The metabolic rate of an organism per unit weight. The metabolic rate of an organism divided by the organism's weight. (If it were physics it would be called specific metabolism, but biologists perhaps fear confusion with species.)

In biology great stress is traditionally laid upon a warm blooded (homoiothermic) animal having to get rid of much of its body heat through its surface. Since the amount of heat it produces increases with the cube of its linear dimension (volume) and its surface increases with the square of its linear dimension the larger it gets the more difficulty it has cooling itself. (See Bergman's Rule.) If this factor determines its metabolic rate - i.e. if homoiotherms keep their heat output per square centimeter constant - then simple mathematics reveals the intrinsic metabolism should be inversely proportional to their linear dimension. Inversely proportional to the cube root of the body weight. In fact it is inversely proportional to the fourth root of the body weight.

(This even prompted a paper, in the Journal of Theoretical Biology, claiming that animals are 4-dimensional.)

Hence plotting a graph of the logarithm of the intrinsic metabolisms of birds against the logarithm of their weights yields a good straight line without too much scatter. Another type of form, say snakes, would give another straight line of different intercept upon the weight axis.

The relationship between metabolism and body size applies, as far as Sporus knows, to all animals including microorganisms, independently of whether they are homoiothermic.

It follows that the intrinsic metabolism of smaller things is greater than that of a larger. For example the metabolic output of a mouse is greater than that of a mouse sized chunk of elephant. A small thing's fires burn more fiercely than those of a large

Metabolism is the sum total of all chemical reactions in an organism, from a bacteria cell to a complex mammal like a human. In science it does not usually refer specifically to the digestion of food. The reason why metabolism can refer to all of the reactions in an organism is because of the conservation of energy in Nature. Nature does not like waste. It makes her extremely unhappy. Thus there are absolutely no superfluous chemical reactions in the body, they all contribute to something. Metabolism can accurately refer to all of them.

The basic subcomponents of metabolism are called pathways. Pathways are sequences of reactions leading towards a single goal. Pathways can either be classified as anabolic or catabolic, but they will always fall into one of those two categories. Pathways are extremely interwoven in an organism, feeding and stimulating each other in a complex pattern. They are, however, fairly similar across species, following certain themes:

  1. Cells maintain a fairly constant internal concentration of substances: inorganic ions, metabolites (basic small molecules important to reactions like pyruvate), and enzymes. The content of substances in the cell is regulated by the cell membrane, which makes sure equilibrium is preserved.
  2. Organisms or cells extract energy from external sources (food) to drive endrogonic reactions (reactions requiring energy)
  3. Metabolic pathways are specified by an organism's genes. DNA is a blueprint for proteins and enzymes, and enzymes catalyze reactions, so one can rightly say that genes determine of which metabolic pathways one is capable.
  4. Cells interact with their environment. Their activity is dependant on their surroundings, their supply of food.
  5. All materials and cells turnover, they recycle themselves and undergo continual degredation and resynthesis. An example of this is the fairly scientific adage Everyone today has at least three atoms of carbon from Julius Caesar. Another is that in seven years, an entire human body's cells have been fully resynthesized. These things recycle themselves quickly and efficiently (remember, Nature's pet peeve is waste).

Metabolism is really just a measure of how efficiently your body runs. All the fancy scientific mumbo jumbo aside, what you're thinking about is why dieting works or doesn't work, or why you can or cannot gain weight.

Everyone's body is a little bit different. Some of us put on muscle easily, while others can't seem to squeeze that bicep tight enough to form any visible bump. There are many different reasons for this, one being hormone levels. Ladies: do you ever notice how you tend to put on a little weight while you're on your period, only to lose whatever was gained instantly following?

When your body is working efficiently, it is using up all the fuel that you put in each day. Your body requires energy to carry out all the normal everyday tasks. When you diet, you are asking your body to use more fuel than you give it. That causes your body to use up some of the reserves it was saving for those times when you might have to go a few extra hours between meals. Your body doesn't like this one little bit. Metaphorically speaking, if you have managed to save a little money in your bank account for a rainy day, aren't you really hoping that there are no rainy days? When this happens, your body does not want to make withdrawals from its deposits. What it would rather do is balance things back down to efficiency. There are some easy ways to do this, and one of it's favorite ways is to digest your muscle. Since muscles require lots of energy, diminishing your muscle mass will also make your body run more efficiently. TA DA! Metabolism has been lowered.

People who claim to have very high metabolic rates claim that they can eat whatever they want and not gain an ounce. All it really means is that their bodies are running very efficiently, and they are using all of the energy that they put into their bodies everyday. Of course, there are limits, and no one can really eat anything without gaining weight unless there is a medical issue. Most people who fall into this category are more balanced than they realize. They eat several small meals throughout the day, and therefore appear to be eating constantly while not consuming any more than someone eating three squares a day. They also tend to be very active - perhaps hyperactive and a little bit fidgety.

In order to increase your metabolism, you must find a way to decrease your body's efficiency. It sounds slightly counterintuitive, but it is in effect what you want to do. This cannot be done with dieting, but may be accompanied by a change in diet. What I mean is that you can't decrease calories for a short period of time, since that decreases your metabolism. You may only make a lifelong dietary change. Eating several small meals throughout the day balances the blood sugar, and makes more of the calories you take in available to your body. It also helps to increase activity and muscle mass. These are the only long term solutions which don't require medical intervention.

Me*tab"o*lism (?), n. (Physiol.)

The act or process, by which living tissues or cells take up and convert into their own proper substance the nutritive material brought to them by the blood, or by which they transform their cell protoplasm into simpler substances, which are fitted either for excretion or for some special purpose, as in the manufacture of the digestive ferments. Hence, metabolism may be either constructive (anabolism), or destructive (katabolism).


© Webster 1913

Me*tab"o*lism (?), n. (Biol.)

The series of chemical changes which take place in an organism, by means of which food is manufactured and utilized and waste materials are eliminated.


© Webster 1913

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