4. Protein
Protein is a critical structural and functional component
of the human body, and commonly represents anywhere from 10 to 30
percent of a person’s daily caloric intake. If you have a 2,400-calorie
diet and 25 percent of it comes from protein, you’re getting 600
calories, or 150 grams, of that macronutrient.
The ability of the body to properly metabolize protein foods
diminishes at an intake of about 35 percent of
calories, which is why you are not advised to go crazy on
protein intake and try to get the majority of your calories from it. A
potentially fatal condition known as “rabbit starvation syndrome” can
occur with a diet dominated by lean protein and deprived of fats and
carbs.
Early American explorers and pioneers discovered that dependence
in the wilderness on meat that didn’t contain enough fat, like rabbits,
caused nausea, diarrhea, and sometimes death.
Vilhjalmur Stefansson, the famous Arctic explorer from Canada (he
ended up his exciting life as Director of Polar Studies at Dartmouth in
Hanover, New Hampshire), also commented on the ill effects and futility
of trying to survive on skinny animals in the far north with the Inuit.
He spent a few years living with the Inuit on Arctic expeditions about
100 years ago, and his writings and self-experiments cast light on the
Inuit’s, and our own, ability to live almost exclusively for months on
meat and fat.
Note
Where did the Inuit derive their vitamins, such as C,
on this seal- and whale-meat diet? “Fediuk [analyzed] the vitamin C
content of 100-gram (3.55-ounce) samples of foods eaten by Inuit women
living in the Canadian Arctic: Raw caribou liver supplied almost 24
milligrams, seal brain close to 15 milligrams, and raw kelp more than
28 milligrams.”
4.1 Proteins Do Important Stuff
The body requires protein in big and small ways. Human
genes are essentially recipes for creating proteins. Skeletal muscle
incorporates proteins called myosin and actin; bones are about
one-fifth composed of the protein collagen, which dominates connective
tissue such as ligaments and tendons. Organ tissue, the gut, skin,
hair (keratin), and blood vessels also contain structural
proteins.
Proteins are also functional; they do important stuff. Examples
of the functional proteins are hormones that are made of protein
(including glucagon, insulin, and growth hormone), immune system cells
such as antibodies, enzymes (the catalysts for the body’s chemical
reactions, and used in digestion itself), as well as red and white
blood cells. Figure 6 shows a
hemoglobin molecule, a protein complex in red blood cells that carries
oxygen throughout the body.
Proteins are like Lego structures, but this time the Lego pieces
are amino acids, the protein’s building blocks.
The body can build thousands of different proteins using numerous
combinations of these almost two dozen ingredients.
Note
The body synthesizes from 500 to 1,000 pounds of
protein, depending on the person’s size, during a
lifetime!
Here is some nomenclature involving proteins: 10 or more amino
acids stuck together form a polypeptide (and two
are a dipeptide and three a tripeptide). A polypeptide with more than
50 amino acids is called a protein, which can be
composed of as many as 10,000 amino acids.
Proteins have to be broken down into their constituent amino
acids to be digested. This happens first in the stomach and then in
the small intestine, employing a number of enzymes, including
peptidases, proteases, and trypsin. The protein building
blocks, the amino acids, make their way to the liver (where some are
actually metabolized for energy), then into the bloodstream, where the
cells take them up to help make more proteins.
