To answer those questions, we need to look at how our bodies
process the foods we consume. While our bodies are generally quite efficient at
absorbing nutrients and energy from the foods we eat, it’s not 100% efficient.
Just because you put 2000 calories into your mouth doesn’t mean your body
absorbs all 2000 calories. How many calories your body actually absorbs depends
on many factors.
For example, the type, amount, and mix of foods you eat
affect nutrient absorption. Certain foods are digested and converted to energy
more readily while others can slow down the digestive process. The health of your
digestive tract can also matter. When you have an intestinal infection or
severe food poisoning, food can go through with little of it being absorbed. Even
the microorganisms in your gut can affect how much you can extract from your
food.
If you consider the ease with which your body digests and
absorbs nutrients from various “whole-grain” products, it becomes pretty clear
which ones can be healthier for you, and which ones are not much different from
refined flour products. Digestion takes place in many phases. We commonly think
of it starting in the mouth when we chew and mix saliva with the food. This
helps break down the food into smaller particles, giving the rest of the
digestive process more surface area to work on. However, if you think about it,
we often “pre-digest” our foods by preparing and cooking them. For instance, a
tough cut of meat can be made quite tender and easy to eat if tenderized or
cooked long enough.
It’s a similar story with whole grains. Left completely
intact, they are quite difficult for our body to digest since each grain has several
fibrous outer layers protecting it. If you don’t chew and break apart the
protective layers, the grains go through you relatively untouched. However, when
whole grains are ground into whole-grain flour, the fiber-rich outer layers are
broken up, exposing the readily-digested carbohydrates inside. White flour takes
this “pre-digestion” a step further and removes all of the fibrous parts,
leaving a fine powder of quickly-absorbed carbohydrates.
Since white flour is so refined, your body will readily
digest and absorb it as if you are eating sugar. This is why your blood sugar can
spike quickly when eating foods made primarily with white flour. Whole-grain
flour is essentially the same as white flour but with the ground-up fibrous
parts left in. The presence of fiber in the whole-grain flour may slow down
absorption slightly but not to a great extent. So, your body will also rapidly
absorb the carbohydrates and cause your blood sugar to spike. Neither option is
as favorable as eating intact whole grains. These take longer to digest and
therefore affect your blood sugar levels less quickly.