DEAR DR. BLONZ: If blood glucose level is so important -- if it gets too low, we get weak or pass out -- why does the body automatically convert the carbohydrates we eat into fat? Why don't we store carbs like we do fat? -- R.S., Dublin, Calif.
DEAR R.S.: Plants are able to capture energy from the sun, photosynthesizing the sun's energy into carbohydrate using carbon dioxide from the atmosphere. The human body, like that of most other animals, can't do this. Our only source of energy comes from the foods we consume.
Fat gets used as the storage form because it is nature's most concentrated form of metabolic energy, containing over twice the energy per unit weight as proteins or carbohydrates such as glucose. A concentrated form of energy is essential for us because of mobility: If we stored the bulk of our energy as carbohydrate, we would be too bulky to move.
That provides a good contrast to plants, where the basic theme is staying put, sending roots into the soil for nutrients and growing leaves that are exposed to the sun. With sunshine comes more energy for growth, the production of flowers, and the eventual production of seeds for the next generation. Plants take in their "meal" of sunshine and convert their energy into carbohydrate because it takes up the most space per calorie, which then speeds their growth.
Although the "mission" of the plant is to grow rapidly while remaining in one spot, that of the seed is very different. A seed must be light enough to be carried by the wind or on the fur of animals, but after falling to the ground, it needs to have enough stored energy to fuel the process of sending out roots and growing shoots. Seeds' need for concentrated energy explains why they store their enery as fat, as opposed to carbohydrates like mature plants.
You asked about carbohydrate storage in our body. It turns out that there is a small amount in the form of glycogen, which is a branched chain of glucose molecules. Glycogen is stored in the liver and muscles. It gets used as a source of emergency muscle energy, or if the blood sugar level drops too low, but it's not a large amount: The average adult has just over 100 grams in the liver and about 250 grams in all the muscles combined.
DEAR DR. BLONZ: Why should you only take 500 milligrams of calcium at a time? My supplement contains 600 milligrams and I had been taking two at bedtime. I had heard that it is better to take them at bedtime, but now I hear differently. Could you please comment? -- S.A., Seattle
DEAR S.A.: The reason for the calcium limit is that the efficiency of absorption tends to decrease when a single dose exceeds 500 milligrams. Taking larger amounts means that more will end up passing through without being absorbed. It is probably wise to find a way to distribute your calcium in more than one dose, and taking it at mealtime is usually best.
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