 level remained unchanged during the diet. The T-3 level did decrease. The reverse T-3 level also increased for the first five weeks and then started to return to base line, and at 10 weeks it was back to the same level as before the diet began. The resting energy requirements also decreased by around 21%, from an average of 1,692 calories to 1,334 calories a day. By studying energy requirements, nonfat bodyweight (lean body mass) and the thyroid functions, the investigators concluded that the decrease in T-3 hormone was not the primary factor influencing the resting energy requirements. In fact, giving supplemental T-3 did not prevent the decline in resting energy requirements.

It is interesting that these subjects were all studied on a relatively high-protein diet, which means a low-carbohydrate diet. There is evidence that carbohydrate restriction may cause the decrease in T-3 levels and that a low-calorie diet that is based on carbohydrates may prevent such a change. More study is needed in this area. It also suggests again that for the long term, a high-protein, very-low-calorie diet may not be as effective as one that contains more carbohydrate.

The decrease in bodyweight decreased the energy requirements for the day's activities and the decrease in lean bodyweight decreased the resting energy requirements of the tissues. But the energy requirements per pound of lean body mass were also decreased. That has to mean that some other factor or factors affected the energy requirements of the cells. What this other influence might be is not explained by the study. However, it is well to remember that energy loss is primarily through loss of calories as heat from the skin. This mechanism is controlled by the brain. The investigators' observations can be explained by changes in loss of calories through the skin, by the brain's control over blood flow through the skin and other mechanisms of heat loss or heat conservation. The brain plays an essential role in controlling energy requirements for the metabolism, the loss or conservation of calories and the mechanisms involved in loss or gain of bodyfat.

The way to stimulate the body to lose calories while on a diet, whether it is a low-carbohydrate diet, or some other means of restricting calories, is to use exercise. Exercise stimulates the brain's heat-loss mechanism and can make a diet program continue to work after it has ceased to be so effective by itself. The Ohio State University research indirectly points to the value of a sensible exercise program as part of any bodyfat loss program.

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