Study Guide

Animal Nutrition and Digestion In the Real World

  • Health

    Warnings on My Diet Coke Can?

    Next time you are cramming for finals and are up studying until midnight, take a glance at your Diet Coke can. Notice the ingredient panel contains a warning saying the drink contains phenylalanine. Phenylalanine is one of the ingredients in aspartame, which is an artificial sugar substitute. But that's is an essential amino acid that we already mentioned. What gives?

    We didn't lie or lead you astray, pinky swear. It's just that some people have a genetic deficiency called phenylketonuria (PKU) where they lack phenylalanine hydroxylase (PAH), the enzyme that degrades the essential amino acid phenylalanine. PAH normally converts phenylalanine to tyrosine, another amino acid. Sithout PAH activity, phenylalanine builds up, creating an acid called phenylpyruvate that can cause mental retardation, seizures, and a musty odor in humans. If phenylketonurics (the name for those people with PKU) continue to eat the amino acid, they'll eventually develop blonde hair and pale skin since this acid will also cause a loss of pigmentation.

    It's a pretty serious disease, so phenylketonurics have to avoid high-protein foods, like meats and beans. They've still got to get those amino acids, so they usually drink a specially designed liquid to supplement their diet. Something like this.

    Phenylketonuria is a homozygous recessive disorder—that means both parents have to have the recessive gene for a child to inherit it. It's now required that all newborn babies receive a screening blood test for PKU before they leave the hospital so the disorder can be caught early.

    If your mind is still thinking clearly after studying all night, you may ask, "Why can't they drink Diet Coke? There's no protein in soda." Aspartame—that's an artificial sugar substitute—is made up of phenylalanine. Aren't you glad you asked?

  • Pop Culture

    Building the Body

    It's not easy being a body builder. They have to train hard, tan hard, and you've really got to know your hormone science. Yes, we did say science.

    Very muscular body builders rely on hormones to make themselves look even more muscular. If their bodies have too much water, they'll be bloated, and their muscles won't be as defined. They could kiss that gold medal goodbye.

    The most logical way to get rid of extra water is to avoid it for a few days, to excrete all possible water and not replace it by drinking anything. Body builders are often told to drink lots of water 8-10 days before a show, and limit themselves only about 8 hours beforehand. The reasoning is in our hormones.

    To start to see what's going on, think about what would happen if a body builder really limits water intake a week before their competition. First, blood osmolarity rises (the same amount of salts are dissolved in less water), and the posterior pituitary gland releases antidiuretic hormone (ADH) that will circulate to the kidney and increase water reabsorption at the nephron's collecting duct. When more water is returned to the blood, water retention increases and urine volume decreases. First off, the body has saved and stored water, but secondly, the body builder can become severely dehydrated and risk severe complications.

    What should the body builder do to win the competition? When they drink plenty of water, their blood osmolarity decreases, and ADH is blocked. Minimal amounts of water will be reabsorbed in the nephron and the urine will become dilute as lots of water is excreted. Another side effect of drinking lots of water is stimulating atrial natriuretic peptide (ANP) release. When the heart senses the increased blood volume (from drinking all that water), ANP will circulate to the glomerulus. This will the release of renin. When renin is blocked, aldosterone is also blocked, and the kidneys won't reabsorb more salts and water. Blood volume and pressure will be limited.

    With their body's hormone primed and the kidneys convinced it will be producing dilute urine, a body builder will stop drinking water a few hours before a competition to ultimately confuse the body. For a while, lots of water will still be eliminated, the blood volume will decrease with less water coming in, and the body builder's muscles appear quite ripped. Just like that, our science-loving body builder wins the competition!

    Some body builders don't follow these guidelines, but simply block ADH with drugs. Artificially blocking this hormone can cause severe dehydration, and some body builders end up getting pretty sick.

    We should mention that you shouldn't try this at home. Body builders have support staff that help them monitor their blood composition, and help decrease the likelihood that they'll mess something up. Unless you have a support staff, keep drinking those eight ice-cold glasses of water a day.

  • History

    A Scientific Window into Digestion

    We didn't always know everything about nutrition and digestion. Scientists and doctors knew that food didn't look the same going in and coming out—we don't need fancy degrees on our wall to figure that out. They knew some mysterious process was happening, but they didn't have a way of studying it since everything was occurring inside our bodies.

    Everything changed one fateful day in 1822 at Mackinaw Island, Michigan. A poor fur trader named Alexis St. Martin was accidentally shot in the stomach from just a few feet away. He managed to survive, but by the time doctors arrived, he had a hole in his gut and his breakfast was hanging out for everyone to see.

    One of his doctors, Dr. William Beaumont, who had only completed one year of medical training, kept a close eye on his patient. With a gaping hole in his guts, anything that St. Martin ate popped right out. Since anal injections of food weren't (and still aren't) incredibly satisfying, Dr. Beaumont put a bandage over the hole to keep food inside.

    The hole never healed or properly sealed. It ended up creating a gastric fistula, or a permanent opening. Normally, an open hole in your belly isn't really thought of as a great thing, but for Beaumont, it offered a window into the world of digestion. The doctor was so curious about digestion that he would dangle pieces of food tied to string into St. Martin's stomach hole, remove it, and then analyze how the food had been digested. He would spoon an egg into the fistula, and remove it after a certain amount of time to see how long it takes for a hardboiled egg or one that's cooked sunny-side-up to be digested.

    Dr. Beaumont ended up paying St. Martin to stay in his house so he could constantly keep experimenting on his stomach hole. Needless to say, they didn't become BFFs. This type of research continued for ten years. Gross.

    What did we learn from these gruesomely interesting scientific experiments? Prior to 1822, and before Beaumont published his study, scientists thought stomach digestion was either mechanical or chemical. Either the stomach ground food into tiny pieces or it had enzymes that broke food down. Beaumont's experiments settled the debate, and he concluded that the stomach had gastric juices and HCl to digest food. Thank goodness for gastric fistulas.

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