Questions 1-11 are based on the following
passage.
This passage is excerpted from Rob Dunn, “Science Reveals Why Calorie Counts Are All Wrong.” © 2015 by Scientific American.
Food is energy for the body. Digestive enzymes in the
mouth, stomach and intestines break up complex food
molecules into simpler structures, such as sugars and amino
acids that travel through the bloodstream to all our tissues.
5 Our cells use the energy stored in the chemical bonds of
these simpler molecules to carry on business as usual. We
calculate the available energy in all foods with a unit known
as the food calorie, or kilocalorie-the amount of energy
required to heat one kilogram of water by one degree Celsius.
10 Fats provide approximately nine calories per gram, whereas
carbohydrates and proteins deliver just four. Fiber offers a
piddling two calories because enzymes in the human
digestive tract have great difficulty chopping it up into
smaller molecules.
15 Every calorie count on every food label you have ever
seen is based on these estimates or on modest derivations
thereof. Yet these approximations assume that the 19th-
century laboratory experiments on which they are based
accurately reflect how much energy different people with
20 different bodies derive from many different kinds of food.
New research has revealed that this assumption is, at best, far
too simplistic. To accurately calculate the total calories that
someone gets out of a given food, you would have to take
into account a dizzying array of factors, including whether
25 that food has evolved to survive digestion; how boiling,
baking, microwaving or flambéing a food changes its
structure and chemistry; how much energy the body expends
to break down different kinds of food; and the extent to
which the billions of bacteria in the gut aid human digestion
30 and, conversely, steal some calories for themselves.
Nutrition scientists are beginning to learn enough to
hypothetically improve calorie labels, but digestion turns out
to be such a fantastically complex and messy affair that we
will probably never derive a formula for an infallible calorie
35 count.
Consider how vegetables vary in their digestibility. We eat
the stems, leaves and roots of hundreds of different plants.
The walls of plant cells in the stems and leaves of some s
pecies are much tougher than those in other species. Even
40 within a single plant, the durability of cell walls can differ.
Older leaves tend to have sturdier cell walls than young ones.
Generally speaking, the weaker or more degraded the cell
walls in the plant material we eat, the more calories we get
from it. Cooking easily ruptures cells in, say, spinach and
45 zucchini, but cassava (Manihot esculenta) or Chinese water
chestnut (Eleocharis dulcis) is much more resistant. When
cell walls hold strong, foods hoard their precious calories and
pass through our body intact (think corn).
Some plant parts have evolved adaptations either to make
50 themselves more appetizing to animals or to evade digestion
altogether. Fruits and nuts first evolved in the Cretaceous
(between 145 and 65 million years ago), not long after
mammals were beginning to run between the legs of
dinosaurs. Evolution favored fruits that were both tasty and
55 easy to digest to better attract animals that could help plants
scatter seeds. It also favored nuts and seeds that were hard to
digest, however. After all, seeds and nuts need to survive the
guts of birds, bats, rodents and monkeys to spread the genes
they contain.
60 Even foods that have not evolved to survive digestion
differ markedly in their digestibility. Proteins may require as
much as five times more energy to digest as fats because our
enzymes must unravel the tightly wound strings of amino
acids from which proteins are built. Yet food labels do not
65 account for this expenditure. Some foods such as honey are
so readily used that our digestive system is hardly put to use.
They break down in our stomach and slip quickly across the
walls of our intestines into the bloodstream: game over.
Finally, some foods prompt the immune system to identify
70 and deal with any hitchhiking pathogens. No one has
seriously evaluated just how many calories this process
involves, but it is probably quite a few. A somewhat raw
piece of meat can harbor lots of potentially dangerous
microbes. Even if our immune system does not attack any of
75 the pathogens in our food, it still uses up energy to take the
first step of distinguishing friend from foe. This is not to
mention the potentially enormous calorie loss if a pathogen
in uncooked meat leads to illness.