The first law of thermodynamics which states that energy can neither be created nor destroyed and that it is only transformed or transferred to its surroundings applies to the human body. This implies that the macronutrients taken in by the body can only either be used or stored. Energy taken in by the body must be equivalent to the body’s energy output if it is to maintain its weight. Energy taken in excess will necessarily be converted to a form of potential energy. In the human body, this potential energy takes the form of fat or adipose.
A higher energy input relative to energy output will naturally favor the formation of fat regardless of the energy source, be it carbohydrates, fat or protein. Such conversion is reflected by an increase in body mass and body weight. On the other hand, a higher energy output will favor the mobilization of energy stores which is reflected by a decrease in body weight (Whitney and Cataldo, 2002). These are the basics of weight control which, unfortunately are the extent of knowledge from which laymen derive their actions when managing their weights.
Nutrition education is after all, limited to dietary guidelines and simple tips that target the general population (Centers for Disease Control and Prevention, 2008). However, awareness, and even a rigid application of such oversimplified principles do not guarantee total control over one’s body as this subject pertains to a whole area of study with its own set of complications that must be fully understood if one is to aim total control over the body weight.
The body’s adaptive mechanisms as well as individual differences in physiology complicate the process of weight control and give birth to certain factors that must be considered in controlling weight (Whitney and Cataldo, 2002). Weight control is therefore not simply a matter of reducing or increasing intake and physical activity. It is a highly technical subject that requires understanding and knowledge of human physiology and the contingencies resulting from individual differences.
The differences in body weight and the rate of weight change are dependent, essentially on the resting metabolic rate. 70% of the body’s total energy expenditure comes from the resting metabolic rate. Only about 20% comes from physical activities while the rest comes from the thermal effect of food which includes all the energy expenditures required in digestion, from mastication in the mouth until the body completely metabolizes the food. In principle, it is this resting metabolic rate that adjusts itself depending on one’s physical activities and body composition.
Any change in the other components of the total energy expenditure, the physical activity and the thermal effect, is balanced by a subsequent adaptive response that manifests itself through the changes in resting metabolic rate (Whitney and Cataldo, 2002). A change in physical activities without any change in energy intake, for example, will result in a sudden increase or decrease in body weight until the resting metabolic rate adjusts itself to bring back the total expenditure and the body to its previous status.
One principle that must be noted in understanding weight control is that the magnitude of energy output does not depend on an individual’s body weight but more on an individual’s body composition. Cunningham (1991) made a synthetic review and confirmed the results of numerous studies regarding the positive relationship between energy expenditure and fat-free or lean body mass. A similar study done by Johnstone, et. al (2005) also confirms such relationship. There is a logarithmic relationship between fat and lean mass and a physiological contribution of lean body mass to weight loss in biliopancreatic diversion patients (Tacchino, 2003).
A negative relationship between percent body fat and energy expenditure is seen in men (Novotny and Rumpler, 2004). An accelerated reduction in lean body mass is concomitant with reduced energy expenditure in menopausal women (Poehlman, et. al. , 1995). To explain this relationship it must be remembered that most of ATP production occurs aerobically, that is with increased oxygen-carrying blood to facilitate energy utilization. In other words, an individual who has more capacity to carry oxygen-rich blood will definitely have a higher basal metabolic rate compared to an individual with less capacity.
Basal metabolic rate, in turn, is dependent on an individual’s number of cells (and mitochondria) to which oxygen can be delivered for the production of energy: Between an obese person and a child with marasmus, the former has the higher energy expenditure; taller people have a generally higher energy expenditure compared to shorter ones with the same body composition; a person who trains regularly has the higher metabolic rate compared to a sedentary (Whitney and Cataldo, et. al. , 2002).
The implication is that individuals may have different energy output despite similar body weights. In the same way, individuals with different body weights may have similar energy outputs. What matters in weight control is the composition, that is, the percentage of lean body mass and fat mass. From these principles, we can derive several inferences. In general, individuals with higher percentage of lean body mass have a higher energy expenditure compared to another individual of the same weight, but with a higher percentage of fat mass (Cunningham, 1991; Johnstone, 2005).
