Food Chemistry Basics: Proteins, Fats, and Carbs (part 6) - Protein - Eating Protein

When you eat protein-containing foods such as meats, fish, eggs, cheese, nuts, and plant foods, as we mentioned, proteases and other enzymes split up the big block of Legos into their separate amino acids, which can then be absorbed in the small intestine and released to the liver and bloodstream.

The body can do a number of things with these amino acids, depending on your physical needs:

Proteins that are already in the cells (not coming in as part of your food) are constantly being degraded or split up into their constituent amino acids and resynthesized into new proteins the body needs. It’s a very complicated and ingenious metabolic process by which the liver finds so many different uses for proteins and amino acids, but one of the important elements of the sequence is that when the liver has to metabolize amino acids (in order to make glucose out of them, for example), the organ breaks down the amino acids into ammonia, which is converted to urea and excreted out via the kidneys in the urine.

Cells can take up the amino acids for their own use, such as new protein synthesis by muscle cells.

If you aim for optimal health, you need to get all of the essential amino acids in high enough quantities from your diet. Meat and fish eaters have an easier time of it than vegetarians, for instance, because these meat constituents are generally complete proteins—they have all of the essential amino acids in greater than trace amounts. Eggs are a great source of essential amino acids for vegetarians (eat copious amounts of pastured eggs), as is the occasional whey protein shake .

To show you how to analyze food for its protein quality, we’ll do a quick check of lamb, salmon, and bananas with NutritionData. This searchable web database for nutrition information has a good section on its result pages for protein. Figure 7 shows the page for a small slab of cooked lamb, which the site gives a good protein score (anything over 100 is a complete protein).

Many of the amino acids are in lamb in a quantity in excess of a gram, and the four ounce piece of meat has 26 grams of protein, a bit less than a quarter of what a training athlete would need in a day. Salmon is also an excellent source of amino acids, as Figure 8 shows. A six ounce filet has about 40 grams of protein, with more than 3 grams of the important muscle-building amino acid leucine. It also has more than 300 mg of tryptophan, an important biochemical precursor in the synthesis of the neurotransmitter serotonin, as well as the hormone melatonin.

Just for the sake of comparison, let’s check out the amino acid content of a starch: bananas. A banana actually contains all the amino acids, but in very small amounts: less than 100 mg each (a large banana, after all, provides just 1.5 grams of protein). As shown in Figure 9, though, the banana scores more than 30 grams of carbs, including over 6 grams of glucose and fructose, and even a little maltose.