Eating

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Ideally, one should eat the kinds and amounts of foods that provide adequate vitamins, minerals, essential amino acids and fatty acids, and calories. Proper caloric intake maintains energy reserves (primarily fat and glycogen) and results in a body weight within an optimum range for health.

Eating behavior appears to be at least partially controlled by areas of the hypothalamus. Lesions (destruction) in the ventromedial area of the hypothalamus produce hyperpha-gia, or overeating, and obesity in experimental animals. Lesions of the lateral hypothalamus, by contrast, produce hypophagia and weight loss. More recent experiments demonstrate that other brain regions are also involved in the control of eating behavior.

Fox: Human Physiology, I 19. Regulation of I Text I I © The McGraw-Hill

Eighth Edition Metabolism Companies, 2003

Regulation of Metabolism 605

Dietary or

energy-reserve

Triglycerides Polysaccharides

Proteins

sources

(in cells)

i- i :

Circulating energy Fatty acids, substrates glycerol,

(in blood) ketone bodies

Glucose, lactic acids

Amino acids

Circulating energy Fatty acids, substrates glycerol,

(in blood) ketone bodies

Glucose, lactic acids

Amino acids

Intermediates of aerobic respiration (in cells)

Pyruvic acid

Acetyl coenzyme A

( Krebs cycle

Urea

■ Figure 19.2 A flowchart of energy pathways in the body. The molecules indicated in the bottom rectangles are those found within cells, while the molecules in the middle rectangle are those that circulate in the blood.

The neurotransmitters that may be involved in neural pathways for eating behavior are being investigated. There is evidence, for example, that endorphins may be involved because injections of naloxone (a morphine-blocking drug) suppress overeating in rats. There is also evidence that the neurotransmitters norepinephrine and serotonin may be involved; injections of norepinephrine into the brain cause overeating in rats, whereas injections of serotonin have the opposite effect. Indeed, the diet pills Redux (D-fenfluramine) and fen-phen (L-fenfluramine and phentermine) work to reduce hunger by elevating brain levels of serotonin. (Both drugs have been taken off the market because of their association with heart valve problems.) The regulation of hunger is discussed shortly in a separate section.

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