2.2 RDAs
The US public health authorities also publish Recommended
Dietary Allowances (RDAs) for each macronutrient (such as the RDA for
carbohydrates—130 grams per day for adults).
You’re probably familiar with the RDAs for vitamins and
minerals. They are essentially a suggested “minimal amount for
health.” For example, the RDA for carbs is based on the brain’s need
for glucose.
Note
Your brain accounts for around 5 percent of your
body weight, but grabs about 20 percent of your calories—mainly
glucose, but also ketones, a byproduct of metabolizing fats for
energy. That’s a very impressive, “greedy” detail of our metabolism;
if you take in 3,200 calories per day, your brain is getting about
640 calories.
Now we’re going to move into a discussion about each of the
three macronutrients.
Food molecules are like Legos. They are
small molecules bonded or stuck together to make more complex ones.
Starch, for example, is a big polymer or chain
containing many monomers, which are glucose or
sugar molecules. When you eat and digest these substances, the
digestive enzymes split apart the connections between the pieces and
separate them so that they can be absorbed into the body through the
small intestine.
The small intestine cannot absorb whole protein, carb polymer,
and fat molecules; they all have to be split up by digestive enzymes
first, so the substances you are actually absorbing are glucose and
other sugar monomers, amino acids (from proteins), and fatty acids
(from triglycerides, the storage form of fat).
Carbs are polymers of glucose molecules. Proteins are made up of
an often-complex configuration of amino acids. Fats, when you eat them
in food, are composed of a glycerol molecule attached to three
fatty acids in an “E” shape. These are the original Lego
structures.
We’ll start our more specific introduction to food chemistry
with carbs.
