From the first article, we know that the energy content of the food we eat per gram (called energy density of the food) influences our appetite (the amount of meal consumed until satiety). In general, the higher the energy density of the meal, the lesser is the appetite and vice versa, the lower the energy density of the meal, the higher is the appetite. This is true for all animals with the possible exceptions of human being since appetite seems to be influenced also by how delectable the presentation of the dessert is after a meal. Remember that this generalization was made after numerous observations in the laboratory. Needless to say, the meal and conditions were under control to make this kind of conclusion.
Animals in general eat to satisfy its energy requirement in tune with their growth rate. Thus, younger animals consume more energy relative to their body weight because they have faster growth rate than do the adult animals. In human context, when you get to be 20 years old or older, you have to eat at maintenance level because your growth has slowed down or has stopped altogether. Growth, again is loosely defined as additional weight at the level of the animal, or increased tissue weight at the tissue level. In human context, before you attain your maximum height in your early 20s, your increase in weight corresponds to your increase in height. This correspondence is almost linear until you approach the end of teenage years where the correspondence become less steep. In effect, biological graphs of growth of animals, including human beings, are depicted as sigmoid. In fish, the plateau of this sigmoid curve could be reached in a matter of weeks or months.
So as the animal and human being approach this plateau of growth, their energy consumption should be at maintenance level. You may now argue that even if additional food energy is consumed when one has reached this plateau, still weight will increase and since we define growth as increase in weight, then there is growth. You are correct. But there is something wrong with our definition of growth.
If Anna is now 23 years old and continue to eat considerably more food energy on a daily basis, on her 24th birthday, she has grown. Yes, that is correct, except that she has grown horizontally rather than vertically and if she continued this in the long run, obesity would result and she would be prone to some cardiovascular diseases and even diabetes.
What about fish? Three month old tilapia are fed continuously with energy-rich formulated diet and in no time will reach marketable size. Isnt this a good idea? Yes, the tilapia will gain weight very fast but it is not a sound animal husbandry practice. For one, you could have saved the extra expense of supplying extra food energy and could have channeled it to your profit. Another is that the tilapia may have accumulated fat in the body. You may argue that fatty tilapia are even tastier for the consumer since if he tried broiling them, oil would come oozing out of the fish's body and it really would be so sumptuous. Fine, your argument is acceptable. Except when you are a big time tilapia farmer and you handle tons of tilapia harvest in a tropical country such as the Philippines. Fatty fish are prone to rancidity when not stored properly after harvest. If you do not have these freezers, you will end up with harvest of very poor quality and thus low profitability. But if you have all these storage facilities, it would be nice to supply the consumers with the much needed highly unsaturated fatty acids instead of taking pills of these daily, which is very expensive.
Lastly, if obesity kept in the long run leads to cardiovascular diseases in human beings, it is also true for tilapia. But then again, before the period of what we call 'long run' is approached, we have already harvested and eaten them.
Then, are we wrong in our definition of growth. This will be tackled in the next post.
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