An individual with a higher percentage of lean body mass may have the same energy expenditure compared to another individual who may have a higher percentage of fat mass, but is relatively taller. This same individual may have a higher or lower basal energy expenditure depending on his relative height and body composition. An individual with a higher fat mass may have a higher basal energy expenditure if it is found out that he too, has a relatively higher total lean body mass because of his height.
Lastly, since males are biologically endowed with a higher lean body mass percentage compared to females, males in general have higher basal energy expenditure than females of the same height (Novotny and Rumpler, 2004; Whitney and Cataldo, 2002). This principle also applies to burned children. The measured energy expenditure was found to be significantly higher in males both during hospitalization and during discharge, compared to females (Micak, et. al. , 2006). Similarly, energy expenditure adjusted for differences in covariates was found to be lower in females than in males (Morio, et.
al. , 1997). The physiological implications of weight change can be best seen in people with eating disorders, the most common being anorexia nervosa, bulimia and binge-eating disorder. Common among these eating disorders is the person’s active refusal to gain or maintain a low BMI. People with any of these disorders restrict their diets to lose body weight by permanently or intermittently reducing their energy intake until the body adapts and produces a very low energy expenditure equivalent to their minimal intake.
Crash dieters may be included in the same category as binge-eaters who alternate severe calorie restriction with heavy intake. During the restrictive phase, the body, initially depleted with glucose, switches to a wasting metabolism by turning on its glycogen stores. Thereafter, if still unable to sustain the demand for energy, it turns on its own protein tissues and fat stores (Whitney and Cataldo, 2002). The immediate results and thus, popularity of crash diets can be attributed to the fact that the perceivable weight loss occurs at a point when the body is wasting its glycogen stores.
As said earlier, during the initial stages of a calorie-restricted diet, the glycogen stores are the first to be utilized to sustain the body’s energy demand (Whitney and Cataldo, 2002). Glycogen is stored with water. The loss of glycogen stores is always accompanied by a loss in body water, that is, about a liter of water is lost for every 1/3 kilogram of lost glycogen (Tesch, et. al. , 1982; . Da Silva and Fernandez, 2003; Eastwood and Eastwood, 1988).
Needless to say, this drastic water loss may be accompanied by loss in body salts and electrolytes as well as water-soluble vitamins which explains the weakness and exhaustion felt by people who use crash diets. Guerra et. al. (2004) recognize this loss in their study on the influence of fluid ingestion on the performance of soccer players. Replenishment of the carbohydrates and water lost during an activity is found to be beneficial in preventing deterioration in performance (Guerra, et. al. , 2004). Especially for endurance sports, the rate of glycogen depletion becomes a determinant of an individual’s performance.
A continuous supply of energy and constant hydration are critical not just to achieve top performance but to reduce risks associated with the physiological changes that happen during the activity and in times of fast (Eastwood and Eastwood, 1988). In the same way, extreme fasting results to dehydration. Hence, what is perceived to be a significant weight loss in crash diets is merely a significant water loss, which may soon be regained upon hydration or refeeding. Upon exhaustion of glycogen stores, the body proceeds to the mobilization of fats (Ati, et.
al. , 1995). It is to this fact that the success of severely low calorie diets can be attributed. Meanwhile, the preference of brain and the red blood cells for glucose necessitates the conversion of protein tissue to pyruvate which would then be converted to glucose. Fats contribute to this demand for glucose by using its glycerol chain. As the fast ensues, the body slows down its use of protein stores and finds a way to use fat to provide for the needs of the brain and the red blood cells while saving the protein tissues from being totally wasted.
This adaptation is done by combining the Acetyl CoA fragments from fatty acids to produce ketone bodies which would then supply energy for the brain cells. However, a high concentration of ketone bodies does not only indicate the depletion of energy stores; it also signals the drop of blood pH (ketosis) which may cause some negative consequences, particularly the further mobilization of proteins (Castaneda, 2002). At such stage, blood ketones may spill into the urine or cause acetone breath (Whitney and Cataldo, 2002).
