Schaum's Easy Outline of Biology by George H. Fried & George J. Hademenos

Author:George H. Fried & George J. Hademenos [Fried, George H. & Hademenos, George J.]
Language: eng
Format: epub
Tags: General, Science, Life Sciences, Biology, Study Aids, Outlines; syllabi; etc, Study Guides
ISBN: 9780071369718
Publisher: McGraw-Hill
Published: 2001-01-30T05:00:00+00:00

Fig. 16.4 Adipocyte

At complete rest under no metabolic stress, an average human of 70 kg expends approximately 68 cal/h. In a sedentary job, such a person might use up about 2300 cal daily. Certainly, the average western diet exceeds that number of calories, and those who do not expend energy in regular exercise programs are threatened with weight gains that are detrimental from both a cosmetic and a health point of view. Simple obesity is not by itself regarded as a pathology, but its presence complicates preexisting disease states, especially cardiovascular conditions, diabetes, arthritis, and even susceptibility to cancer.

The ability to convert newly arrived foodstuffs into long-term storage fuel (fat) is regarded as a significant evolutionary advance in birds and especially in mammals. The efficiency of this process is connected to the elegant metabolic versatility of adipose tissue, which was earlier misconceived as a mere storage bin for lipids manufactured elsewhere. However, this capacity to convert and store calories as fat, though of importance to survival under conditions of scarcity, becomes a threat to health in an environment in which tempting foods are freely available and seductively displayed and advertised. In humans, a complex of subtle desires and responses determines whether feeding will be initiated, continued, and, finally, terminated. Both appetite and hunger are involved. According to the eminent nutritionist Jean Mayer, appetite is a complex of usually pleasant sensations “by which one is aware of desire for and the anticipation of ingestion of palatable food.” Hunger is a far more intense, unfocused craving for any kind of food following acute deprivation. Satisfaction of both hunger and appetite induces a sensation of surfeit, or fullness, which cuts off further eating. These sensations are called satiety and are mediated by satiety centers in the hypothalamus of the brain. Other hypothalamic centers are involved in hunger and appetite.

The normal regulation of food intake appears to be dependent on physiological events arising in the gut (stomach and intestine), brain, and circulatory system. It has also been shown that physical activity is related to the appropriate control of feeding. Long bouts of overexertion may cause an adjustment of caloric intake to a level below that necessary for weight maintenance and thus produce an underweight condition. Too little exercise may be associated with excess food intake and thus may produce obesity. Only within appropriate ranges of physical activity does the “appestat” work effectively. Research in obesity appears to confirm that obese individuals are not merely gluttonous or lacking in self-control but are the victims of altered homeostatic mechanisms which are difficult to defy.

Closely related to obesity in terms of its malfunctioning feeding control system is anorexia nervosa. In this serious illness, found largely among adolescent girls in western societies, a deep-seated conviction of threatening obesity causes the normal-weight or underweight victim literally to starve to death. Although it seems a simple matter to intervene and force-feed these people, intervention often occurs too late to reverse the process of starvation. Anorexia appears to arise from psychological problems, but it has been extremely resistant to conventional therapeutic procedures.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.