DEAR DR. BLONZ: I have a question about carbohydrates. You have stated that once a carbohydrate is converted to fat, it can never become a carbohydrate again. If glucose is so important to the human body, why have we evolved to store excess energy as fat and not carbohydrate? -- R.D., San Francisco
DEAR R.D.: The human body can't run on batteries, and it has no mechanism for the continuous capture of energy from nonfood external sources, such as the sun, as plants do. Our only sources of energy are the proteins, fats and carbohydrates we eat. And because we are not eating 24/7, there is a need for energy storage.
Fat is the ideal storage substance because it is nature's most concentrated form of metabolic energy. There are approximately 9 calories per gram of fat, compared with 4 calories per gram of protein or carbohydrate. (Alcohol, it turns out, provides 7 calories per gram, which is something those concerned with their body weight should consider.) You find energy stored as fat throughout the animal world, whenever mobility is needed. If we stored the bulk of our energy as carbohydrate, we would be too bulky to move.
This takes on more meaning when we consider plants. By contrast, their basic theme is not mobility: They stay put, sending roots into the soil for water and nutrients, and leaves up to the sun. Plants are able to utilize sunshine directly, converting it into energy for growth and functionality -- including the production of flowers and of seeds for the next generation. As plants take in their "meal" of sunshine, they turn the energy into carbohydrate because it takes up the most space per calorie.
Although the "mission" of the plant is to grow rapidly while remaining in one spot, that of the seed is much different. It must first travel away from the parent plant, being carried by winds or on the fur of animals. In many fruits, the seed is inside a hard-coated pit that is surrounded by tasty flesh that becomes sweet and gives off enticing aromas once the seed is fully developed and ready to go. Animals do the harvesting and rely on the flesh for their own nourishment, and the pit enters their digestive system and gets dropped in a new area where the plant can develop.
Once the seed falls upon a new patch of ground, it must have enough energy to send out roots and produce that first shoot so that it can begin gathering new energy from the sun. The seed, unlike the parent plant, requires an "energy-dense" material so it can be lightweight and compact in size. This explains why in seeds we find energy stored as fat. Nature: fascinating stuff.
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