Note the utilization of protein tissues for energy during fast. Protein tissues, aside from constituting most of the body weight, are normally reserved by the body for purposes related to the creation of essential proteins necessary to facilitate the normal body functioning. The symptoms of chronic obstructive pulmonary disease, for example, can be attributed to the loss of skeletal muscle mass (Jagoe and Engelen, 2003).
The depletion of protein tissues in restrictive diets, combined with ketosis causes both weight loss resulting to a loss of a higher percentage of body mass compared to what would have been lost physiologically by applying less restrictive intakes, and the disruption of the body system (Biourge, et. al. , 2008). The loss in lean body mass means that for individuals who have eating disorders or are using crash diets to induce rapid weight loss, the body composition changes to that containing less lean body mass and more fat mass (Krzywicki, et.
al. , 1968). In other words, compared to an individual with the same body weight, the one who uses crash diets have less basal metabolic rate. In people who have undergone a gastric bypass surgery, the loss in their body weight resulted in significant decreases in energy expenditure (Das, et. al. , 2003). This implies a greater reduction in metabolic rate than what would have been lost physiologically as more tissues are utilized for energy with the continuous depletion of energy sources.
As a result of the reduction of basal metabolic rate, less weight may be lost compared to what would have been lost if a reducing diet that did not have to utilize body tissues was applied. In addition, less food must be taken in order to maintain weight compared to an individual with the same weight or who lost the same amount of weight but didn’t use crash diets. Given that body composition determines the rate of weight change, it appears that with the effects of the use of crash diets is the progressive difficulty in weight control.
Hence, after a crash diet, less lean body mass is gained and more fat is stored compared to what was lost in the diet. Accordingly, weight maintenance requires a relatively less caloric consumption compared to others with the same weight but did not use the diet. In a study by Bjorntrop and Yang in 1982, there was a greater increase in body weight per gram food consumed when the subjects were refed after 3 days of fast. This indicates the persistence of a starvation-induced energy conservation process after fasting. Any attempt to go back to the previous diet may result in a progressive weight gain as the body stabilizes at a higher weight.
It is understood that basal metabolic rate that determines the extent of weight change that an individual may experience. Aging has a general effect of reducing an individual’s lean body mass especially during weight loss (Kehayian, et. al. , 1997; Newman, et. al. , 2005). Meaning, individuals must continuously decrease their caloric intake in order to maintain the body weight. Given the principle that prior body composition determines the rate, amount and composition of future weight gain or loss, the fact that aging progressively reduces lean body mass means that energy expenditure and caloric intake are also progressively reduced with age.
As the ratio of lean/fat ratio in weight loss increases, each amount of lean body mass subsequently lost takes a progressively increasing percentage of the remaining mass, requiring an equivalent proportion of caloric intake reduction to maintain energy balance. Conclusion The body has a mechanism that delicately adjusts the system in order to maintain homeostasis. It adapts to any weight gain or loss by altering its energy expenditure to a point that would restore its previous status. This explains the difficulty in gaining or losing weight for people who are underweight and obese.
Weight control is therefore, the manipulation of body mass and composition, not only by arbitrarily changing the diet to be more or less than the previous intake, but with consideration of the body’s adaptive mechanisms and predisposition to settle on its original status.
Works Cited
Ati JE. , et. al. (1995) Increased fat oxidation during Ramadan fasting in healthy women: an adaptive mechanism for body weight maintenance. American Journal of Clinical Nutrition, 62, 302-307. Biourge V. , et. al. , (2008).Longe-term voluntary fasting in adult obese cats: nitrogen balance, plasma amino acid concentrations and urinary orotic acid excretion. Journal of Nutrition, 124, 2680S-82S. Bjorntorp P and Yang MU. (1982). Refeeding after fasting in rat: effects on body composition and food efficiency. American Journal of Clinical Nutrition, 36, 444-59. Castaneda C. (2002). Muscle wasting and protein metabolism. Journal of Animal Science, 80, E98-E105. Center for Disease Control and Prevention. (2008). Healthy Weight. Retrieved 26 April 2008 from http://www. cdc. gov/nccdphp/dnpa/nutrition/nutrition_for_everyone/healthy_weight.