Metabolism

Simply stated, metabolism refers to all of the chemical reactions that occur in the living body. This ranges from breaking down calorie containing components such as proteins, carbohydrates, and fats to provide energy, to converting the various nutrients in foods into the protein needed for making muscles, hair and a variety of other structures.

Homeostasis

First proposed in the late 1800's, it refers to the ability of an animal to maintain its internal environment, despite changes in its external environment. Of course, there are limits to homeostasis as will be seen in the following examples. Think of a dog or cat with a body temperature of 101 degrees Fahrenheit. The animal easily adjusts to changes in the temperature of its environment without changing its own temperature. If it's too cold, the animal will shiver in an effort to produce heat and maintain its body temperature. If it's too hot, the animal sweats or pants in an effort to lose heat and stay cool. Obviously, there are limits to the conditions to which an animal can adapt , but across a fairly wide range, the animal can maintain its normal body temperature. A nutritional example is calcium metabolism. An adult dog can do well on diets containing anywhere from about 0.8 to 1.8% calcium. By metabolically controlling how much calcium is absorbed from its diet, the dog can make sure it absorbs approximately the same amount of calcium when fed a diet containing 0.8% calcium as when fed a diet containing 1.8% calcium. Although, at very low levels the dog can't get enough calcium even if absorption is 100%, and at high levels it cannot turn down the system enough to prevent it from absorbing too much calcium. Thus, both minimum and maximum dietary levels must be known to formulate the proper diet.

Nutrient Requirements

The next concept we will review is nutrient requirements. Each animal requires essential nutrients in its food in order to live. If the diet contains too little, the animal is considered deficient and usually shows signs of the deficiency. If the diet contains too much of a nutrient, then the animal can develop signs of an excess or toxic amounts. Between those two extremes are levels, which support life and good health and lead to neither a deficiency nor an excess. This is a key point and is the result of the process of homeostasis mentioned earlier. The range of levels of a nutrient required to go from a deficient state to a toxic state can be expressed by Figure 1 below. Two important points to be noted from this figure are as follows: First, the level of nutrients required for normal health is not a single point, but a range as represented by the plateau region shown in the figure. Oftentimes it is inferred that requirements are quite narrow and that only small changes will lead to catastrophic consequences. For most nutrients, animals can adapt to different levels through the process of homeostasis. Second, the size or length of the plateau depends on the species of the animal, the age of the animal and the nutrient in question. Protein,minerals, and most vitamins have a plateau region that is quite wide and so toxicity in some cases has never been observed (figure 1). In contrast, the trace minerals zinc, selenium and vitamin D can be quite toxic and the plateau region is very narrow (figure 2). For these nutrients great care must be taken when formulating diets. As mentioned earlier, other factors also play a role in the shape of this graph and the response. Growing, reproducing, sick or old animals may have a much narrower plateau for many of the nutrients than normal healthy adult animals.

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