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Metabolism: Balancing the Body’s Ledger
To make sense of metabolism, it’s easiest to think of the body as a machine with countless functions and very specific fuel requirements. When we eat, enzymes in our digestive system break down food into its component ingredients: proteins devolve into amino acids, fats into fatty acids, and carbohydrates into sugars such as glucose. These then seep into the blood stream and spread throughout the body, so that thousands of metabolic reactions happen at the same time. Some of the energy from these acids and sugars gets used for specific bodily maintenance functions, such as pumping the heart or yelling to scare away a mean-looking squirrel. Whatever isn’t needed right away will be stored as fat for later use.
To avoid gaining extra fat deposits, we must use all the calories that we ingest. The speed at which energy is burned while at rest is called the Basal Metabolic Rate, or BMR, and it’s affected by a person’s age, sex, and proportion of muscle to fat. The older a person is, the lower their BMR typically is; men generally have a higher BMR than women; and the body burns more calories to maintain muscle than fat. (After all, it would be a rather poor design if the body had to use a great deal of energy just to maintain its energy reserves.) Our BMR adjusts according to our lifestyle, slowing down when food is scarce and speeding up when we have to eat our way out of the wreckage of a popcorn-factory explosion. This is why eating five smaller meals throughout the day can help people lose weight, whereas starvation diets often result in little to no weight loss.
How the body uses its fuel was a perplexing mystery for generations of thinkers. In the 1600s, a man by the name of Santorio Santorio took it upon himself to study body weight. For 30 years he weighed everything that went into his body and everything that came out, often eating, working, and sleeping on a scale. What he found was that his total visible excreta was less than the amount of food he ingested, but the difference couldn’t be accounted for by his weight gain. This led him to come up with the theory of insensible perspiration—the unpalatable idea that we constantly expel waste through our skin.